Inoculum production and evaluation of temperature and leaf wetness for Passalora sojina inoculation in soybean

ABSTRACT The objective of this study was to develop a methodology for inoculum production and inoculation of Passalora sojina in soybean to assess the reaction to frogeye leaf spot. Thus, sporulation of fungal races was quantified in five substrates under two light regimes. The temperature and the leaf wetness period that resulted in greater infection efficiency were also determined. Petri dishes containing the culture media Oat Flour Agar (OFA), Tomato Juice (FTJ), V8 agar juice (V8), Infant Food (IF) and Potato Sucrose Agar (PSA) plus isolates of P. sojina races 23, 24 and 25 were subjected to 12-h photoperiod and 24-h continuous dark. Inoculated plants were incubated at temperatures of 15, 20, 25, 30 and 35 ºC and leaf wetness periods of 12, 24, 36, 48 and 72 hours. Experimental design was completely randomized with five replicates. There was interaction among isolate, culture medium and photoperiod. The highest sporulation (conidia/cm2) was obtained in the culture media FTJ (race 23) and V8 (races 24 and 25) under 12-h photoperiod. The optimum temperature for the disease development was 27oC under 72 hours of continuous leaf wetness.