Biological diversity of cauliflower mosaic virus isolates expressed in two Brassica species

Abstract

Infection characteristics and symptom pathology of 39 isolates of cauliflower mosaic virus (CaMV), obtained worldwide, were compared following infection of a susceptible host (Brassica‐rapa‐rapifera‐turnip) and a tolerant host (Brassica oleracea‐gongylodes‐kohlrabi). Considerable variation in symptom expression between isolates was observed in turnip plants. Local lesions appeared between 4 and >10 days post inoculation (dpi) and systemic symptoms appeared from 10 to 20 dpi depending upon the CaMV isolate. Leaf coloration effects due to systemic infection ranged from generation of dark green tissue through various patterns of vein clearing and vein banding to severe chlorosis. Plant distortions in turnip varied from mild to severe stunting, and various types of leaf wrinkling and rugosity. In contrast, symptoms caused by the same CaMV isolates in kohlrabi were restricted to older leaves and were much milder, showing vein clearing, some chlorosis, but no plant distortions. Kohlrabi plants infected by all isolates tested eventually showed recovery and became asymptomatic. One symptom character in turnip, veinlet necrosis, was dependent upon time of inoculation rather than infection by a particular isolate. Virus titre was estimated by dot‐blot analysis of CaMV DNA. The greatest difference in viral DNA amounts was observed between turnip and kohlrabi plants for all isolates. A significant decline in viral DNA occurred in kohlrabi, concomitant with plant recovery from infection. Variations in virus titre observed early in infection (10 dpi) tended to become more uniform with time. We observed no clear correlation between virus titre and symptom type and severity in turnip, although plants infected with viral isolates producing mild symptoms generally, but not always, contained less viral DNA than isolates causing more severe symptoms. The great variety of CaMV biotypes we have characterized offer considerable scope for investigating mechanisms of plant viral pathogenesis.