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.

As most grapes are in bloom now, we should remember that the bloom and the post-bloom periods are critical for disease control in grapes. During these growth stages, the young clusters are highly susceptible to diseases, including black rot, downy mildew, powdery mildew and Phomopsis, and most of the fungi are active at this time of year. The risk is especially great if we have a lot of rain, like we’ve had recently, and moderate to warm temperatures during this time. Prolonged wet conditions during bloom can also allow Botrytis to get a foothold in the clusters of susceptible varieties by promoting growth on senescing flower parts.

The main aim for fungicide sprays at this time is to protect the clusters from infection by these pathogens while simultaneously protecting the foliage as well. Some infections that occur during this period may remain dormant (invisible) until the berries are close to veraison (black rot) or ripen (Phomopsis, Botrytis). As the berries grow and mature, they become naturally resistant to black rot, downy mildew and powdery mildew, and the need for protection diminishes after the susceptible period ends. This happens quite rapidly for downy mildew (two to three weeks after bloom), whereas for powdery mildew it is about three to four weeks after bloom. Concord grapes become resistant to black rot four to five weeks after bloom, but some wine grape varieties may remain susceptible to black rot for up to eight weeks postbloom.

However, be aware that the cluster stem (rachis) and berry stems can remain susceptible longer than the berries in most cases. The only disease to which berries remain susceptible throughout their development is Phomopsis, but the risk of infection diminishes after bunch closing because inoculum levels drop off then. Botrytis is just the opposite in that berries actually become more susceptible as they get closer to harvest, especially in tight-clustered varieties. In general, aim to protect the clusters from the major diseases from immediate pre-bloom until four to five weeks after bloom.

Black rot

Temperatures in the high 70’s and low 80’s are perfect for black rot. At these temperatures, only six to seven hours of wetness are needed for infection. Black rot is a tricky disease because infections can remain latent (invisible) for a long period of time, so you won’t know that you have the disease until is it too late to do anything about it. However, one can scout for the small, round leaf spots – a lot of black rot leaf lesions indicate high disease pressure from ascospore inoculum and also contribute to fruit infections. In a field with a history of black rot, old fruit cluster remnants left hanging in the trellis are major contributors to infection. Fruit infections can take place anytime from bloom onwards, but only become apparent sometime between bunch closure and veraison. The period from immediate pre-bloom through early fruit development is crucial to protect grapes against black rot infection.

The approach to black rot control now focuses primarily on protecting the clusters from infection. EBDC sprays applied earlier in the season for Phomopsis will also control black rot leaf infections, and therefore no sprays are recommended specifically for black rot on the foliage early in the season. In five years of trials in New York, good black rot control was achieved with one immediate pre-bloom and one to two post-bloom fungicide sprays. A second post-bloom application is strongly advised if black rot has been a problem in the vineyard the previous year, and should be considered prudent if wet weather is anticipated. During three years of fungicide trials in a ‘Concord’ vineyard in Fennville, Michigan, just two post-bloom applications of sterol-inhibitor fungicides (Nova, Elite) have provided very good control under high black rot pressure. An immediate pre-bloom application is advised only if black rot was severe in the vineyard in question in previous years.

Sterol-inhibitor (SI) fungicides (e.g., Nova and Elite) continue to provide outstanding control of black rot, and provide several days of post-infection activity. Currently, there are various “generic” tebuconazole products on the market, e.g., Orius and Tebuzol, that may be more cost-effective. When using SI fungicides on a post-infection schedule, use the highest label rates because post-infection activity is strongly rate-dependent, particularly when extended “kickback” activity is required. The strobilurin fungicides (Abound, Flint, Sovran, Pristine) are excellent protectants, but provide only limited post-infection activity (probably less than 24 hours). Flint and Pristine should not be used on Concord grapes because of potential phytotoxicity.

Phomopsis

Cane and leaf lesions have been showing up in high numbers in susceptible varieties. Each rainfall event will lead to spore dispersal and can also lead to successful infection if the tissue remains wet for a sufficient amount of time. The optimum temperature for infection is 59 to 68ºF, at which time about six to 10 hours of wetness are needed for infection. The longer the tissue stays wet, the more severe the symptoms will be. Since rachis and flower clusters are now fully exposed, we should be concerned with preventing Phomopsis infection of the rachis and fruit, especially in mechanically pruned vineyards and vineyards with a history of the disease. Rachis infections are most closely correlated with yield losses at harvest.

If at this time you find a lot of lesions on the leaves and canes, infection pressure will be high for the fruit also. It is not too late to apply fungicides for cluster protection from Phomopsis. Best fungicide options for control of Phomopsis during and after bloom are Abound, Sovran or Pristine (do not use Pristine on Concord grapes). Phosphorous acid fungicides, such as ProPhyt and Phostrol, are also good and cost-effective alternatives. These are systemic and will most likely provide some kick-back activity. In trials done in Michigan, ProPhyt provided very good control of Phomopsis when sprayed on a 14-day schedule. Tighten the schedule and increase the rate if disease pressure is high. Ziram is a moderate to good protectant against Phomopsis and can be a tank-mix partner with any of the phosphorous acid fungicides. EBDC fungicides are good protectants, but cannot be applied after bloom has started in grapes grown for the National Grape Cooperative. EBDC’s have a 66-day pre-harvest interval.

Powdery mildew

No powdery mildew has been sighted in vineyards yet. However, we have had multiple occasions for primary ascospore release this spring and trap plants placed in a high disease-pressure vineyard showed evidence of infection. Ascospore discharge is initiated in the spring if 0.1 inch or rain at an average temperature of 50ºF or more. This results in thorough wetting of the bark where the cleistothecia have overwintered. When the cleistothecia are sufficiently wetted, infectious ascospores are discharged within four to eight hours and are carried by wind to susceptible plant tissues. They can infect any green surface on the developing vine and do not need water for infection. The fungus then grows on the plant surface and produces a second type of spore (conidia) that are windborne and cause secondary infections. Under optimal conditions, the disease can spread rapidly, as the time from infection to production of conidia can be as short as seven days. Although infections can occur at temperatures from 59 to 90ºF, temperatures between 68 and 77ºF are optimal for disease development. Temperatures above 95ºF inhibit spore germination, and the fungus may be killed at temperatures above 104ºF.

Berry age has a marked effect on susceptibility to powdery mildew. Researchers in New York showed that when clusters of ‘Chardonnay,’ ‘Riesling,’ ‘Gewürtztraminer’ and ‘Pinot noir’ were inoculated from pre-bloom to six weeks post-bloom, only fruit inoculated within two weeks of bloom developed severe powdery mildew. Berries became substantially resistant to infection by three to four weeks after bloom, resulting in diffuse, non-sporulating colonies on berries, and were virtually immune at six to eight weeks after bloom. Therefore, early sprays (from immediate pre-bloom until three to four weeks after bloom) are critical for preventing powdery mildew on the clusters. This usually coincides with critical sprays for black rot. For wine grapes, control of diffuse infections is also important as these can predispose the grapes Botrytis bunch rot and sour rot later in the season.

Sulfur remains an effective and inexpensive protectant fungicide for powdery mildew control on non-sulfur-sensitive grape varieties. The most effective systemic fungicides for powdery mildew control are the sterol inhibitors (Nova, Elite, Vintage, etc.) and the strobilurin fungicides (Pristine, Sovran, Abound and Flint). Luckily, we do not have any reports of fungicide resistance to strobilurins in the powdery mildew fungus in Michigan, but in some vineyards where sterol inhibitors have been heavily used for many years, they appear to be less effective than they used to be. Newer fungicide options that provide excellent control of powdery mildew are Quintec, Endura, and Adament. Therefore it would be best to not entirely rely on SI’s during the most critical period for fruit infection (immediate pre-bloom until three weeks after bloom), but alternate or tankmix with other effective fungicides. Over the past two years, we have noticed that Ziram as a tank-mix partner did improve control of powdery mildew in a spray program.

Downy mildew

Downy mildew has already been sighted several weeks ago in Chancellor and wild grapes. Currently, ‘Chancellor’ clusters in an unsprayed vineyard in Fennville are showing heavy infection pressure. We have had several opportunities for primary infection already. These occur with rain (at least 0.4 inches) and temperatures are above 50ºF over a 24-hour period. Check the recent weather conditions at or near your location at Enviro-weather. It takes seven to 12 days for the lesions to form after infection has taken place, so keep an eye out for downy mildew. Early in the season, downy mildew lesions may be confused with low-concentration Gramoxone and possibly Chateau herbicide injury, which also cause yellow spots on leaves. However, if no herbicide was used and no herbicide spots are present on lower leaves, the spots may be downy mildew. To confirm that, you can enclose a leaf with lesion in a ziplock bag with a moist paper towel and leave it out in the dark overnight. If white sporulation appears on the underside of the leaf, it is downy mildew.

A spray for downy mildew before or just after bloom is recommended for susceptible varieties, especially in vineyards with a history of disease. Early infections can lead to severe downy mildew infection and premature defoliation of the vine. Ridomil Gold MZ and Ridomil Gold Copper have excellent curative and protectant activity against downy mildew. Under moderate infection pressure, they will provide three to four weeks of protection. Of the strobilurins, Pristine, Abound, and Sovran are good choices. Other effective fungicides are mancozeb, ziram, and fixed coppers. ProPhyt and Phostrol are also good alternatives: they provides excellent curative and about seven to 10 days of protective activity. Under high disease pressure or when spraying after an infection period, use higher rates.

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