Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
Soft rot, which is a widespread disease of vegetables caused primarily by Pectobacterium spp. (formerly calledErwinia),has been observed this past week in cabbage. Numerous vegetable crops are susceptible to this disease, including cabbage, broccoli, onion, pepper, melon, cucumber, bean, and beet (for potato, see below).
The pathogen is a bacterium and overwinters in infected tissue, farm implements, and plant debris. The pathogen invades primarily through wounds. Therefore, it is important to reduce wounding during planting, cultivation, harvesting, and subsequent transport. In addition, uninjured tissue can be infected through natural openings when free moisture is present. Avoid waterlogged soils. The pathogen prefers high temperatures and multiplies quickly at high temperatures, although disease can progress rapidly when conditions are not optimum for plant growth, even if the temperature is low.
Additional control measures include use of disease free seed and transplants; rotation with crops such as corn, small grains, or alfalfa; and avoidance of insect feeding. Insect feeding creates wounds and insects can carry the bacterium from plant to plant. As the crop grows avoid wounding during cultivation and field work. Also take care to minimize wounding in harvest, and store and ship produce in clean and cool conditions. Hold plants at 39 to 45°F to minimize spread following harvest.
Potato
Potato is also susceptible to Pectobacterium and a related bacterium, Dickeya (also formerly classified as Erwinia). Potato diseases include blackleg, aerial stem rot, and tuber rots. Blackleg always develops from a seed piece, whereas aerial stem rot develops from wound in the stem. Different bacteria cause the two stem diseases on potato and tuber rots are caused by several strains of these bacteria. The inoculum for these diseases commonly originates in infected seed pieces.
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). 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 ten to 14 days for wound healing. Following healing the temperature can be lowered below 50 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.
Tuber soft rot and blackleg of potato.
Images from Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org
Onion soft rot.
Images from Howard F. Schwartz, Colorado State University, and S. K. Mohan, Bugwood.org
Soft rot on cabbage.
Image from Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org