Gray mold occurs in all climatic zones in field crops, under covers, and in storage. Its wide range of host plants (over 200 species) makes it economically significant for many crops. In potatoes, gray mold is not a economically important disease. The initial symptoms are usually observed at the leaf tips, presenting wedge-shaped necrotic spots of various sizes, tending to enlarge. Their development is often constrained by the main leaf veins. Although the initial symptoms typically begin at the leaf margin, under favorable conditions for disease development, infection can also occur elsewhere. The appearance of the necrotic spot, with its wide concentric zonation, can sometimes be misleadingly diagnosed as symptoms of potato early blight or later symptoms of late blight. The disease gets its name from the formation of gray spores on the necrotic spot, in contrast to late blight, where the fungal growth with spores forms around the necrotic spot. The source of infectious material is the spores, which, like in the case of late blight and early blight, can be dispersed by wind over long distances. Leaf infection always occurs in the presence of water, rain, fog, or irrigation, and high humidity (above 90%). The optimal temperature for infection is 18 to 24°C. Under such conditions, infection can occur within 5 hours. In addition to the characteristic appearance of the necrotic spot, another distinctive symptom of the disease is the infection starting from the infected flowers falling on the leaves and stems.

The pathogen can also affect tubers, but these cases are rare.

Prevention There are no chemical means for control or limitation. Avoid overly dense planting. Avoid excessive watering. After harvest, destroy crop residues to prevent overwintering of inoculum for the next season. Use increased potassium fertilization.

Compiled by: Dr. Eng. Jerzy Osowski