2022 Pennsylvania Vegetable Disease Update for September 14

Later-season crops, especially greenhouse or high tunnel cucumbers, should continue to be scouted regularly and actively managed for downy mildew. Downy mildew has yet to be confirmed in PA on hosts preferred by clade 1 of the pathogen, which includes jack-o-lantern pumpkin, summer squash, winter squashes, and watermelon. Still, there are several reports in the surrounding states. In PA, downy mildew has been confirmed in western PA on cucumber in Washington Co. and cantaloupe in Mercer Co. and on cucumber in Adams, Cambria, Centre, Huntingdon, Mifflin, Dauphin, Lehigh, Juniata, Snyder, Lancaster, Berks, Bucks, and Delaware Co. If you suspect cucurbit downy mildew on your farm, please let me know either by email at bkgugino@psu.edu or by phone at 814-865-7328 or contact your local Extension Office. For the latest reports and forecasts, check out https://cdm.ipmpipe.org.

Figure 2. As of 9/14/2022, reports of downy mildew on cucumber and cantaloupe (left) and all other cucurbits (right) from the cdm.ipmpipe.org website.

Late blight was confirmed in two commercial potato fields in Michigan in early September. The infected fields were vine-killed to minimize disease spread and larger area impact. Once the plant tissue is dead, so is the pathogen. The isolates are in the process of being genotyped. Cool fall conditions and long overnight dew periods are ideal for the development of late blight both in the field and high tunnel. 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.

Be on the lookout for Botrytis gray mold in late-season tomato high tunnels with cooler temperatures. Although high tunnels provide direct protection from rain, high relative humidity and dew can still provide the moisture necessary for disease development. Gray mold affects many types of vegetables and ornamentals, so there are many potential sources of this pathogen. It quickly grows on weakened or senescing (dying) plant tissue such as old flower blossoms or leaf litter; however, it can still cause lesions on the leaves and stems. These lesions are whitish, very soft, and watery and typically develop near the stem end. Foliar lesions can be confused for late blight since both are irregular in shape; however, gray mold will develop more of a concentric ring pattern, and the fuzzy growth (sporulation) is darker and grayer in color and can develop on both the upper and lower leaf surface as well as the surface of stem lesions. Stems can become girdled, and break and foliar symptoms can become severe enough to cause defoliation. The fruit can become infected by dying flower petals that hang on as the fruit develop. This completely contrasts with late blight, which causes dark-brown greasy, firm lesions on the fruit.

Figure 3. Irregular zonate lesions are characteristic of Botrytis gray mold. Dense gray sporulation can often be seen on all symptomatic surfaces in humid conditions. Photo: Beth K. Gugino, Penn State

For gray mold, general sanitation is important since it is a very good saprophyte. Also, maximize and maintain good air circulation through cultural practices. Disease development is favored by temperatures from 64 to 75°F and typically develops on more mature plants with dense canopies. Foliar applications of products such as Scala (FRAC code 9, 1-day PHI), Botran (FRAC code 14, 0-day PHI), and Fontelis (FRAC code 7, 0-day PHI) will help manage the disease and are labeled for use in greenhouse (and high tunnel) tomato production.

This past season despite the dry weather, issues with bacterial canker on tomato were reported in several regions of the state. If bacterial canker was a problem on your farm this year, now is the time to think back through the production season while it is fresh in your mind and identify points in the crop production cycle where disease management could be improved. Managing bacterial diseases starts with the seed and ends post-harvest, requiring a season-long approach. As you clean up from this season, remember that wooden stakes can harbor the bacteria even when exposed to freezing temperatures overwinter. Therefore, stakes from symptomatic fields should not be used again to produce solanaceous crops like tomato and pepper. Stakes from asymptomatic fields should be sanitized before use by power washing and soaking in a sanitizing solution like sodium hypochlorite, Oxidate, Zerotol, etc., or subjecting them to high temperatures through kiln drying or steaming. Bacteria are more susceptible to hot rather than cold temperatures.

Also, when planning crop rotations this winter, allow 3 to 4 years between tomato/pepper crops to facilitate crop residue decomposition. The bacterial canker pathogen can also be seedborne, so ensure that you are sourcing high-quality seed from a reputable source, purchasing transplants from a supplier aware of the disease, and implementing a good greenhouse IPM program. These recommendations and strict sanitation practices in the high tunnel and field can help reduce potential losses.

Clinic Corner

Submitting Digital Samples to Plant Disease Clinic

Did you know that you can submit photos to the Penn State Plant Disease Clinic? Photos can accompany a physical sample sent to the clinic or submitted independently without a physical sample. Make sure to include a completed Specimen Information Form when submitting pictures. Email the form and digital images to plantclinic@psu.edu. Printed photos can be mailed to:

Plant Disease Clinic, The Pennsylvania State University, 220 Buckhout Lab, University Park, PA 16802

Please provide several photos of the site/field at large and in-focus close-ups of the affected plant part(s). The best images are those that provide context. For example, receiving this photo along with a physical sample of the affected plant provided the context needed to make an herbicide injury diagnosis. Figure 4 is a great photo showing an ornamental plant damaged by contact herbicide. The photo shows plant death in areas where herbicide was improperly applied.

Figure 4. Photo: Tosh Mazzone, Penn State

We do not want blurry photos or photos in which we cannot see symptoms (Figure 5, left). Blurry images are not helpful in diagnosis. Especially when taking pictures with your phone, check the image's clarity before hitting send in an email. Long-distance photos are also not useful if they do not provide context or show symptoms (Figure 5, right).

Figure 5. Photo: Jennie Mazzone, Penn State

If you have further questions regarding submitting photos or samples, check our Plant Disease Clinic website, which has additional instructions. Your local extension office may also be able to answer your questions. Contact the Clinic for all other questions at plantclinic@psu.edu or 814-865-2204.