Southwest Michigan grape scouting report – July 26, 2016

Wine grapes are now out of the critical period of disease control, and the third generation control window for grape berry moth begins this weekend or early next week.

Weather and development

Last week was hot with highs in the 90s, which provided good heat for fruit development. Most areas received 2 inches or less of rain in multiple events last week. Concord berries in Berrien County are at berry touch, and wine grapes range from tight cluster to berry touch.

Growing degree-days base 50 accumulations for 2016

Location

July 26

Aug. 1 (projected)

Berrien Springs

1,685

1,820

Lawton

1,838

1,982

Diseases

Dry conditions continue and disease symptoms are at very low levels in the vineyards we are scouting, with 0-2 percent of clusters with visible infections and 1-10 percent of leaves with visible infections. Much of the disease symptoms we observed were on older leaves and appeared to be remnants of now-dead infections of black rot or Phomopsis.

Juice and wine grape clusters are now resistant to new infections from major diseases. Leaves can still be infected with downy mildew or powdery mildew, and new infections can still form on the rachises and stems of some wine grape varieties. Bunch closure is a critical time for application of materials for Botrytis in wine grapes, however pressure appears to be very low from this disease in 2016.

Michigan State University Extension advises growers continue scouting vineyards for development of these diseases. As morning dews become more common in August, the risk of downy mildew will increase, particularly in Niagara grapes.

Insects

Second flight of grape berry moth is ending, and third flight is about to begin. Most of the moths we captured this week were in traps placed near vineyard edges. Approximately 6 to 12 percent of clusters had evidence of grape berry moth damage in Berrien County vineyards, and 15 to 25 percent in Van Buren County vineyards. However, most of the damaged clusters had only a single berry sting. This suggests the applications made during the early parts of the egglaying period were effective at preventing larvae from infesting multiple berries.

According to the grape berry moth Enviroweather model, we can expect egglaying to begin around July 30 near Benton Harbor, July 31 in Lawton and Aug. 1 or 2 in Berrien Springs. Because of this early start to the third generation due to warm temperatures, it is very likely we will have a fourth generation of grape berry moth in 2016.

Growing degree-day (GDD) base 47.5 accumulations for grape berry moth

Location

Wild grape bloom (approx.)

GDD 47.5 as of July 25

Beginning of control window (1,620 GDD)

Berrien Springs

May 28

1,415

Aug. 2

Benton Harbor

May 28

1,504

July 30

Lawton

May 30

1,413

July 31

It is important for growers and vineyard managers to use a different mode of action in their third generation spray than they did for their second generation spray. Ingestion insecticides like Intrepid, Altacor or Belt should be applied at the beginning of egglaying (see dates in table) to be most effective. Contact insecticides such as Imidan, Sevin or pyrethroids (Baythroid, Mustang Max, etc.) should be timed for egg hatch, which occurs several days later. Often, a second application of a contact insecticide is needed to control the entire third generation. Due to berry sizing, good coverage of clusters is essential for an effective third generation spray. For those planning on using Intrepid, spraying a few days early to get into the clusters before they close can be better than waiting until egglaying begins.

Potato leafhoppers and grape leafhoppers are still mostly absent from vineyards across southwest Michigan. Japanese beetles are increasing at some southwest Michigan vineyards, but numbers are still very low where we are scouting. However, growers and scouts should continue monitoring for these pests, particularly where there is a history of problems.

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