Evaluation of relatve resistance of cotton cultivars to Alternaria alternata

Cotton leaf spot disease caused by Alternaria alternata is one of the most common and important diseases in the cotton growth areas. The use of tolerant cultivars is one of the most economic means for the disease control. This study was conducted to evaluate the response of cotton cultivars against the fungus.In this research the fungus were isolated in seedling stage and purified in seedling stage from the cotton fields of Golestan province. and then fungus was detected using morphological data. Confirmed Alternaria species was performed using nucleotide sequence data of ITS – rDNA region and sequencing of the part of endo poly galacturonase gene. Accordingly, isolated Kb2 isolate with fungus
A. alternata were in one group phylogeneticlly. The relative resistance of seven cotton cultivars consist of Sepid, Golestan, B - 557, Sahel, Thermus-14, Bakhtegan and Armaqan to A.alternata Kb2were evaluated in greenhouse conditions and seedling stage with completely randomized design in three replications.The results revealed that the reaction of cultivars to disease were different, however, none of the cultivars showed high levels of resistance. Termus- 14 cultivar had the highest of leaf spot severity and B-557 and bakhtegan cultivars had the lowest of leaf spot severity. In other words, these two cultivars were introduced as the tolerant cultivars to leaf spot disease at seedling stage.