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wrdcccGRAPE AND WINE RESEARCH ANDDEVELOPMENT CORPORATION

Downy MildewQuestions and Answers

3 December 2010


2/9Downy Mildew | Questions and Answers 01

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)
http://www.growcare.com.au/http://www.growcare.com.au/



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
http://www.gwrdc.com.au/webdata/resources/files/DownyMildewFactShee.pdfhttp://www.gwrdc.com.au/webdata/resources/files/DownyMildewFactShee.pdfhttp://www.gwrdc.com.au/http://www.gwrdc.com.au/http://www.gwrdc.com.au/webdata/resources/files/DownyMildewFactShee.pdfhttp://www.gwrdc.com.au/webdata/resources/files/DownyMildewFactShee.pdf




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.




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.



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.




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.


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.
http://www.growcare.com.au/http://www.growcare.com.au/http://www.growcare.com.au/http://www.growcare.com.au/http://www.growcare.com.au/http://www.growcare.com.au/



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)

http://www.awri.com.au/industry_support/viticulture/agrochemicals/agrochemical_booklet/booklet.pdfhttp://www.awri.com.au/industry_support/viticulture/agrochemicals/agrochemical_booklet/booklet.pdfhttp://www.awri.com.au/industry_support/viticulture/agrochemicals/agrochemical_booklet/booklet.pdfhttp://www.awri.com.au/industry_support/viticulture/agrochemicals/agrochemical_booklet/booklet.pdfhttp://www.awri.com.au/industry_support/viticulture/agrochemicals/agrochemical_booklet/booklet.pdfhttp://www.awri.com.au/industry_support/viticulture/agrochemicals/agrochemical_booklet/booklet.pdfhttp://www.awri.com.au/industry_support/viticulture/agrochemicals/agrochemical_booklet/booklet.pdf


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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)

wrdccc

GRAPE AND WINE RESEARCH AND

DEVELOPMENT CORPORATION
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