No. 16 — July 24, 2019


Deciphering Downy Mildew

Downy mildew symptoms on the top of the leaf porulating colonies on the bottom of the leaf
Downy mildew symptoms on the top of the leaf (A) and sporulating colonies on the bottom of the leaf (B). Photo credit: D. Volenberg.

I reiterate from last week that having two grape pathogens with mildew in its name is confusing. These two pathogens are not related. By far and away downy mildew is by far a much bigger problem than powdery mildew for grape growers. If I still have your attention, maybe you will make note of the following. Downy mildew belongs to the oomycetes. Another highly destructive oomycetes is Phytophthora infestans that caused the Irish potato famine in the 1840’s. Further, downy mildew from North America almost wiped out the French wine industry in 1870. It was Bordeaux mixture that was discovered to control downy mildew. The point being, downy mildew is a serious pathogen and can quickly defoliate your vines.

The cooler temperatures coupled with morning dew increase the potential for downy mildew infections. The weather forecast has rain coming in on Sunday night into Monday. The seven-day forecast suggests rain is likely next week. Continued wetting events, coupled with high humidity, provide the environmental component for downy mildew infection periods.

Year after year, I will receive a phone call or email that describes a downy mildew outbreak in a vineyard. Sometimes the vineyard operator panics and applies a fungicide that may not be appropriate.

Before applying any fungicide, take some time to educate yourself and learn from the experience. Look back in your spray records and determine what fungicide and rate was applied in your last cover spray. Think about the past weather pattern, and specifically focus on extended foliage wetting periods. Also learn about the biology and ecology of downy mildew, as this will provide insight on management. There are a number of best management practices to keep your vineyard relatively free of downy mildew infections.

Know Your Grape Cultivar's Susceptibility

Many of the grape cultivars that are grown in Missouri are slightly to moderately susceptible to downy mildew. For example, Chambourcin is slightly susceptible to downy mildew. In contrast, Chardonel, Norton, Noiret, Traminette, Vidal blanc and Vignoles are moderately susceptible to downy mildew infections. Not sure about the susceptibility of your grape cultivars to downy mildew? See page 94-95, Relative disease susceptibility and chemical sensitivity among grape cultivars in the 2019-2020 Midwest Small Fruit and Grape Spray Guide.

If you are growing a grape cultivar that is highly susceptible to downy mildew, your disease management plan that includes cultural methods and fungicides will likely differ from grape cultivars that are only slightly susceptible to downy mildew.

A Quick Downy Mildew Biology Primer

Downy mildew overwinters on infected leaves on the vineyard floor. Spores (zoospores) are released when standing water is present. Grape tissue needs to be wet for infection to result. Infection takes place when a zoospore enters a stomata on the underside of grape leaves. The first visible symptoms of a downy mildew infection are yellow oily spots that appear on the tops of leaves. As the disease progresses unchecked, a downy white mass will appear on the undersides of the leaves. These downy masses release spores that cause secondary infections. This cycle will continue throughout the growing season if fungicides are not used to manage downy mildew. Severe downy mildew outbreaks can result in premature defoliation. Vineyards that experience severe downy mildew out breaks should remove and destroy leaf litter or at a minimum use a lawn mower to grind fallen leaves into small pieces to help promote the breakdown of the leaf tissue.

Canopy Management To Reduce Grape Tissue Wetness

There are a number of ways to improve air-flow in through vineyard that will result in reducing the tissue wetness duration. Air-flow through the vines can be increased using canopy management. This typically begins during dormant pruning and leaving a pre-determined number of buds per spur. Also at this time, the selection of spurs and position on the cordon determines your fruit cluster positions. Ideally, having the spurs equal distant will result in no overlapping clusters. Canopy management continues with shoot thinning when shoots are 3 to 4 inches in length. As the season progresses, hedging shoots that shade the fruiting zone in VSP trellised vineyards will improve air-flow through the canopy. Similarly, hedging the shoots that begin to reach the ground in high-wire trellised vineyards will improve air-flow through the canopy. Besides managing the canopy, the fruit zone should also be managed to improve air flow. This may involve cluster thinning and leaf-pulling. Leaf-pulling should be completed first on grape cultivars that are highly sensitive to downy mildew.

Leaf-pulling should begin early in the season when berries are buckshot to pea-sized. Take into consideration the row orientation when leaf-pulling. Possibly removing more leaves in the fruiting zone that receives morning sun and less on the side of the fruiting zone that receives afternoon sun.

Canopy management changes microsites within the fruiting zone and canopy to reduce humidity, and thereby reducing the time the grape tissues remain wet. This alone will help reduce the incidence of downy mildew infections, but fungicides will still need to be applied in wet humid growing areas that includes all of Missouri.

Fungicides and Downy Mildew

A number of fungicides are available to control downy mildew. Early in the growing season, when the shoots are at 3 to 4 inches to immediate pre-bloom, most grape growers are using Mancozeb to manage downy mildew. Mancozeb is a preventative fungicide, meaning it needs to be applied prior to infection to protect the grape tissue. At immediate pre-bloom to bloom, Mancozeb applications are typically stopped because of the 66-day pre-harvest interval. The critical time to manage downy mildew, as well as most diseases of grapes, is the period of immediate pre-bloom through four to five weeks after bloom. During these phenological stages, a number of fungicides can be applied to manage downy mildew (see page 91-92 of the 2019-2020 Midwest Small fruit and Grape Spray Guide.) Typically, a systemic fungicide (Abound, Pristine, Sovran, Forum, Presidio, Quadris Top, or Ranman, Reason, Re-vus, Revus Top, Tanos or Zampro) is tank mixed with Captan or Captan tank mixed with phosphorous acid products to manage downy mildew. Tank mixing two fungicides with different modes of action will help delay the development of fungicide resistant downy mildew strains.

When you need to rescue a crop from downy mildew, then Ridomil is the fungicide of choice. Never apply Ridomil more than one time in the growing season, as downy mildew is very prone to developing resistance to Ridomil. Be aware of the PHI of Ridomil Gold MZ or Ridomil Gold SL of 66 and 60 days, respectively. Ridomil Gold Copper PHI is 42 days.

To combat the development of fungicide resistant downy mildew, do not apply fungicides from the same chemical class more than two times in a row. The fungicides Abound, Sovran, Flint and Pristine all are in the same chemical class – Strobilurins. Using the same fungicide repeatedly may result in the development of fungicide resistant strains of downy mildew. In Kentucky during 2012, downy mildew developed resistance in a grape vineyard that had been treated with Abound and Pristine repeatedly over a two-year period. Using tank mixes of fungicides from different classes and rotating fungicides in your spray program will prevent or delay the development of fungicide resistant downy mildew.

Applying a highly selective fungicide such as a Strobilurin to a vineyard that has a serious outbreak of downy mildew increases the risk of development of downy mildew resistant strains. The larger the population of downy mildew, the greater chance that just one of those individuals will be resistant to the fungicide.

Once downy mildew begins to sporulate on the undersides of leaves, the more difficult it becomes to control. Although most all the fungicides used to manage downy mildew should be considered preventative, some fungicides have curative, eradicative, and have anti-sporulant activity.

Phosphorous acid fungicides applied during an outbreak of downy mildew helps reduce sporulation by burning out active downy mildew colonies. Tank mixing of phosphorous acid fungicides with a protective fungicide such as Captan often results in bringing the pathogen under control. After achieving control of downy mildew, the vineyard needs to be scouted on a short rotation of every 3 to 4 days to make sure lingering downy mildew colonies are not present.

Be forewarned that the downy mildew season extends into the cool fall months of September and October. During these months, downy mildew infections can explode, especially if temperatures nighttime and morning temperatures range between 65-77°F, cloudy conditions persist, and moisture from rainfall or heavy dew occur.

What Is It: Downy Mildew or Powdery Mildew?

If you suspect that a mildew is impacting your vineyard, but you are unsure if the culprit is downy or powdery mildew, then try the following simple moist incubation test. Rinse a 1-gallon ziplock bag with water, leaving a few water droplets behind in the bag. Place your suspect leaves in the bag. Breathe a little air into the bag and close the bag. Place the bag in a dark place overnight. A kitchen drawer works well for this. Within 24 hours, you may see the following:

  1. If white colonies are apparent on the bottom side of the leaf only: This is most likely downy mildew. Powdery mildew can form colonies on either the top or the bottom of the leaf. Downy mildew will only form colonies on the bottom of a leaf.
  2. If white colonies are on the top of the leaf only: This is powdery mildew.
  3. If white colonies are on the top and bottom of the leaf: This could be powdery mildew or this could be a mixed infection of powdery and downy mildew. This is when you have to think about the past weather conditions in the vineyard. If the weather has been hot and humid but rainfall and dew have been mainly absent, then most likely the problem is powdery mildew. If weather conditions have been variable with extended wetting periods, then it is possible to have a leaf sample with both downy and powdery mildew. View the colonies with a 20X-hand lens. Downy mildew will be multi-branched whereas powdery mildew will be a single stalk or conidiophore. See more details here: ViNews: IA 20x Hand Lens Can Save You A Lot Of Misfortune

In summary, the ideal management of downy mildew is to have protective sprays applied so downy mildew primary infections never occur. Once downy mildew primary infections take hold in a vineyard, it becomes a serious battle managing secondary infections. When wet weather is forecast, a cover spray needs to be applied prior to the rainfall event. If you are relying on Captan as your primary fungicide for downy mildew, be sure shorten your cover spray duration when rainfall amounts are more than 1 inch. Captan is prone to wash-off by rainfall events.

Cumulative Growing Degree Days for the Seven Grape Growing Regions of Missouri from April 1 to July 22, 2019

Region Location by County Growing Degree Days1
2019 2018 30-year Average
Augusta St. Charles 2023 2199 1984
Hermann Gasconade 1944 2066 1894
Ozark Highland Phelps 2048 2276 2044
Ozark Mountain Lawrence 2039 2263 1996
Southeast Ste. Genevieve 2063 2216 2050
Boone 2025 2244 1957
Western Ray 1886 2114 1903

1 Growing degree days at base 50 from April 1 to July 22, 2019. Data compiled from Useful and Useable at Click on link below to determine growing degree days in your area.

To determine the number of growing degree days accumulated in your area since April 1, use this tool.