Abstract
Rhizoctonia solani causes stem canker and significantly impacts potato production. Conventional assay to evaluate potato resistance using R. solani inoculum is time consuming. To establish an effective and fast assay, 20 potato cultivars/lines were examined using both R. solani AG2-1 as an inoculum and its derived toxin (RS toxin). In field trials of 2009 and 2010, wheat-bran-mediated inoculum of R. solani AG2-1 was incorporated at either 0, 2, 3, 4 or 5 g per seed piece in the soil before sowing in the furrow. Stem canker was evaluated sixty-five days after planting. Inoculation at 2, 3, or 4 g R. solani AG2-1 well distinguished resistance of potato cultivars/lines. For a quick assay of resistance screening, a toxin-based method was established by treating potato seedlings with RS toxin of R. solani AG2-1. RS toxin was prepared by heating R. solani AG2-1 culture. Potato seedlings were obtained through tissue culture and grown in Murashige and Skoog medium. Seedlings at the stage of 12 cm in height were transferred into agar amended with RS toxin and incubated for eight days. The level of inhibition caused by RS toxin was positively correlated with toxin concentration. By evaluating various potato cultivars/liens that have different sensitivities to toxin, the inhibition was from 33% to 100% on potato stems sections and from 32% to 148% on seedlings. The toxin-based evaluation was highly correlated with field data using pathogen inoculum (r = 0.731, P < 0.01). Inoculation with wheat bran-mediated R. solani AG2-1 of 2, 3 or 4 g per seed piece was effective in field evaluations. The toxin-based assay improved the efficiency of disease resistance evaluation. Both assays showed that none of the 20 potato cultivars/liens was completely resistant to R. solani AG2-1. However, cultivar ‘Desiree’ had the lowest level of disease, whereas ‘Atlantic’, ‘Favorita’, and ‘Shepody’ were the most susceptible cultivars.
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