As the florets on the inflorescence begin to separate this is a visual cue that the grapevines are entering the critical period of disease management (Figure 1). The period of time from immediate pre-bloom to 5-weeks post-bloom is when disease pathogens can cause catastrophic damage. A number of pathogens can infect a grape crop during this critical period. Some of these pathogens could be coined invisible invaders since the damage from an infection at bloom will not appear for a long period of time. Both Phomopsis cane and leaf blight, Phomopsis viticola and Bittter rot Greenaria uvicola can infect at bloom with delayed symptom development.
After infection at bloom these two pathogens become quiescent and as the fruit enters veraison the pathogens are awakened. Once symptoms are apparent on the fruit, future disease management is to reduce the spread. In the case of Phomopsis infection at bloom there is no secondary spread as grape berries will shatter and fall to the ground. In contrast, Bitter rot infection at bloom will result in infected berries producing secondary spread. In addition, to Phomopsis viticola and Greenaria uvicola the more common pathogens all remain a threat during the critical period.
Without question Black rot, Guignardia bidwelli and downy mildew, Plasmopara viticola are far and above grave threats during the critical period. This is not new news and was known as far back as 1883 in Missouri. Back then when grape growers spoke of early June with heavy thunderstorms and morning dew their focus on Black rot was summed up as:
"We may look and wonder, powerless yet knowing"
…"That the bright hopes of to-day may be dispelled by the next morn"
- Bushberg Catalog 1883, page 50, Jefferson, MO
Fortunately, today we have management tools to help protect the crop from these insidious diseases. They key word in the last sentence is "protect", meaning the management needs to be applied prior to infection. I believe some of us can relate to being…"dispelled by the next morn". The grape crop looks perfect one day but by the next day Black rot has ravaged the fruit (Figure 2).
Right now we are entering the period where attention to detail will pay dividends at harvest. The immediate pre-bloom to 5 weeks post-bloom is when you should consider using your best disease management tools to control the grape disease complex.
Select fungicide products that will provide protection from phomopsis, black rot, downy mildew, powdery mildew, and anthracnose. These are the five major diseases of concern. Take some time now to plan for your next three cover sprays. This will help to avoid applying the same products repeatedly which can result in the selection for fungicide resistant pathogens. Depending on your past disease problems from last season as well as disease outbreaks that may have occurred this season consider using both systemic fungicides as well as protectant fungicides in a tank mix to protect your crop.
Phomopsis remains a threat, especially during the bloom period. During bloom Phomopsis spores can infect developing flowers. Phomopsis development remains dormant in developing berries until brix levels begin to rise starting at approximately 15 brix. Infected berries begin the rotting process and some infected berries shatter. The primary inoculum of Phomopsis is depleted shortly after bloom. Although grape berries are susceptible to Phomopsis throughout the growing season, reduced primary inoculum coupled with increasing temperature conditions reduce the threat of Phomopsis infections after the bloom period.
During the critical period (pre-bloom to 5 weeks post-bloom) fungicides from two chemical classes are often applied. Fungicides from these two classes provide good protection to a broad spectrum of pathogens. The two classes of fungicides are the strobilurins and the sterol-inhibitors. Both powdery mildew (Michigan)) and downy mildew (Virginia, North Carolina) have been identified that are resistant to these two classes of fungicides Therefore when planning your critical period spray program implement a spray program that avoids sequential applications of the same product or products within the same chemical class. Many of the labels of both strobilurins (Frac 11: Abound, Sovran, Flint Extra) and sterol-inhibitors (Frac 3: Mettle, Procure, Rally or Tebustar) limits the number of applications during the growing season. For example, the Rally label states that only 2 sequential applications of Rally or Rally and another product containing the same active ingredient as Rally or another Sterol inhibitor can be applied. Fungicides that contain more than one active ingredient (Pristine, Inspire Super, Revus Top, or Quadris Top) are also effective for broad spectrum disease management.
As the grape berries age they become resistant to some fungal diseases. Grape berries develop resistance to Black rot, downy mildew and powdery mildew approximately 3 to 5 weeks after bloom. Therefore, early season disease management through postbloom disease management are critical. Although berries develop resistance other green tissue remains susceptible throughout the growing season.
For more viticulture information check out The MissourI Grape and Wine Institue IPM reports: https://gwi.missouri.edu/IPMreports/index.htm