Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
Our wet cool weather this past week has been favorable for two stem diseases of potato, which are caused by bacteria in the Pectobacterium group. They are aerial stem rot and blackleg. Blackleg begins on tubers and progresses up the stem and aerial stem rot begins with an infection at a wound or natural opening such as a leaf scar on the stem. Blackleg and aerial stem rot inoculum originates from infected tubers, infested soil, or other contamination.
Even though the infections may come from infected tubers, the symptoms may not occur until the plants have begun to flower. Symptoms are black to brown slimy (or mushy) decay of the stems. The conditions that favor potato plant growth are less favorable for blackleg or tuber rots (i.e. more disease will occur if the weather is either very cool or hot). Management of these diseases focuses on reducing inoculum development. To minimize the disease use certified potato seed pieces. Use either small tubers for planting or allow the seed pieces to heal (cork) over before planting. Don’t plant in waterlogged or low-fertility soils, and space plants so that air can move around the plants to reduce moisture. Wet soil promotes infection of tubers through lenticels (small natural openings in the potato skin). Limit irrigation to the morning and apply longer, less frequent irrigation rather than short frequent irrigations. During storage, keep the tubers at 50 to 55°F for 10-14 days for wound healing. Following healing the temperature can be lowered below 50°F to reduce bacterial growth (though temperatures should not be low enough to promote conversion of starches to sugars).
A free chlorine wash maintained at 25 ppm chlorine or a fresh chlorine rinse maintained at 50 ppm chlorine may help reduce soft rot in storage.
Blackleg of potato.
Image from Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org