Potato plant and tuber infection and soil colonization byVerticillium tricorpusandVerticillium albo-atrum"group 2"

Verticillium wilt of potato (Solanum tuberosum) is a vascular disease that causes reduced tuber yields and diseased seed tubers. Several species of Verticillium are associated with the disease complex. To better understand the role of Verticillium tricorpus and Verticillium albo-atrum "group 2" in the verticillium-wilt complex, the occurrence and population levels of these two fungi species were studied in the potato host. In field and greenhouse experiments, potato plants were wound inoculated through the soil with a suspension of 12 × 106 spores/mL of V. tricorpus or V. albo-atrum group 2. Destructive sampling were performed on five dates, during which plants were separated into six parts: roots; lower, middle, and upper stem; and the stem-end and eye-end of the tuber. Soil samples were also collected. DNA extractions were performed, and polymerase chain reaction (PCR) techniques were used, with species-specific primers, to determine the population levels of each of the two pathogens, relative to a series of standards of known concentrations for the respective pathogens. Results showed that V. tricorpus and V. albo-atrum group 2 infected and colonized all parts of actively growing potato plants and tubers, even though visually assessed foliar-wilt incidences were 0%. In addition, relative population levels within each of the two species were low to moderate in the plant stems but high in the soil's rhizosphere. Our results demonstrate that these two species are associated with the verticillium-wilt complex, but they do not appear to be aggressive pathogens.