Turnip mosaic virus resistance breaking strain in canola: Symptomatology, seed and contact transmission, yield losses and seed germination

Turnip mosaic virus (TuMV) causes serious yield-limiting diseases in Brassica crops including the oilseed B. napus (canola). Further spread of TuMV's canola resistance breaking strain poses a threat to global canola oilseed production. Isolate phenotyping found that, like isolate 12.1 from canola, isolate BRSB1 from B. rapa belongs to this resistance breaking strain. New biological information was obtained on the responses of canola cultivars to infection with isolates BRSB1 and 12.1, including about symptoms they elicit in infected plants, their seed-to-seedling and contact transmission, and, for isolate 12.1, effects on yield limiting parameters and seed germination. In 12/13 canola cultivars with diverse TuMV resistance traits, isolate BRSB1 reached lower virus concentrations and caused milder symptoms than isolate 12.1. However, because isolate BRSB1 overcame TuMV temperature sensitive invasion resistance, it reached higher virus concentrations in cv. Scoop. Late TuMV symptoms in five cultivars included axillary shoot growth proliferation, flower abortion, pod discoloration, deformation and size reduction, and within-pod seed germination sometimes causing pod splitting. TuMV seed-to-seedling transmission was demonstrated unequivocally in canola. For isolate 12.1, its rate was enhanced from 0.1% after growing infected-mother plants under natural light to 0.5 - 3.3% after growing them under artificial light, and to 2.4 - 3.5% when there was mixed infection with cucumber mosaic virus (CMV). Low rates (0.1-0.9%) of CMV seed-to-seedling transmission occurred following mixed infection. TuMV seed-to-seedling transmission rates of 0.3-1.5% were obtained with seed from isolate BRSB1-infected mother plants under natural light. Both TuMV isolates were contact-transmitted, which occurred more readily with abrasive dusted leaves. Isolate 12.1-infection reduced average pod number/plant, total pod dry weight/plant and individual dry weight/pod by 59%, 84% and 62%, respectively. Seed yield loss/plant was 71% with pregerminated seeds included, but 87% when they were excluded. When pregerminated seeds were included, yield loss was due to fewer seeds, but, with pregerminated seeds excluded, individual seed weight reduction also contributed. Decline in seed germination rate during storage was faster with seeds from TuMV-infected plants. These findings concerning the biological properties of TuMV's canola resistance breaking strain emphasise not only its potential to cause major losses to global canola oilseed industries, but also provide important clues towards improving its management.