Apple (Malus × domestica) is an important fruit crop in Chile, with a cultivated area of 32,313 ha concentrated (63%) in Maule Region (35°25' S). Unusual core rot on 'Fuji' apples was observed at harvest in a commercial orchard in Curicó, Maule Region, with an incidence ranged between 22 to 35% in 2018 and 2019. Previously, in 2017, an incidence of 30% was estimated on 'Fuji' fruits destined to the Asian market. Internal decay symptoms consisted of dry, corky light to dark-brown tissue, within the seed locules initially. In moderate to severe cases, the necrotic lesion progresses deeper into the mesoderm. External symptoms were quite subtle, and typically, the disease goes unnoticed. However, infected fruit ripen earlier. Small pieces (2-3 mm) from the internal lesion margin of symptomatic apples (n = 50) were placed on Potato Dextrose Agar (PDA) (2%) and incubated at 20°C for 10 days. Pure cultures (n = 41) were obtained and transferred to Malt Extract Agar (MEA) (2%). Colonies on MEA produced an even to slight undulating buff margin with white woolly aerial mycelium, and immerse ochreous in the center, changing gray to olivaceous aerial mycelium with age. On the underside, colonies were umber and buff in the center and margin, respectively. After 10 days, numerous densely aggregate dark-brown mature pycnidia were observed. Aseptate conidia were subglobose to cylindrical, straight, and sometimes curved with rounded at both ends, that was initially hyaline to pale olive, thin, smooth wall with mean dimensions of (2.9-) 3.4 (-4.4) x (1.5-) 1.8 (2.2) µm (n=50). Based on morphology, the fungus was identified as Kalmusia variispora (Verkley et al. 2014). The internal transcribed space (ITS), portion of β-tubulin (TUB), and large subunits of the nuclear ribosomal RNA (LSU) loci were used for molecular identification, using primers ITS4/ITS5, Bt2a/Bt2b, and LR0R/LR5 (Ariyawansa et al., 2014). BLAST searches indicated 100% identity with K. variispora (ex-type CBS 121517). The maximum parsimony phylogenetic analysis placed Chilean isolates in the K. variispora clade. Sequences were deposited in GenBank (OL711706 to OL711709, OL739499 to OL739502 and OL711710 to OL711713 for ITS, TUB and LSU, respectively). Pathogenicity tests were conducted using four K. variispora isolates. 'Fuji' apples (n = 20) were surface disinfested (75% ethanol, 30 s) and then wounded and inoculated with conidial suspension (50 L of 106 conidia/mL) deposited in the middle and into the core region using a sterile fine-tipped micropipette. Additionally, 20 one-year dormant rooted cuttings 'Fuji' and 'Cripps Pink' were pruned and immediately inoculated on the pruning wound. An equal number of apples and rooted cuttings treated with sterile water were used as controls. The experiments were repeated once. All inoculated fruits developed lateral lesions (22 to 37 mm) and dry core rot (18 to 36 mm) symptoms identical to those described in the original outbreak, after 20 days at 20°C in a commercial packing box. The inoculated cuttings produced canker lesions of 10 to 21 mm in length, and dieback symptoms were observed after 3 months. No symptoms were observed on the negative controls. Koch's postulates were fulfilled by 100% reisolating K. variispora. Previously, Alternaria spp. have been reported as the primary pathogen associated with moldy core and dry core rot of apples worldwide (McLeod et al., 2014) and in Chile (Elfar et al., 2018). However, Kalmusia spp. have been associated with dry core rot in apples (McLeod et al., 2014) and have been isolated from canker symptoms on apples in Chile (Díaz et al. 2021). To our knowledge, this is the first report of a severe outbreak of K. variispora causing dry core rot in apples in Chile and worldwide.
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Sep 29, 2024
Tianci Huang + 4
Open Access