MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

A number of diseases have started showing up in Michigan vineyards in the past few weeks, including powdery mildew and downy mildew. The warm, dry conditions this summer, combined with moderate to high relative humidity have been ideal for powdery mildew development. However, the number of primary infection events (less than 0.1 inches of rain at 50ºF or more) in spring and early summer were less frequent than last year, reducing overall disease pressure. Interspersed rains and nightly dew have promoted other diseases as well. Rains in June have promoted black rot in unsprayed or poorly sprayed vineyards. Downy mildew has been sighted on leaves and clusters of some cultivated and wild grapes, though warm dry conditions have generally kept this disease in check. This can change as the recent rains and expected dew events will likely increase downy mildew pressure in the coming weeks. Phomopsis lesions are present on leaves and canes in many vineyards and also can be seen on some rachises (cluster stems).

At this point, we are beyond the critical period for black rot control in juice grapes; this period runs from bloom to about five to six weeks after bloom, at which time the berries become naturally resistant to infection, and sprays are no longer needed. We have to keep in mind, however, that due to the spring frost, there may be clusters present in various stages of development. This means that we have to protect the clusters until the last ones have reached the point of becoming naturally resistant. Also, in some wine and table grape varieties, the berries do not become fully resistant to black rot infection until eight weeks after bloom. As they develop, grape berries also become naturally resistant to powdery mildew and downy mildew infection also (within three to four weeks after bloom); however, the berry and cluster stems remain susceptible a bit longer. By now we don’t expect any new Phomopsis infections, however, the symptoms that are already present on the clusters will increase in severity as we get closer to harvest. Lesions on the rachis will then rapidly expand and also infect berries and berry stems, resulting in fruit rot and premature fruit drop.

Botrytis bunch rot symptoms are promoted by late-season rains, particularly in tight-clustered varieties. A rapidly spreading brown rot and gray mold often becomes apparent on the clusters after veraison. It can easily be confused with sour bunch rot, however, the latter smells of vinegar, which is a product of Acetobacter bacteria. Yeast and other undesirable fungi are often also involved in sour rot development. Any injury to the berries, whether by insect feeding, hail, fruit cracking, or powdery mildew infection can increase the likelihood of sour Botrytis and sour bunch rot. Fruit flies also seem to play a role in spreading sour bunch rot. Bunch rot severity can be reduced by improving air flow through leaf and shoot removal in the fruit zone. Be careful that you do not remove too many leaves, as the berries may get sunburned if fully exposed to the sun.

There are still many late-season fungicide options for grapes; for descriptions, efficacy ratings, and pre-harvest intervals, see the Fruit Management Guide (http://web1.msue.msu.edu/pestpubs/E154/) to choose the most appropriate fungicide for your conditions. The chemical classes are provided so that growers can decide what fungicide rotations to use. During rainy periods it is better to use systemic products because they are more rainfast than products that are strictly protectants. Also, due to their penetration of plant tissues, they may be able to stop incipient infections that may have started during recent rain or dew events.

Fungicides for downy mildew control

Ridomil Gold Cu (Phenylamides and inorganics; systemic + protectant; 42-day PHI), and Ridomil Gold MZ (Phenylamides and EBDCs; systemic + protectant; 66-day PHI). These fungicides have excellent preventive and curative activity, and are considered the “big guns” when you want to “smoke out” downy mildew. Under light disease pressure, it may be better to use other products though. In addition, the pre-harvest interval may preclude their use at this time of the season, especially for juice grapes, which will start to be harvested in less than a month.

ProPhyt, Phostrol, Agri-Fos, Aliette (Phosphonates; highly systemic; 0-day PHI), good preventive and curative activity. Use a higher rate if applying after infection period. Possible phytotoxicity when tank-mixed with sulfur, surfactants, foliar fertilizers, some pesticides and when applied during high temperatures.

Abound, Pristine, Sovran (Strobilurins; systemic/locally systemic; 14-day PHI). Excellent preventive activity (less than 14 days), limited post-infection activity. Abound is phytotoxic to apples, Pristine is phytotoxic to ‘Concord’ and some other Labrusca-type grapes; Sovran is phytotoxic to some sweet cherry varieties.

Ziram (Dithiocarbamates; protectant; 21-day PHI): good preventive activity. Can be tank-mixed at a lower rate with phosphorous acids.

Dithane, Penncozeb (EBDC’s; protectant; 66-day PHI): good preventive activity; however, the long PHI precludes their use at this point in the season. Also, EBDCs are not allowed on juice grapes after bloom by some processors.

Captan (Phthalimides; protectant; 0-day PHI): good preventive activity; not allowed on juice grapes by some processors.

Copper (Inorganics; protectant; 0-day PHI): good preventive activity, some grape varieties are sensitive to copper, especially under cool, slow-drying conditions. Specific formulations can be used in organic vineyards.

Serenade (Biological control agent; protectant; 0-day PHI): moderate preventive activity. Organic formulation can be used in organic vineyards.

Fungicides for powdery mildew control

Nova, Elite, Rubigan, Bayleton, Procure (Sterol inhibitors, systemic;14-day PHI, except for Procure, which has a 7-day PHI; and Rubigan, which has a 30-day PHI). Excellent preventive activity and good post-infection activity.

Abound, Sovran, Flint, Pristine (Strobilurins; systemic/locally systemic; 14-day PHI). Good to excellent preventive activity, limited post-infection activity. Remember that Abound is phytotoxic to apples, Sovran is phytotoxic to some sweet cherry varieties, Flint is phytotoxic to ‘Concord’ grapes, and Pristine is phytotoxic to ‘Concord’ and some other Labrusca-type grapes; Niagara seems to be less affected.

Endura (Carboxamides; systemic; 14-day PHI): good preventive some post-infection activity.

Quintec (Quinolines; locally systemic; 14-day PHI): good preventive activity, little to no curative activity.

Topsin M (Benzimidazoles; systemic; 14-day PHI): good preventive and post-infection activity.

Sulfur (Inorganics; protectant; 0-day PHI): very good preventive and eradicant activity. Some grape varieties are sensitive to sulfur, especially under hot conditions. May be used in organic vineyards.

JMS Stylet oil (Oils; protectant; 0-day PHI): good preventive and eradicant activity, needs thorough coverage. Organic formulation may be used in organic vineyards. Be careful when applying Stylet oil, as more than a couple of applications may reduce brix accumulation. Also, do not tank-mix with copper or apply with or after a Captan spray; do not apply within 14 days after a sulfur application or apply sulfur within 10 days of an oil application. Thorough coverage is important for control.

Copper (Inorganics; protectant; 0-day PHI): moderate preventive activity. Some grape varieties are sensitive to copper, especially under cool, slow-drying conditions. Specific formulations can be used in organic vineyards.

Armicarb/Kaligreen/MilStop (Inorganics (salts); protectant; 0-day PHI). Moderate preventive and eradicant activity, needs thorough coverage. May be used in organic vineyards.

Serenade (Biological control agent; protectant; 0-day PHI). Moderate preventive activity. Organic formulation may be used in organic vineyards. Thorough coverage is important for control.

Fungicides for Botrytis bunch rot control

Elevate (Hydroxyanilides; locally systemic; 0-day PHI): good to excellent preventive and limited post-infection activity.

Vangard (Anilinopyrimidines; systemic, 7-day PHI): good to excellent preventive and post-infection activity.

Scala (Analinopyrimidines; systemic; 7-day PHI): good to excellent preventive and post-infection activity.

Endura (Carboxamides; systemic; 14-day PHI): good to excellent preventive and post-infection activity. Use at 8-oz rate for Botrytis control.

Rovral (Dicarboximides; locally systemic; 7-day PHI): moderate to good preventive activity; activity is improved by addition of spray adjuvant. Some vineyards may have resistant strains if Rovral was used a lot in the past.

Topsin M (Benzimidazoles; systemic; 14-day PHI): moderate preventive and post-infection activity.

Serenade (Biological control agent; protectant; 0-day PHI): fair to moderate preventive activity. Organic formulation can be used in organic vineyards.