A comparative study of virulent and avirulent isolates of Chalara elegans on roots and shoots of bean (Phaseolus vulgaris)

Chalara elegans Nag Raj and Kendrick, also known by the name Thielaviopsis basicola (Berk. and Br.) Ferr., causes black root rot disease on many plants including bean (Phaseolus vulgaris). This thesis describes the infection process and host response using virulent (BK28) and avirulent (BK28R) isolates of C. elegans on root and shoot tissues of two cultivars of bean. A study of the relationship of inoculum level to lesion occurrence showed that 10-36 phialospores were required to initiate infection at point inoculation sites. Lesions appeared on both hypocotyls and primary roots inoculated with BK28 at 3-4 days after inoculation, and continued to enlarge after their initial appearance. The BK28 isolate penetrated directly, and infection hyphae were observed in epidermal cells of bean roots by 30 to 36 hours after inoculation and by 36 to 42 hours in hypocotyl tissues. Isolate BK28 produced secondary phialospores germination and penetration by secondary phialospores were observed only on roots. The avirulent isolate BK28R rarely penetrated either hypocotyl or root tissues. Where penetration occurred, infection hyphae of BK28R were observed only in epidermal cells of hypocotyl tissues, and in both epidermal and cortical cells of roots. Sporulation by BK28R was not observed on either root or hypocotyl tissues, whereas BK28 sporulated at infection sites on both roots and hypocotyls. Appressorium-like enlargements were produced infrequently by the two isolates. Isolate BK28 penetrated at a significantly higher frequency than BK28R, on both root and hypocotyl tissues. Both isolates caused browning of colonized cells at infection sites. (Abstract shortened by UMI.)...