With bud break taking place over the span of next two-weeks depending on the cultivar and location, your concern should turn to protecting young tissue after bud break from Phomopsis cane and leaf spot (Phomopsis viticola) infections. Phomopsis spore release and infections occur during cool wet weather. The optimum temperature for shoot and leaf infections is between 60 to 68 degrees F with at least 6 hours of tissue wetness for infection. Average April high temperatures are near 60 degrees F. The extended temperature weather forecast throughout Missouri for April is for near-normal temperatures (Figure 2). The extended precipitation weather forecast throughout Missouri for April is for above normal precipitation (Figure 3). Coinciding with rainfall events is cloudy conditions that result in extending the wetting period of grape tissue. If these weather forecasts hold-up over time, there is a very good chance of extended wetting periods of grape tissue. Near normal April high temperatures coupled with precipitation events is ideal for Phomopsis infections.
To protect grape leaves and shoots from Phomopsis infections a protective fungicide should be applied starting at ½- to 1-inch shoots (Figure1). Fungicide products that provide good protection contain the active ingredient mancozeb (Table 1). Mancozeb also provides excellent protection to black rot (Guignardia bidwellii) and downy mildew (Plasmopara viticola). Some common trade names of mancozeb fungicides are, Dithane M45, Dithane F-45 Rainshield, Manzate Pro-Stick, and Penncozeb 75DF. According to the label of the product, a second application should be made at 3-to 5-inch shoots followed by a third application at 8-to10-inch shoots. Further the label states that applications should continue at 7- to 10-day intervals until fruit set. All mancozeb fungicides have a 66-day pre-harvest interval.
Be good stewards and adjust your sprayers for early season spray applications. Spray nozzles should be directed at the cordons. Other nozzles out of the range of the cordons can be turned off. Reduce fan speed for early season applications. A total of 25 gallons of spray solution should cover an acre of grapes for early season spray applications. A total of 1.5 lbs of a mancozeb product in 25 gallons of water is approximately the same rate as applying 4 lbs of mancozeb product in 75 gallons of water later in the growing season. By applying less mancozeb product in a reduced carrier volume in the early season provides growers a buffer in running up against the maximum use restriction per acre later in the growing season (Table 1). — By Dean Volenberg, University of Missouri Grape & Wine Institute
Article By
Dean S. Volenberg
Viticulture Program Leader, University of Missouri Grape and Wine Institute
Dean Volenberg is Viticulture Program Leader and an Extension Assistant Professor in the Division of Plant Science and Technology at the University of Missouri, where he works with the Grape and Wine Institute. His applied research examines how canopy management, vine balance and integrated pest management affect grape quality, and he provides viticulture and winery extension throughout Missouri. He holds a Ph.D. in agronomy and an M.S. in horticulture from the University of Wisconsin.