Identification Of Suspected Botryosphaeria Spp. And Observation Of Symptoms Caused In Current-year Peach Shoots

Peach tree gummosis, which has affected the healthy development of the peach industry, is widely distributed in the Yangtze River Basin and the south region. Botryosphaeria spp. are the predominant causal agents of peach tree gummosis and Fusicoccum aesculi is the most common one in China. Peach tree gummosis pathogen can enter into the tree through lenticels, pores or small openings. A comprehensive investigation and study on the initial infection symptoms of the pathogen in the host can help us grasp the best time to control the disease. The present study mainly focused on the milky bubble spots on the current-year peach shoots initially. We observed the morphological characteristics of the spots in different stages using paraffin section the corresponding period. Based on morphological characteristics, molecular systematic analysis and pathogenicity determination, we identified the pathogens isolated from these spots. The results are summarized as follows:1. Characteristics of spot development stage The development process of milky bubble spots caused by suspected peach tree gummosis pathogen was divided into five stages:(Ⅰ) Diameter of milky bubble was less than 1 mm, and the cells under the epidermis divided actively, causing the epidermis bulges outward.(Ⅱ) The center of spot became brown after 3 days, while the middle cortical cells began to suberification.(Ⅲ) The whole spot(diameter about 1 mm) became brown with after 10 days or thereabout. Paraffin section showed that all cells of the spot became purple.(Ⅳ) The whole spot has become brown while the central part cracked(diameter ranged from 1 mm to 2 mm) in 20 th day. The suberification extended from the center to the periphery gradually, middle cortex cells collapsed and cracked.(Ⅴ) After 40 days, the surrounding formation of brown spots(diameter greater than 2 mm). Cortical cells surrounding the spot center(1 mm to 2 mm) became purple, while cortex partially scatted.2. Isolation of pathogens at different stages Pathogens were isolated from different stages of the spots, 120 samples of the spots at each stage were collected. The obtained isolates were sorted by the different characters in order to collect the suspected Botryosphaeria spp. fungal. The results showed that: in culture, isolates of fungal belonged to one type. The isolation rates of the five stages were different(0%, 20.8%, 60%, 75.8%, 80.8%) which increased with the development of separating periods.3. Identification and pathogenicity of isolates Isolates were identified based on morphology characteristics and DNA sequence analysis, and nine of them were identified as Fusicoccum aesculi. As for WH-9, WH-31, WH-76 and WH-X5 isolates, phylogenetic studies showed that WH-31 and WH-X5 could cause peach tree gummosis. And the same fungal specie was also isolated from the disease tissue after inoculation.