Don’t let downy mildew get you down

Downy mildew has been sighted in southwest Michigan, so scout and keep an eye on vineyards with susceptible cultivars. Know the symptoms, biology and management of downy mildew in grapes.

Downy mildew has gotten a relatively early start this year in southwest Michigan with first sightings from mid- to late June, depending on location and cultivar. So far, the most affected cultivar has been Chancellor with anywhere from 1 to over 40 infected clusters per vine in unprotected vineyards. Often, downy mildew will be visible on clusters before leaves in Chancellor. Downy mildew was also noticed on leaves of unsprayed ‘Niagara’ vines and can be found on wild grapes especially those growing in humid locations, such as near ponds or in ditches.

Downy mildew is caused by the fungal-like organism Plasmopara viticola and can seriously damage leaves and clusters of susceptible cultivars. Leaf infections may lead to premature defoliation, which can reduce winter hardiness and sugar accumulation in the fruit in severe cases. Cluster infections usually translate into direct losses, as the infected cluster stems and berries will become necrotic and fail to develop. This is often the case with Chancellor, which is highly susceptible to downy mildew. First symptoms on the leaves may be yellow or light-green spots that may have a greasy appearance (oil spots). On older leaves, lesions are smaller and more angular as they are delimited by leaf veins. White sporulation usually develops on the underside of the leaf after warm nights with high relative humidity. Infected clusters and tendrils may also be covered with a fluffy white growth.

Biology of the pathogen

The pathogen overwinters as thick-walled spores (oospores) in fallen infected leaves on the ground. Oospore germination is favored by moist soils and temperatures over 50ºF, and typically starts several weeks before bloom in this region. Oospores develop a second spore type, sporangia, which are splashed by rain or carried by wind to young leaf and shoot tissues. The sporangia release zoospores (swimming spores) that need a film of water (rain or dew) to infect plant tissues. Infection by zoospores is relatively rapid and a wetting period of two to three hours is often sufficient. Zoospores infect the plant exclusively through the stomates, or breathing pores on the leaf, which are mostly located on the lower leaf surface. Young leaves and berries are particularly susceptible, but become resistant to infection as they age.

Lesions appear within 5 to 17 days after infection, depending on the temperature. The fungus then sporulates on infected tissues under warm, humid conditions (greater than 98 percent humidity and greater than 55°F) at night. The optimal temperature for sporulation is 65 to 72ºF. On leaves, sporulation typically occurs on the underside of the leaf or rarely along veins on the upper leaf surface. This is in contrast to powdery mildew, where sporulation mostly occurs on the upper surface. Lesions typically sporulate three times before they turn brown and die. Rain is the principal factor driving epidemics. Temperature plays a less important role by retarding or accelerating the development of the disease.

The most serious epidemics occur when a wet winter is followed by a wet spring and a warm summer with cloudy days and intermittent rainstorms every 8 to 15 days. Since the generation time of the fungus can be as short as five days under optimal conditions, this can lead to “explosive” disease development. Once the weather turns warm and dry, the downy mildew fungus goes “on vacation” and may not be very active until favorable conditions return in late summer and early fall. Often, in late summer, heavy dews at night promote disease development.

Disease monitoring

Since downy mildew can develop explosively under conducive conditions, frequent disease monitoring is important even when fungicide sprays have been applied. Scout several rows in various places in a vineyard. Visually scan leaves and clusters, and also look for symptoms on tendrils and shoots. Early in the season, lesions may be most visible on leaves and shoots close to the ground, but later on, they may appear higher in the canopy. If you see yellow lesions, turn the leaf over to look for white sporulation on the lower leaf surface. If no sporulation is present, it may be that the lesions are still young and conditions have not been right yet for sporulation. Occasionally, low-level paraquat herbicide injury may resemble downy mildew lesions but these spots do not show sporulation. Also, in the case of herbicide injury you’ll see typical necrotic lesions associated with paraquat injury on the same or nearby leaves. If you are not sure of the cause, remove symptomatic leaves and place them in a plastic bag with a moist paper towel at room temperature (68 to 75°F) overnight. If it is downy mildew, white sporulation should become visible on the underside of the leaf within one or two days.

Control options

Fungicide sprays for downy mildew are recommended for susceptible varieties, especially in vineyards where the disease has been found. Be careful with young vines as downy mildew can defoliate and greatly decrease winter survival. Be extra careful with young vines in grow tubes as the tubes provide excellent conditions for disease development. Keeping the disease from defoliating vines may also be important after harvest to allow the vines to build up maximum reserves for the winter. In general, juice grapes are able to withstand more disease than wine grapes, especially if the crop is light. If the crop is heavy, the vines will be more stressed and brix and cane cold-hardiness may be reduced by downy mildew destroying part of the leaf area. If downy mildew has been found in your vineyard, don’t allow the disease to develop to epidemic proportions before taking action. Listed below are some characteristics of fungicides that may help you decide which ones are most appropriate. At this point, it may be too late to use fungicides with extended pre-harvest intervals (PHI).

  • Abound (azoxystrobin), Pristine (pyraclostrobin + boscalid), Sovran (kresoxim-methyl) (strobilurins/Quinone outside inhibitors; systemic or locally systemic; 14-day PHI). Very good to excellent preventive activity (~14 days), limited post-infection activity so would be better applied on a preventative basis. Strobilurins will reduce sporulation in existing lesions, thus slowing the epidemic. Abound is phytotoxic to apples, Pristine is phytotoxic to ‘Concord’ and some other Labrusca-type grapes; Sovran is phytotoxic to some sweet cherry varieties.
  • Aliette, ProPhyt, Phostrol, Agri-Fos (salts of phosphorous acid) (phosphites; highly systemic; 0-day PHI; Aliette: 15-day PHI), good to excellent preventive and curative activity. systemic and highly mobile within the plant. They have at least 4 days of curative activity and 7-10 days of protective activity. These products do not eradicate active lesions, but can reduce spore production. Use higher rate if applying after an infection period. Research in New York has shown good to excellent disease control on a 14-day schedule, except on highly susceptible varieties, which may require more frequent sprays. When using after infection, applying a booster spray 5 days after the first spray improves efficacy. There is a risk of phytotoxicity when applied to plants under stress or at high temperatures. Do not tank-mix with copper products, Quintec, surfactants or foliar fertilizers. There are many other generic versions available – compare by looking at the phosphorous acid equivalent).
  • Captan (captan) (phthalimides; protectant; 0-day PHI): good preventive activity; not allowed on juice grapes after bloom by some processors; suspected carcinogen.
  • Copper (copper) (inorganics; protectant; 0-day PHI; 24-day REI): good preventive activity, some grape varieties are sensitive to copper, especially under cool, slow-drying conditions. Specific formulations can be used in organic vineyards.
  • Dithane, Penncozeb, Manzate (mancozeb) (EBDC’s; protectant; 66-day PHI): good preventive activity; however, the long PHI precludes their use late in the season. Also, EBDCs are not allowed on juice grapes after bloom by some processors; suspected carcinogens.
  • Forum (dimethomorph) (carboxylic acid amines; systemic, 28-day PHI): new fungicide for control of downy mildew in grapes. Use Forum as a preventive application before infection occurs. The minimum application interval is 7 days. Performance may be improved by using Forum as a tank mix with another fungicide. The addition of a spreading/penetrating adjuvant is prohibited. Do not make more than 5 applications per year, and no more than one application before switching to a fungicide with a different mode of action. Forum has not been evaluated for disease control in Michigan but is used widely in Europe for control of downy mildew.
  • Gavel (zoxamide + mancozeb) (benzamides and EBDC’s; protectant; 66-day PHI): broad-spectrum protectant fungicide. Addition of an agricultural surfactant will improve fungicide performance. Do not make more than 8 applications per acre per season. Consider Gavel and all other EBDC fungicides in observing the maximum seasonal use rate recommendations for mancozeb. Gavel was effective against downy mildew in grape trials in Michigan but its use is limited later in the season because of the 66-day pre-harvest interval.
  • Presidio (fluopicolide) (acylpicolides; systemic, 21-day PHI) is a new fungicide which very good protective, curative, eradicative, and antisporulant properties. Presidio is compatible with many fungicides and insecticides and is rainfast in 2 hours. No more than two sequential applications are allowed. A tankmix with another fungicide with a different mode of action must be used with Presidio for fungicide resistance management.
  • Ranman (cyazofamid) (Quinone outside inhibitors; locally systemic, 30-day PHI) is a new fungicide for control of downy mildew in grapes. Ranman has limited systemic activity, so it should be applied in a preventive mode. Apply on a 10-14 day schedule when conditions are favorable for disease development.
  • Reason (fenamidone) (Quinone outside inhibitors; systemic, 30-day PHI) is a new broad-spectrum fungicide related to the strobilurins, which may result in cross resistance. Reason has not been evaluated in Michigan yet, but has shown good control of downy mildew in other states.
  • Regalia (extract of Reynoutria sachalinensis = giant knotweed) (plant extract; protectant, 0-day PHI) is a biofungicide that is OMRI approved for organic production. It is labeled for broad-spectrum disease control in grapes. The proposed mode of action is by increasing the plant’s natural defenses. This induced resistance is not systemic throughout the plant but limited to the leaf it is applied to. The resistance reaction takes 1 to 2 days to develop. Regalia should therefore be used as a preventative treatment. In past trials in grapes with a different formulation, Regalia showed moderate control of downy mildew.
  • Revus (mandipropamid) (carboxylic acid amines; systemic, 14-day PHI) is a fungicide which is active against diseases caused by downy mildew. Revus Top is a pre-mix of mandipropamid and difenoconazole, a powdery mildew fungicide. It has preventative and limited curative properties. A maximum of four sprays and two sequential sprays is allowed. The addition of a spreading/penetrating type adjuvant such as a non-ionic based surfactant or crop oil concentrate is recommended. Do not apply Revus Top to Concord, Thomcord or Noiret grapes due to phytotoxicity concerns.
  • Ridomil Gold Cu (mefenoxam + copper) (phenylamides and inorganics; systemic + protectant; 42-day PHI), Ridomil Gold MZ (mefenoxam + mancozeb) (phenylamides and EBDCs; systemic + protectant; 66-day PHI). Ridomil Gold has excellent preventive and curative activity (i.e., it will stop development of lesions before and after symptoms start to show). It also stops or reduces sporulation in developing and existing lesions. It has up to 21 days of protective activity. However, the pre-harvest interval may preclude their use at this time of the season. Consider your earliest estimated harvest date to decide if these are still an option.
  • Serenade Max (Bacillus subtilis: biocontrol agent; protectant; 0-day PHI): moderate to good preventive activity, especially when applied with Nu-Film-P or similar spreader-sticker. Good coverage is important for control. Serenade has no maximum seasonal application rate. Approved for use in organic vineyards.
  • Tanos (famoxadone and cymoxanil) (strobilurins and cyanoacetamide-oximes; systemic, 30-day PHI) has curative and locally systemic properties against downy mildews. Tanos rapidly penetrates into plant tissues and is rainfast within 1 hour of application. It must be tank-mixed with a contact fungicide labeled for that crop (e.g., mancozeb, captan or copper). A maximum of 9 applications of Tanos including other group 11 (strobilurin).
  • Ziram (ziram) (dithiocarbamates; protectant; 21-day PHI): good preventive activity. Apply on a preventive basis. Susceptible to wash-off by rain.

Dr. Schilder's work is funded in part by MSU's AgBioResearch.

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