Fusarium solani: A New Pathogen Causing Postharvest Lemon Rot in Changchun, China

Lemon (Citrus limon) is an excellent fruit renowned for its high citric acid and vitamin C, and widely used in food and cosmetic industries. Postharvest fruit loss is a major factor leading to China’s rapid decline in lemon production, after recording unprecedented high yields from 2007 to 2009. During 2015 and 2016, we collected rotting lemons from Chenxing market (43.48°N, 125.24°E), Changchun, Jilin Province. The incidence was about 5 to 10%. Symptoms initially appeared as light- to dark-brown blemishes with small amounts of white mycelium on the lemons’ blossom ends. Lesions gradually expanded to encircle the blossom ends. Finally, the whole fruit rotted; white mycelium covered the entire fruit, emitting a fermenting-like odor. We isolated pathogens from 30 samples. A single spore from each sample was cultured on potato dextrose agar (PDA) and incubated at 25°C. Colonies appeared oyster-white with loose, fluffy mycelium. To produce conidia, the pathogen was cultured on carnation leaf agar (CLA) (Liu et al. 2016) under a 12/12-h dark/light photoperiod at 25°C. Microconidia were abundant and oval, with 0 to 1 septum and dimensions ranging from 6.1 to 11.7 × 3.1 to 5.5 μm (average = 8.9 × 3.9 μm). Macroconidia were sickle-shaped, with 1 to 3 septa and typical foot-shaped basal cells; dimensions ranged from 18.1 to 45.5 × 3.6 to 7.3 μm (average = 33.9 × 5.4 μm). Chlamydospores were circular- to ovate-shaped, formed singly or in pairs, and exhibited both smooth and unsmooth textures. Based on morphological characteristics (Nelson et al. 1983), the pathogen was identified as Fusarium solani. We performed PCR amplifications of the internal transcribed spacer (ITS) region of rDNA and the translation elongation factor 1-alpha (TEF-1α) genes, using genomic DNA of representative isolate CL12 with primer pairs ITS4/ITS5 and EF1/EF2. Results of ITS sequencing (562 bp, KX497027) demonstrated 99% positive identification of F. solani (AM412637). The TEF-1α sequencing (719 bp, KX497028) also demonstrated 99% positive identification of F. solani (KF255497). Thus, based on morphological and molecular characteristics, we confirmed the pathogen as F. solani. Three independent pathogenicity tests were performed. For each test, we surface sterilized 10 fresh, nonblemished lemons and then inserted sterile needles to make 1-mm deep wounds. Lemons were then surface disinfected before inoculation with a spore suspension concentrated at 106conidia/ml of CL12 isolate (grown at 25°C for 7 days on PDA). Each inoculation site was covered with wet cotton wool. Lemons were wrapped in plastic bags and stored in a growth chamber at 25°C, 90% relative humidity, and a daily 12-h photoperiod. After 5 days, inoculated lemons displayed symptoms similar to the lemons observed in our initial study. Pathogens were consistently reisolated from infected lemons. Control lemons displayed no symptoms. An Argentina study reported postharvest lemon rot caused by F. oxysporum (Fogliata et al. 2013). To our knowledge, we are the first to report F. solani-caused postharvest lemon rot in China. An effective monitoring program and appropriate postharvest handling and storage methods are needed to prevent further economic losses. © 2017, American Phytopathological Society. All rights reserved.