Microdrops: A method for inoculation with Rhizoctonia solani Kühn for evaluation of bean (Phaseolus vulgaris L.) genotypes

Abstract

Three methods, namely, bulk, microdrops, and 0.5-cm discs of potato dextrose agar (PDA) with fungal mycelium, were evaluated in order to identify a suitable methodology to inoculate bean with Rhizoctonia solani. In the microdrop method, the combination of dilution and volume of inoculum was examined.The effect of age of inoculum and of the leaf on symptom expression was also tested. All inoculations were done on the upper side of the leaf. The best method for symptom expression was the combination of 10 µl with dilutions of 100 ml of sterilized distilled water per Petri dish (60-mm diam).The bulk and disc methods induced severe symptoms causing defoliation 72 h after the inoculation. Four-day-old colonies as source of inoculum were best for symptom expression. With older colonies, lesion size was smaller and the induction of symptoms less successful. Age of trifoliate did not influence the success of the inoculations. Validation tests showed the potential of microdrops as a useful technique for the evaluation of bean genotypes.