Sep 24, 2020
Late blight seen in Pennsylvania tomatoes; pests still a problem

September in the Northeast region has been dominated by warmer and drier than normal conditions, which has been layered on top of a warmer than average July and August.

The hottest summer on record was recorded in two of the seven climate sites monitored in Pennsylvania by the Northeast Regional Climate Center. Also, as of Sept. 10, 2020, the Department of Environmental Protection expanded its Drought Watch to include 18 counties in Pennsylvania and declared a drought warning for Potter County. Over the past several nights, light to moderate frost events across much of the state has signaled that the growing season is approaching an end, and that harvest is well underway if not already done for the season.

Field production update

Today, late blight was confirmed in a seven-acre tomato field in Lancaster County. The field had been maintained with a regular fungicide program, so symptoms were not severe, and the foliar lesions had minimal sporulation. Although field production may be winding up, late blight can still develop in high tunnel tomatoes. However, once all the plant tissue is dead, so is the pathogen. There have been several reports of powdery mildew on tomato in high tunnels. Powdery mildew on tomato looks very similar to that on pumpkin; however, it is caused by a different fungal pathogen specific to tomato. Powdery mildew could be confused with leaf mold; however, lesions from leaf mold will be yellow on the upper leaf surface with dense dark sporulation on the leaf’s underside. Unlike leaf mold that requires very high relative humidity, powdery mildew is favored by mild temperatures and moderate humidity. Very high relative humidity above 95% suppresses disease development.

Harlequin bug, a member of the stink bug family that feeds on cole crops, is typically more of a problem in states to the south of Pennsylvania, but it is showing up in large numbers in Lancaster County on mustard greens. We also expect to see adults of the fall generation of allium leafminer (ALM) at this time of year. In the past several years, when scouting on leeks in Lancaster, adults of ALM’s fall generation first appeared anywhere from the last week of September to the last week of November.

These are adults emerging from pupae that have aestivated, which is like a hibernation state through the summer. Scout fall allium crops for the oviposition marks. Weekly sprays during the adult flight period – as measured by the presence of new oviposition marks – have controlled ALM on fall leeks. Effective foliar materials that have been the most consistent include dinotefuran, cyantraniliprole, and spinetoram. Control has also been achieved with foliar sprays of abamectin, acetamiprid, cyromazine, imidacloprid, lambda-cyhalothrin, methomyl, and spinosad. A surfactant is recommended whenever applying foliar sprays on alliums due to their waxy coating on the leaves.

Left: Harlequin bug feeding on mustard greens. Photo: Debra Deis, Seedway. Right: Oviposition marks of allium leafminer. Photo: B. Lingbeek, Penn State

In sweet corn harvest evaluations at Rock Springs, GE cultivars that express the VIP (vegetative insecticidal protein), such as Remedy, or others in the Attribute II series, once again gave excellent (100%) control of corn earworm. In contrast, cultivars with various Cry (CryIAb, CryII, etc.) failed (control was only 40% or less).

Spider mites and thrips continue to be a problem, and aphids are showing up in multiple crops. Tomato spotted wilt virus, vectored by western flower thrips, caused field scale damage in peppers in western PA, and tomatoes in New Jersey. Striped cucumber beetle adults will cause scarring of fruit at this time of year. Cleaning up cucurbit fields should help reduce cucumber beetle pressure next spring.

Rind feeding on watermelon, probably from striped cucumber beetle. Photo: Penn State Insect ID Lab

Berry crops

Most plasticulture plantings of strawberries are establishing well. However, what is becoming apparent is that the shipment of strawberry plant material from place to place makes it very hard to predict which diseases are likely to show up in a given year. Growers should be ready for some of the recurring problems such as anthracnose fruit and crown rot, which will be present on susceptible varieties, while others such as Pestalotiopsis (aka Neopestalotiopsis) warrant keeping an eye peeled.

Growers should remove spent canes of floricane-fruiting (summer-bearing) raspberries and blackberries if this has not yet been done. Spent floricanes should be entirely cut to the ground, as this will remove inoculum of diseases such as cane anthracnose.

Likewise, canes of blueberry plants showing symptoms of flagging (cane death with leaves still attached) should be removed now as this may be a sign of cane diseases such as Botryosphaeria and Phomopsis. Lime sulfur can be applied after leaves have fallen off to help with Phomopsis control.

Spotted wing drosophila remains present in plantings of primocane-fruiting raspberries and blackberries and day-neutral strawberries, requiring continuing applications of effective pesticides. Bright sunshine during the fall frequently results in sunscald on primocane-fruiting raspberries. This should not be mistaken for a disease, and symptoms are generally present on berries for only a few days.

White druplets are a symptom of sunscald which appears following days with bright sun. Photo: K. Demchak, Penn State

Beth K. Gugino, Kathy Demchak and Shelby Fleischer, Penn State University

Photo at top: Powdery mildew lesions on the upper leaf surface of tomato. Photo: Beth K. Gugino/Penn State

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