Kernefrugt

Nematodproblemer og jordtræthed Forskningsleder i nematologi Christer Magnusson Planteforsk Plantevernet, Afd. Skadedyr, Høgskoleveien, N-1432 Ås, Norge Tlf. +47 7 64 94 92 88, e-mail: christer.magnusson@planteforsk.no

ABSTRACT Tired soil is characterised by poor growth of trees, reduced yields and premature death. Root parasitic nematodes may be an important factor causing tree deterioration. Nematodes capable of damaging fruit trees include root lesion nematodes (Pratylenchus spp.), ring nematodes (fam. Criconematidae), pin nematodes (Paratylenchus spp.), spiral nematodes (fam. Hoplolaimidae), dagger nematodes (Xiphinema spp.) and needle nematodes (Longidorus spp.). Nematodes are powerful parasites, but the damage may also involve interactions with micro-organisms (including virus) and abiotic factors. The nematode multiplication rate and the damaging threshold are important parameters in estimating the level of damage to be expected. The relationships between nematode numbers and damage to trees recorded in field studies in Norway demonstrate that there is a need to strengthen the focus on nematodes in Nordic fruit production. Successful control of nematodes relies on the optimal integration of plant hygiene, kind of plant material, type of cover crop and management strategies that recognise the importance of monitoring nematode populations. There is an urgent need to develop nematode controllants that pose no threat to man and the environment.

INTRODUCTION The problem of tired soil refers to a situation characterized by poor growth of trees, reduced yields and premature death. This complex condition contains at least two main disorders: a) The “specific replant problem” which refers to the difficulties in replanting an orchard site when the crop to be planted is closely related systematically to the preceding tree species, and b) The “non-specific replant problem” which is not confined to the preceding plant species. Nematodes have been implicated as causal factors in both situations, but have probably a more important etiological role in the latter “non-specific replant problem (Zehr 1979). Few studies have been done on nematodes as root pathogens of trees. Nematode groups containing important parasites of cherry, apple and pear are listed in Table 1. Root lesion nematodes (Pratylenchus spp.) are common in agricultural soil. The species P. penetrans is wide spread and is known to be a serious pest of fruit trees world wide and especially in cooler regions (Nyczepir & Halbrendt, 1993). P. penetrans has caused serious damage especially on apple and cherry trees (Mai & Parker 1967). The nematode attacks on feeder roots result in necroses and root mortality. These nematodes migrate freely between the soil and the root tissue. Apple roots attacked by root lesion nematodes may contain densities of more than 100 nematodes per gram of root (McElroy 1972). The cavities caused in the root tissue would enhance secondary infections by soil micro-organisms.

Table 1. Groups of plant parasitic nematodes recorded

as being important parasites of cherry, apple and pear (McElroy 1972, Nyczepir & Halbrendt, 1993, Kunz & Bertschinger 1998, Buser 1999)

NEMATODE GROUP CHERRY APPLE PEAR Root lesion nematodes

(Pratylenchus)

X X X

Ring nematodes (fam. Criconematidae)

X

Pin nematodes

(Paratylenchus)

X X

Spiral nematodes (fam. Hoplolaimidae)

X

Dagger nematodes (Xiphinema)

X* X** X

Needle nematodes (Longidorus)

X***

* Tomato ringspot virus, Cherry raspleaf virus ** Tomato ringspot virus, Cherry raspleaf virus *** Raspberry ringspot virus, Cherry rosett virus Other important nematodes on fruit trees are found among pin nematodes (Paratylenchus) and ring nematodes (fam. Criconematidae). According to our observations, spiral nematodes (fam. Hoplolaimidae) also seem to be involved in tree deterioration. Our observations relate to sweet cherry, but it cannot be excluded that other fruit trees could be damaged. Dagger nematodes (Xiphinema spp.) are reported to damage sweet cherry, apple and pear, while most reports on damage by needle nematodes (Longidorus spp.) concern cherry. In general, the first year after planting would be the most critical year for plants attacked by nematodes, because in this period those roots which later will become the frame roots of the tree develop (McElroy 1972). Attacks on new-planted trees may therefore have long lasting effects on the productivity (Mai et al. 1994). Nematode damage on fruit trees may have a complex etiology including interactions with micro-organisms. The ring nematode Criconemella xenoplax is well known for its capability to interact with the bacterium Pseudomonas syringae on peach (Lownsbery et al. 1973). There is also a strong suspicion that P. penetrans may be involved in similar interactions (Mai & Abawi 1978), and that replant disorders to a certain degree may involve nematode interactions with weak parasitic fungi and bacteria. The nematode genera Xipninema and Longidorus contain species, which are powerful parasites of fruit trees. The situation, however, is complicated further by the ability of these nematodes to transmit plant virus (Table 1), causing serious diseases on fruit trees. In Switzerland, Longidorus arthensis transmit the Cherry Rosette Virus (CRV) causing the highly destructive Rosette Disease of cherry (Kunz & Bertschinger 1998). Again in Switzerland, the Pfeffinger Disease of cherry, caused by the Raspberry Ringspot Virus (RRV) transmitted by the nematode Longidorus macrosoma (Buser 1999).

Nematodes may also influence the response of trees to abiotic factors. For cherry it has been demonstrated that plants attacked by P. penetrans are more vulnerable to cold or winter injury (Mai et al. 1994), and for peach, plants attacked by the ring nematode Criconemoides xenoplax have so high levels of the growth hormone indole-3-acetic acid (IAA) that dormancy is not induced properly, resulting in cold injury and winter-kill (Zehr 1979). The objectives of this paper are to give some basic information on the nematode plant relationship, to present two case studies from Norway on nematode involvement in growth disorders of fruit trees, and to consider some options for nematode control.

NEMATODES AND TREES As mentioned above, the nematode attack may change the entire physiology of the plant. Analyses on the relationships of fruit trees and root parasitic nematodes require information on the reproductive rate of a particular nematode species on the root stock, and its pathogenicity both to the rootstock and the cultivar. Some principles of this relationship are shown in Figure 1 and 2. Figure 1 shows the fundamental relationship between the initial nematode population density (Pi) and the final population density (Pf). The quotient Pf/Pi is the rate of multiplication, or the multiplication factor. For a plant that maintains the population (Fig.1, curve A) Pf/Pi=1. For a good host plant the relationship is different (Fig.1, curve B). In this case, increasing Pi levels will give higher Pf levels, resulting in Pf/Pi > 1. This is true until a point is reached where the nematode initial density is so high that the food source is damaged and any further increases in Pi will result in successively falling Pf levels. Ultimately a situation is reached with multiplication factors Pf/Pi < 1. For resistant plants increasing Pi levels always will give lower Pf levels and Pf/Pi<1. The relationship of Pi and Pf is affected by abiotic and biotic factors and this relationship is unique for each stand. The relationship between Pi and the relative yield is described in Figure 2. With increasing Pi levels, there will be a gradual decrease in the relative yield. At a certain point, i.e. ”the damaging threshold”, any increase in Pi will lead to a decrease in yield. As for the multiplication factor, the value for the damaging threshold will vary with abiotic and biotic conditions. Local conditions like soil type, soil temperatures, soil humidity, type of tree and age, as well as nematode species (Zepp & Szczygiel 1985) and pathotype (Zepp and Szczygiel 1986) are all expected to influence the position of the damaging threshold. Investigations on the damaging threshold for Pratylenchus penetrans on young apple trees have demonstrated damage to occur at Pi values of 38-75 ind./250 gram of soil (Zepp and Szczygiel 1986). It has also been demonstrated that apple and cherry are more susceptible than pear, which again is more susceptible than plum (Parker et al. 1966).

0

Pf (i

ncre

asin

g)

Nematode population dynamics

Pi (increasing)

A

B

C

Figure1. Relationships between initial population (Pi) and final population (Pf) host plants that maintain (A), increase (B) and reduce the nematode population.

Relative yield=(f)Pi

90

75

55

38

28

100 100 97

0

10

20

30

40

50

60

70

80

90

100

Increasing initial population Pi

Rel

ativ

e yi

eld Damaging treshold

Figure 2. The relationship between initial nematode population and the relative yield.

NORWEGIAN CASE STUDIES In general, good correlations between tree growth, yield and nematode numbers are difficult to demonstrate in the field. This is partly due to the long lifespan of trees and the rapid and changing dynamics of nematodes in time and space. The time of nematode damage to the root system normally precedes considerable in time the development of aboveground symptoms. Nematodes that cause severe damage to the root system may, as a consequence of destroying their food source, fall drastically in population density. In this case it would be impossible to relate nematode numbers to the damage observed without previous information on the population dynamics. In Norway several field studies have been conducted in fruit orchards. Two examples will be given on the involvement of plant parasitic in growth disorders. The material presented below will concern apple and sweet cherry. 1. Chemical control and growth responses in apple at Dømmesmoen. In 1980-1981 an experiment was conducted on the effect of chemical treatments on growth parameters of apple seedling rootstocks (unknown cultivar). Each treatment plot measured 5 x 4,2 m, and all registrations included 2 rows of plants 3,5 m long. All treatments were replicated 4 times. At the time of planting, the nematicide aldicarb (Temik 10G) was applied at a rate of 10 gram/m2, and the fungicides Benomyl (Benlate) and Quintozene (Brassicol 20) were applied at the rates of 20 and 40 gram/m2. All chemicals were applied as granulates and incorporated into the soil to a depth of 20 cm. The control was not treated. The experiment was initiated in May 1980. Budding was performed in June the same year. Nematodes were sampled in May and October 1980, and also in May and October 1981. A nematode sample was made up of 20 sub-samples taken with a soil auger between the plant rows. The samples were extracted with a Seinhorst elutriator (Seinhorst 1988), counted in a stereomicroscope, and when necessary more detailed studies were made under oil immersion in a compound research microscope. The data was tested with Students´ t-test.

Table 2. Changes recorded in the mean densities of root lesion

.

nematodes on seedling rootstocks of apple after pre-plant treatments of soil. Field trial at Dømmesmoen, Norway 1980-1981 (n=4). Numbers followed by similar capitals in columns indicate

a statistically significant difference (p≤0,05), similarly small letters show a statistical

tendency (0,10≥p>0,05). * denotes a statistically significant difference between

numbers in rows

Pi Pf Pi Pf ind./250g ind./250g ind./250g ind./250g soil soil soil soil

TREAT-MENT

1980 1980 1981 1981Control 341 398a 352 178* Temik 198 163B 190 333Benlate 231* 754aBc* 377 210Brassicol 240 320c 289 277

Data on nematode mean densities is presented in Table 2. At the time of planting there was no statistically significant difference in the initial nematode population (Pi) between treatments. In October 1980, however, differences were recorded between treatments in the final nematode population (Pf). Although, no statistically significant difference was present between the control treatment and the treatment with Temik the p-value (p=0,109) almost qualifies as a statistical tendency (0,10 ≥ p >0,05). Compared with the control, the nematode numbers in the treatment with Benlate tended (p=0,095) to be higher. This is also true when comparing Benlate with Brassicol (p=0,063). The only statistically significant difference in Pf values is the lower nematode density in treatments with Temik compared with Benlate (p=0.022). The only statistically significant increase in nematode densities from May to October 1980 occurred in the treatment with Benlate (p=0,041). The nematode multiplication factor is shown in Table 3. The data from single plots demonstrates the dramatic effect of Temik on the nematode multiplication rate in 1980. The treatment with Temik reduces the multiplication factor to values below 1 in 3 out of 4 plots. In the control, 3 out of 4 plots have multiplication factors larger then 1. This is also true of the Brassicol treatment. Benlate comes out as the treatment where nematodes in all plots have multiplication factors larger than 1. With regard to the mean multiplication rate there are, however, no statistically significant differences between the treatments. The mean value for Temik is the lowest, reflecting the nematode decrease in most of the single plots. Table 3. Changes recorded in the multiplication factor (Pf/Pi) of root lesion nematodes on seedling rootstocks of apple after pre-plant treatments of soil. Field trial at Dømmesmoen, Norway 1980-1981 (n=4).

TREATMENT Plot 1 1980

Plot 2 1980

Plot 31980

Plot 41980

MEAN 1981

Plot 11981

Plot 21981

Plot 3 1981

Plot 4 1981

MEAN 1981

Control 0,37 1,56 2,06 1,56 1,39 0,40 1,05 0,37 0,39 0,55 Temik 0,47 0,51 0,42 2,14 0,89 7,50 1,81 1,11 0,94 2,84 Benlate 3,26 1,37 8,90 2,94 4,12 0,73 0,82 0,31 0,63 0,62 Brassicol 0,74 2,38 2,89 1,19 1,80 0,45 2,15 0,84 0,82 1,07

There were no additional chemical treatments in 1981. In this year in the control treatment multiplication factors lower than 1 were recorded in 3 out of 4 plots, which indicates that the nematodes progressively destroy their food source. For the Temik treatment, 3 out of 4 plots have multiplication factors larger than 1, an indication of the limited duration of the controlling effect of Temik. In the Benlate treatment all plots show nematode multiplication factors smaller than 1. The strong reproduction in 1980 has most probably resulted in “over grassing” and a subsequent population decrease in 1981. The nematode multiplication rate in the treatment with Brassicol is also different in 1981, with 3 out of 4 plots showing nematode multiplication factors below 1. This nicely demonstrates the intensive dynamics of nematode populations in time. Data on some growth parameters is displayed in Table 4. The total tree growth for 1980 was measured in 1981, there is a statistically significant 61 % increase in tree growth in the treatment with Temik compared to the control (p=0,019). Trees in the Temik plots also show a statistically significant 51 % increased growth compared to trees in plots treated with Brassicol (p=0,036), and a tendency of better growth compared to trees in the Benlate treatment (p=0,098). There is also a tendency for trees treated with Benlate to grow better than trees in the control treatment (p=0,054).

Table 4. Some growth parameters of apple recorded after pre-plant treatments of soil. Field trial at Dømmesmoen, Norway 1980-1981

(n=4). Numbers followed by similar capitals in columns indicate a statistically significant difference (p≤0,05), similarly small letters show a statistical tendency (0,10≥p>0,05). .

TREATMENT TOTAL TREE GROWTH 1980 (cm)

TOP SHOOT GROWTH (cm) 1980

LIVING BUD SHOOTS % 1981

Control 103Ab 25Aa 7,4A Temik 166ABa 52ABC 49,7ABC Benlate 129ab 32aB 18,7B Brassicol 110B 27C 10,6C

For top shoot growth (Table 4) there is a similar pattern. Trees in plots treated with Temik show a 108% statistically significant increase compared to control (p=0,011). Trees in the Temik treatment also have statistically significantly higher top shoot growth than trees treated with Benlate (63%) (p=0,019) and Brassicol (93%) (p=0,007). Compared to the control, there is also a tendency for trees treated with Benlate to have higher top shoot growth (p=0,075). Living bud shoots from the 1980 budding were counted in 1981 (Table 4). For this parameter there is a strong statistically significant effect of the Temik treatment with 49,7 % living bud shoots compared to only 7,4 % in the control (p=0,001). The relative frequency of living bud shoots in the Temik treatment is also statistically significantly higher compared to the Benlate treatment (18,7 %) (p=0,013) and the treatment with Brassicol (10,6 %) (p=0,008). 2. Nematode distribution and growth of sweet cherry, Ullenvang Research Station In 1994-1995 the nematode distribution and tree growth in an 11 year-old orchard of sweet cherry (Prunus avium), cvs. Van, Kristin and Stella on the rootstocks GM61, GM69, Colt, P. avium and Sandøya, was investigated. In May 1994 nematode samples of 200-300 gram of soil were taken within a radius of 0,5 m around the stem of each individual tree and down to a depth of 20 cm. The nematodes were extracted from 50 gram of soil by the Seinhorst elutriator (Seinhorst 1988). The nematodes were counted in a stereomicroscope, relaxed in hot fixative, mounted on slides and identified further in a research microscope under oil immersion. In May 1994 and in May 1995 the growth of each tree was measured as the total length of 10 shoots. The orchard is situated on the western slope facing the Hardangerfjord (Sørfjorden). The distribution of root lesion nematodes (Pratylenchus spp.), spiral nematodes (genera Rotylenchus and Helicotylenchus) and ring nematodes (family Criconematidae) in May 1994 is shown in Figure 3. The northwestern corner of the orchard has high populations of root lesion nematodes (Figure 3A) and spiral nematodes (Figure 3B). There are also high densities of spiral nematodes in the northeastern quarter of the orchard (Figure 3B). Ring nematodes have a more sparse distribution with the highest densities in the first rows, and down hill in the northeastern quarter of the orchard (Figure 3C).

1 2 3 4 5 6 7 8

0500

1000

1500

2000

ind./250 g.soil

Root Lesion Nematodes 1994

N

A

1 2 3 4 5 6 7 8

0

200

400

600

ind./250 g.soil

Spiral Nematodes 1994

B

1 2 3 4 5 6 7 8

0

50

100

ind./250 g. soil

Ring Nematodes 1994

C

Figure 3. Distribution of some important groups of plant parasitic nematodes in an 11 year-old stand of sweet cherry (Prunus avium) at Ullensvang Research Station, Norway. A. Root lesion nematodes (Pratylenchus penetrans, P. fallax and P.crenatus). B. Spiral nematodes (genera Rotylenchus and Helicotylenchus). C. Ring nematodes (family Criconematidae).

12

34

56

78

0 5001000

cm

Growth 1993 length ot 10 shoots

N

12

34

56

78

0 5001000

cm

Growth 1994 length of 10 shoots

Figure 4. Growth in an 11 year-old stand of sweet cherry (Prunus avium), cvs. Van, Kristin and Stella on the root stocks GM61, GM69, Colt, P.avium and Sandøya, Ullensvang Research Station, Norway. A. Shoot growth in 1993. B. Shoot growth in 1994. Dead or dying plants are shown in black.

The growth of trees and mortality in 1993 and 1994 is shown in Figure 4. The measurements of the growth for 1993 were made in May 1994, and the situation is shown in Figure 4 A. The orchard has 76 trees out of which 12 are dead or dying. Nine of the dead or dying trees are situated in northwestern and the northeastern quarter of the orchard. These parts of the orchard also have high numbers of nematodes (Figure 3). In 1995 the situation has deteriorated further (Figure 4B). The orchard now has 58 trees, out of which 6 trees are dead or dying. Between 1994 and 1995 18 trees have been removed. The number of living trees has decreased from 64 in 1994 to 52 in 1995, a decrease with 19 %. In the northwestern corner two more trees have died or been removed in 1995. This may relate to the high Pi-values of root lesion nematodes in this area of the orchard (Figure 3A). In 1995 the whole upper eastern line of trees has been removed. In addition, 3 of 4 trees, that were dead or dying in 1994 in the northeastern quarter of the orchard, have also been cleared out. This is an area with high Pi-levels of spiral nematodes in 1994 (Figure 3B), but also ring nematodes occur in association with dead or dying trees (Figure 3C). In the southern tree row one additional tree has died in 1995, and this mortality is associated with the occurrence of ring nematodes (Figure 3C). Clearly, it is difficult to pinpoint the contribution of nematodes to deterioration and death of the individual trees in the field situation. The general pattern is, however, important and suggests an active involvement of several types of nematodes in tree death. This is further emphasised if growth classes is considered. The population of trees was divided into two classes, ex. trees showing a growth above and trees showing growth below the 1993 and 1994 average for the orchard. If the densities of root lesion nematodes of each tree in 1994 also is grouped into densities above and below 500 ind./250 gram of soil an interesting pattern emerges (Figure 5). For the growth in 1993, there is no difference between classes in the proportion of trees having nematode densities > 500 ind./250 gram soil (Figure 5 A and B). For the growth in 1994, however, 18 % of the trees in the growth class “below average” have nematode densities > 500 ind./250 gram soil, compared to 7 % of the trees in the growth class “above average” (Figure 5 C and D). As the nematode densities found in may 1994 influence the growth of 1994, it is suggested that for this orchard and for this year, root lesion nematode Pi levels of 500 or more ind./250 gram of soil could cause trees to grow less well than the average for the orchard.

Growth below mean 1993

88 %

12 %

<500>500

Growth above mean 19939 %

<500>500

Figuind./P. fachershooD) th

Conclusi The dataa degreegrowth, ttree growfungi. Thand areanematodfield situa There is productiosupportecherry ornematod

A

Growth below mean 1994

82 %

18 %

<500>500

re 5. Relative frequency of trees with high (>250 g. soil) population density of root lesion llax and P.crenatus) as measured in may 19ry (Prunus avium) at Ullensvang Research Sts/tree) is divided in the two growth classes e mean growth recorded for the years 1993

ons and comments on the case studies.

from the apple experiment demonstrate of control on the root lesion nematode phe top shoot growth, and the survival of bth in the one of the fungicide treatments

e data from the cherry tree orchard shows of tree deterioration seem to coincide ine densities above 500 ind./250 gram soil tion.

a need to strengthen the focus on plant pn also in the Nordic region. The results pd by a Norwegian team study on the causchards in the Lofthus area (Meland et ales to be an important causal factor in the

B

91 %

Growth above mean 1994

7 %

93 %

<500>500

C

D

500 ind./250 g. soil) and low (<500 nematodes (Pratylenchus penetrans, 94, in an 11 year-old stand of sweet tation, Norway. The growth (10 “below” (A and C) and “above” (B and and 1994.

that a treatment (Temik) that exerts opulation increases the total shoot ud shoots. The tendency of better suggests a possible involvement of s that tops of nematode density space, and that root lesion appear to decrease growth in this

arasitic nematodes in fruit resented here are further al factors of tree mortality in sweet

. 2001). This study indicates tree mortality.

CONTROL Chemical treatments against nematodes are getting increasingly uncommon, due to the concern for environmental and human safety. So, control strategies of tired soils and nematodes relying on pre-plant treatments like methyl-bromide fumigation need to be abandoned. In the future, nematode control in fruit production would require a focus on several key-areas simultaneously. Some important focal areas would be:

• Site surveys • Nematode free planting material • Plant material resistant or tolerant against nematodes. • Cover crops • Environment-friendly treatments

Site surveys Pre-planting surveys for nematodes would be an important standard tool in the preparation of an orchard site, and would be of equal importance as analyses of soil nutrients. In general, for fruit trees on sandy soils Pi values for P. penetrans of 63-100 ind./250 gram of soil at planting could lead to growth reductions. The corresponding value for clay soil would be 113-200 ind./250 gram of soil. (pers.comm. Schoemaker). Sites heavily infested with potentially harmful nematodes need to be fallowed. The time in fallow that is required to reduce the root lesion nematode P. penetrans is not exactly known. Any fallowing regime would need to include a thorough extraction of the root systems and keeping the soil free of vegetation.

Nematode free planting stock In the entire production chain for seedling rootstocks, measures need to be taken not to contaminate the plant material with nematodes. Root lesion nematodes are of special importance because they colonise the root system. Nurseries should only accept nematode free material, and locate the production on land sampled and found free from root lesion nematodes and other harmful root parasitic nematodes. It is also possible to clean nematode dormant planting stock infested with root lesion nematode by hot-water dips at 46,1-46,7 oC for 30 minutes (apple) and at 50,0-51,1 oC for 10 minutes (cherry) (McElroy 1972).

Plant material resistant or tolerant against nematodes It is known that rootstocks vary in their host status for nematodes (Hoestra and Oostenbrink 1962, Nyczepir & Halbrendt 1993), but so far there are no commercial rootstocks of apple and pear that are resistant to root lesion nematodes (Nyczepir & Halbrendt 1993). Systematic screenings for resistance and/or tolerance need to be intensified, but is complicated by the variation in virulence occurring within certain species of nematodes, as was demonstrated by Zepp and Szczygiel (1986) for the root lesion nematode P. penetrans.

Cover crops Like in natural forest, the cover vegetation under trees may have crucial importance for the densities of plant parasitic nematodes (Magnusson 1983). This is certainly also true for orchard sites. In view of the more than 350 plant species recorded as host for P. penetrans (Corbett 1973), it would be important to identify and use cover crops, which are poor hosts for the nematode. In green house experiments (MacDonald & Mai 1963) Sudan grass (Sorghum vulgare var. sudanense), Perennial rye grass (Lolium perrenne), Domestic rye grass (L. multiflorum) and Brom grass (Bromus inermis cv. Saratoga) were classified poor host plants for P.penetrans. Kauri-Pääsuke (1973) reported Italian rye grass (Lolium italicum), Westerworlds rye grass (Lolium rigidum), Poa annua and Stellaria media as poor hosts for P. penetrans. Marigolds (Tagetes erecta) are well known for their nematicidal action against P. penetrans. Preliminary observations in some Norwegian crop systems also indicate Brom grass as negative for root lesion nematodes (Planteforsk unpubl. data). It is conceivable that the effect of cover crops could vary with locality. The optimal use of cover crops against nematodes also seems to require information on optimal pruning regimes, as MacDonald & Mai (1963) demonstrated the pruning frequency of cover crop species to affect the host status for P. penetrans. Environment-friendly treatments. The use of plant preparations to treat human and animal diseases dates back thousands of years (Porter & Fox 1993), but the recognition of plant-derived products as deterrents or toxins against plant parasitic nematodes is very recent. The discoveries of nematicidal thienyl-derivatives from Tagetes and the benzofurans from Helenium were made in 1958 and 1971 respectively (Gommers 1981). Some natural substances like asparagusic acid (from Asparagus officinale), Furfural (from agricultural residues), Avermectin (from Streptomyces spp), Fenazin (from Pseudomonas spp.) and Ricin (from Ricinus communis) all have been reported to interfere or have toxic effects on plant parasitic nematodes (Gommers 1981, Stirling 1991, Rodriguez-Kabana et al 1993). The leaves and seeds of the neem tree Azadirachta indica, are well known to have a nematicidal activity (Mojumder 1995). Oil cakes from neem, castor bean (Ricinus communis), sesame (Sesamum indicum), mahuva (Madhuca indica), rape seed (Brassica campestris), mustard etc., frequently are used as soil amendments against plant parasitic nematodes. This is often carried out in combination with other measures of integrated pest control. Also seaweed extracts have a potential as nematode controllants (Whapham et al 1994). It can be concluded that much work remains to be done in the area of integrated pest management of nematodes in fruit orchards. The success of the control relies entirely on the optimal integration hygiene, kind of plant material, type of cover crop, and management strategies that recognise the need for a regular monitoring of nematode populations. There is also an urgent need to develop nematode controllants that pose no threat to man and the environment.

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plant parasitic nematodes with furfural - a naturally occurring fumigant. Nematropica 23: 63-73.

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Zepp, A. L. & Szczygiel, A. 1985. Pathogenicity of Pratylenchus crenatus and P. neglectus (Pratylenchidae, Nematoda) to three fruit tree seedling rootstocks. Fruit Science Reports XII: 109-117.

Zepp, A. L. & Szczygiel, A. 1986. Pathogenicity of five populations of Pratylenchus penetrans to three fruit tree seedling rootstocks. Zeszyty Problemowe Postepow Nauk Rolniczych z. 323: 73-89.

Optimum pomefruit plant material, which quality can the growers expect from nurseries? (Optimalt plantemateriale, herunder hvilke kvalitetskrav der bør stilles til planteskolerne) Planteskoleejer Koen Carolus, B.V.B.A. Carolus C, Heuvelstratt 50, B-3850 Nieuwerkerken, Belgien Tlf. +32 1168 8701, e-mail: info@carolustrees.com

Hvordan etableres en optimal plantage? Forsøgsleder Birgitte A. Pedersen, Fejø Forsøgsplantage A.m.b.A., Slettervej 27, 4944 Fejø, Danmark Tlf. +45 5471 3466, e-mail: Fejoe.Forsoegsplantage@get2net.dk Forudsætningerne skal være i orden Forud for plantning af en æbleplantage skal flere ting være i orden. Lokaliteten skal være velvalgt, læforholdene være i orden, grundgødskningen være gennemført, flerårigt ukrudt være behandlet samt forberedelser vedr. evt. drypvanding være foretaget. Dertil kommer beslutninger når der skal vælges sorter, planteafstand og støttesystem. Gennem mange år er der i Danmark blevet anvendt trykimprægnerede pæle til kernefrugt. Og i en række år gav pælene træerne tilfredsstillende støtte, men de seneste år har mange frugtavlere brugt mange timer på at udskifte knækkede pæle med nye pæle. Flere avlere er samtidig blevet interesserede i at få højere træer, dels for at opnå en bedre balance i træernes vækst dels for at opnå et merudbytte. Et alternativ til de trykimprægnerede pæle er valg af et støttestativ. Støttestativer kan etableres på mange måder, men de består ofte af kraftige pæle placeret med 6-8 meters mellemrum og forbundet med 2-3 tråde, hvorpå der fastgøres tynde stokke, der bruges til støtte for det enkelte frugttræ. Hvorvidt et støttestativ prismæssigt er konkurrencedygtigt med brugen af trykimprægnerede pæle afhænger i høj grad af, hvor mange træer der støttes pr. bærende pæl. Altså bedre rentabilitet ved lille planteafstand. I Danmark er der endnu kun få års erfaringer med støttestativer. Nogle erfaringer fra Fejø Forsøgsplantage vil blive omtalt her. Overvejelser ved valg af stativ Støttepæle Der findes en række brugbare pæletyper. Det er bl.a. vigtigt at pælene er stabile, findes i passende dimensioner og kan skaffes til en rimelig pris. Som rettesnor skal pælene 0,9-1 m i jorden og min. 2 m over jorden. Ved brug af azobépæle bør dimensionen mindst være 5cm x 5cm x 300cm. Gerne med kraftigere og evt. længere pæle som endepæle. Afstand mellem de bærende pæle Afstanden mellem de bærende pæle anbefales at være omkring 6-7 meter. Prismæssigt er det naturligvis attraktivt med så stor afstand mellem pælene som muligt – men stor afstand går ud over stabiliteten. Tråd Flere typer kan vælges men brudstyrken skal være høj og elasticiteten lav. 2,8 mm Crapal wire er en alu-belagt tråd, der er meget lidt elastisk og dermed behøver tråden ikke strammes op særlig tit. En tråd placeres så højt som muligt på pælene, tråd nr. 2 kan placeres ca. 1 meter under øverste tråd. I de fleste systemer er det tilstrækkeligt med 2 tråde. Det kan være relevant med en ekstra tråd til montering af drypslanger eller ved stativer til særligt høje træer. Montering af tråd

Trådene skal fastgøres på pælene enten ved brug af klemmer eller store bøjler der holder tråden fast på pælene. På træpæle kan der bruges øskner som tråden trækkes igennem eller der kan bores huller til tråden i pælene. Ved trækning af tråde sparer en trådvinde meget tid og besvær (pris ca. 350 kr.). En god tang kan ligeledes lette arbejdet meget. Valg af trådstrammere Der findes flere typer af trådstrammere. Et hjul der sættes midt på den hele tråd eller Gripple samleled, der strammer over to trådender. Gripple-samleled kræver en Gripple tang der koster ca. 355 kr. Bruges betonpæle, fastgøres trådene ved endepælene i en særlig trådstrammer. Valg af stokke ved træerne Bambus er mest velegnet til pæretræer på grund af pærernes meget stabile og oprette vækst. Ved brug af bankirai-stokke bør stokke med afrundede kanter eller helt runde stokke foretrækkes. Placering af pæle Afstanden mellem de bærende pæle bør vælges så den passer med planteafstanden og det forventede udbytteniveau. Eksempelvis kan 10 ’Discovery’ træer plantet med 75 cm afstand stå mellem de bærende pæle (dvs. 7,5 m) mens de bærende pæle ved en frugtbar sort som ’Jonagold’ bør stå tættere. De yderste træer i rækkerne kan plantes ved enkelt -pæle så de beskytter jordanker + tråde. Den første pæl bør have hældning væk fra rækken ud mod forland. Placering af jordanker Jordankeret skal placeres med samme vinkel som den øverste tråd kommer ned til jorden i. Der skal være ca. 2 m mellem endepæl og jordanker. Ved at benytte en separat tråd fra endepæl til jordanker mindskes risikoen for større skade hvis de skrå tråde beskadiges. Opsætning af stokke Inden stokkene sættes, skal trådene strammes op. Vær opmærksom på, at når stokkene sættes bruger de lidt tråd fordi stebofixen bøjer tråden lidt. Stokken sættes det ønskede stykke i jorden og gøres fast på trådene. Stebofix er en metalbøjle der kan fiksere stokke på metaltrådene. De findes i flere størrelser, hvor den mest almindelige er nr. 1 (16-26 mm stokke). Det kræver en form for hjælpeværktøj at sætte Stebofix på. Plantning Der er flere brugbare arbejdsgange: Plantning før der trækkes tråd, giver god plads til plantearbejdet, men træerne står let i vejen når tråden skal monteres. Plantning når der er trukket tråd og inden der sættes stokke er også en mulighed, men plantning efter der er trukket tråd og monteret stokke giver den mest præcise plantning. Træer og stokke sættes på samme side af tråden. På Fejø Forsøgsplantage sættes begge dele vest for tråden, når der kommer vind, presses træ og stok ind mod tråden og ikke væk fra den. Vælg en ensartet plantedybde for at sikre en ensartet vækst. Træer der ligger klar til plantning skal beskyttes så rødderne ikke tørrer ud. Brug eksempelvis en vintermåtte eller et stykke plastic til at lægge omkring.

Etableringsudgifter Materiale til støttesystem på 1 ha plantet på 0,8 m x 3,25 m 3846 træer. Priseksempel beregnet på 15 rækker a 250 + 6 træer og forventet træhøjde 2,6 m. Materialer til 1 række Dimension Antal Ca. stk. pris Ca. pris

Endepæle 6x6x325 cm 2 Stk. 45 Kr. 90 Kr.Pæle 5x5x300 cm 32 Stk. 30 Kr. 960 Kr.Jordanker 14cmx100cm 2 Stk. 22 Kr. 44 Kr.Tråd alu-belagt 2,8 mm dia, 500 m /rl. 420 m 0,5 Kr. 210 Kr.Stokke- bankirai 22mmx22mmx270cm 250 Stk. 9 Kr. 2250 Kr.Tilbinding stok Stebofix nr. 1 500 Stk. 0,2 Kr. 100 Kr.Trådstrammere 2 Stk. 22 Kr. 44 Kr.Tilbinding træ Gummislange 512 Stk. 0,2 Kr. 102 Kr.Enkelt pæle 6 Stk. 15 Kr. 90 Kr. 3890 Kr. 1 ha =15 rækker 15 rk. x 3.890 kr. 58.350 Kr.Støtte til 1 træ 58.350 kr. / 3846 træer 15.17 Kr.(33 mellemrum à 6 meter = 198 meter). Der er ikke taget højde for engangsudgifter til trådvinde, Gripple-tang eller special bidetang. Afrunding Der er stadig fordele ved at anvende trykimprægnerede pæle til kernefrugt, først og fremmest fordi det er enklere at bestille og etablere. Sætning af enkeltpæle vil for de flestes vedkommende være det system der er lettest og hurtigst at etablere. Fordelene ved støttestativ ligger i muligheden for at støtte træer højere end 2,25 m og for et mere stabilt system. Desuden er der ved tættere plantning mulighed for at bringe etableringsomkostningerne ned under niveauet for brug af enkeltpæle. Dertil giver de fleste bærende pæle mulighed for genanvendelse herunder især azobé- og betonpæle. Som ulemper kan det nævnes, at betonpæle udmærker sig ved en ensartet kvalitet, men er meget dyre og tunge at håndtere. Azobé-materialer kan give ubehagelige splinter med kraftig allergisk reaktion i huden. Både for de creosot- og trykimprægnerede pæle bør man imødese udgifter i forbindelse med bortskaffelse af materialerne.

Thinning, main points from trials and other experiences from Altesland (Udtynding – uddrag af mange års forsøg I Altesland) Researcher M. Clever Obstbauversuchs und Beratungszentrup, Moorende 53, D-21635 Jork, Niedersachsen, Tyskland Tlf. +49 416 260 16 111, e-mail: Clever.Michael@Lawikhan.de Summery At the Lower Elbe thinning is an essential task in the apple cultivation. Because more than one third of the area is covered with Elstar and Boskoop the biennial bearing fight and protection is taken an important part. Trials of the last 15 years have shown that at the Lower Elbe by Elstar and Boskoop with strong biennial bearing in the year of high bloom 0.5 to 1.0 l Ethephon/ha/mKh are necessary. Therefore the fruit amount is reduced around 50 % and the harvest around one third. In other areas in Germany and foreign countries such high amounts of Ethephon are not common. In Elstar and Boskoop orchards with heavy bloom but no or less biennial bearing before, 0.15 – 0.3 l Ethephon/ha/mKh are in most cases successful. Because the temperature at lower Elbe in spring often is too low for a chemical thinning the advising is to thin between begin of bloom and end of full bloom when temperature is good (> 15°C). For a better fruit quality a thinning with 1 to 3 times 1.0% ATS or 3.0 to 5.0% urea is advised. In this case an additional hand thinning is necessary. For Varity’s with small fruit size (Cox, Gala) an additional early hand thinning gives the best results. NAA doesn’t have a licence admission in Germany right now but would be desired. Young orchards shouldn’t be thinned with Ethephon. Here applications with ATS are advised. If an biennial bearing is possible a one time hand thinning of blooms can help. In the year of planting a thinning with ATS can reduce the number of fruits and so decrease a better growing (brunch length), which is wished.

Thinning, main points from trials and other experiences from Altesland (Udtynding – uddrag af mange års forsøg I Altesland) 1. Titel 2. Structure 3. Organisation Chart 4. Map 5. Apple area 6. Etephon trials 7. At the beginning something fundamental about biennial bearing. Here you can see a trial with the variety Elstar with a heavy bloom. The trees were thinned differently heavy at flowering time. The untreated control brought 400 fruits by harvest. This is equivalent to 24 Kg/tree. In the following year these trees showed a flowering index of 1.9. Flower index 1 = no bloom 9 = full bloom 6 = optimal 8. During flowering time some trees were just thinned in a way that the bearing was 330 fruits equivalent to 22 Kg/tree. These trees gained a flowering index of 3.5. 9. Other trees were thinned even heavier and showed 215 fruits by harvest. This is a reduction of 50 %. The yield of these trees was 16 Kg. This is a reduction of 33 %. A flower index of 5,5 was reached here. 10. A more extremely thinning reduced the number of fruits below 50 %. Now the yield decreased higher. The next year the flowering of these trees was higher than 6.0 and they bore a full yield. If the aim of the thinning is the prevention of biennial bearing you have to thin so heavy during flowering that the number of fruits at harvest will be cut in half. In this case the yield will be reduced by 30 %. With the end of bloom the effect of the flower induction is decreased significant. After that point the differentiation can only be influenced. 11. Thinnigtrial Elstar 1995 Control reached 40 – 45 Kg equivalent to 60 t/ha NAAm no effect 3 % KS (limesalpeter) no effect 0,5 l Ethephon in full bloom no effect 1,0 l Ethephon showed a small decrease 2,0 l Ethephon decreased the yield by 30 % equivalent to 30 Kg 0,25 l Ethephon beginning of June equivalent to 1,0 l in full bloom 0,5 l Ethephon beginning of June equivalent to 2,0 l in full bloom

12. Fruit size in control was 67,5 mm KS the same NAAm plus 1,5 mm (almost always) With rising amount of Ethephon the fruit size is improved, at 2,0 l we gain 6.0 mm. Ethephon in June doesn’t make a better fruit size than Ethephon in bloom. 13. Flower index in 1996 in control 3.3 equivalent to a middle biennial bearing. KS, NAAm and 0.5 l Ethephon showed no significant effect 1.0 l was enough for a reasonable flower index. 0.25 Ethephon in June reached with 5.3 more than the control but less than 1.0 l during flower with 6.0 14. The shown trial of 2001 we visited together in spring 2002. Control reached 40 Kg equivalent to 80 t 0.5 l Ethephon no effect 1.0 l and 1.5 l reached with 30 Kg the desired yield. 0.3 l Ethephon in June hardly any effect 15. Fruit size in control 67 mm 1 and 1.5 l reached a 4 – 5 mm better fruit size 16. Flower index in control was 2.9 (middle to heavy biennial bearing) 0.5 and 1.0 l weren’t sufficient enough 1.5 l reached with 5.0 just enough 0.3 l Ethephon in June brought no effect 17. Thinning with urea, ATS and NAA in order to increase the fruit quality. In this case a good thinner should decrease yield by 20 %. This is equal to a fruit reduction of 30 %. A additional hand thinning should always happen. 18. Thinning with urea (summary of different trials over 3 years) The influence of rain is shown after the application of urea. You can recognise a 15 % yield reduction by the application of urea zero to two days before rain. If rain occurs only after 3 days or later hardly any yield influence is obtained. The same effect happens with fruit size. Urea generally works unsecured. 19. Results with ATS (COX 1998) Control 27 Kg, 1.0 % ATS at begin of bloom reduced the yield to 22 Kg (optimal) 1.0 % ATS in the middle and at the end of bloom had no effect 20. Fruit size was improved adequate to yield reduction.

21. Trial with ATS by Gala Must 1999 The control showed more than 200 fruits ( a good thinning should reduce number of fruits by 30 %) 3 x 1.0 % ATS gained such a result Decrease of ATS showed at 1.75 % a good result In other years 1.5 % have been too much The hand thinning reached 80 fruits equivalent to 13 Kg which equals 33 t. 22. Fruit size in control was at 65 mm, 3 x ATS reached 71 mm, hand thinning was with 76 mm too strong. 23. Results with NAA (Promoxon) (Elstar 2001) Control reached 40 Kg equivalent to 80 t NAA between 0.02 and 0.08% were not successful 24. The fruit size was only improved by 2.0 mm. 25. Therefore in 2002 the concentration was set higher. Control reached 32 Kg equivalent to 40 t. A higher concentration of 0,15% reduced the yield to the desired amount. 2.0 l Ethephon was responsible for a reduction of 30 %. 26. The fruit size was „increased“ from 83 to 86 mm, with Ethephon even 87 mm. Here the control was optimal, which is a problem of the user. The correct judgement of the orchard is necessary for a successful thinning. In 2002 we got an bad fruit setting and so in many cases thinning was not good. A other reason are the good fruit growing in 2002 which brought 2 – 3 mm better fruit size than in normal years. 27.

Advisement for Lower Elbe 2003

Kan man med vækstregulering, drypvanding og gødskning påvirke udbytte, blad- og frugtkvalitet samt vekselbæring i positiv retning? Kernefrugtskonsulent Lene Baarts Frugt og Grønt Rådgivningen, Industrivej 31 C, 4230 Skælskør, Danmark Tlf. +45 5816 0998, e-mail: lba@skl.dk Kvalitet står før kvantitet i tidens diskussioner om frugtavl, og det er tydeligt at indkøberne af frugt lægger mere og mere vægt på håndfaste krav om kvalitet af frugten. I konkurrencen med den udenlandske frugt er det et punkt vi bør ofre mere opmærksomhed. Frugtavleren kan gøre en del i plantagen for at fremme både udbytte og kvalitet. Det starter allerede med indkøb af træerne, fortsætter med formningen af dem, men også dyrkningsmæssige tiltag som kemisk udtynding, væksthæmning og gødningstilførsel på det rigtige tidspunkt er vigtige faktorer. Det er vigtigt for kvaliteten af frugten at de får luft og lys og at der er et tilstrækkeligt areal med fotosynteseaktive blade passende til antallet af frugterne i træet. Det har indflydelse på frugtens kvalitet i form af bedre dækfarve, højere tørstofindhold, højere sukkerindhold samt på frugtens størrelse og smag. Kvalitetskriterier som betyder meget for frasorteringsresultatet både i plantagen og på frugtlagrene. For at opnå dette i praksis ved nyetablering af en plantage skal man starte med træer af god kvalitet – det gør beskæring og vækstregulering nemmere! I en etableret plantage beskæres efter at få et træ som er slankt i toppen og som har en krans af gode, produktive basisgrene. Vigtigt er det at der beskæres under hensyntagen til træernes vækstniveau. I kraftigt voksende træer lægges der vægt på, at der foretages få, store klip frem for mange små klip. Vækstregulering Udover at tilpasse beskæringen til antallet af blomsterknopper og med hensyntagen til om der er vekselbæring er rodbeskæring er blevet en fast del af moderne dyrkningssystemer i kernefrugt, og kan med fordel bruges i træer i vekselbæring. I år hvor vekselbærende træer har stor blomstring går væksten sent i gang og 1 års skud vokser ofte langt ind i sæsonen. De afsluttes for sent til at endeknoppen kan nå at udvikle sig til en blomsterknop. Samtidig bruger træet mange ressourcer på blomsterknopperne og på frugtudviklingen. Strategien til at få gode blomsterknopper i skuddenes endeknopper involverer såvel gødningstildeling som vækstregulering og udtynding. I praksis vil det sige at man skal holde øje med 1.års skuddene, så man får stoppet dem i tide. Der skal ikke rodbeskæres hårdt i marts, for det er ønskeligt at der sker en vis vækst i skuddene for at fremme junifaldet. Hen på sommeren er det vigtigste at man får stoppet skuddene tidsnok til at de danner blomsterknop – det vil sige ofte i juli måned, når skuddene har nået en længde på en 10-15 cm. Gødningsstrategien går på at man giver træerne gødning efter behov. Man deler gødskningen op i to dele, en basisgødskning i marts og en sommergødskning som vurderes efter vækst, frugtmængde samt bladenes indhold af næringsstoffer. Det indebær at man tager en tidlig bladanalyse fra tidligt afsluttede årsskud.

Samtidig er det vigtigt at være opmærksom på at træerne ikke bliver for tørkestressede som følge af rodbeskæringen. Drypvanding er en god måde at sikre at træerne kan rodbeskæres i den grad de har behov for det. Hvis træerne i forsommeren bliver tørkestressede og af den grund afslutter væksten for tidligt, er der en større chance for at der kommer genvækst. Dermed vil antallet af blomsterknopper blive mindre. Noget tørkestress hen på sommeren for at få afsluttet kraftig vækst er ønskelig, men det er en fin balance og man bør have mulighed for at måle jordens vandindhold gennem sæsonen for at styre drypvandingen. Bladkvalitet og bladgødskning Bladkvaliteten en forudsætning for at der er ressourcer nok i træet til både at bære frugten og sætte nye stærke blomsterknopper til sæsonen efter. Den sikres ved at forsyne træerne optimalt med næringsstoffer. Udover det normale gødningsprogram, som gives over jorden, er det vigtigt at sørge for en hurtigtvirkende forsyning med mikronæringsstoffer. Træerne forsynes ved adskillige bladgødskninger gennem sæsonen, baseret på behov ifølge den ovenfornævnte bladanalyse. I praksis er der en barriere for den optimale forsyning af bladene ved bladgødskning i at der skal køres mange gange med sprøjten gennem sæsonen. Vi ved ikke meget om hvad der sker når man blander flere gødningsstoffer, ligesom der mangler viden om hvordan blandinger af gødningsstoffer med pesticider virker. Selv om det kan blandes i tanken og kan sprøjtes ud uden at det volder problemer er det ikke ensbetydende med at pesticiderne er lige så effektivt i blandingen, som hvis de blev sprøjtet ud separat. Udtynding Udtyndingen er første forudsætning for at træerne ikke kommer i vekselbæring. Udtyndingen skal ske så tidligt som muligt, dvs. kemisk udtynding kan ikke undværes. Men det er også vigtigt at der efterfølgende bliver håndudtyndet i tide hvor dette er nødvendigt. Det kan være svært at finde grænsen for hvor meget træet kan bære uden at det går ud over næste års høst. Nu hvor vi lokalt har store problemer med spindemider kan det være nødvendigt at udtynde igen i løbet af sæsonen hvis bladkvaliteten bliver for ringe i forhold til antallet af frugter på træet. Specielt i Jonagold kloner er det observeret at frugtfarven lider under for mange frugter til for lille bladareal pr frugt og/eller for dårlig bladkvalitet. Det gælder altså om: • At foretage en så god som mulig kemisk udtynding • At foretage en tidlig håndudtynding • Ikke at have flere frugter på træet end det kan bære uden at det går ud over

kvaliteten/går i vekselbæring • At holde øje med bladkvaliteten gennem sæsonen og evt. foretage en håndudtynding

mere Den enkelte frugtavler kan altså reelt gøre en del for at påvirke udbytte og kvalitet i sin plantage, men det kræver en meget målrettet indsats at følge et dyrkningskoncept så konsekvent at det får en mærkbar effekt.

Forskelle mellem dyrkningssystemer Præsentation af resultater fra dyrkningssystemerne i forsøgene på Fejø Forsker ph.d. Marianne Bertelsen Danmarks JordbrugsForskning, Forskningscenter Årslev, Forskergruppe for Frugt og Bær, Kirstinebjergvej 10, Postboks 102, 5792 Årslev, Danmark Tlf. +45 6390 4163, e-mail: marianne.bertelsen@agrsci.dk På Fejø Forsøgsplantage blev der i midten af 1990’erne plantet en række forsøg med forskellige dyrkningssystemer til kernefrugt. De foreløbige udbyttemæssige og økonomiske resultater af 2 af disse forsøg vil blive præsenteret. V-system æbler I efteråret 1998 blev der på Fejø forsøgsplantage plantet et forsøg med tætplantning af æbler i V-system. 7 forskellige æblesorter afprøves på planteafstandene 0.25, 0.50, 0.75 og 1.0 m . Sorterne er valgt så der både indgår kraftigt voksende sorter som fx Holsteiner Cox og Rød Gråsten og svagtvoksende sorter som Pigeon. Delcorf er med for at teste hvordan de intensive plantesystemer påvirker sorter, som modner uens. Derudover indgår flere nye sorter, hvis dyrkningspotentiale vi endnu ikke kender under danske forhold, det drejer sig om sorterne Gala, Topaz og Santana. Efter det 3. høstår, er der i gennemsnit af sorter høstet 20% mere ved at gå fra 1 m planteafstand til 0.75 m, godt 50% mere ved at gå ned på en 0.5 m afstand, mens udbyttet er godt fordoblet på den tætteste afstand. En af betænkelighederne ved især den tætteste planteafstand, er om frugtstørrelsen og især frugtfarven påvirkes negativt. Disse betænkeligheder er til dels blevet bekræftet, idet andelen af store velfarvede frugter falder, men det er dog stadig kun undtagelsesvis, at der totalt set høstes færre store og velfarvede frugter ved den tætte afstand. I 2002 var frugtstørrelse generelt så stor at det tilmed var fordelagtigt med den generelt mindre frugtstørrelse i de tætte plantninger. Udbyttemæssigt hører de 3 nye sorter til blandt topscorerne, mens Rød Gråsten og Pigeon har givet de ringeste udbytter. Sammenligning af V-system og spindeltræer i Clara Frijs. I 1996 blev der på Fejø forsøgsplantage plantet forsøg med Clara Frijs i 2 forskellige V-systemer på planteafstanden 1.0, 1.5 og 2 m. Formålet var at sammenligne V-systemerne med almindelige spindeltræer, da der i udenlandske forsøg har kunne påvises store produktivitetsstigninger ved V-systemer. Første nævneværdige udbytte på i gennemsnit 15 tons/ha blev opnået i 2000, 4 år efter træerne blev plantet. I det første udbytteår var der ingen forskel mellem dyrkningssystemer, men udbyttet fra træerne på 1 m afstand var 60% højere (18 versus 11 tons/ha). I andet høstår gik produktionen på spindeltræerne lidt tilbage, mens V-systemerne nåede op på en produktion på i gennemsnit 25 tons/ha. Tredje høstår (2002) gav en produktionsforøgelse i spindeltræerne til 25 tons/ha, mens V-systemerne nåede op på mellem 30 og 40 tons/ha.

Planteafstanden 1 m har over de 3 produktionsår givet det højeste udbytte for alle dyrkningssystemer, men relativt set har plantetætheden haft størst udbyttemæssig betydning for spindeltræerne. Til gengæld er det også i spindeltræerne i den tætteste plantning der har haft den største negative påvirkning af frugtstørrelsen, i gennemsnit er frugtstørrelsen reduceret med knapt 20%. I V-systemerne har vi ikke med sikkerhed kunne påvise en negativ effekt af plantetætheden på frugtstørrelsen. I de to år med væsentligt højere udbytter i V-systemerne var frugtstørrelsen gennemsnitlig mindre i disse systemer end det var tilfældet for frugt fra spindeltræene. Det skyldes den langt større frugtbelastning på træerne, men totalt set kunne der høstes lige så mange store frugter (>65mm) i disse systemer som der kunne i spindeltræerne