Comparison of Greenhouse-Based Inoculation Methods to Study Aggressiveness of Diaporthe helianthi Isolates Causing Phomopsis Stem Canker of Sunflower (Helianthus annuus)

Phomopsis stem canker is an economically important disease of sunflower (Helianthus annuus), and Diaporthe helianthi is one of the primary causal agents of the disease in the United States. The objective of this study was to evaluate inoculation methods of D. helianthi isolates on sunflower in the greenhouse. Four isolates of D. helianthi were selected to test the effectiveness of four inoculation methods using mycelial plugs as the inoculum, including stem wound, wound inoculation, petiole wound, and straw test. Infection was established by the D. helianthi isolates at 14 days after inoculation for all inoculation methods used. However, recovery of the pathogen from the inoculated plants differed significantly (P \u3c 0.0001) among inoculation methods. Given higher mean recovery of D. helianthi isolates from the inoculated plants and the size of the lesions caused by the pathogen, the stem-wound inoculation method was found to be the most user friendly of the four inoculation methods