First Report of Anthracnose Crown Rot on Strawberry (Fragaria × ananassa Duch.) Caused by Colletotrichum pandanicola in Yunnan Provence, China

Strawberry (Fragaria × ananassa Duch.), a widely grown octoploid species, is one of the most important economic fruit crops and has been widely cultivated in the world, including China. In December 2021, a serious crown rot disease (approximately 50% incidence) was observed in strawberry (cultivar Miaoxiang) plantations in Qujing City, Yunnan Province, China. Symptoms observed on aboveground part withered rapidly, reddish-brown marbled necrosis on crown. The roots were healthy and strong, but the plants finally died. To isolate the causal agent of this disease, crown tissues from five strawberry plants showing typical symptoms were cut into pieces of 5×5 mm, and the pieces were surface-sterilized with 75% ethanol for 45 s followed by 2.5% NaClO for 3 min and rinsed thrice with sterile water, and then placed onto potato dextrose agar (PDA) for 7 days at 25 ºC. After 3 to 4 days, extended single hyphal tips from the tissues were transferred to PDA and incubated for 7 days at 25 ºC. The colonies were initially white, later became somewhat zonate, velvety, cyan gray on the upper side and cyan ink pigment ring on the reverse side of plates, with concentric rings of salmon sporodochia. Many yellowish or orange creamy conidial droplets formed on PDA after 14 days at 25 ºC. Fifty-nine isolates were obtained, and three isolates QLYRR1, QLMCR9, and QLMCR39 were selected for further experiments. Conidia were hyaline, cylindrical with rounded ends, 12.17-19.35×3.71-6.30 μm (average±SD, 15.24±1.37×5.09±0.45 μm, n=150), L/W ratio = 2.99. The three isolates were molecularly identified using the genomic regions of internal transcribed spacer (ITS), actin (ACT), chitin synthase (CHS-1), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and beta-tubulin (TUB2) genes, and the sequences were deposited in GenBank (accession nos. QLYRR1, QLMCR9, QLMCR39: ON668272, ON668256, ON668257[ITS], ON684302, ON684300, ON684301[ACT], ON684316, ON684314, ON684315[CHS-1], ON684292, ON684290, ON684291[GAPDH], ON684286, ON684284, ON684285[TUB2]). The phylogenetic analysis of experimental strains was performed by Maximum-likelihood (ML) tree and Bayesian inference (BI) method. Nucleotide sequences exhibited three isolates were clustered with the ex-type strain C. pandanicola strain MFLUCC 170571T found in Thailand, C. pandanicola strains (SAUCC201152, SAUCC200204) found in Shandong Province, and the holotype stain C. parvisporum YMF 1.06942T found in Guangxi Province, China. Morphologically, isolates were easily distinguished from C. parvisporum by the colony on PDA and the size of conidia (Yu et al. 2022). Morphological characteristics and phylogenetic analyses revealed that QLYRR1, QLMCR9, and QLMCR39 belong to C. pandanicola, the members of the C. gloeosporioides species complex (Tibpromma et al. 2018; Mu et al. 2021). Koch's postulates were tested by strawberry plants (two cultivars, Akihime and Miaoxiang) in vivo, strawberry plants were tested for the three isolates by spraying 1×106 conidia/mL suspension on three seedlings. Three seedlings sprayed with sterile distilled water were served as control. All of the plants were transferred to a glasshouse with a 28/20 °C day/night temperature range and natural sunlight. After 6 weeks, QLYRR1-, QLMCR9-, and QLMCR39-sprayed seedlings were stunted and developed typical wilt symptoms similar to those observed in the field with the incidence for 3, 3, and 3 seedlings, respectively. The negative control remained asymptomatic. The fungi were reisolated again from lesions of diseased plants and leaves with 100% frequency, and morphological characteristics and tested gene sequences were identical to the original isolates in this note, thus fulfilling Koch's postulates. C. pandanicola was described from the healthy leaves of Pandanus sp. and the lesion fruits of Juglans regia. To our knowledge, this is the first report confirming C. pandanicola causes anthracnose crown rot on strawberries in China. C. pandanicola has the potential for causing serious losses to the strawberry industry, and research is needed on management strategies to minimize losses.