Southwest Michigan grape scouting report for August 11, 2015

Third generation grape berry moth flight is occurring now. Some leaf infections of downy mildew are appearing in Niagara grapes.

Sunscald on Chambourcin berries. This symptom sometimes occurs when leaf removal or shoot positioning happens before very hot, sunny weather. All photos by Brad Baughman, MSU Extension

Sunscald on Chambourcin berries. This symptom sometimes occurs when leaf removal or shoot positioning happens before very hot, sunny weather. All photos by Brad Baughman, MSU Extension

Weekly scouting report

Weather over the past two weeks has been relatively hot and dry, with rain showers in some areas Monday morning, Aug. 10. Soils are dry, and those growers using irrigation on young grapevines are running it now. Grape berry moth egglaying for the third generation will begin one or two days later than originally predicted. Traps are beginning to pick up the third generation flight now. This is the time to apply ingestion poisons for this generation; wait a few days to apply contact or “broad-spectrum” poisons.

Pressure from late summer leaf-feeding pests, including Japanese beetles, potato leafhoppers and grape leafhoppers, has been low or nonexistent in all vineyards we scout. Some downy mildew lesions on leaves have begun to appear. We expect downy infections that began during recent wet or dewy periods to spread on leaves in the coming weeks. Growers with susceptible varieties or a history of downy mildew pressure should consider an eradicative spray for this fungus. Juice grape and wine grape fruit are approaching veraison, and potential for new infection of downy mildew, powdery mildew or black rot in fruit has passed.

Enviro-weather degree day summary for 2015 (GDD50 from March 1)

GDD summary since March 1

Location

Aug. 10

Aug. 17 (projected)

Berrien Springs

1,870

2,025

Lawton

1,857

2,018

Enviro-weather grape berry moth model summary (GDD47 from wild grape bloom)

Grape berry moth egglaying dates

County

Wild grape bloom

Current GDD after wild grape bloom (as of Aug. 10)

Predicted start of third generation egglaying (1,620 GDD after wild grape bloom)

Berrien

May 26

1662

Aug. 9

Van Buren (Lawton)

May 28

1620

Aug.11

Van Buren (Mattawan)

May 29

1600

Aug.11

Fennville

June 3

1544

Aug.14

Berrien County farms visited Monday, Aug. 10

Concord is at berry touch and berry diameters range from 1.5 to 1.75 centimeters. Vignoles are at tight cluster with the largest berries at 0.75 to 1 centimeter.

The number of grape berry moth males caught in traps has remained relatively low, and on the last scouting visits to these farms captures ranged from zero to eight moths per trap. This slight increase in captures is a signal that the third flight is now underway. The MSU grape berry moth model predicted the start of egglaying for the third generation as Aug. 9 at the sites we scout in Berrien County, and the growers at these farms have applied insecticides that target eggs and young larvae to control this early activity.

Grape berry moth control appears to be very good in the vineyards we visited this week. In vineyard hotspots, some clusters have signs of grape berry moth damage, but most of this damage involves only a small entry wound on a single berry with no further increase in the severity of damage. In the vineyards we scouted Aug. 10, infestation ranged from 14 to 21 percent of clusters with grape berry moth feeding damage on vines adjacent to woods containing wild grape. Little to no damage was recorded on vines in vineyard interiors.

Very few Japanese beetles were seen during scouting this week, but some light feeding is visible on leaves. There have been reports of high numbers of this pest at some sites, but overall, beetle numbers are low and vineyards with a healthy canopy do not need treatment, even with a heavy crop load. Growers of susceptible vinifera and hybrid varieties should still scout regularly to detect possible build-up of Japanese beetle populations and plan to treat if canopy protection is needed. Potato leafhoppers and eastern grape leafhoppers were not seen during scouting at these farms, but growers and scouts should continue to look for these pests on the foliage.

Japanese beetle damage 
Japanese beetle feeding damage on leaf. Damage from this pest has been very slight in most vineyards this year.

The incidence of disease symptoms at the Berrien County scouting sites remains low. In Concord, 8 percent of leaves had signs of Phomopsis, but these symptoms are not expected to spread. No black rot lesions were seen on Concord leaves or clusters in vineyards that achieved good coverage during the wet periods that occurred during bloom. Downy mildew is beginning to appear though, with lesions on 5 percent of leaves. In Vignoles, a similar percentage of leaves were infected with Phomopsis, and old infections from this disease were seen on 4 percent of clusters. Single black rot-infected berries were seen on 3 percent of clusters, but no further infection is expected. Very low incidence of downy mildew (1 percent infected leaves) and powdery mildew (1 percent of clusters) was also observed in Vignoles. In all of these vineyards, these disease symptoms are not widespread and all infections appear to be old and not expected to increase. Michigan State University Extension recommends continuing to monitor leaves for downy mildew and powdery mildew. Treatment will be necessary in many cases to ensure the canopy remains healthy and able to ripen the crop.

Van Buren County farms visited Monday, Aug. 10

Niagara clusters are beginning to tighten and berry diameters are now 1.5 to 1.75 centimeters. Chancellor berries are 0.75 centimeters in diameter and clusters are tightening as this variety begins veraison. The number of grape berry moth males caught in traps ranged from zero to six moths per trap, showing that the third flight is also underway in this region. Grape berry moth damage in these vineyards has increased over the past two weeks and this late feeding by second generation larvae ranged from 42 to 49 percent of clusters with damage. As was observed in Berrien County vineyards, the incidence of damage is highest at vineyard borders, but the severity of damage in this region is higher, and over half of the damaged clusters have multiple berries webbed together.

The MSU grape berry moth model predicts egglaying by the third generation to begin Aug. 11 in Van Buren County. Growers in this area that choose to apply insecticides like Intrepid, Altacor or Belt that target eggs and young larvae should do so as soon as possible to manage this early activity. The best timing for broad spectrum insecticides, on the other hand, is 100 growing degree days from now, or about three to five days. Depending on the length of the third generation, or if a fourth generation occurs, an additional insecticide application may be required before harvest; be sure and check pre-harvest intervals (PHI) on any insecticide products used going forward.

Potato leafhoppers and grape leafhoppers were not seen during scouting Aug. 10th, and no Japanese beetles were observed. However, growers should continue scouting for these pests from now until harvest.

Diseases appear to be well-managed in the vineyards we scouted. Downy mildew and powdery mildew symptoms were absent from clusters in Niagara or Chancellor, even though these varieties are susceptible to these pathogens. Downy mildew leaf lesions, or “oil spots,” were seen in the Niagara vineyards, but the incidence of infected leaves remains low with 9 percent of leaves with symptoms for the time being. The disease has not moved onto the fruit in vineyards we scout. Low incidence of black rot leaf lesions were observed in Chancellor with 2 percent of leaves infected, but no infections were seen on the fruit in project vineyards. A few black rot lesions were seen on fruit in Niagara, and it appears these infections were held in check by fungicide applications. Phomopsis lesions were seen on leaves in Niagara with 3 percent of leaves with symptoms and Chancellor with 6 percent leaves with symptoms, but symptoms were not observed on any clusters. A low number of Botrytis leaf lesions were seen in Chancellor with 6 percent of leaves with lesions, but only one berry infected with Botrytis was seen on 100 clusters during scouting.

Please consult the “Michigan Fruit Management Guide” (E0154) for management options for insects, diseases and weeds.

This scouting report is supported by the Michigan Grape and Wine Industry Council and the National Grape Cooperative.

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