2022 Pennsylvania Vegetable Disease Update for August 31

There have been no new reports on butternut squash (previously reported in MA) or jack-o-lantern pumpkin (reported in OH, VA, and NC). In PA, downy mildew was most recently confirmed in western PA on cucumber in Washington County and cantaloupe in Mercer County and previously on cucumber in Adams, Cambria, Centre, Huntingdon, Mifflin, Dauphin, Lehigh, Juniata, Snyder, Lancaster, Berks, Bucks, and Delaware Counties. If you suspect cucurbit downy mildew on your farm, please let me know by email at bkgugino@psu.edu or by phone at 814-865-7328 or contact your local Extension Office. Check out the Cucurbit Downy Mildew Forecast website for the latest reports and forecasts. 

No new cases of late blight have been reported on tomato or potato across the region this past week. If you suspect late blight on your farm, please let me know by email at bkgugino@psu.edu or by phone at 814-865-7328 or contact your local Extension Office. We are also always interested in a sample to genotype. For over the past decade, US-23 has been the predominant genotype affecting both tomato and potato in the northeastern and mid-Atlantic regions of the US.

Figure 2. Alternaria leaf spot lesions with characteristic concentric lesions on a cabbage leaf. Photo: Beth K. Gugino, Penn State

Be on the lookout for Alternaria leaf spot on fall-planted cole crops. The fungal species of Alternaria that affects cole crops is specific to cash crops, cover crops, and weeds in that plant family. Like other diseases caused by Alternaria, concentric ring patterns will develop as the lesions expand, and often the centers can crack and fall out. The disease is favored by moderate temperatures ranging from 75 to 82°F. The disease can also affect the heads of cole crops, causing small black to discolored brown spots easily colonized by secondary soft rotting bacteria. Selecting cultivars that have tighter heads and more readily shed water will tend to be less susceptible.

During the season, fungicides are the most effective tool. Check the 2022-23 Mid-Atlantic Commercial Vegetable Recommendations for a list of recommended products. Be sure to read the fungicide label to ensure that the specific cole crop you treat is labeled. Some products, for example, Luna Experience, are only labeled for brassica leaf greens and rotate between Fungicide Resistance Action Committee (FRAC) codes for resistance management.

Clinic Corner

Diagnosing Viruses on Vegetables

Viruses make plants look strange and can often resemble herbicide drift injury. Viruses are submicroscopic, which means they are so small that you cannot see them with the naked eye and require a specialized microscope called an electron microscope to visualize. Therefore, we rely on plant symptoms to give us clues when they are infected with a virus. Symptoms associated with virus infections include:

• Reduced growth resulting in stunting
• Mosaic pattern of light and dark green (or yellow and green) on leaves
• Malformation of leaves or growing points
• Yellow streaking of leaves (especially monocots)
• Yellow spotting on leaves
• Ringspots (circular line patterns on leaves)
• Cup-shaped leaves
• Uniform yellowing, bronzing, or reddening of foliage
• Flower color breaking
• Distinct yellowing only of veins
• Crinkling or curling of margins of leaves

Viruses can enter healthy plants through wounds made by vectors or by equipment, such as pruning shears. Vectors transmit viruses from one plant to the next, including sucking insects, such as aphids, leafhoppers, thrips, and whiteflies, and chewing insects, such as beetles. Some viruses, such as cucumber mosaic virus (CMV), die quickly if outside a cell or if the cell dies. Still, other viruses, such as tobacco mosaic virus (TMV), retain their ability to infect for years after the infected plant part dies and can survive on hard surfaces.

Figure 3. Cucumber mosaic virus causes a yellow mosaic pattern on cucumber leaves (left) which is vectored by many species of aphids (right). Photos: Jennie Mazzone, Penn State

Weed management also plays a big part in managing viral diseases because perennial weeds often serve as a reservoir for viruses to survive when other host plants are absent. Viruses can also be carried in seed, cuttings, tubers, graft rootstocks, scions, or buds when propagating a virus-infected plant. For this reason, purchasing certified virus-free seed and planting stock is essential to avoid problems in many vegetables.

Figure 4. Ringspots, yellowing, and necrotic leaf spots on tomato are caused by tomato spotted wilt virus (left) and thrips larva insect vector (right). Photos: Jennie Mazzone, Penn State

If you think you may be dealing with a plant virus, send a sample to the Penn State Plant Disease Clinic. We can test for many of the common viruses that occur in PA.