2022 Pennsylvania Vegetable Disease Update for September 7

It has also been reported on summer squash in New Jersey and watermelon on Long Island, NY, in the past few days. These hosts are primarily affected by clade 1 isolates of the pathogen. Up until the past couple of weeks, downy mildew has primarily been affecting cucumber and cantaloupe, which are largely affected by clade 2 isolates. It is essential to continue to manage for downy mildew on all cucurbit hosts, especially if trying to maintain the foliage until later in the season. Downy mildew will not directly affect the fruit but defoliate the plant prematurely. The current wet conditions have been very favorable for downy mildew development across Pennsylvania. In PA, downy mildew has been confirmed in western PA on cucumber in Washington County, cantaloupe in Mercer County, and 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.

Figure 2. White sporulation characteristic of late blight on the underside of potato leaflets. Photo: Beth K. Gugino, Penn State

No new cases of late blight have been reported on tomato or potato across the region this past week. On potatoes, with these recent rains, it is crucial to maintain a regular fungicide program to protect the leaves as the tubers enlarge. Protectants being applied to help manage other foliar diseases, such as early blight, will also help protect against late blight. 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 U.S.

Post-harvest cucurbit fruit rots are likely rising with the recent wet weather. Infection frequently occurs in the field, and the symptoms develop post-harvest in the bin. Minimizing injury during harvest is critical. Injuries create a crucial opening that many pathogens can use to enter the fruit. Also, focus on harvesting mature fruit. Fruit harvested due to plant decline rather than maturity will have reduced storage life. If the temperatures are warm enough, curing harvested pumpkins and winter squash in the shade at 80 to 85°F for 7 to 10 days will help the skin harden. If there are fruit rot issues, this will also allow them to be detected before going to market. Pumpkins and winter squash are best stored between 50 and 55°F with 50 to 75% relative humidity for longer-term storage. Some common cucurbit fruit rots include Fusarium fruit rot, Phytophthora fruit rot, Pythium fruit rot, black rot (gummy stem blight), and anthracnose.

Figure 3. Fruit rots caused by (A) Phytophthora blight and (B) Fusarium. Photos: (A) Beth K. Gugino and (B) Tom Ford, Penn State, respectively).

If you observe fruit rot occurring in the field or post-harvest, take steps to identify the cause so that it can be factored into your crop rotation and subsequent management. Different fruit rotting pathogens have different host ranges, so crop rotation can be one strategy for reducing disease pressure in the future. For example, gummy stem blight/black rot only affects cucurbit crops, so rotating affected fields to crops such as sweet corn, cauliflower, and tomato (different crop families) before coming back in with butternut squash would be recommended. Also, remember that it is very easy for secondary soft rotting bacteria and fungi to infect damaged fruit and lead to additional losses. When done with harvest, disk under the crop to facilitate decomposition of the crop residue. Next season, consider growing cucurbits in a reduced tillage system or use straw mulch in the row middles to reduce direct contact of the fruit with the soil and reduce soil splashing onto the fruit.

Clinic Corner

What Type of Sample Does the Plant Disease Clinic Want?

Are you having trouble deciding what to send to the Penn State Plant Disease Clinic? Instructions to select and send plant disease specimens are available on the Penn State Plant Disease Clinic website. Most pathogens are present in the transition between living and symptomatic/dead tissue. Our general recommendations include collecting several cuttings (or entire plants) showing the symptoms of concern and sample from the transition zone. Prepare a sample showing a range of symptoms (e.g., early, intermediate, late, or more advanced). Fill a gallon-sized plastic bag with plant material, if possible. Fill out the entire specimen information form, both front and back, and include one with each sample.

Figure 4. Wrap roots and soil separate from foliage, add a dry paper towel and place the sample in a plastic bag to prepare for shipping. Photo: Jennie Mazzone, Penn State

Figure 5. Submit at least 1 cup of roots collected from multiple locations around the plant, or submit whole plants, for root rot analysis. Photo: Tosh Mazzone

Package samples to minimize degradation and damage during shipping. Package samples carefully and separately wrap roots/soil to prevent soil from getting onto the upper part of the plant and causing deterioration during shipping. Wrap each specimen in a dry, absorbent paper towel and place it in a plastic bag. If possible, do not expose samples to high temperatures and store them in the refrigerator before mailing them. It is best to avoid shipping samples at the end of the week to avoid having them deteriorate in shipping over the weekend.

If you suspect root disease, send the entire plant, or at the very least, the crown and roots. We request this because we need the lower, main stem of tomatoes/potatoes, etc., to isolate for vascular wilt fungi like Verticillium and check for discoloration of the vascular and stem tissues. If the sample is from a perennial plant, collect a handful of roots (about 1 cup) from multiple locations around the plant. Select a mix of large roots and fine feeder roots showing root rot symptoms (necrosis, sloughing). Sample live and dead roots. If the roots are entirely dead, keep in mind that we may no longer be able to detect root rot pathogens, even if they were responsible for plant death.

Additional instructions on selecting and sending plant disease specimens are available on the Plant Disease Clinic website. If you have further questions about how to handle a sample, first check with your local extension office. Contact the Clinic for all other questions plantclinic@psu.edu or 814-865-2204.