Boeremia Exigua Leaf Spot: A New Emerging Threat to Gossypium hirsutum L. in Pakistan

Cotton is grown all over the world including Asian countries. Severe foliar disease symptoms were observed on cotton in fields from different districts of cotton zone of the province of Punjab, Pakistan. Infected plant leaves were sampled from various areas and processed for the isolation of the causal organism. Fungal isolates were identified morphologically. The conidia of the fungus were observed oblong to ellipsoid, hyaline, and aseptate, but occasionally 1-septate with dimensions ranging from 2.7 to 7.8 × 1.6 to 3.5 μm. A total of two isolates were characterized further for genetic diversity. Both the isolates showed moderate reactions to the NaOH test for E metabolite. The pathogen was morphologically identified as Boeremia exigua. Further, the internal transcribed spacer region (ITS) and TUB gene of the fungal isolates were amplified and analyzed. The sequence of partial ITS and TUB showed 99% homology with the isolates of B. exigua. The pathogenicity test was performed on three months old cotton plants by using two isolates of B. exigua. After 15 days of inoculation, necrotic spots started developing on the leaves that were very similar to those observed in the field. The fungal pathogen was re-isolated from the leaves of all the inoculated plants, identified morphologically following Koch's postulates. This disease might be a potential threat to cotton production in Pakistan in the future. However, further studies are required to know the virulence, behavior, alternate hosts, and spreading nature of the pathogen.