Grape growth is a bit ahead of normal this year. Grapes are at the critical early shoot growth stage when Phomopsis cane and leaf spot infections occur. This is a critical time to take preventative control measures for this disease. If left unchecked, the early shoot infections will spread to cluster stems and developing berries. The organism remains latent in those lesions during most of the season. As fruit maturity begins the fungus becomes active and damages berries or cluster stems, causing a soft, brown berry rot. Secondary spoilage organisms usually become involved at that time, resulting in very poor fruit quality. Losses can be 50% or higher and overall fruit quality is greatly diminished. Many of the popular varieties grown in Indiana such as Traminette, Seyval, Chardonel, La Crescent, and Marquette are highly susceptible to Phomopsis.
Control of Phomopsis is relatively easy with captan or mancozeb. Mancozeb is the preferred fungicide because it is broad-spectrum and also controls black rot and downy mildew. It is important to understand that captan and mancozeb are protectant fungicides and must be on the plant before any infection periods occur. So they must be applied before the next rain event and reapplied after a major rain event. Fungicide applications are needed on a 7-10 day interval through bloom. This is usually about four sprays. Addition of a sterol inhibitor fungicide in the final early season sprays (immediate pre-bloom, bloom and post bloom) will provide additional control of black rot. A simplified version, a Suggested Grape Spray Schedule is available on my web site, along with the full spray program in the 2017 Midwest Fruit Pest Management Guide. Find both at: https://ag.purdue.edu/hla/fruitveg/Pages/Grapes.aspx
Grape shoot at 3 inch stage