First Report of Diaporthe stewartii Causing Phomopsis Stem Canker of Sunflower (Helianthus annuus) in Minnesota

Phomopsis stem canker is one of the most economically important sunflower diseases in the northern Great Plains (Mathew et al. 2015). In October 2015, lesions consistent with Phomopsis stem canker were observed on sunflower (Helianthus annuus L.) in a commercial field in Polk County, MN (47°46′12″ N, 96°24′00″ W). Five plants displaying elongated, brown stem lesions were obtained. Stems were cut into small pieces (10 mm), surface-sterilized, and plated onto potato dextrose agar (PDA). The plates were incubated for 10 days at 22°C under 12 h of alternating light/dark conditions. Two isolates of brown colonies were hyphal-tipped and transferred to fresh PDA plates. The isolates were detected according to their morphology (conidial formation and stromata pattern); Diaporthe helianthi Muntañola-Cvetkovic, Mihaljcevic and Petrov (isolate DH1), a known causal agent of Phomopsis stem canker (Mathew et al. 2015), and an unknown Diaporthe sp. (isolate MN1) DNA was extracted from the mycelium of isolate MN1 and sequenced using the ITS (White et al. 1990) and EF1-α primers (Carbone and Kohn 1999). Using BLAST analysis, the sequence of isolate MN1 (GenBank accession nos. ITS: KX668416 and EF1-α: KX852355) showed 100% identity to D. stewartii Harrison isolate CBS 193.36 from Cosmos bipinnatus. To verify the pathogenicity, six 4-week-old sunflower (cv. HA 288) plants were inoculated with isolates DH1 and MN1 by the stem wound method (Mathew et al. 2015). Inoculation was performed at the second internode by placing infested plugs into a wound created with a micropipette tip (1,000 μl) and wrapped with Parafilm. At 14 days after inoculation, all inoculated plants developed significant necrosis and wilting based on the Phomopsis disease severity rating scale (rated 4 or 5 using a 0-to-5 rating scale), and it was impossible to differentiate between the symptoms produced by the two Diaporthe species. In contrast, control plants displayed no symptoms. D. stewartii was reisolated from the inoculated plants and confirmed by sequencing the ITS region. Previously, Mathew et al. (2011) detected D. stewartii as a Phomopsis stem canker pathogen on sunflower using DNA sequence analysis, but Koch’s postulates were not completed. To the best of our knowledge, this is the first confirmed report of D. stewartii causing Phomopsis stem canker of sunflower. It remains to be established if D. stewartii is capable of causing yield loss similar to previously identified causal agents, such as D. helianthi. Importantly, it is unknown if genotypes resistant to D. helianthi will also be resistant to D. stewartii. © The American Phytopathological Society.