Virulence of Chalara elegans on bean leaves, and host-tissue responses to infection

Chalara elegans (syn. Thielaviopsis basicola) is a soilborne plant pathogen with a wide host range, which initially establishes a hemibiotrophic relationship, following infection of its host. To assess the differences in virulence among isolates and to study colonization of host tissues, detached bean leaves were used in an inoculation assay. Two-week-old ‘Kentucky Wonder’ leaves were sprayed with 0.8–1.0 mL of inoculum (105 phialospores/mL) and incubated under moist conditions for 5 days at 20–23 °C. Necrotic lesions that developed were enumerated and measured for 27 isolates of C. elegans from a broad range of hosts and geographic locations. Lesion numbers were affected by culture age, with 2-week-old cultures providing the most lesions compared with older cultures. There were pronounced differences in virulence among isolates, which were correlated with differences in colony morphology. The isolates were grouped into high, moderate, and low virulence classes based on the number of lesions produced. Highly virulent isolates were darkly pigmented, while moderately and weakly virulent isolates had reduced colony pigmentation. Bean cultivars influenced the number of lesions that developed, and ‘Kentucky Wonder’ was the most susceptible compared with ‘Royal Burgundy’ and ‘Kentucky Blue’. In infected leaf tissues, the pathogen was initially localized to epidermal cells, which became necrotic and accumulated phenolic compounds. Callose deposits, cell-wall thickening, and accumulation of polyphenol oxidase were also observed in these epidermal cells after 36–48 h. Spread of hyphae to underlying mesophyll cells occurred after 48–72 h. Chlamydospores and phialospores developed on lesions within 5 days after inoculation. Most lesions were limited in size to less than 1 mm and resembled local lesions resulting from a virus inoculation. The ability of C. elegans to infect and induce defense responses in bean leaves has not been previously reported. This host-pathogen system provides an interesting model for further studies on the cellular responses and potentially induced resistance in beans, resulting from a soilborne pathogen.