gwr 074downymildewqa pdf r3
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wrdcccGRAPE AND WINE RESEARCH ANDDEVELOPMENT CORPORATION
Downy MildewQuestions and Answers
3 December 2010
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Q 1.: How do I know i the spots I see in myvineyard are downy mildew oilspots?
Downy mildew produces golden-yellow, oily spots on leaves,
which are best seen on the top side o the leaves. When young
(less than say 45 days old), the spots are oten surrounded by
a chocolate halo, which ades as the oilspot ages. The youngest
spots rst appear as circular spots 810mm in diameter. In
susceptible young leaves they grow rapidly bigger, reaching
2030 mm in diameter in 1014 days. (For more detail on
distinguishing downy rom other symptoms, go to the Disease
Diagnosis module at www.GrowCare.com.au).
Q 2.: What is the white down o downy mildew?
It is the sporangia (spores) o the disease. They are produced
only on the undersides o oilspots and on inected green tissue
but then only when the conditions are right during a warm,
humid night. These spores can spread downy at very rapid rates.
Q 3.: I get downy and powdery mildewconused. Is there a test to tell i an oilspotis downy mildew and i it is active?
Yes, do the bag test. Lightly moisten a sealable plastic bag by
adding a little water then shaking the bag vigorously to spread
the water evenly around the inside o the bag. Then up-end the
bag to drain out any excess water. Place the leaves with suspect
oilspots or inected bunch material (it is best to take the sample
in the aternoon) in the bag. Make sure that the undersides o
the leaves are acing upwards and are not wet. Seal the bag and
incubate it at 20250C in the dark - a kitchen cupboard is a good
spot. In the morning, active downy mildew tissue will show resh
white down on the undersides o the oilspots, on the surace o
bunch stems and on berries smaller than pea-size
(57 mm diameter).
Identifying the disease
Q1
Oilspots are circular and light yellow when young
Q3
Active viable downy mildew oilspots will show the characteris tic resh
white down in the morning ater the bag test.
(Photo: Richard Hamilton)
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Q 4.: What is the dierence between downyprimary and secondary inections?
A primary inection occurs when the downy spores move rom the
soil to the vine. It produces a ew oilspots in the oliage usually
about 13 oilspots in every 50 metres o canopy row.
Secondary inection occurs when downy spreads rom oilspots in
the canopy. Spores move rom lea to lea or lea to bunch and,
because each oilspot can produce many 1,000s o spores, this can
lead to an explosive increase in disease levels causing many more
new generation oilspots and bunch inections.
Q 5.: I know downy is driven by wet weatherbut can you summarise the conditionsneeded or each step?
A primary inection requires the soil to be wet or 16 hours at
temperatures at or near 100C or overwintering spores (called
oospores) to release zoospores in the soil. These swimmingzoospores then need rainall to splash them up to the vine oliage
and the leaves need to be wet or a urther 23 hours or inection
to occur. Because these conditions are quite specic, primary
inections usually only occur a ew times a season. Ater the
primary inection, the downy mildew pathogen will incubate or
ve or more days beore oilspots appear in the canopy.
Once these spots have appeared, spores can be produced on their
under-suraces as part o the secondary inection process. This
requires a warm, humid night and the leaves need to be wet in the
morning or inection to occur. This event may occur many times in
a season. A second incubation period is then needed beore a newgeneration o oilspots will appear (or more detail on the lie cycle
o downy mildew, go to the GWRDC website http://www.gwrdc.
com.au/webdata/resources/les/DownyMildewFactShee.pdor
www.GrowCare.com.au)
The disease
Q 6.: I have not ound any oil spots in myvineyard. Does that mean that I am sae?
Have you looked? Have you really looked inside the canopy?
Eective monitoring requires time, eort and experience. Follow
the three Ts o good monitoring: Target, Timing and Technique.
Know the symptoms you are looking or (target); when to look
or them (timing) and how to monitor (technique). For instance,
be sure to look inside the canopy or bunch inection and when
looking or oilspots, assess up to 200 vines.
Primary inections are easy to miss and can go untreated. It is
possible or inection to go straight to bunches but this is rare.
A secondary inection is more noticeable and shows up as small
clusters o oilspots on leaves.
Oten the primary oilspot will be ound as an aged spot in the
middle o this new cluster. Inected bunches will usually be the
result o secondary inec tion (see Q15).
Q 7.: In on-going wet conditions that causerepeat inection events, what must I do?
Be sure to look closely in your vine canopy. Search or oilspots
in the oliage and especially look within the canopy or inected
bunches (See Q1.) It is important to know what level o disease is
present in your vineyard and to know what stage the epidemic is
up to (Q4 and 5. See www.GrowCare.com.au or urther details).
This allows good decisions to be made when considering which
ungicides to apply.
Managing the disease
Q6
Older oilspots are dark yellow with or without
the white down on the undersides
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Managing the disease
Q 8.: What is the dierence between apre- and post-inection ungicide?
A pre-inection ungicide is a product that deends against downy.
It must be applied as close as possible beore inection begins to
ensure that new growth is well covered. These products include
the copper-based products mancozeb (e.g. Dithane) and captan.
They are otherwise known as protectants or cover sprays.
A post-inection ungicide has the capacity to kill o downy whileit is developing, ater inection has occurred. They are best applied
as soon as possible ater an inection event has occurred. These
products include metalaxyl (Ridomil, Axiom and Medley)
and phosphorous acid (phos acid). The strobilurins Cabrio and
Amistar have some e ect also. These products are otherwise
reerred to as eradicants or systemics.
Q 9.: Are the dierent pre-inection (protectant)ungicides dierent in their control o downy?
All the pre-inection sprays will do a good job i they are sprayed
correctly (Q12). This includes the dierent copper ormulations,
mancozeb and the like. Generally, i it is registered, it will work!
Q8
Downy mildew rapidly kills highly susceptible young bunches (let),
but berries are immune well beore pea-size (E-L 31) (right).
Downy is best controlled at or beore primary inection because
in suitably warm, humid weather the disease spreads rapidly in
secondary inection.
Q 10.: I sprayed with copper last weekbeore the recent rains. How much copperdid the rain wash o?
Understandably, the rain washes some chemical o the sprayed
oliage, but most pre-inection sprays stick quite well. The chie
actor in a spray ailing to endure is not so much the amount o
rain but more the rate o growth o new vine tissue. Leaves and
shoots expand and critically, at the time o fowering, the bunches
grow rapidly. Because the pre-inection sprays act on the surace
o the vine tissue, they need to be present in sucient coverage
and dose beore a spore lands on that surace so that the ungus
makes contact with the chemical and is killed. As the vine grows, it
expands. This spreads out the layer o ungicide like dots spread on
a balloon when it is blown up. With expanding tissue, the downy
spores have a much better chance o landing in a gap in the spray
cover and growing into and inecting the vine.
When the vine is rapidly growing at, or near, fowering, aprotective spray cover remains eective or as ew as 35 days
(see Q15). Later, as vine growth slows, a spray cover might last
1014 days.
Q 11.: How can I measure vine growth?
A simple way is to tie a bread bag twist-tie at the tip o 23 shoots
at the time you spray. Come back a ew days later and you will see
how much the shoots have grown. This gives you a measure o
how much new and thereore unsprayed vine surace is present
beore the next inection event.
Q 12.: Im told that the pre-inection spraysare very eective against downy. How come Isprayed just beore the last rain and still ound anew generation o downy oilspots on my vines?
Your spray Timing seemed good but good spray application is
essential or good control. This means getting the chemical Type
and Treatment right. Good treatment oten means using the
highest label rate o product and matching the amount (volume) o
water and chemical applied to the canopy size (volume) at the time
o spraying. This also means spraying at suitable speeds with best
sprayer set-up to maximise coverage o the young bunches and the
lower lea suraces where the downy inects the leaves. These arethe places where it is most dicult to achieve good coverage, so
canopy management such as trimming is oten essential i sprays
are needed ater fowering in dense canopies.
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Managing the disease
Q 13.: I want to make sure I also control powderywhen I am ocussing on downy.Can I tank mix several products when I spray?
This is a good idea. It is easy to ocus on downy and orget
powdery mildew, botrytis and other bunch rots. The latter two
disease groups spread abundantly in conditions that avour
downy. Tank mixes are OK or most o the products used or the
three disease groups but it is a good idea not to mix more than
three products in the one tank.
Q 14.: Do I need to add a wetteror sticker to my sprays?
Generally, no. Most ungicides are ormulated with these already
incorporated into the product you buy. They are ormulated to
spread and stick well. Follow the manuacturers advice on the
label. Too much wetter can lead to the spray spreading too much
and running o the vine. This reduces the eective dose thatremains to prevent inection and can lead to inection occurring
even though you sprayed.
Q 15.: I sprayed my vines at the right timeand with good coverage but ater the rains,I ound downy on my bunches and hardlyany on the leaves. What is going on?
This has been showing up in a number o v ineyards. Nearly all o
these have been sprayed just beore an inection at the time o
fowering. The sprays covered the oliage and the young bunches
but in the 3 or 4 days beore the rain, lots o bunch fower caps ell
o, leaving the developing fowers exposed to inection. Ater the
incubation period o downy inside the vines, inection showed up
wherever the coverage was incomplete (Q12). This means that the
very susceptible young bunches that were exposed were diseased
despite the spraying.
Q13
Secondary inect ion can be explosive, producing many 1000s o
new generation inection sites overnight . Early season, when the
bunches are very susceptible, this phase o the disease can be rapidly
destructive.
Q15
Good timing o spray applications in relation to inectionevents, especially primary inection, is a major actor in
eective spraying or downy mildew. Young bunches are
highly susceptible so good control o primary inection will
prevent the risk o secondary inection and reduce the need
or more sprays.
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Q 16.: What can I do i downy mildewinection has occurred? Can I stop it?
The post-inection sprays, such as metalaxyl and phos acid, are
very eective in controlling downy ater an inection event. There
are some excellent examples o inection developing, then a post-
inection ungicide being applied and the disease being eectively
stopped in its tracks.
There are several actors involved in the capacity o downy to
multiply in the vineyard. The number, size and age o oilspots
determines the capacity o the disease to produce spores in a
secondary inection event. The suitability o the conditions is
another actor. On a warm, humid night suitable or secondary
inection (see Q5), downy produces thousands o spores rom
each oilspot. I the temperature is around 200C, an oilspot will
produce many more spores than i the temperature is near the
lower limit o 130C. Once the spores are produced, temperature
and lea wetness are critical actors in determining how muchnew inection will occur. The post-inection ungicides are active
against most o these actors preventing oilspots being produced
and reducing sporangial production and viability.
These ungicides are best applied as soon as possible ater
inection to prevent oilspots developing. I they are applied just
ater oilspots appear (not recommended!), they sometimes have
the capacit y to kill out that oilspot or, i the spot persists, the post-
inection ungicide has capacity to reduce the number o spores
produced and/or it may reduce the viability o any spores that do
develop. This multiaceted activity o the post-inection ungicides
are the reason why they are so eective, especially i applied
within the rst 45 days ater an inection event.
Q16
Post-inect ion ungicides like metalaxyl can kill out oilspots i timed
correctl y. The brown oilspot (let) has died and the yellow oilspot
(right) did not produce new spores despite avourable weather
conditions it can no longer spread the disease.
Managing the disease
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Q 17.: How can I tell when to spray,especially in wet weather?
In wet conditions, there are two approaches. One is to spray
regularly and oten with a pre-inection (protective) ungicide. The
eectiveness o control will depend on achieving a good spray
cover beore an inection event occurs (see Q10). The spray interval
is best determined by assessing vine growth (see Q11) and, bymonitoring the weather orecast s or the conditions that avour
inection. For some regions, the Bureau o Meteorology works in
liaison with regional industry bodies to provide a Downy Mildew
Alert, warning growers o impending conditions avourable to
inection (see www.GrowCare.com.au).
The second approach is to apply a ew cover sprays at critical times
early in the season (beore the canopy closes over and makes spray
penetration more dicult), then ollowing up with post-inection
sprays i and when needed, as soon as possible ater an inection
period (see Q16).
Q17
Automatic weather stations (AWS) monitor the vine canopy or
leawetness, rainall, relative humidity (RH) and temperature, critical
to the lie cycle o downy mildew. Some AWS, like this Model T
MetStation, signal inection events and help rene optimum
spray timing.
Managing the disease
For the maximum eectiveness o both approaches, spray timing
will rely more on fying by instruments than by observing oilspots
in the vineyard. The pre-inection strategy is best guided by
orecasts o wet weather. This means spraying beore you can see
the downy inection. The post-inection strategy is best guided
by use o a weather station to provide weather data or a sitenear you. This permits analysis o the data to determine the just
triggered risk o disease rom a recent weather event though
the resultant disease outcome is still unseen in the vineyard (see
examples o this approach in www.GrowCare.com.au ).
Q 18.: What i my vines were at or justpast fowering and unprotected when theconditions avoured downy in my vineyard?
As the words o the Downy Rap song say, at this critical time, Give
it no room. Because young bunches are so susceptible to downy
(Q15), it is essential to gain and maintain good control. Apply a
post-inection ungicide as soon as possible and ensure good
spray coverage by using well-calibrated equipment and optimising
a high water rate or thorough coverage (Q12). (Note: For those
interested in the novelty disease management story The Downy
Rap, the song is available rom www.GrowCare.com.au).
Q 19.: I have had a downy mildew inection,the disease is spreading and I cant purchaseany new metalaxyl ... but I have some 5-yearold Ridomil in the shed. Can I use it?
Many companies only guarantee products with metalaxyl and
copper or ve years and metalaxyl and mancozeb products orthree years. As long as it is kept dry, and preerably cool, the
metalaxyl is more stable than the pre-inection actives copper and
mancozeb, which are added to reduce the risk o resistance. Also
note that some industry quality assurance programs prevent the
use o out-dated chemicals in the vineyard. Please consult your
winery or more specic advice about chemical use.
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Q 20.: Just recently, some ormulations ometalaxyl used in pineapples and othercrops have been approved by the APVMA oruse in vineyards. They are not ormulatedwith a pre-inection ungicide such as
copper or mancozeb. Is this sae to use?
Yes but, used alone, metalaxyl will only provide limited protection
to the vines. It will work well as a post-inection ungicide but to
provide a high level o protectant (pre-inection) activity and to
prevent resistance developing to the metalaxyl, make sure you
tank mix a pre-inection ungicide with the metalaxyl.
Q21.Can I use phos acid on my vines?I have nothing else I can use tostop the downy spreading.
Although phos acid is extremely eective in reducing the vineyard
disease potential rom inected vines, its use will almost certainlyresult in a detectable residue in wine. Because some international
markets have no MRL (Maximum Residue Level) or phos acid, ask
your winery or their policy prior to using phos acid.
An alternative, when all else ails, is to maintain a requent spray
program using pre-inection (protectant) ungicides. Trim vines to
make or easier penetration o sprays. The Group Y ungicides (e.g.
copper, mancozeb etc) are very eective and cheaper than the
Group K strobilurins (e.g. Amistar and Cabrio), but the latter
also give control o powdery mildew. Keep a look out or powdery
in your canopies too because now is a good time to make sure it is
under control!
Q 22.: Can I use products that arenot registered or grapevines?
No! Chemical applicators should only apply agrochemical
products that are currently registered or use on grapes or have
a current o-label permit. An expired permit does not constitute
permission to use a product. Reer to the AWRIs Dog Book (http://
www.awri.com.au/industry_support/viticulture/agrochemicals/
agrochemical_booklet/booklet.pd) or contact the APVMA, AWRI
or your grape purchaser i in any doubt.
Q 23.: When do berries gain resistanceand what about the berry stalks?
The berries gain resistance to downy mildew as they cease
unctioning as green tissue and begin to unction as a berry.
The stomates are where downy enters to cause inection and the
stomates on the berry close up permanently sometime beore
berries reach pea size (57mm diameter). Note though, that the
berry sta lks and bunch stalks (rachis) continue to
remain susceptible.
Q23
Berries gain resistance to downy mildew when near pea-size but
stalks remain susceptible. Note the resh white down on the inected
stalks (Photo: Andrew Weeks)
Managing the disease
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Q 24.: How long do oilspots remainactive in my vineyard?
About 90 days. Though the centre o the spots may die out when
the oilspot sporulates and produces the characteris tic resh white
down, untreated oilspots will continue to grow rom an outer
ring o yellowed (inected) tissue. This maintains the active downy
mildew and gives the disease the potential to spread in suitable
conditions or much o the rest o the season.
Q 25.: Will skirting andtrimming the vines stop downy inection?
No, but it will help airfow to dry out the canopy more quickly and
it will assist the penetration o ungicide sprays. Both skirting and
trimming will assist in controlling downy (and powdery) mildew
and bunch rots. Desuckering will remove susceptible lea material
rom near the soil where the disease overwinters shoots low to
the ground oten show the disease rst.
Q 26.: Sometimes post-inection ungicideslike metalaxyl seem to kill o establishedoilspots. How can I tell i the spots are dead?
Post-inection ungicides like metalaxyl and phos acid both have
capacity to kill existing oilspots i applied soon ater the spots
appear (Q16). The bag test (Q3) is one way o checking i the
oilspots are dead. Another is to check the spots ater a warm,
humid night and look or resh white down in the morning. I
unsprayed oilspots produce the down and the treated spots do
not, then you can assume the post-inection spray has been
very eective.
This season has again shown some clear examples o this success
story when a ull epidemic has been eectively quelled by well-
timed applications o either metalaxyl or phos acid.
Dr Peter A MagareyMagarey Plant PathologyLoxton SA 5333December 2010
AcknowledgementsManaging the disease
The author thanks the many who assisted with swit practical
suggestions or the production o this text, particularly Andrew
Weeks and the members o the Riverland Viticultural Technical
Group, Mark Krstic and Kate Harvey rom the GWRDC, Duncan
Farquhar rom the NWGIC, Marcel Essling rom AWRI, Warren
Burgess rom VBH Supplies, Liz Riley rom Vitibit Pty Ltd, andRichard Hamilton rom Hamilton Viticulture.
For further information:
1. The Grape and Wine Research and Development Corporation
(GWRDC) website www.gwrdc.com.au. Follow the links under
Resources and Innovator Network Resources tabs to:
TheDownyMildewfactsheets
TheDownyMildewtechbook
Otherfactsheets
2. The web-site www.GrowCare.com.au, provides inormation
or Australian grapegrowers seeking details on the diagnosis,epidemiology and management o the mildews o grapevine.
It hosts written inormation, interactive disease and weather
inormation, and songs on the mildews and hosts a teaching
video on downy mildew.
3. Nicholas, P.R., Magarey, P.A. and Wachtel, M.F. (1994). (Editors).
Diseases and Pests, Vol. I. Grape Production Series. Winetitles,
Adelaide, South Australia. 106 pp. ISBN 1-875130-15-2
4. Magarey, P.A., MacGregor, A.M., Wachtel, M.F. and Kelly, M.C.
(1999). (Editors). The Australian and New Zealand Field Guide
to Diseases, Pests and Disorders of Grapes. A companion to
Diseases and Pests, Grape Production Series No.1. Winetitles,Adelaide, South Australia. 108 pp. ISBN 1 875130 33 0. (634.82)
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GRAPE AND WINE RESEARCH AND
DEVELOPMENT CORPORATION
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