First Report of <i>Fusarium acuminatum</i> Causing White Fruit Disease of Mulberry in China

Mulberry (<i>Morus alba</i> L.) is an excellent fruit tree species, which is widely planted all over China. In April 2023, a white fruit disease was observed on Mulberry plants in Hunan Province (27°22'13.80"N, 112°1'32.19"E), China. The disease incidence was nearly 50% in a surveyed area of 2000 m², which contained nearly 400 plants. Symptoms appeared on the fruit, which was grayish white, with dark brown spots on the surface, and the small fruit core was dehydrated. A total of 20 symptomatic samples were randomly collected. To identify the pathogen, symptomatic fruit tissues (0.3 cm × 0.3 cm) from the lesion margin surface were sterilized with 75% ethanol for 30 s and 2% NaClO for 1 min, rinsed three times with sterile water, air dried, the sterile fruit pieces were placed on potato dextrose agar (PDA) and incubated at 26°C. The five colonies obtained showed the similar characteristics, and two representative isolates (MS3, MS6-1) were used for further identification. After 5 days of cultivation on PDA, colonies developed a slightly white-pink to red color with abundant white aerial mycelia and then gradually developed rose pigmentation with a slightly brown color in the center and a light gray rose at the periphery. To induce sporulation, colonies were cultured in a carboxymethylcellulose (CMC) broth for enhanced conidia production (Zhang et al. 2020). Macroconidia were sickle-shaped, slender, and distinctively curved in the bottom half of the apical cell, with 1 to 5 septa, 25.8 to 39.6 μm ×2.7 to 4.8 μm (n=30). Microconidia were absent. Morphological characteristics of the isolates matched the description of Fusarium sp (Leslie and Summerell 2006). For molecular identification, four loci, ITS (White et al. 1990), RPB1, RPB2, and TEF1 (O'Donnell K, 2022), were sequenced from two representative isolates (MS3, MS6-1). Compared with a reference isolate, <i>F. acuminatum</i> strain NRRL 39702, GenBank accession nos. OL832228.1 (ITS), OL772800.1 (TEF1), OL772952.1 (RPB1), OL773104.1 (RPB2). The similarity of ITS sequence, TEF1 sequence, RPB1 sequence and RPB2 sequence of MS3 and MS6-1 were 521/523 (99%) and 520/524 (99%), 610/611 (99%) and 621/625 (99%), 713/713 (100%) and 780/781 (99%), 871/878 (99%) and 871/878 (99%), respectively. The phylogenetic tree constructed by the maximum likelihood method using MEGA 7 software showed that the isolates ( MS3, MS6-1 ) and <i>F. acuminatum</i> strains were in the same evolutionary branch. According to morphological characteristics and molecular phylogenetic analysis, MS3 and MS6-1 were identified as <i>F. acuminatum</i>. Pathogenicity testing was conducted on there-year-old healthy plants with fruits, (cv. Hongguo No.2), two fruits on healthy plants were inoculated by spraying spore solution (1 × 10⁶ conidia/mL), sprayed with sterile water as a control. The experiment was conducted twice. Plants were maintained at 28°C and >80% RH. Plants showed symptoms after 10 days, symptoms were observed similar to those of the original infected plants, control plants were asymptomatic. The fungus was reisolated, confirmed as <i>F. acuminatum</i> based on conidial characteristics, no pathogenic fungus was isolated from the control plants. <i>F. acuminatum</i> has been reported that cause fruit rot on Postharvest Pumpkins (Li et al. 2019). To our knowledge, this is the first report of <i>F. acuminatum</i> causing white fruit disease of Mulberry in China. This result may expand the etiological study of <i>F. acuminatum</i> and the control strategy of white fruit disease of Mulberry.