Rhizoctonia solani Kuhn [teleomorph: Thanatephorus cucumeris (Frank) Donk] causing root rot and web blight (WB) on common bean (Phaseolus vulgaris L) have become very important in Puerto Rico. The objective of this research was to test a new methodology for screening for web blight resistance. Several methods for screening common bean for web blight reaction have been reported in Colombia, Costa Rica and Puerto Rico. These greenhouse conditions were effective for inheritance studies but are impractical and tedious when a. large number of lines need to be evaluated. We used a detached leaf method recently proposed by Steadman et al., as a technique to evaluated common bean lines and cultivars for resistance to white moid (Sclerotinia sclerotiorum). The youngest fully expanded trifoliolate leaves from bean plants grown in the field were detached and placed in orchid tubes containing 10 ml of tab water. Aluminum pans (30.5 χ 39 χ 9 cm) were lined with paper towels, and four 10 cm-petri dishes were inverted and placed on the bottom of each pan. These petri dishes were used to prevent the trifoliolate from touching the wet paper towel. Four 4mm-disks of potato dextrose agar (PDA) from 6 old-day cultures of Rs 011 and 012 were placed near the center of two foliolates of each leaf, the third foliolate was inoculated with PDA only. Water was added to the bottom of each pan and then covered tightly with a plastic bag to provide a high humidity environment. A constant room temperature of 24"C and diurnal room lighting with daylight were the incubation conditions. The length and width of the lesions were measured at 24, 36 and 48 hours after inoculation. Two bean lines from Puerto Rico, 9418-2 and 9607-27 and two cultivars from CIAT, Talamanca and Mus 138 were evaluated in each pan. A randomized complete block design with 4 replications was used. Significant differences in lesion length and width (P<0,05) among hencitypes were obtained. Mus 138 showed the highest level of resistance, followed by 9607-27 and Talamanca. 9418-2 line was highly susceptible to WB. In spite that there were not significant differences for WB lesions between isolates, the Rs 012 produced larger lesions than those of the Rs 011 isolate. No lesions were observed on the foliolate inoculated with PDA alone. This inoculation method should provide an effective and rapid technique for WB screening of bean lines and cultivars.


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