Growth rate and pathogenicity of isolates of Diaporthe phaseolorum var. caulivora

Diaporthe phaseolorum var. caulivora is the major causal agent of stem canker in soybean (CTS) in Argentina. It has appeared mainly in the Southern Pampeana sub-region, but also in warm areas, and it has been observed with different levels of incidence, severity and virulence. The objective of this work was to study the growth rate in vitro and the pathogenicity of isolates from soybean plants at different temperatures. Twenty isolates from Buenos Aires Province were selected. The daily growth rate was measured in nine of them and in two reference isolates in potato dextrose agar (APD), under different light/darkness conditions at four temperatures (15, 20, 25 and 30 °C). The toothpick technique was used to inoculate four isolates on 10 plants of a susceptible soybean cultivar, which were incubated in growth chambers at 15, 20 and 25 °C. Another 17 isolates were inoculated in the field and in a greenhouse. The number of dead plants was recorded. The growth rate of the isolates was greater with higher temperature: at 15 °C they were similar, at 20 °C there were small differences, at 25 °C two groups were detected whereas at 30 °C there was no growth. All the isolates were pathogenic. The different incubation temperatures induced different levels of plant death: at 15 and 20 °C in both the chamber and greenhouse (at 20 °C), the percentage of dead plants was less than 40%. It is suggested that temperatures lower than 20 °C would not allow to observe physiological differences between the different varieties which cause CTS. The similarity of growth and the high number of dead plants indicate that the incidence and severity of the disease would be due to environmental factors.