MSU Extension

Weather and crop development

Last week brought some cooler weather, with highs in the upper 70s and low 80s, along with cooler nighttime temperatures. Most areas received over 2 inches of rain in multiple events last week. All varieties are in veraison, and harvest of some early wine grape varieties as well as most table grapes is likely to occur in the next two weeks. Niagara harvest is set to begin in mid-September.

Diseases

Symptoms of common diseases are still quite scarce, but recent storms have increased the risk of powdery mildew and downy mildew on leaves in most varieties and Botrytis and sour rots in clusters of varieties with compact clusters. Most of the disease symptoms we observed do not appear to be actively growing, but rather they look like remnants of infections that have been controlled. We are seeing some latent symptoms from earlier infections of black rot and Phomopsis. Michigan State University Extension advises growers and scouts to continue monitoring for Botrytis and sour rot in clusters, as well as downy mildew or powdery mildew on leaves to determine if treatment is needed.

In Vignoles in Berrien County, Botrytis was seen on a few berries on 8 percent of clusters, and 6 percent of clusters had signs of black rot infection. Powdery mildew was observed on 2 percent of leaves, and some yellow spots, indicative of downy mildew, were observed on 1 percent of leaves. However, there were no spores on the underside of the leaves, indicating that the infection was not active.

In Concord in Berrien County, neither Phomopsis nor black rot were observed on clusters of the vineyards scouted, but signs of downy mildew were seen on 9 percent of leaves during scouting. As in the Vignoles vineyards, there were no spores on the underside of the leaves and it appears the infection is not progressing.

In Chancellor in Van Buren County, we are continuing to closely monitor downy mildew and powdery mildew. Yellow spots similar to those seen in other vineyards were observed on 3 percent of leaves, but again no spores are being produced on these leaves. No signs of Phomopsis or black rot were found on the clusters in these vineyards, but Botrytis was found in 1 percent of clusters.

In Niagara, Phomopsis symptoms were not seen, but black rot was found on 7 percent of clusters. Signs of downy mildew were seen on 7 percent of leaves, but as was observed in the other vineyards we scout, no spores were developing on the leaves with these symptoms.

Insects

The third flight of grape berry moth is continuing and we are likely near the peak of this flight. Males were caught at all the farms we scouted in Berrien and Van Buren counties, with zero to three moths per trap in Berrien County and one to 25 moths per trap in Van Buren County.

Insecticide wash-off caused by recent heavy rains is likely to reduce the activity and effectiveness of previous insecticide applications, so the risk of damage caused by this generation is high. Across the sites we are monitoring, there is a range of third generation feeding damage. In Berrien County, 2 to 10 percent of clusters had evidence of grape berry moth damage, and 6 to 40 percent of clusters at vineyards in Van Buren County contained evidence of damage from this pest. These numbers have remained steady over the last two weeks, which suggests managing third generation grape berry moth has been successful in these vineyards.

Most of the observed grape berry moth damage has been found on the outermost vines that are adjacent to woodlots containing wild grapes. Growers and vineyard managers should be checking clusters on vineyard borders to determine if an additional insecticide application is needed as harvest approaches. A border spray using a contact insecticide such as Imidan (note the 14 day re-entry interval!), Sevin or a pyrethroid (e.g., Baythroid or Mustang Maxx) can be used to help clean up cluster infestations. It is very important for growers and vineyard managers to use an insecticide that has a different mode of action than the insecticide that was used for the second generation; this rotation of insecticide classes is an important part of managing insecticide resistance.

We predict that a fourth generation of grape berry moth is highly likely to occur this year, beginning before or during harvest. Growers and vineyard managers should monitor MSU Enviro-weather’s grape berry moth model. In vineyards with a history of high infestation at harvest, growers should plan to protect their clusters starting at 2,430 GDD from wild grape bloom in any vineyards with more than a week to harvest. Currently, Enviro-weather predicts fourth generation egglaying to begin around the end of August in warmer areas such as Benton Harbor and Lawton, and a few days later in vineyards around Berrien Springs.

As we are now nearing harvest for many vineyards, be aware of pre-harvest and re-entry intervals and consider applications only to the border 10 rows where infestations are generally greatest. Please refer to MSU Extension’s “2016 Fruit Management Guide” (E0154) for additional information about insecticide modes of action and choices for grape berry moth management.

Potato leafhoppers, grape leafhoppers and Japanese beetles are still mostly absent from the vineyards we scout across southwest Michigan. However, growers and scouts should continue monitoring for these pests, particularly if there is a history of problems with these insects in previous years. Additionally, as fruit ripen there is increasing potential for grape clusters to be fed upon by bees, wasps, ants and Asian lady bugs. These are rarely at levels high enough to cause economic loss, but can be a major nuisance during hand-harvest. It’s much better to be regularly scouting vineyards to check on these potential pests during the lead-up to harvest rather than finding them on harvest day. Keep ahead of the pests to help with a smooth harvest!

Immature stages (nymphs) of brown marmorated stink bugs have been caught in traps in southwest Michigan vineyards, and these insects have also been observed on vines during scouting. No damage from this pest has been found, but growers and scouts should know what these insects look like. Brown marmorated stink bugs have been a late-season pest in grapes in the northeastern U.S., and they could pose a new problem around harvest in Michigan vineyards. Visit MSU’s Brown Marmorated Stink Bug website for pictures and more information.