VIEW ARTICLE

Effect of Propagule Size of Rhizoctonia solani on Saprophytic Growth, Infectivity, and Virulence on Bean Seedlings. Y. Henis, Associate Professor, Department of Phytopathology and Microbiology, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot, Israel; Y. Ben-Yephet, Assistant, Department of Phytopathology and Microbiology, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot, Israel. Phytopathology 60:1351-1356. Accepted for publication 6 April 1970. DOI: 10.1094/Phyto-60-1351.

The effect of inoculum concn and propagule size of Rhizoctonia solani Kuehn on rate of infection of bean and on damping-off disease in artificially infested soil was tested. A linear relationship between inoculum concn, rate of infection, and disease intensity was observed at concn up to 0.25 g/kg soil. Virulence of mycelial homogenates prepared from a culture grown on yeast dextrose broth was higher than that grown on potato-dextrose broth. Mycelial homogenate was fractionated by precipitation and sieving procedures, and fractions composed of propagules of similar size were tested for saprophytic growth and virulence towards bean seedlings. In the absence of external nutrients, saprophytic growth was positively correlated with propagule size and volume. No symptoms were observed in bean seedlings grown in soil infested with propagules smaller than 250 µ in size at a rate of four/g, whereas with propagules of 250-1,000 µ in diam, 100% of the host plants showed symptoms 7 days after inoculation. When bean seeds were directly inoculated with the 400 to 500-µ fraction, 67% and 93% of the host plants showed symptoms 21 days after inoculation with two and four propagules/seed, respectively. When placed in groups of eight or more at a distance of 2 cm from each seed, a significant proportion (30% or more) of the host plants developed symptoms. The possible fitness of these results to some mathematical models is discussed.