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Potato Mop Top Virus (PMTV) – Biology

PMTV was first reported in Britain in 1966 and has subsequently been found in Europe, South America, North America and Asia. In North America it has been confirmed in Colorado, Idaho, Maine, New Mexico, North Dakota, Oregon, Washington and Canada. It was first reported in the USA in Maine in 2002. It likely resides in other production areas where powdery scab is present.

In the US, field infection of PMTV appears to be limited to potato and a few weed species including hairy nightshade, black nightshade and lamb’s quarters. Other potential crop hosts include sugar beet, tomato and pepper.

PMTV is transmitted by Spongospora subterranea f. sp. subterranea, the causal agent of powdery scab. Growers who find PMTV in their potatoes have not one, but two, diseases to contend with. The protozoan organism that causes powdery scab, once thought to be a fungus, occurs in most potato production areas. It persists in the soil due to the production of resting spores (cytosori) that remain viable in the absence of a host for more than a decade. PMTV is taken up during the development of resting spores and remains infectious inside the spore. When a host plant is present and environmental conditions are favorable, the resting spores germinate and release swimming spores (zoospores) that carry the virus. The zoospores infect root hairs, roots, stems, stolons and tubers and release the virus inside the plant. The virus can then reproduce and spread within the plant host. Potatoes are most susceptible to powdery scab during tuber initiation and early bulking, a period which generally lasts 3-4 weeks. Inoculum is produced by scabs on tubers as well as by galls on roots and builds up in the soil over time. Because spore balls contain hundreds of resting spores, even one spore ball is enough to cause significant disease. S. subterranea f. sp. subterranea can infect more plants species than PMTV, but it only produces long-lived resting spores on a few hosts including potato, tomato, black nightshade and oat. PMTV can survive within the resting spores for up to 18 years.

Environmental conditions are critical for the development of powdery scab. Development of powdery scab disease requires cool, wet weather conditions. Even with high levels of inoculum in the soil, disease will not develop on even the most susceptible cultivars unless conditions are right. Favorable temperatures are between 52-65⁰F, with 60⁰F being optimal. Soil moisture is particularly critical as zoospores require water to swim to new host plants. Powdery scab rarely develops in areas with less than 30 inches of rainfall per year but may occur in warmer, drier climates where irrigation is used. Sandy soils or soils with poor drainage are conducive to development of the disease. Under favorable conditions, multiple infection cycles can occur during the growing season.

PMTV and powdery scab are primarily spread in infested soil adhered to the tuber and to equipment, and in scab lesions on the surface of the tuber. These inoculum sources are important considerations for the management of the pathogens. Spore balls can also be distributed by wind from neighboring fields and in manure from animals that have been fed with infected potato debris. Transmission of the virus from mother plant to daughter tubers depends on cultivar and environment. In many cultivars, transmission appears to decrease with each subsequent generation resulting in a self-limiting infection.

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