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The following symptoms appear on young potato plants propagated from phytoplasma-infected tubers. They characteristically include formation of many axillary and basal branches with short internodes and narrow leaves. Typical witches'-broom symptoms include plant stunting with proliferated branches carrying small round leaves. No flowers develop at plant maturity and infected plants produce hairy sprouts. Before harvesting, adventitious aerial tubers are evident on infected potato plants. Tubers collected from infected plants become less dormant and commonly produce hairy sprouts. Infected plants produce numerous tiny tubers and sometimes no tubers at all. Severe and typical witches'-broom symptoms develop on plants grown under greenhouse conditions.
These symptoms could be attributed to the insect transmission of PWB phytoplasma from infected to healthy plants. Potato seedlings exposed to native leafhoppers started to show symptoms a few weeks later. Symptoms on seedlings resembled those of PWB on mature plants and included formation of proliferated branches with narrow leaves on new growth. Stems were shortened and tender and, later, affected plants were stunted. Plants produced no flowers but formed small and medium-sized tubers.
Phytoplasmas are obligate pathogens, similar to plant viruses, which require living hosts to reproduce and survive. So far, there is no appropriate method to control PWB disease. Although virus-resistant potato cultivars have been produced, information concerning resistance to phytoplasmas is lacking. However, preventive measures may help to reduce the incidence of PWB disease. These include controlling and breaking the life cycle of insect vectors. This approach can be implemented before planting potato tubers by destroying weeds that harbour adult leafhoppers or their eggs. Poaceous weeds like wild oats are favourite hosts for M. quadrilineatus. Spraying with insecticides will eliminate leafhoppers in the weedy edges of the field. Members of the Asteraceae family are susceptible to yellows-type diseases and are attractive to leafhoppers transmitting these diseases. Avoidance of growing plant species belonging to this family adjacent to potato fields will reduce the vector insect population and eliminate a potential source of phytoplasma inoculum.
Another recommended cultural practice is to remove symptomatic plants (stunted with narrow leaves) during roguing operations in potato fields. Elimination of very small tubers before planting potatoes will reduce phytoplasma infection attributable to tuber transmission.
Maintenance of appropriate cultural practices and planting potato tubers obtained from certified sources ensures a potato crop with high yield and quality for commercial production. In the long term, biotechnology could be the only reliable approach to develop resistant potato cultivars against PWB disease.
Potato witches'-broom disease may cause serious losses in potato yield. An initial study indicated that small and tiny tubers with a total weight of <500 g per plant were collected from phytoplasma-infected potato plants. This type of yield reduction is commonly associated with PWB phytoplasma transmitted by potato tubers. Lack of epidemiological information on this disease prevents any accurate estimate of the economic impact of PWB disease on potato yield.