Grapes (Vitis vinifera L.) are very popular in China as fresh fruit. Due to its storability, some grape varieties can be kept fresh until winter, increasing the popularity of fruit grapes. However, in 2019, rot symptoms were observed on cv. Crimson in Wuhan, Hubei (30°52'N, 114°31'E), and Chengdu, Sichuan (30°67'N, 104°06'E). Subsequently, from 2019 to 2021, Liangshan (28°33'N, 102°42'E, cv.Crimson), Ya'an (29°40'N, 102°66'E, cv. Red globe), and Nanchong (30°80'N, 106°06'E, cv. Victoria), Sichuan also experienced the same decay symptoms. Initial symptoms of this disease were slightly sunken lesions on the berries 5 to 7 days in storage at 28℃, and then white mycelial growth on the surface of lesions. The growth became bluish-green following the occurrence of abundant sporulation, along with softening and collapsing the whole berry (Fig. 1a). Twenty symptomatic berries from each city were collected (100 samples in total) and twenty isolates were obtained using the single spore isolation technique developed by Chomnunti et al. (2014). The colony on PDA media initially appeared as white mycelium, and later developed into greenish-gray to grayish-green sporulation with white margins, the colony diameter reached 32.5 to 34.5 mm after ten days of incubation at 28±1℃. The reverse side of the colony was oblive-brown or grayish-yellow. Morphological characteristics of the twenty isolates showed that the conidiophores were broom-shaped and verticillate, the stipes smooth-walled and measured 120 to 300 × 2.5 to 4.0 μm; the ramus (n = 2 to 3) measured 6.0 to 15 × 2.5 to 3.6 μm; the metulas (n = 2 to 4) were verticillate, with sizes ranging from 8.7 to 9.8 × 2.0 to 3.2 μm; the phialides (n = 3 to 7) were elongate and ampulliform, with sizes ranging from 2.0 to 3.5 × 2.0 to 2.4 μm; the conidia (2.0 to 3.5 × 2.0 to 2.4 μm) were sub-globose to ellipsoidal in shape, with thick and finely roughened walls. Based on these cultural and morphological characteristics, the isolates were identified as Penicillium olsonii Bainier & Sartory (Frisvad et al., 1990). A multi-locus approach was performed to accurately identify a representative WHG5 isolate. The internal transcribed spacer regions (ITS), calmodulin (CaM,), beta tubulin (BenA), and 18S ribosomal RNA (18S) of isolate WHG5 were amplified and sequenced as described by Walker et al. (2012). The pairwise alignments of ITS, CaM, BenA, and 18S sequences was nearly 100% identical to Penicillium olsonii with GenBank accession numbers KX056230.1 (524/524 bp, 100%), DQ645807.1 (570/572 bp, 99%), AY674444.1 (472/472 bp, 99%), and FJ717701.1 (1299/1301 bp, 99%), respectively. The resulting sequences were deposited in GenBank (Accession no. ITS: MW192867; CaM: MZ936474; BenA: MZ936475; and 18S: MZ936476). The phylogenetic analysis performed with the Neighbor-Joining method classified WGH5 into the P. olsonii clade with a posterior probability of 100% based on the concatenated sequences of the ITS CaM, BenA, and 18S (Fig. 2). Combined with the above morphological characteristics, we finally confirmed the identity of isolate WGH5 as P. olsonii. To fulfill Koch's postulates and confirm the pathogenicity of WGH5, a 10 μL conidial suspension (1 × 106 spores/mL) aliquot was inoculated into the healthy grape berry (cv. Crimson) while using sterile distilled water as a control. Thirty berries were surface disinfected with 2% sodium hypochlorite then artificially wounded prior to inoculation with the conidial suspension. The artificial wound was made using a sterilized steel needle with a diameter of 0.5 mm and a depth of 0.3 cm. All the inoculated fruits were placed in sealed and sterilized Petri dishes and incubated at 28±1℃. The experiments were done in triplicate. After five days, the inoculated grape berries showed typical symptoms (Fig. 1b) while the control remained asymptomatic. Using the same protocol as above, the fungus P. olsonii was re-isolated from the symptomatic inoculated berries but not successfully from mock-inoculated berries. Previously, P. olsonii has been reported from Portuguese wine grapes (Serra et al., 2007). This study is the first time that P. olsonii was reported as a plant pathogen in China. Since the grapes were collected from grocery stores, details of post-harvest management that could have affected disease presence and progression of rotting were not available.
Read full abstract
Mar 22, 2023
J.O Akowuah + 3
Open Access
