Symptom Identification and Control Methods of Potato Blackleg

[Pathogen scientific name] Erwinia carotovora var. atroseptica (Van Hall) Dye.

[Host crops] Many crops in the Solanaceae, Cucurbitaceae, Fabaceae, Apiaceae and other families.

[Disease diagnosis] It can occur throughout the entire growth period, mainly damaging the root-stem area and tubers. When seed potatoes are heavily infected, sprouts rot before emergence, or emerged seedlings die quickly; in mild cases, symptoms appear successively when seedlings are 10-20 cm tall. Diseased plants are stunted, yellowed and dwarfed; the cortex and pith of the underground root-stem area turn black and undergo water-soaked rot, hence the name blackleg. In severe cases, the blackened part extends to the mother tuber, causing the tuber tissue to become wet, soft and putrid, giving off a sour odor. The root system of diseased plants is completely destroyed, and they are easily pulled from the soil and die quickly. Sometimes the disease develops more slowly; diseased plants remain stiff and upright, with grayish-green leaves, recurved leaf margins, and the top leaves bearing a slight metallic sheen, and eventually the plants wilt and die. When the diseased stem is cut lengthwise, the vascular bundles turn brown; when the diseased tuber is cut lengthwise, the diseased tissue is soft and often forms black cavities, with a distinct black line at the boundary between diseased and healthy tissue. When dry, the stem end of the tuber shrinks and hardens. The pathogen is Erwinia carotovora subsp. atroseptica. The bacterial cells are short rod-shaped, with bluntly rounded ends, bearing 4-5 flagella arranged around the cell, without spores, and Gram-stain negative. On agar medium, colonies are grayish-white and circular.

[Occurrence pattern] The pathogen is mainly transmitted through infected seed potatoes and enters through wounds. Infection via the cutting knife during seed-piece cutting is the main route of disease spread. After invasion, the pathogen develops in the intercellular spaces of the seed potato's epidermal tissue, moves bidirectionally along the vascular bundles into the stem, roots and tubers, secretes pectinase, dissolves the middle lamella of the cell wall, causing cells to separate and tissue to disintegrate, leading to rot. Reinfection between tubers in the field occurs mainly through vectors such as seed flies or nematodes that cause damage and carry the pathogen. Temperature and humidity levels are the main factors affecting disease epidemics. Warm, humid conditions favor bacterial invasion. Poor drainage, excessive and frequent irrigation, wet and cold soil, and weakened plants affect wound-healing ability and reduce resistance, thus worsening the disease. After harvest, storage under poor ventilation and high temperature and humidity favors disease development.