First Report of Root and Crown Rot Caused by Fusarium oxysporum on Sweet Cherry (Prunus avium) in British Columbia

British Columbia is the largest producer of sweet cherries in Canada. In July 2012, gumming was observed from leaf scar and crown areas in self-rooted, 1-year-old cv. Hartland × Staccato (H×S) cherry trees planted in a nursery block at the Pacific Agri-Food Research Centre, Summerland, BC. Gumming was followed by wilting of the leaves and symptomatic trees collapsed by end of August. Light-brown discoloration of the cambium was observed on the crown and in the roots. Roots and crown from 10 symptomatic trees were collected, surface sterilized in 4.5% NaClO for 5 min, and rinsed twice with sterile distilled water. Small pieces of wood from the margin of discolored tissue were plated on potato dextrose agar (PDA) amended with 0.01% tetracycline hydrochloride. After 5 days incubation at 22°C in the dark, all wood pieces yielded a white and tinge pink colony with abundant cottony aerial mycelium. Oval-ellipsoid, hyaline and aseptate micro- (3.0-)5.8(-13.8) × (1.0-)2.0(-3.7) μm (n = 50) and fusoid-subulate, hyaline and 3-5 septa macroconidia (15-)17.9(-21.1) × (2.3-)3.0(-4.4) μm (n = 50) were observed from pure cultures after 4 weeks and isolates were tentatively identified as Fusarium Link. Genomic DNA was extracted from 3 isolates and amplified with primers ITS1/ITS4 (White et al. 1990) and EF-1/EF-2 (O’Donnell et al. 1998). ITS and EF sequences from British Columbia (GenBank Accession Nos. KT455374 to KT455379) showed 99% identity with Fusarium oxysporum Schltdl. isolates KP942931 and KM025417 from GenBank, respectively. Isolate PARC682 was deposited at the Department of Agriculture Ottawa Mycology Collection (DAOMC 250239) and used to conduct pathogenicity studies in 1-year-old scion cvs. H×S, H, S, and Lapins (L), and rootstock cv. Mazzard (M) detached dormant shoots. Shoots of 20-cm length were disinfected and a 5-mm-diameter hole was drilled in the center. For each cultivar, 3 shoots received a PDA colonized plug taken from a 7-day-old culture and 3 received 50 μl of a 1.8 × 106 conidia/ml spore suspension. Noncolonized PDA plugs and sterile distilled water were used for negative controls. Cuttings were placed on metal grids over water in crispers at room temperature. Thirty-one days after inoculation, discoloration similar to the one observed in roots and crowns from trees in the field was observed in all inoculated shoots. Necrosis length (mm) averaged (mycelium/conidia) 185.5/46.8 in H×S, 21.5/19.8 in M, 8.1/3.3 in S, 5.0/2.0 in L, and 0.5/0.2 in H. Fusarium oxysporum recovery from all inoculated cuttings varied but was always higher than 60% and it was not reisolated from the controls. Results show H×S and S to be highly susceptible to F. oxysporum. However, cherries are planted on rootstocks and thus, the most significant finding of this study is the high susceptibility to F. oxysporum shown by M, the most widely planted rootstock in British Columbia. Fusarium spp. have been associated with root rot of Prunus spp. in California (Ogawa and English 1991) and a Fusarium sp. has been reported to cause M infections and consequent tree dieback in the Czech Republic (Blaẑková 2004). To our knowledge, this is the first report of F. oxysporum causing root and crown rot of sweet cherry in British Columbia. © 2016, American Phytopathological Society. All rights reserved.