Southwest Michigan grape scouting report – September 6, 2016

A heavy crop is accumulating sugar. Fourth generation of grape berry moth continues. Expect botrytis in vulnerable varieties.

Weather and crop development

The coming week’s weather is predicted to be hot with highs in the 90s, and we can expect some rain through the week and into the weekend. We are currently two weeks ahead of average in heat accumulation.

Concord berries in Berrien County have or are nearing full color. Clusters of all varieties are continuing to soften and accumulate sugar, and Niagara harvest is scheduled to begin Sept. 19. The earliest harvests are beginning now.

Growing degree-days (GDD) base 50 accumulations since March 1, 2016

Location

Sept. 6

Sept. 12 (projected)

Berrien Springs

2,653

2,765

Lawton

2,917

3,038

Insects

We are in the middle of the fourth flight of grape berry moth across southwest Michigan; males were caught at all the farms we scouted (zero to 13 moths per trap). Evidence of active feeding by grape berry moth was found in 3-20 percent of clusters. Fourth generation damage has, for the most part, been limited to one to two berries per cluster, and limited to the rows adjacent to woods. This lower-than-expected damage is perhaps due to recent cooler weather.

GDD base 47 accumulations for grape berry moth model

Location

Wild grape bloom (approx.)

Current GDD47

Beginning of fourth generation treatment window

Berrien Springs

May 28

2,508

Sept. 4

Benton Harbor

May 28

2,698

Aug. 28

Lawton

May 30

2,705

Aug. 28

A border spray using a contact insecticide such as Imidan, Sevin or a pyrethroid (e.g., Baythroid or Mustang Maxx) can still be used to help clean up cluster infestations of grape berry moth. The treatment window for this spray opened Aug. 28 in Benton Harbor and Lawton, and Sept. 4 in Berrien Springs. At the same time, fruit flies, ants and yellow jackets are appearing on ripe clusters with split berries, so a clean-up spray before harvest may be needed for these pests. For this clean-up spray, managers should use an insecticide that has a different mode of action (i.e., a different insecticide class) than the insecticide that was used for the third generation grape berry moth.

Continue monitoring for potato leafhoppersgrape leafhoppers and Japanese beetles, though damage from these pests has been absent or only occasional in most vineyards this year. Monitoring for brown marmorated stink bugs should also be occurring; we are finding immatures in traps and during vineyard scouting. So far we have not seen damage from this late-season pest, but they could pose a new problem around harvest, as has been reported in some vineyards in the northeastern U.S.

Diseases

Cluster infections of Botrytis and sour rots in vulnerable varieties are expanding, especially where there is some grape berry moth damage; up to 20 percent of clusters with some infection in the vineyards we scout. Early harvest may be necessary in some locations and varieties, especially if we continue to have moisture.

Symptoms of other common infections are still very low in the vineyards we scout (under 10 percent of leaves infected). Downy mildew infections don’t appear to be actively growing or expanding, and we are not seeing much new spore development. With rain forecast for later this week, there is increased risk of expanding downy mildew on leaves in most varieties, and Botrytis and sour rots in clusters in varieties with compact clusters. Michigan State University Extension encourages growers and scouts to continue monitoring clusters and leaves, and treat if necessary.

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