Greenhouse Data Suggest That Growth Stage Impacts Phomopsis Stem Canker Severity Associated with Diaporthe gulyae on Sunflower (Helianthus annuus)

Diaporthe gulyae Shivas et al. was initially described as causing Phomopsis stem canker of sunflower (Helianthus annuus L.) in the United States in 2010. In this study, the impact of growth stage on disease severity associated with D. gulyae in sunflower accessions was investigated in the greenhouse. One hundred thirty-nine accessions were inoculated with a single isolate of D. gulyae using the mycelium contact inoculation method at V4 to V6 (four to six true leaves) and R4 to R6 (inflorescence to ray flower wilting) growth stages. The experiment was conducted in a completely randomized design with six replications and repeated once. Disease severity was assessed at 14 days postinoculation using a 0-to-5 rating scale. A significant interaction between growth stage and accessions (P<0.0001) was observed for disease severity (relative treatment effect [RTE]) caused by D. gulyae. Among the 139 accessions, 24, three, and two accessions had significantly lower RTE when inoculated at V4 to V6, R4 to R6, and both growth stages, respectively, upon comparing with the inbred susceptible check 'HA 288'. This study suggests that accessions differ in their susceptibility to D. gulyae, possibly because of changes in resistance response during host development. Future studies on sunflower-D. gulyae interaction are warranted for developing commercial hybrids with resistance to the causal fungus.