Evaluating Malus germplasm for resistance or tolerance to apple replant problems.

摘要

Apple replant problems (ARP) cause widespread and serious difficulties in replanting orchards. Our objective involved evaluating apple (Malus domestica Borkh.) germplasm--40 seedling accessions, 17 clones, 30 transgenics with cecropin, attacin, hen egg white lysozyme or chitinase transgenes--for reaction to ARP. We used a composite Field Soil (FS) collected from 10 ARP-infested New York orchards. Data were recorded after growing three to six-week-old plants for {dollar}{bsol}approx{dollar}60 days in a controlled environment greenhouse. Relative biomass index (RBI) was calculated as Field Soil plant biomass subtracted from Pasteurized Soil (PS) biomass divided by PS biomass. Tolerance referred to RBI ranging from 0.01-0.4, root-lesion nematodes (RLN) {dollar}{bsol}ge{dollar}50/g of roots plus 100 cm{dollar}{bsol}sp3{dollar} of soil, and good plant growth in FS and PS. Susceptibility referred to RBI ranging from 0.41-1.0, RLN {dollar}{bsol}ge{dollar}50/g of roots plus 100 cm{dollar}{bsol}sp3{dollar} of soil, poor growth in FS, and good growth in PS. Unspecified response referred to variable RBI and RLN values, and poor growth in FS and PS. Tolerant accessions included 2 of 40 seedling accessions (M. sieversii--3530.01 and M. sieversii--3674.01), and 7 of 17 clones (74M27BY--C1(CG65), M. baccata--1883H, 75O3R5--C4(CG210), M. xanthocarpa--Xan, M. spectabilis--1844, M. mandshurica--364S, and 74R5M9--C2(CG30)). The remaining accessions exhibited a susceptible or an unspecified response to ARP. Unlike dagger nematodes (DN), RLN generally increased, but were affected by relative resistance, tolerance, susceptibility or intolerance to ARP. The significant variation in responses suggested that genetic resistance could be identified and developed for integrated management of ARP. We isolated Fusarium, Pythium, Cylindrocarpon, Rhizoctonia and Phytophthora species from ARP-infested apple roots, then associated them, RLN and DN with the ARP symptoms observed in our bioassays. There was no significant growth improvement in any transgenic compared to the control parents. Eggs and vermiforms of RLN were extracted from roots of chitinase transgenics, which had chitinase activity that was low and close to the basal level in control apple. This result suggested testing the effect of individual ARP pathogens on transgenics, and that gene-pyramiding and high gene expression could be necessary to obtain useful resistance to the ARP complex.