Histopathology, toxin and secondary metabolites of Alternariaster helianthi in sunflower

Leaf blight is one important fungal disease of sunflower and it is caused by Alternariaster helianthi. Scanning electron microscopy showed various histological changes in leaf tissues of infected plants. A putative phytotoxin was isolated from the culture filtrate of A. helianthi and was tested on germinating plants. The toxin of highly aggressive A. helianthi isolates inhibited root and shoot lengths and reduced vigor index, while the toxin of less aggressive isolates showed comparatively more root, shoot lengths and higher vigor index of seedlings. The detached leaf technique was used to check pathogenicity of different fungal isolates by comparing leaf spot symptoms on sunflower leaves. A. helianthi isolates Ah-18, Ah-38, Ah-125 of the highly aggressive group resulted in > 50% disease severity, Ah-157, Ah-142, Ah-12 of moderately aggressive resulted in 35-50% and less aggressive isolates Ah-92, Ah-160, Ah-158 resulted in < 35% leaf blight severity. Representatives of each group were profiled for variation in the production of secondary metabolites and proteins. In moderately aggressive Ah-157, Ah-142 isolates, an extra fifth band specific to these isolates was observed under TLC fingerprinting. Maximum retention factor was recorded in Ah-92, 160, 158, while minimum retention factor observed for isolates Ah-18, 38, 125,157 and 142. The only exception was Ah-12 that had a minimum retention factor of 0.394 among the moderately and highly aggressive groups. Secondary metabolite analysis under HPLC of Ah-92, 157, 125 isolates recorded 22 peaks. Isolate Ah-125 with high aggressiveness has a specific peak at 33.86 min in HPLC that was absent in other isolates. Six different masses were observed in MALDI-TOF with a maximum peak at 663.5 with Ah-125 isolate. Highly aggressive isolates showed similar profiling in both native and dissociated proteins with maximum identical banding patterns.