Correlation between Reaction to Potassium Chlorate of some Iranian Isolates of Macrophomina phaseolina, the Causal Agent of Soybean Charcoal Rot, and its Pathogenicity

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An experiment was carried out in 2008-2009 to study the correlation between the reaction to potassium chlorate and pathogenicity on some Iranian isolates of Macrophomina phaseolina, the causal agent of soybean charcoal rot. From among a total of 48 collected isolates from the major soybean cultivation areas of Golestan, Mazandaran, Ardabil, and Lorestan provinces, twenty four isolates were selected as representative their geographical origins. The isolates were cultured on Minimal Medium of 120 mM potassium chlorate. Five isolates (about 20%) were chlorate sensitive while 19 (about 80%) chlorate resistant. Pathogenicity test was carried out under in vitro (through seed inoculation) as well as under greenhouse (stem and soil inoculation) conditions. In any of the three tests, all the studied isolates were pathogenic to the soybean cultivar Williams. Mean comparison of treatments showed highly significant differences (p<0.01) among virulence of isolates. Moreover, in in vitro as well as in the greenhouse test of the stem inoculation, sensitive to chlorate isolates were less virulent on seeds and stems of the soybean sensitive cultivar Williams, whereas in greenhouse test involving soil inoculation, a higher level of pathogenicity was observed in sensitive to chlorate isolates in comparison with the other tests. According to the obtained results it seems that the inoculation method can affect the correlation between sensitivity to chlorate and pathogenicity as regards from soybean isolated Macrophomina phaseolina.

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