First report of brown rot caused by Monilia mumecola on Chinese plum (Prunus salicina) in Fujian, China.

Brown rot caused by Monilinia spp. (anamorph Monilia) is a common disease in stone fruits worldwide, but the species in different hosts or regions may vary. Monilia mumecola is a recently identified species, only reported in some regions. Although the pathogen has been found on plum in Yunnan, Hubei, and Zhejiang provinces, and Chongqing municipality in China (Yin et al. 2015), it has not been reported in southeast coast of China. In May 2022, brown rot with grey spores on fruit was observed in a plum orchard with 15% disease incidence in Sanming City, Fujian Province, located in southeast coast of China. Four single-spored isolates were obtained from germinating conidia on the water agar for further investigation. The colonies on potato dextrose agar (PDA) were initially white, gradually turned gray to brown, with lobbed margins and rare sporulation. Average mycelial growth rate ranged from 0.74 to 1.08 cm/day at 25 oC. Conidia were lemon-shaped or subglobose, hyaline, with an average size of 17.64 to 19.35×11.14 to 14.44 µm (n=30). Each isolate produced one to three or four germ tubes. Such characteristics are similar to M. mumecola (Yin et al. 2015). To confirm the identity of the isolates, genomic DNA was extracted and species-specific primers of Hu et al. (2011) were used to amplify the 712 bp sequence. In addition, the ITS region, partial genes of glyceraldehyde-3- phosphate dehydrogenase (GAPDH) and beta-tubulin (TUB2) were also amplified using primers sets ITS1/ITS4 (Glass and Donaldson 1995), Mon-G3pdhF/Mon-G3pdhR and Mon-TubF1/ Mon-TubR1, respectively (Hu et al. 2011). Sequences obtained for those three regions were 478, 762 and 1527 bp, respectively. Each region of all four isolates was identical, so one sequence for each region was submitted to GenBank with accession numbers OQ207672, OQ225251 and OQ225252, respectively, which had 100% identity with M. mumecola HQ908786 (ITS), HQ908784 (GAPDH), and HQ908775 (TUB2) using BLAST analysis in NCBI database, respectively. Pathogenicity was conducted with mycelium plugs from the edge of 7-day-old colony on three mature 'Angeleno' plums fruit (Prunus salicina) creating nine inoculations with three wounds per fruit, and each wound was 5 mm in diameter and 2 mm in depth. The same amount fruit and wounds inoculated with PDA plugs without fungi were used as a control. Brown rot symptoms were observed on all inoculated plums 4 days post-inoculation under room temperature with 100% humidity, whereas control plums remained symptomless. Fungal colonies re-isolated from the lesions showed the same morphological features as the original isolate , thus fulfilling Koch's postulates. To our knowledge, this is the first report of M. mumecola on plum in Fujian Province of China. The findings in this studies have important management implications for local plum growers because more than one Monilia species have been reported, where only M. fructicola was present in this region.

Prevalence of H6Y mutation in ß-tubulin causing thiophanate-methyl resistant in Monilinia fructicola from Fujian, China.

Brown rot disease broke out in stone fruit orchards of Fujian, China in 2019, despite pre-harvest application of methyl benzimidazole carbamate (MBC). To determine the reason, a total of 44 Monilinia fructicola strains were collected from nectarine, plum and peach fruits in this study, among which 79.5% strains were resistant to thiophanate-methyl, indicated by discriminatory dose of 5 µg/mL. The resistance of these strains was confirmed by treating detached peach fruit with label rates of formulated thiophanate-methyl which only completely inhibit infection of the sensitive strains, but not the resistant strains. Further analysis of the mechanism of MBC resistance revealed that all resistant strains carry a H6Y mutation in ß-tubulin protein Tub2, which was only reported previously in the M. fructicola strains from California, USA, and do not display obvious fitness penalties, as no significant defects in mycelial growth rate, sporulation, conidia germination, aggressiveness on detached peach fruit and temperature sensitivity was detected. In addition, we found that diethofencarb, the agent for managing MBC-resistance strains, was unable to inhibit growth of the H6Y strains. Taken together, our study, for the first time, identified a mutation form of H6Y in the ß-tubulin protein of M. fructicola in China, rendering the strains wide resistance to thiophanate-methyl. This mechanism of M. fructicola gaining resistance to MBC fungicides needs to be fully considered, when designing management strategies to control brown rot disease in stone fruit orchards.