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Virulence of Phytophthora asparagi, susceptibility of asparagus cultigens to spear and root rot, and the efficacy of seed treatments

In 2016, Michigan led the United States in production (11.7 million kg) and hectares (3,723.1 ha) of asparagus spears grown for fresh market and processing, a crop worth $20.2 million to Michigan growers. Phytophthora asparagi, causal agent of spear and root rot of asparagus, contributes to crop decline in Michigan. Management strategies are limited, and identifying resistant cultigens and seed treatments could provide valuable strategies. Nine isolates of P. asparagi were tested for virulence using spears of five cultivars and seedlings of three cultivars. On spears, differences in isolate virulence and host susceptibility were observed with ‘Jersey Knight’ being the least susceptible and ‘Jersey Supreme’ the most susceptible. No significant differences were detected among cultivars or isolates in the seedling root rot experiment. Two virulent isolates were selected to inoculate seedlings of 18 cultigens. Isolate SP317 was significantly more virulent than SP316, and ‘Jersey Giant’ and ‘UG009’ were the least susceptible cultigens. The interaction between isolate virulence and cultigen susceptibility significantly affected root rot severity. ‘Pacific 2000’ and ‘Jersey Giant’ were the least susceptible cultigens for isolates SP316 and SP317, respectively. Seven seed treatments, including experimental fungicides and biocontrol agents, were screened for efficacy against two isolates of P. asparagi on ‘Millennium’ in vitro. Seeds treated with mefenoxam and Bacillus subtilis had the highest germination overall and on plates inoculated with SP326. When seeds were inoculated with SP316, mefenoxam, oxathiapiprolin, and B. subtilis treatments germinated at the highest percentages. Antagonism of mycelial growth was observed in the B. subtilis treatment, which resulted in significantly decreased disease incidence compared to all other treatments. These results are consistent with previous research suggesting that virulence of isolates of Phytophthora spp. is a combination of host and pathogen genetics and that some asparagus cultigens are less susceptible than others to P. asparagi.