No. 9 — May 20, 2019

Immediate Pre-Bloom: Critical Period of Disease Management

close-up of florets on pre-bloom plant
Photo: Immediate Pre-bloom when florets on inflorescence separate. Photo credit: D.S. Volenberg

The period from immediate pre-bloom through 5 weeks post bloom is the most critical time to protect your crop from pathogens. This period is when your crop is the most susceptible to fungal pathogens.

As the phenology of the vines progress from vegetative growth to reproductive growth, the potential for disease increases. Prior to pre-bloom disease management focused on keeping the shoots and leaves protected from disease. Depending on your vineyard location the inflorescence will soon begin flowering and this calls for increased vigilance to protect the crop from pathogens.

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.

Use The Best Fungicides To Protect The Crop: Avoid Repeat Applications Of The Same Fungicide And Fungicides From The Same Chemical Class

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) and sterol-inhibitors (Frac 3: Mettle, Procure, Rally or Tebuzol) limits the number of applications during the growing season. For example, the Rally label states that only two 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 post-bloom disease management are critical. Although berries develop resistance other green tissue remains susceptible throughout the growing season.

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