Preliminary studies on resistance phenotypes to Turnip mosaic virus in Brassica napus and B. carinata from different continents and effects of temperature on their expression

Preliminary studies were undertaken to establish the occurrence of Turnip mosaic virus (TuMV) resistance phenotypes in 99 Brassica napus and 32 B. carinata accessions, breeding lines and cultivars mostly from Africa, Australia, the Indian sub-continent or China. An isolate of TuMV pathotype 8 (WA-Ap1) was used in most inoculations. The influence of temperature on expression of resistance phenotypes was determined. Those identified were O (extreme resistance), R_N (localised hypersensitivity), R (resistance to systemic movement without necrosis), +_N (systemic infection with some necrosis), + (susceptibility), and R_N/+ (systemic infection with necrosis limited to inoculated leaves). In the initial glasshouse evaluations with B. napus, 18 lines developed phenotype O, 42 R_N, two + (both from Australia), and 30 segregated for O and R_N, two for R and R_N, one for O and R, two for O, R and R_N, one for O, R_N and +_N, and one for R_N and +_N. Phenotype +_N only occurred in two lines from India. In the initial glasshouse evaluations with B. carinata, 14 lines developed O, two R_N or R, and one +; the 13 remaining lines segregated for phenotypes O and R (12), or R_N and R (1). In B. carinata, phenotype R_N only occurred in African lines, and phenotype + only in a line from Pakistan. The 18 B. napus and 14 B. carinata lines that developed phenotype O uniformly were re-evaluated at low (16 and/or 18 °C) and high (25 °C) temperatures in the glasshouse three or two times, respectively, and again under controlled environment room conditions at 16 and at 28 °C. While in most lines phenotype O was replaced by other phenotypes or segregated with other phenotypes at the higher temperatures, it occurred uniformly regardless of temperature in Chinese B. napus line (06-6-3777) and three African B. carinata lines (IP 117, ST 18 and ST 50). Graft inoculations confirmed extreme resistance in these four lines. Other promising lines which displayed phenotype O in >75 % of inoculated plants included for B. napus Chinese lines 06-p71-1 and 06-p74-4 and French line Cresor, and for B. carinata African lines Mbeya Green, ML-EM-1, ML-EM-7 and ML-EM-8, and Australian lines P 195923.3 and 30200533. Five promising lines segregated for phenotype R_N, B. napus Ding 110, Hyola 42, Fan 028, ZY 007 and Qu 1104. Chinese B. napus line Ding 110 developed phenotype R_N uniformly at high (25 and 28 °C) temperatures. Thus, in plants developing phenotypes R_N, R and +, low temperature retarded virus multiplication in inoculated leaves to below the level at which it can be identified using ELISA. No clear phenotypic differences attributable to TuMV pathotype were found in tests in which isolates from pathotypes 1, 7 and 8 were used. This highlights the need for future evaluations for TuMV resistance in Brassica germplasm to be done at higher temperatures. This study also highlights the need to identify additional genes responsible for the different resistance and susceptibility reactions found, especially in B. carinata. The four lines that developed phenotype O uniformly at all temperatures and also withstood graft-inoculation will be particularly valuable for developing new TuMV-resistant cultivars of oilseed and forage Brassicas.