Gummy stem blight, or black rot, caused by the fungus Didymella bryonia, is an important disease of cucurbits, particularly pumpkins and squashes.    

Symptoms

Gummy stem blight, or black rot, can occur at any growth stage and on any aboveground plant part. Infection of vines and leaves, which is usually associated presence of gum on vines, is referred to gummy stem blight. When the disease occur on fruit, it is called black rot. Leaf symptoms appear as dark yellow or reddish brown lesions of various shapes. Lesions begin at leaf margins but rapidly extend back into the leaf blade, causing curling, shriveling, and leaf death. In contrast to bacterial diseases, tiny, black fruiting bodies may be seen (using a hand lens) within leaf lesions caused by the gummy stem blight pathogen. Vine infections appear as water-soaked, cracked, brownish cankers that produce a reddish gum. Fruit symptoms vary among crops but generally appear as a brown to black rot of the rind, flesh, and seed cavity, accompanied by heavy white and black fungal growth. Lesions may develop anywhere on the fruit, first as water-soaked areas dotted with fruiting bodies, as described above. Fruit may decay at the site of attachment as a result of the fungus invading the stem.

Life Cycle

The pathogen can be seedborne or can survive on plant debris from previously infected cucurbits or on wild or volunteer cucurbits. Following primary infection, secondary spores are released in a gummy substance, which makes them adapted for short-distance spread by splashing water. Infection of fruit commonly occurs through wounds or through dying flowers. The optimum conditions for growth of the organism are temperatures from 61 to 75 F and free moisture in the form of dew, fog, or rain.  

Management

• Crop rotation and clean tillage (for example, plowing) help reduce the risk of disease by reducing the amount of primary inoculum (spores) in the immediate area.
• Resistant varieties are not currently available.
• Use only pathogen-free seed produced in arid western locations.
• Apply a recommended fungicide when conditions favor disease.