Vegetable Production Updates
- Downy mildew was reported on cucumbers, cantaloupe, and butternut squash in sentinel plots at the Mountain Research Station in Haywood County. Downy mildew was also reported on cucumbers in sentinel plots in Henderson County.
- Southern blight was reported on cucurbits, peppers, and tomatoes in Western North Carolina. In most cases, southern blight has been reported in fields that were not fumigated.
- Powdery mildew was reported on squash in Western North Carolina.
- Magnesium deficiency was reported on cucumbers in Western North Carolina. Magnesium deficiencies are common in sandy soils with low PH or with low concentrations of magnesium (less than 70 ppm). Symptoms will appear a few weeks before harvest, when vines are rapidly growing, and fruit begin to enlarge. A gray-green discoloration of the interveinal tissue will occur on the crown leaves, which will expand and become brown or tan over time. In severe cases, the leaf tissue will disintegrate. For management, test soils regularly and amend with dolomitic lime when appropriate.
Cucurbit Downy Mildew
Cucurbit Downy Mildew is caused by the fungus-like oomycete pathogen Pseudoperonospora cubensis. It infects forty species in twenty genera within the Cucurbitaceae family including cucumber, watermelon, melon, cantaloupe, squash, and pumpkin. High-humidity and moisture, and cool temperatures (~60 ℉) favor disease development. In North Carolina, the disease typically begins in June and lasts throughout the growing season.
Symptoms
Leaf lesions appear as angular, yellow to brown spots on the upper side of the leaf. Leaf lesions are restricted by leaf veins. Under favorable disease conditions, the undersides of leaves may be covered in a mass of dark-colored spores which is also called “downy growth”. Downy mildew can be confused with other foliar diseases such as Alternaria leaf blight, angular leaf spot, anthracnose, powdery mildew, and Phytophthora leaf blight.
Signs
Under favorable disease conditions, the undersides of leaves may be covered in a mass of dark-colored spores which is also called “downy growth”. However, the color of the masses can range from colorless to gray-brown to deep purple depending on the density and age of the sporangia. Sporulation can be seen in the field with a 20x hand lens. They are most noticeable in early in the morning when dew is present or immediately following rainfall. Under a compound microscope, P. cubensis forms large (20-40 x 14-25 mm in diameter), lemon-shaped sporangia with a conspicuous papilla.
Management
Plant early in the season to escape high disease pressure. Do not allow water to remain on leaves for long periods of time as this can favor disease development. Scout plants often and remove infected plants. If possible, plant tolerant varieties and protect the crop with fungicides. For the latest fungicide recommendations for cucurbit downy mildew see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels. Resources: Quesada-Ocampo, L. 2013. Cucurbit Downy Mildew. North Carolina State University. Plant Disease Fact Sheet. Retrieved June 14, 2018. Colucci, S.J. and G.J. Holmes. 2010. Downy Mildew of Cucurbits. The Plant Health Instructor. Retrieved June 14, 2018.
