Vegetable Disease Update: September 6, 2018

New reports of cucurbit downy mildew have decreased across the region, but variable weather conditions are forecasted through the weekend.
Vegetable Disease Update: September 6, 2018 - News

Updated: September 6, 2018

Vegetable Disease Update: September 6, 2018

V-shaped lesions characteristic of black rot on the leaves of cole crops. Photo: Beth K. Gugino, Penn State

This will likely lead to further disease development and spread in fields not being managed with a protectant or downy mildew specific fungicides. Over the past couple of weeks, reports have originated from fields that were being sprayed regularly, but due to the weather, extended spray intervals and wet field conditions limited the ability to get good coverage. Good coverage is critical!

We are still interested in reports so if you suspect cucurbit downy mildew on your farm, please contact your local Penn State Extension Office or let Beth Gugino know via email at or by phone at 814-865-7328. Every confirmed report of downy mildew enables us to improve disease forecasting accuracy for the benefit of cucurbit growers not only in Pennsylvania but all along the east coast. This includes reports made from previously reported counties. The latest information on reports of cucurbit downy mildew can be found at the CDM ipmPIPE website.

Phytophthora blight

Phytophthora blight is showing up in some cucurbit and pepper fields that have been affected by flooding over the past several weeks. It can cause crown, stem, and root rot, as well as fruit rot. On maturing fruit, water-soaked lesions develop a thick powder sugar-like coating on the fruit, especially fruit touching or close to the soil surface.

In fields continually exposed to rain, spray programs have been unable to slow disease progression. It is essential to keep detailed field notes of where this disease develops on the farm. This information can be used to determine future crop rotations and limit the number of susceptible crops in the rotation. Also, augmenting water drainage to restrict run-off from these fields will be essential to reduce spread between fields. Once the disease gets going in the field, it is challenging to manage. Consider disking in heavily infected portions of the fields to slow the spread of the pathogen during heavy rain events, and facilitate decomposition of the crop residue.

Powder sugar-like sporulation from Phytophthora blight on the surface of rotting pumpkin fruit. Photo: Submitted to B.K. Gugino for diagnosis.

Late blight

Late blight was confirmed on potato at the Penn State research farm in Centre County late last week, not far from the previous report on tomato which was determined to be US23. Cooler evening temperatures and longer dew periods will favor late blight development on crops being harvested later into the season, including high tunnel tomatoes.

If you suspect late blight, please contact your local Penn State Extension Office or let Beth Gugino know via email at or by phone at 814-865-7328. It is crucial that we collect a sample to characterize the genotype. The presence of a new genotype could alter our current and future management recommendations. For the latest reports visit USAblight.org.

Black Rot in Cole Crops

Be on the lookout for black rot on fall cole crops including cauliflower, kale, broccoli, etc. The application of copper-based products is typically recommended to reduce further spread by protecting the surfaces of adjacent plants from splash-dispersed bacteria. However, under very wet weather conditions the efficacy can be more limited.

Black rot is caused by the bacterial pathogen Xanthomonas campestris pv.campestris and has been a common problem for growers for over 100 years. It is primarily a disease of the above-ground portion of the plant. However, the bacteria can enter through the roots and move systemically within the plant. Plants can become infected at any stage of growth.

Typically symptoms first appear at the margin of the leaves where the bacteria enter into the hydathodes (natural opening) located along the leaf edge. However, the bacteria can also enter through wounds following heavy rain, hail, insect feeding, or mechanical injury. Depending on the weather conditions, symptoms may be visible within 8 to 12 days, or it may take up to 40 days for symptom expression.

Foliar lesions are usually yellow and V-shaped from the leaf margin toward the mid-rib. As the disease progresses, the veins in the yellow tissue will become dark in color and can extend from the leaves into the main vein. Optimal conditions for disease development are higher temperatures from 77 to 86°F and free moisture from rain, fog, dew, or irrigation. Extended periods of warm wet weather favor rapid pathogen spread and disease development.

The bacterial pathogen can survive season to season in crop residue, cruciferous weeds, and on seed. The bacteria are not thought to survive long in the soil in the absence of host tissue. The bacteria associated with the seed will infect the cotyledon leaves (first leaves following germination) and then the first true leaves through the hydathodes. As the bacteria multiply inside the leaf, they move through the xylem (water-conducting tissue) towards the stem. During the growing season, the bacteria are moved between plants through rain or irrigation splashing, blowing of detached leaves, insects, cultivation equipment, or people working in the field—especially when the plants are wet.

Since seed can be an important source of the pathogen, it is important to select high quality, pathogen-free seed. As few as two or three infected seeds in 10,000 are enough to cause a severe epidemic. Hot water seed treatment (broccoli and cauliflower at 122°F for 20 min) can help eliminate bacteria from the surface of the seed and under the seed coat. However, cole crop seed is prone to seed coat splitting and needs to be planted promptly.

Keep in mind that seed companies may treat for bacterial diseases like black rot and hot water treating the seed will void any agreements made with the seed company regarding seed performance. Surface disinfesting the seed with sodium hypochlorite will help minimize bacteria on the surface of the seed but not under the seed coat.

Scout transplants and rogue symptomatic ones as well as those in the surrounding flats which are likely infected, but not yet showing symptoms. Keep the transplant production area clean and disinfest transplant trays and other equipment between uses. Harden plants off by reducing water and fertilizer rather than by topping them mechanically. If you are not growing your transplants talk with your supplier to understand what measures are in place to manage black rot.

Once planted in the field, copper is the primary management tool and provides minimal efficacy when conditions are favorable. The emphasis needs to be placed on management before planting:

  • In the field, rotate a minimum of three years between cole crops to allow the crop residue to thoroughly decompose.
  • Eliminate cruciferous weeds in and around the field which can harbor the bacteria.
  • Implement practices that reduce potential leaf wetness and water splash during the season
  • Avoid field activities when the plants are wet.

As in bacterial disease on other vegetable crops, fixed copper will help slow disease spread in the cole crop fields from splash-dispersed bacteria, but will not help manage disease development once the bacteria are inside the plant.

Insect Observations

Be on the lookout for Allium leafminer in all allium crops . In past years, the second flights of the season have started in mid-September. Neonicotinoid insecticides are showing promise as a management tool. Stink bugs, Harlequin bugs, and other members of the Pentatomid insect family are now in the adult stage. Corn earworm trap counts are really high while fall armyworm populations are mostly up in the western part of PA.

Authors

Integrated vegetable disease management Plant pathogen diagnosis Disease monitoring and forecasting Sustainable crop production

More by Beth K. Gugino, Ph.D.