Gummy Stem Blight (Black Rot)

HOSTS

• Cucurbits

DESCRIPTION

Gummy stem blight or black rot when affecting squash fruit, is caused by the fungus Didymella bryoniae. It affects squash, cucumber, melon, and watermelon.

BIOLOGY

D. bryoniae is seedborne and soilborne. The fungus can survive on infected plant debris for up to 2 years in the soil. Seedlings can become infected if the seed is contaminated. Optimum conditions for infection range from 65 °F to 75 °F (depending on the crop) and moisture of up to 10 hours. The fungus can produce fruiting structures on decaying tissue after infection, and the spores move to new leaves and neighboring plants through splashing water.

SYMPTOMS

• Stem lesions that form a brown, cracked area and may ooze a red to amber gummy substance.
• Dark brown circular spots on leaves that start at the margins.
• Large necrotic lesions on squash fruit.
• Distinct concentric rings on butternut squash fruit.

GENERAL MANAGEMENT

This disease is most common in Utah on fruit in the field and seedlings produced in greenhouses, where humidity is higher.

• Use certified disease-free seeds as seedlings can be infected but show no symptoms.
• Maintain good sanitation in growing areas by disinfecting trays, pots, tables, and ceilings.
• Rotate out of cucurbit crops for 2-3 years.
  
• Remove and destroy infected plants.
  

FUNGICIDES

Several fungicides are available, but should only be used upon confirmed diagnosis. There have been reports of Didymella bryionae having developed resistance to some fungicides in other states, so rotation between fungicide classes is critical.