Identification of the pathogen causing soft rot of peach and screening of highly effective fungicides

Abstract:【Objective】Peaches (Prunus persica L.) are known for their palatable flavor and abundant nutrients. However, peaches are perishable and soft rot of peach is a key deterioration factor in the post- harvest storage process. Previous studies showed the species of the genus Monilinia, Penicillium andRhizopus were the major pathogens causing soft rots and decays in peach. This study aimed to identify the pathogen causing soft rot of peach in Henan province based on molecular and morphological identi- fication, as well as the pathogenicity test. To this end, we performed the screening of highly effective fungicides by inhibitory activity test in vitro and relative control efficacy trials on detached peaches. Our work will provide scientific basis for the reasonable prevention and control of soft rot of peach.【Methods】The soft rot peach fruits collected from Yuanyang county of Henan province were used as samples. After isolation and purification, the isolates were identified by using the methods of morpho- logical characteristic analysis. Morphological parameters, including colony, mycelia, sporangia and conidiophores, were observed under a microscope. The genomic DNA of pathogenic fungi was extracted and identified based on molecular biology (rDNA-ITS and rDNA-LSU). The pathogens were inoculated on peaches, and their pathogenicity was determined and compared among 5 different peach cultivars. The virulence of 15 fungicides was tested by mycelium growth rate method.【Results】Twenty-seven strains with the similar morphology were isolated in total, producing white mycelia and growing rapidly on PDA. Sporangia were initially white but became brown and then black on 2-day cultures. Sporangio- phores were hyaline to light brown, unbranched and aseptate. The ITS sequence (Accession No. MK301176.1) and the LSU sequence (Accession No. MH858452.1) showed 99% and 98% homology with Gilbertella persicaria, respectively. Phylogenetic tree analysis confirmed that the isolates belonged to G. persicaria and can be grouped into the same clade as other taxa of Choanephoraceae. Soft lesions were observed at 1 day post-inoculation (dpi), and then the whole fruit was rotted and covered with white mycelia and black sporangia at 2 dpi. Finally, fruits were often juice leaked. Non-wounded-inocu- lated fruit and wounded control fruit did not show the disease. Re-isolation was performed from symp- tomatic peaches, and their identities were confirmed using the above-mentioned methods. We also eval- uated the genetic variability of resistance to this disease by artificial inoculation. The test of the culti- vars studied exhibited a range of susceptibility to G. persicaria, the disease indexes of peach cultivars CP10-7 and CN19 were significantly lower than those of CN18, CN13 and CP9 at 2 dpi. In the toxicity test of fungicides, 250 g·L-1 pyraclostrobin EC had the most obvious inhibitory effect, in which the EC50was 0.618 mg · L- 1. The followed fungicides were 60% pyraclostrobin · metiram WG, 10% difenocon- azole WG, 95% natamycin TC, 96% lauroyl arginine ethyl hydrochloride TC and 450 g·L-1 prochloraz EW with the EC50 as 4.203 mg·L-1, 9.920 mg·L-1, 12.297 mg·L-1, 13.846 mg·L-1 and 39.978 mg·L-1, re- spectively. 500 g · L- 1 iprodione SC, 400 g · L- 1 pyrimethanil SC, 20% imazalil EW and 50% dimetho- morph WP showed poor inhibitory effect with EC50 of 133.982 mg·L-1, 140.509 mg·L-1, 266.796 mg·L-1and 816.936 mg · L- 1, respectively. Metalaxyl · hymexazol, 1- MCP, 7- hydroxycoumarin, thiophanate- methyl and streptococcus lactis showed no effect on G. persicaria【. Conclusion】The major pathogenic fungus of soft rot of peach in Henan province was identified as Gilbertella persicaria. The peach culti- vars CN19 and CP10-7 showed lower susceptibility to G. persicaria than CN18, CN13 and CP9. Pyra- clostrobin·metiram and pyraclostrobin showed excellent inhibitory activity against G. persicaria, mak- ing these fungicides become potential candidates for the control of soft rot disease of peach in the field.