Nonhost selective toxins produced by Diaporthe helianthi and D. gulyae infecting sunflowers (Helianthus annuus L.)

Phomopsis stem canker of sunflowers, mainly caused by Diaporthe helianthi and D. gulyae in the U.S., cause yield losses up to 40%. Previous studies suggested that D. helianthi produces phytotoxins like 4,6-dihydroxymellein during infection in sunflowers. A study was conducted to screen 15 isolates each of D. helianthi and D. gulyae for the production of phytotoxins by mass spectroscopy. The analysis confirmed 4,6-dihydroxymellein in 76.67% of the isolates. However, there is limited information on the sensitivity of these toxins on soybeans, another host of Diaporthe. Current study evaluated the response of toxic filtrates of D. helianthi and D. gulyae on soybean variety, RG200RR. Two isolates of each fungus were grown in PDB at 23+2°C. After 28 days, cultures were filtered using Whatman No. 1 filter paper to remove mycelium. The resulting filtrate was infiltrated on the topmost fully opened leaves of soybean plants using a needleless syringe, kept in a greenhouse at 23°C and monitored for phytotoxic symptoms. Necrosis started 48 hours post infiltration indicating that the toxins produced by D. helianthi and D. gulyae are not host selective. This highlights the role of toxins in the pathogenicity of these fungi across multiple hosts, potentially complicating disease management.