Cucurbit Alternaria Leaf Blight

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 Control:

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

Chemical Control:

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.

References

 


Disease profile written by Ariana Abbrescia and Amanda Gevens. Last updated Apr 2023.

alternaria lesions on cantaloupe leaf
Several Alternaria leaf blight lesions on cantaloupe. Photo credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, via: Bugwood.org
close-up of alternaria lesions on cantaloupe leaf
A closer view of Alternaria leaf blight lesions on cantaloupe. Photo credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, via: Bugwood.org
alternaria damage on mukmelon plant
Significant progression of Alternaria leaf blight on muskmelon - whole leaves become brown and dry. Photo credit: Paul Bachi, University of Kentucky Research and Education Center, via: Bugwood.org
microscope view of alternaria spores
Cucurbit Alternaria leaf blight spores (conidia). Photo credit: Paul Bachi, University of Kentucky Research and Education Center, via: Bugwood.org