Michigan grape growers, gain knowledge
by Courtney Llewellyn
Grapes can be finicky. As a crop, they require somewhat specific conditions to grow to the best of their ability, and even then, all of their growers’ efforts can be sidelined by diseases and insect pests. That’s why it’s important to stay abreast of what’s going on in those two realms.
Back-to-back grape disease and insect update webinars were part of the recent Great Lakes Expo, focusing mainly on Michigan. Tim Miles, Ph.D., of the Department of Plant, Soil and Microbial Sciences at Michigan State University, started by reviewing the diseases being actively managed in Wolverine State vineyards.
Miles noted that Michigan growers are currently combatting black rot, phomopsis, powdery mildew, downy mildew, botrytis bunch rot and sour rot. “We spend a lot of money managing these,” he said. “We do have an awful lot of fungicides available in grapes by mode of action. We do have a lot of tools in grapes. It would be nice if we had a few more because we do have some resistance issues.”
- Black rot is caused by the fungus G. bidwellii. Growers need to culturally remove mummies from the previous season. Dormant sprays are critical in its control, and in-season management should focus on early season sprays up until bloom. The ideal conditions for black rot infection are temperatures from 70º – 80º F, but the initial inoculum is released at much cooler temperatures (around bud break).
- Phomopsis cane and leaf spot is the primary disease of concern, found between one and 10 inches of shoot growth. Pre-bloom and bloom sprays are used to control it, as it comes from spores from the previous year’s infections. To fight it culturally, increase air circulation in vineyards and remove diseased canes during normal pruning. Phomopsis can be controlled by applying protective fungicides before rainy weather begins. Chemically, contact minerals (copper, sulfur, Ziram, Mancozeb and Maneb) are helpful. (Miles noted that black rot can be mistaken for phomopsis, as both will rot fruit and make it unsellable.)
- Powdery mildew/downy mildew are well-known to many grape growers. Miles said, “It’s great if you can plant cultivars that are less susceptible, but a lot of them are susceptible. It’s important to use a diverse program” when managing them. East of the Mississippi, downy mildew is one of the biggest issues in grapes. The defoliation can be “pretty dramatic,” Miles said, and it can really hurt the ability of fruits to ripen and vines to survive the winter. Initially, both leaves and fruit are susceptible. The fruit becomes resistant over time – but leaves can become infected all season long. Miles noted the recent documentation of downy mildew resistance to Group 40 fungicides.
- Botrytis bunch rot is one of the hardest disease to control. To suppress it, prevent excessive vine growth with judicious use of water and fertilizer and practice intense canopy management. Varieties that ripen early and/or split are the most susceptible. Synthetic fungicides are critical (FRACs 2, 7, 9, 11, 12 and 17). A new fact sheet on botrytis is available at canr.msu.edu/grapes.
- Sour rot tends to happen when it’s warmer, and it’s caused by many different yeasts and bacteria. Fruit becomes highly susceptible to infection when they reach about 15º Brix. “It’s not a totally clear-cut disease, but we’ve seen that it is tied to certain insects,” Miles said – notably, fruit flies and yellow jackets. He mentioned Fracture/Mustang Maxx and Switch/Mustang Maxx as the best fungicide combinations for controlling it. (He also noted the Michigan Craft Beverage Council project on sour rot management, which has investigating alternative fungicide/insecticide combinations for control as its main goal.)
Michigan vineyards see fungicide resistance already, with powdery mildew having widespread FRAC 11 resistance, and botrytis is found throughout the state. However, detection of resistance does not always equate to control failure. Cultural practices can make a big difference, specifically those that promote good air circulation via canopy management and leaf pulling.
Scouting is a critical component to managing these diseases. Miles also offered these general recommendations for managing fungicide resistance: don’t apply half rates; rotate FRAC codes; keep inoculum low in vineyards and target sprays around bloom; and use dormant sprays. Alternative FRAC codes will be important in managing resistance, he stated.
Dr. Rufus Isaacs, director of the Berry Crops Entomology program at Michigan State, took over to talk about insect and mite management in vineyards. The “bugs that don’t know about the pandemic” were quite active last year, he said, before reiterating that scouting is crucial.
He first covered his lab’s Erinium mite trial, which took place at two vineyard sites on Old Mission Peninsula in 2020. Both sites were treated at bud swell before being evaluated in August. When comparing the performance of specific miticides, his team found that fenazaquin was active on eggs, immature mites and adults over a wide temperature range. Spirodiclofen was active on all life stages with good IPM compatibility. Additionally, calcium polysulfides were useful as miticides, insecticides and fungicides, and refined mineral oil, which acts by smothering mites, was also active on powdery mildew, botrytis and mealybugs.
The potato leafhopper has made the leap to wine grapes, and tends to blow in on early spring storms. Isaacs noted the pest has a broad host range (more than 100 crop and non-crop plants) and in vineyards has a preference for new vine growth. There is a hypersensitive response in sensitive cultivars, causing stunted growth, yellowing and cupping. It’s typically managed with resistance and pesticides.
Another pest growing in strength is the grape berry moth, which led to a bad year for grape growers in 2019. “We’re losing a significant amount of potential yield from high infestation sites,” Isaacs said. The keys to effective control are scouting vineyards where the pressure is highest (usually the borders), timing sprays to protect clusters during key activity periods (post-bloom, late June/early July, early to mid-August and pre-harvest), rotating chemical classes to prevent resistance, using effective, long-lasting insecticides and getting excellent cluster coverage.
Isaacs provided some good news: eastern grape leafhopper damage is now quite rare. Scouting the inside leaves of the canopy helps growers find it more easily, and it’s very susceptible to neonicotinoids. He also said states to the west of Michigan are now starting to have more of an issue with Japanese beetles, the bane of many leafy plants, which means controls in the state are working.
Another silver lining was the fact that only dead spotted lanternflies, the dreaded sapsuckers first found in Pennsylvania, have been detected in Michigan, “but the potential for spread is there,” he said. “Early detection is critical for slowing the spread.”