Evaluation of resistance of 38 peaches and their wild relatives to crown gall

Abstract:【Objective】Crown gall disease is induced by Agrobacterium tumefaciens and would cause sig⁃nificantly economic loss in both the nursery and commercial stone fruit production including peach. Biochemical and phenotypic analyses were previously used to subdivide Agrobacterium into biovar 1, biovar 2and biovar 3, and the first two were the causal agents for crown gall disease in peach. Soil fumigation withmethyl bromide had been prohibited due to the destruction to natural environment, although it iwas effec⁃tive for controlling the disease. Biological agents such as strain K84 of A. radiobacter var. tumefacienscould only control the disease to some extent, therefore plant resistance was considered to be a good option for controlling this disease. The past researches had found some immune or highly resistant plants incertain peach accessions and its relative species including Prunus mongolica, P. ferganensis, P. subhirtella, P. davidiana var potaninii, P. persica, etc. These highly resistant plants need further test to confirmtheir resistance to virulent strains of the pathogen. In this study, we aimed at identifying the highly resis⁃tant germplasm resources from different peaches and wild relatives in order to provide a foundation for re⁃sistance breeding in the future.【Methods】The degree of resistance of the peaches and the wild germplasmresources were assessed via artificial inoculation of virulent strain of Agrobacterium tumefaciens on shoots infield. Bacteria were cultured in yeast extract and beef extract (YEB) medium on a rotary shaker (200 r·min-1)at 28 ℃ for about 16 h, then collected by centrifugation at 5 000 r·min^-1 for 10 min and suspended in ster⁃ilized distilled water. The density of the suspension was adjusted to 109 colony-forming units per mL for innoculation. The shoots were wounded at three sites by cutting into the cambial area with a scalpel and re⁃moving a piece of tissue about 1 cm long from the stem surface. One drop of prepared bacterial suspension(about 20 μL) was injected onto each wound site, which was wrapped with parafilm to prevent drying. Themaximum diameter of tumor was measured by vernier caliper 60 days after inoculation. Hierarchical clus⁃tering analysis was used to determine the threshold value of each resistant type.【Results】Based on sun⁃flower seedlings and peach assay, the AT4-3 (biovar 2) strain was chosen from 21 wild strains which wereisolated from peach tumor tissues for its strong fertility and high pathogenicity, and was used in followinginoculation. Crown gall disease resistance was evaluated of 972 individuals derived from 38 populations ofopen pollination of the peach cultivars and wild germplasm resources. The results showed that resistantvariation existed among different individuals of the 8 populations in the field in 2013. The immune plantswere found within 30 populations, but not in the other 8 populations. The proportion of immune plants in‘Shou Fen’population was 88.9%, and the residual plants were all highly susceptible. There were nohighly susceptible plants in‘Honggengansutao 1’population. In‘Hongxinguanghetao’and‘PDPP13-1’populations, 43.6% and 41.0% plants were highly susceptible, respectively, and no immune plant wasfound. Highly susceptible plants accounted for 37.5%, and immune and highly resistant individuals didnot appear in the offsprings of rootstock accession‘Okinawa’. The 223 immune plants were re-innoculat⁃ed in 2014 and all of them formed tumors. The gall size of the re-innoculated plants were in 4.61-7.24mm and smaller than those in 2013.‘Funiushanwang 10’,‘Menggubiantao’,‘Sidaolingyeshengli’and‘Shou Fen’were highly resistant.‘Guangyuan 1’and other 13 populations were moderately resistant.‘Siberian C’and other 15 populations were moderately susceptible, and‘Linzhoutianpingshan-1’andother 3 population were highly susceptible. The population coefficients of variations were between 24.3%and 201.4% and the variations of 4 highly resistant populations were extremely large. Acoording to the hi⁃erarchical clustering analysis, the peach resistances to crown gall were divided into 4 classes, that is, high⁃ly resistant (Ⅱ), moderately resistant (Ⅰ), moderately susceptible (Ⅳ) and highly susceptible (Ⅲ) . TheⅠ, Ⅱ, Ⅲ and Ⅳ classes included 14, 4, 4, 16 populations, respectively. The highly resistant (Ⅱ) classhad apricot, almond, plum and dwarf peach and highly susceptible (Ⅲ) class mainly included P. davidiana and their descendants.【Conclusion】There were no immune individuals in inoculated plants. Fourhighly resistant populations including‘Funiushanwang 10’,‘Menggubiantao’,‘Sidaolingyeshengli’and‘Shou Fen’were found. These 4 highly resistant materials were P. armeniaca (apricot), P. mongolica (almond), P. salicina (plum) and P. persica var. densa (dwarf peach) and most of them were relatively far frompeach, which suggested that resistance germplasm resources could exist in peach plants with a wider genetic background. Therefore, assessment should be done in a larger number of peach germplasm resourcesin the future.