Characterization of Neonectria, Nectria, and Sarocladium spp. on pome fruit in the Pacific Northwest

Joseph Katon Mellow

doi:10.7273/000006955

Neonectria fruit rot (NFR) is known to be caused mainly by Neonectria ditissima, the causal agent of European canker in many regions where apples are grown worldwide. NFR-like symptoms were observed in about 35% of 600 grower lots surveyed from 24 apple and pear packinghouses in Washington and Oregon from 2013 to 2018. We used morphological characteristics on several agar media under various photoperiods, molecular phylogenetic analyses, and pathogenicity tests to characterize 56 Neonectria and Nectria isolates collected from 6 apple and 3 pear cultivars. Multisequence analyses of four DNA regions, i.e., ß-TUB, TEF1, LSU, and ITS rDNA, confirmed that NFR of apple and pear in the PNW is caused by N. ramulariae (syn. N. candida, anamorph Cylindrocarpon obtusiusculum). This species was pathogenic on Fuji apples and Green d’Anjou pears at 1.5 and 0.5°C, respectively, and at 22°C with NFR incidences ranging from 6 to 100%. We report for the first time that three Nectria species, i.e., Nectria nigrescens, Nectria cinnabarina, and Nectria dematiosa, can cause NFR-like rot on detached apples and pear at both cold and ambient temperatures. The effects of lighting conditions and agar medium on the growth and sporulation of representative isolates of N. ramulariae and Nectria spp. were investigated to better understand the biology of these species. Although some differences were observed, N. ramulariae and Nectria spp. isolates showed the ability to grow and sporulate on nutrient-poor and nutrient-rich media under different photoperiods. In this study, we also investigated and characterized 85 Sarocladium spp. isolates collected from apples and pears. Phylogenetic analyses revealed occurrence of Sarocladium kiliense and S. mali on several apple and pear cultivars. S. kiliense and S. mali caused brown halos of 2.8 mm diameter on Fuji apple and Green d’Anjou pear at 0 to 41% incidences when incubated at 0.5 and 1.5°C, and 0 to 55% incidences when incubated at 22°C. It is not clear whether Sarocladium spp. are true pathogens or part of endophytic populations on pome fruit. The potential for emergence of N. ramulariae and Sarocladium spp. as additional postharvest pathogens in the PNW may constitute additional challenges to fruit growers and packers. This study is the first step towards species characterization and understanding their biology and epidemiology to develop effective management approaches.

Characterization of Neonectria, Nectria, and Sarocladium spp. on pome fruit in the Pacific Northwest

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