Blackleg Plant Disease of Potato
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Blackleg is a plant disease of potato caused by pectolytic bacteria that can result in stunting, wilting, chlorosis of leaves, necrosis of several tissues, a decline in yield, and at times the death of the potato plant. The term "blackleg" originates from the typical blackening and decay of the lower stem portion, or "leg", of the plant.
Blackleg in potatoes is most commonly caused by Pectobacterium atrosepticum (older synonym: Erwinia carotovora subsp. astroseptica), a gram-negative, nonsporulating, facultative anaerobe that is also associated with soft rot of potatoes. While other bacterial species such as Pectobacterium carotovorum and Dickeya dadantii can exhibit symptoms similar to blackleg of potato, these pathogens exhibit broader host ranges, are present in different climates, and typically are more associated with soft rot diseases. Early blackleg symptoms develop in the growing season soon after the plants emerge. They are characterized by stunted, yellowish foliage that has a stiff, upright habit. The lower part of the below ground stem of such plants is dark brown to black in color and extensively decayed. When infected, the pith region of the stem is particularly susceptible to decay and may extend upward in the stem far beyond the tissue with externally visible symptoms. Young plants affected by blackleg are particularly susceptible, typically dying after a halt in development.
Blackleg symptoms may develop in more mature plants during the latter part of the growing season, and are distinguished from those that develop earlier in the season. Blackleg appears as a black discoloration of previously healthy stems, accompanied by a rapid wilting, and sometimes yellowing, of the leaves. Starting below ground, black discoloration moves up the stem, often until the entire stem is black and wilted. However, in some cases of early disease development, mature stems may turn yellow and wilt even before black decay is evident. However, after the entire stem exhibits disease symptoms, the wilted plant can be lost from view in the healthy potato plant canopy. The pathogen P. atrosepticum thrives in moist, cool conditions, typically causing symptoms at temperatures below 25°C. It is vulnerable to temperatures above 36°C and dry conditions, and thus survives best in potato tuber tissues, although it is known to survive in other plant tissues. Unlike other pectolytic bacteria, evidence shows that P. atrosepticum does not survive well in soil outside its host tissue.
Disease symptoms are not necessarily uniformly exhibited from both shoots originating from a single tuber or in a field infested with P. atrosepticum. Additionally, presence of P. atrosepticum in the soil is not necessarily associated with disease symptoms.[3] This is partly explained by the narrow environmental conditions needed for pathogenicity, although new findings in research are showing strong evidence of density dependent quorum sensing signals used by P. atrosepticum in exhibiting virulence. Given the success with cultural control practices in managing the disease, cultivars resistance is better characterized in the U.S. by susceptible varieties. Washington State University, which has posted a large comprehensive list of potato cultivars available in North America, only calls out two blackleg susceptible varieties: Monona and Superior. In the U.K., and more specifically in Scotland, where the disease has been an issue, they better characterize blackleg-resistant varieties. Varieties with resistance values of 6-9 on a scale of 1-9 include Avondale, Axona, Bonnie, Cara, Emma, Isle of Jura, Orla, Osprey, Sarpo Mira, Saxon, Sebastian and Vales Sovereign.
Regards
Tony Wilson
Journal Coordinator
Research Journal of Plant Pathology