Relationship between Disease Incidence and Severity of Soybean Charcoal Rot in Golestan Province

Charcoal rot, caused by Macrophomina phaseolina, is one of the most important diseases of soybean [Glycine max (L.) Merr.] throughout the world, including Iran. Host resistance may be the only feasible method to manage this disease, but soybean genotypes with high levels of resistance have not been identified because of the lack of reliable disease assessment methods. Two disease assessment methods were compared and the I-S relationship of soybean charcoal rot was studied in Golestan province. Comparison between percent height of stem discoloration (PHSD) at R7 and root and stem severity (RSS) indicated that PHSD couldn’t represent the actual disease intensity in the fields. But RSS had higher correlation with root incidence (r =0. 84). In order to quantify the relationship between disease incidence and severity, statistical analyses were performed based on comparison of model statistics. Result showed that square root transformation achieved the best linear adjustment to combined data of incidence and severity as determined by 2 Ra = 0. 83, MSe= 0. 0125, random pattern in residual plot and R2٭= 0. 72. Data analysis of Gorgan, Aliabad, Kordkoy and Ramian was also confirmed the appropriateness of square root transformation. Thus the assessed incidence could be used to estimate charcoal rot severity based on the model.