Ethiology of Cotton Boll Rot Disease and its Distribution in South Khorasan Province.

Cotton is one of the main cash crops grown in more than eighty countries including Iran. Southern Khorasan province ranks second in cotton production in Iran. Lint and boll rot diseases cause significant economic losses wherever cotton is grown. During surveys of cotton fields since 2008 along the southern Khorasan province, symptomatic samples of both lint and bolls were collected. Symptomatic tissues were excised, surface-sterilised and cultured onto potato dextrose agar (PDA), malt extract agar (MEA), semi-selctive PARPH medium for fungal and nutrient agar (NA) and King's B media for bacterial causal agents isolation. Based on morphology and culture characteristics, several fungal taxa were identified. Exserohilum rostratum and Penicillium expansum are reported as new pathogens of cotton worldwide and Fusarium semitectum, Nigrospora oryzae (associated to Siteroptes sp.) were also recorded as lint or boll rot pathogens for the first time from Iran. Aspergillus niger and Rhizopus stolonifer were isolated frequently from lint parts of cotton prior to harvesting. There was a positive correlation between disease occurrence and Spiny cotton bollworm damage. The pathogenicity of fungal and bacterial isolates associated with diseased boll and lint was characterised by inoculating the attached and detached bolls of cotton plants. Inoculated fungi were always recovered from the decayed tissues. Pathogenicity test of bacterial isolates on bolls yielded negative results. This to our knowledge, is the first report of E. rostratum and P. expansum on cotton in the world and F. semitectum and N. oryzae on cotton lint or boll in Iran. Keywords: Cotton, ethiology, boll and lint rot, southern Khorasan