In some potato varieties bunchy or rosetted terminal leaves may appear on young plants. Terminals appear darker green than the rest of the plant. Foliar symptoms of bacterial ring rot generally appear at midseason or later. Yellow areas develop on leaf margins or between veins and later turn brown, giving the leaves a burned appearance. The leaflet margin rolls up and becomes necrotic. Plants with advanced symptoms show vascular discoloration and milky, viscous bacteria may be forced from cut stems. In tubers the rot begins as a brown necrosis in the vascular ring and progresses to the surface. Cracks may appear on surfaces of tubers, which are frequently nothing more than hollow shells.
Life cycle
The bacterial ring rot bacterium overwinters in infected tubers. It does not live freely in the soil, but it can survive for several years as dried slime on equipment or potato sacks. The bacterium is highly contagious. It infects tubers through wounds that reach into the vascular ring. Seed cutting is the principal means of spreading.
The ring rot bacterium is a vascular parasite and moves up in the water-conducting tissues and produces toxins that cause foliar symptoms. Rosetting and other early symptoms of dwarfing occur when bacteria proliferate in very young stems of certain varieties. About half to three-fourths of the daughter tubers of an infected plant will be infected with the ring rot bacteria, but may not develop symptoms. Ring rot develops in tubers most rapidly at 64 to 75°F and only slightly at 37°F.
Solutions
Use only certified seed tubers, rotate out of potatoes for at least 1 year, and follow strict sanitation procedures when cutting seed. Periodically disinfect cutting tools in a 1% solution of calcium hypochlorite.
