Cucurbit Alternaria leaf blight is a fungal disease of cucurbits caused by Alternaria cucumerina. Cucurbit crops include watermelon, cantaloupe, cucumber, zucchini, pumpkin, and winter squashes. This pathogen causes brown lesions on the leaves that can develop concentric, target-like rings and a yellow surrounding halo. Lesions can also develop on the fruit, appearing first as sunken, brown spots that can become dark and fuzzy as the lesion produces spores (sporulation). Fruit can also experience sunscald from leaf loss, as well as reduced yield. Alternaria cucumerina survives on infected soil-bound plant debris, the spores of the pathogen spread on wind, splashing water, workers, equipment, or insects.
Infection & Disease Cycle
The primary source of inoculum for A. cucumerina is infected soil-bound plant debris. The fungus can survive for up to two years in this debris as dormant fungal threads called “mycelia”. Once temperatures warm and leaf wetness increases, these soil-bound mycelia produce spores called “conidia” that can infect living plant tissue. These conidia can spread via wind, splashing water, equipment, or insects, causing lesions where they contact and infect the plant. These lesions can produce new conidia that infect new plant tissue, spreading through the same process. The pathogen may also survive on infected seed. Favorable conditions for spread and infection include warm temperatures (70-90ºF), high & prolonged leaf wetness, early & late-season plants, and poor plant nutrition especially low nitrogen.
Cultural management includes scouting regularly to identify the presence of the disease early, before it has had a chance to spread and cause significant damage. The following practices can also help prevent disease development:
- Plant resistant varieties when possible
- Plant certified disease-free seed
- Rotate away from susceptible cucurbit crops (>2 years)
- Destroy or deep-plow infested plant debris
- Maximize distance between susceptible cucurbit fields
- Avoid overhead irrigation
- Avoid working in fields when plants are wet
- Maintain proper nutrition, especially nitrogen
Properly-timed fungicides can also be used to manage disease. For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.
- Commercial Vegetable Production in Wisconsin (A3422) from the UW Extension Learning Store. This guide offers the latest recommendations for disease, insect, and weed management in Wisconsin’s most common commercial vegetable crops. Also included are lime and fertilizer recommendations as well as insect identification information and keys.
- UW Plant Disease Diagnostics Clinic. The University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) provides assistance in identifying plant diseases and provides educational information on plant diseases and their control.
- UMass Extension Vegetable Program. “Cucurbits, Alternaria Leaf Spot.” University of Massachusetts: Amherst: Center for Agriculture, Food, and the Environment, January 2013. https://ag.umass.edu/vegetable/fact-sheets/cucurbits-alternaria-leaf-spot.
- Watt, Bruce A. “Alternaria Leaf Blight of Cucurbits.” University of Maine: Cooperative Extension. Cooperative Extension: Insect Pests, Ticks and Plant Diseases, 2013. https://extension.umaine.edu/ipm/ipddl/publications/5086e/.
Written by Ariana Abbrescia and Amanda Gevens. Last updated Aug 2023