Morphological and molecular characterization of Alternaria spp. causing leafspot and flower blight of marigold (Tagetes spp.)

Globally, leaf spot and flower blight disease, caused by Alternaria spp., is the most devastating disease of marigold, especially during rainy season when moderate temperature and high relative humidity prevails. Disease is characterized by appearance of irregular black sunken lesions on the surface of lower leaves which enlarge and progress to stems, buds and flowers causing the death of plants under congenial environment conditions. The pathogen infecting Tagetes spp. was identified as Alternaria tagetica based on the symptomatology and conidial morphology. This finding was further confirmed by sequencing the nuclear ribosomal internal transcribed spacer regions (ITS) of the fungus which showed ~100% homology with the earlier reported Alternaria tagetica isolates affecting marigold across China, Netherlands and USA. Further, ten different growing media were evaluated to study the fungal growth patterns including sporulation to aid large-scale phenotyping of germplasm under controlled conditions. The radial colony and other colony parameters were largely influenced by the type of growing media used. Potato dextrose agar, oat meal agar and V8-juice agar showed desirable colony parameters and showed maximum sporulation of A. tagetica.