Occurrence of Nectria dematiosa causing cankers in sweet cherry in Chile

Sweet cherry (Prunus avium L.) has become the most important fruit crop in Chile, making the country the world's leading exporter, with 414,000 ton in 2023. Fungal wood diseases are a major threat to this crop, with novel pathogens emerging in recent years (Auger et al., 2020; Grinbergs et al., 2021). New symptoms were observed in winter 2023 in a commercial orchard (40°19'46.1"S 72°54'22.4"W) near Río Bueno, Los Ríos Region, on varieties Sylvia, Kordia, Regina and Schneider. These included cankers along the trunk and main branches, with red pycnidia and yellowish sporodochia growing on the symptomatic tissue. A survey was conducted from winter 2023 to fall 2024, during which symptomatic samples (n=63) presenting cankers, internal discoloration, and signs of the probable pathogen, were collected from sweet cherry orchards (n=8), located from Maule to Los Lagos Regions (36°02'26.6"S 71°39'29.1"W to 40°20'14.4"S 72°41'49.8"W). Most of the symptomatic samples exhibited yellowish to orange/red, sessile, semi-immersed to erumpent, mostly solitary, pulvinate sporodochia. Additionally, samples collected in winter 2023 contained reddish, warted, globose, predominantly caespitose perithecia. Microscopic analysis of the sporodochia showed cylindrical to fusiform, hyaline, non-septate conidia, 4.8-5.9 × 2.2-2.9 µm (n=25). The perithecia contained cylindrical to clavate asci 95,6-68 x 13,3-11,8 µm (n=25), with eight, ellipsoidal, hyaline, one septum, ascospores 20-25 × 3-6 µm (n=25). The macro and micro morphometric features of the fungus accorded with the description of Hirooka et al. (2011; 2012) for Nectria dematiosa. To isolate the fungus, wood sections 0.5-cm were cut-off from the margin of discolored area, disinfected (0.5% v/v sodium hypochlorite) and plated on a quarter-strength potato dextrose agar amended with 1mg/L tetracycline (PDA-tet). Plates were incubated at 25°C until mycelial development and subsequently were purified transferring mycelia to PDA. Colonies developed whitish, radial, slightly cottony aerial mycelium, typical of N. dematiosa. DNA was extracted from representative isolates (n=6) and maximum likelihood phylogenetic analysis clustered them with N. dematiosa reference strains in a phylogenetic tree. The reference sequences for isolate RGM 3594 include: ITS PP862960 (White et al., 1990), LSU PP862961 (Rehner & Samuels, 1994; Vilgalys & Hester, 1990), tef1-α PP869349 (Carbone & Kohn, 1999; Rehner, 2001), tub2 PP869350 (O'Donnell & Cigelnik, 1997). For pathogenicity tests, 30 cm sweet cherry cuttings of vars. Lapins, Sweetheart, Regina, Kordia, Bing and Skeena were inoculated with six isolates of N. dematiosa and incubated at 25°C for 60-d in distilled water, at a depth of 5 cm. Moreover, nursery 3 years potted plants var. Lapins were inoculated with isolate RGM 3594 and incubated for 230-d at a shade house. Controls were inoculated with sterile agar for both tests. After incubation, internal discoloration was measured and compared. The varieties differed in their susceptibility to the isolates, with Sweetheart being the most susceptible (25.9 cm) and Bing the least (13.3 cm). The pathogen was also able to produce symptoms on nursery plants (4 to 18 cm). Furthermore, sporodochia developed on inoculated tissue. In both pathogenicity tests, it was possible to reisolate the fungus from the inoculated wood, fulfilling the Koch's postulates. This work contributes to understanding the etiology of cherry wood diseases and the biology of N. dematiosa, a new pathogen for sweet cherry in Chile and the southern hemisphere.