Considering the poor juice grape crop and extended warm, dry weather, the need for fungicide sprays during the rest of this season in juice grapes is minimal, but wine grapes are still at risk of foliar and fruit diseases. However, the weather conditions have definitely helped to keep fungal diseases at bay, for which we can be grateful. The main concern is if the weather changes and we will have to deal with lots of rain in the weeks before harvest, which would promote Botrytis bunch rot and sour rot. Below are some considerations which may help in making a spray decisions for the rest of the season.
Powdery mildew is showing up on leaves here and there on susceptible varieties and in unsprayed plots. This disease does not need a lot of moisture and can continue its development despite the drought. However, since there was not a lot of rain to get the disease started in the spring and early summer, and high radiation and heat can kill young colonies, powdery mildew pressure has been light this year. High temperatures that do not harm the plant can harm the fungus; spores and mildew colonies can be killed after extended durations of temperatures above 91 degrees Fahrenheit. The fungus is killed completely when air temperatures rise above 95 degrees Celsius for 12 hours or more if colonies are directly exposed to UV light.
One thing to remember is that leaf temperatures are usually higher than air temperatures during the day. Protection against powdery mildew now is not necessary in Concords or Niagara, as many vineyards have excess foliage and little fruit. The vine can withstand a fair bit of disease anyway, especially with a reduced fruit load. At this point, most fruit clusters have developed resistance to powdery mildew infection, as berries are susceptible to powdery mildew infection for four to six weeks, with the time of highest susceptibility being from bloom to two to three weeks after bloom. Even wine grape growers can consider easing up on powdery mildew sprays and applying eradicative sprays (i.e., with JMS Stylet Oil or bicarbonate salts) only if powdery mildew were to show up on the leaves between now and harvest.
At this point, there is little risk of fruit infection unless there are berries out there that are still relatively young, which could be the case with uneven bloom after spring frost damage. An eradicative spray is recommended anyway in late August and early September to knock back overwintering inoculum for next year.
Downy mildew, black rot
Downy mildew symptoms were seen on wild grapes in mid-June but have not been seen in juice or wine grapes. The dry weather has kept this disease at bay. During hot, dry weather, downy mildew “goes on vacation.” Were the weather to change, there could still be some downy mildew development; however, it is unlikely to be very damaging. In August and September, heavy dews can aid downy mildew development. Continued monitoring of the vines and weather is advised.
Black rot is showing up on the fruit in some vineyards where black rot was a problem last year and fungicide sprays may have been missed. Fruit symptoms include a sharply delineated brown area that expands quickly, finally shriveling up the fruit to hard, blue-black mummies. These symptoms are the result of infections that took place sometime between bloom and four to six weeks after bloom and have remained dormant until now. Any symptoms showing up now are an indication that you missed an infection period during which preventive or curative sprays should have been applied.
The susceptible period ranges from bloom to about five weeks after bloom in juice grapes and up to eight weeks after bloom in some wine grape cultivars, at which point the berries become naturally resistant to infection. Since there were so few rainfall events during which clusters would have been wetted for a sufficiently long period to get infection, it may be beneficial to look back at the black rot Enviro-weather model to see when infection periods occurred in relation to fungicide sprays. If black rot is showing up in wine grapes and there are still relatively young berries (stragglers), there might still be a chance of infection, potentially from already infected berries. With the high temperatures, six to seven hours of fruit wetness would be sufficient for infection.
Consult the black rot Enviro-weather model of a nearby weather station, and if an infection period has occurred, apply an SI fungicide (Elite, Rally, Orius, etc.) within 24 to 48 hours if possible. This fungicide spray will also aid in powdery mildew control.
Phomopsis lesions are present on canes, leaves, petioles and even some rachises to some extent in many vineyards, but levels are lower than in previous years. Fruit infections will become apparent a few weeks before harvest. During warm, dry years, there may be remaining spores that can still be released in July and August were rainy weather to prevail later in the season. In susceptible cultivars (e.g., Vignoles), it may be helpful to apply a broad-spectrum product (e.g., Pristine) to catch any late-season Phomopsis and other diseases. In wine grapes, there can be berry-to-berry spread by Phomopsis as harvest approaches as rotten berries remain attached more than in juice grapes. Therefore, later-season sprays for Phomopsis may still have benefits in wine grapes.
Botrytis, fungicide use
As we approach veraison, Botrytis becomes a concern in susceptible varieties, especially tight-clustered grapes. However, continued dry, warm weather will also keep Botrytis at bay; the concern is if the weather changes and cool, wet conditions prevail. If needed, good control can be achieved with the reduced-risk fungicides Vangard and Elevate. These fungicides can be alternated for fungicide resistance management. A spray at veraison and one to two weeks prior to harvest is recommended.
Rainy weather in the weeks before harvest will increase chances of sour rot. Work by MSU’s Paolo Sabbatini has shown that berries can take up water rapidly through their skins, swell up and burst, leading to injuries that allow acetic acid bacteria and yeasts to enter the fruit and cause sour rot. For both Botrytis and sour rot management, it is not yet too late to pull leaves around fruit clusters, which will help to reduce disease pressure. However, be careful not to fully expose previously shaded clusters as strong radiation and high temperatures can cause severe scalding of berries.
With respect to fungicide use during hot, dry conditions, remember that in the absence of rain, high temperatures and solar radiation can still break down fungicides on the plant surface and, even though fungicide residues may still be visible, the active ingredient may have been reduced to non-effective levels. In addition, be careful with phosphites (Phostrol, ProPhyt) as leaf burning may occur when applied to drought – and heat-stressed vines and at high temperatures. Sulfur also should not be applied when temperatures are at or anticipated to be over 85 F. Remember that leaf temperatures can be substantially higher than air temperatures during the day.
The best times to apply systemic fungicides during this period is at night or in the early morning, especially when the soil is moist (i.e., after a rain event) and the cuticle is swelled up and permeable to fungicides. Avoid applying systemic fungicides during hot, dry conditions as they will not be taken up by the leavess and can be lost to evaporation and UV breakdown. Protectant fungicides, on the other hand, are OK to be applied during warm, dry conditions, as we want them to quickly dry onto and adhere to the leaves. If a tank-mix is applied, choose the conditions that favor uptake of the systemic component.