BIOLOGICAL DIVERSITY OF CAULIFLOWER MOSAIC-VIRUS ISOLATES EXPRESSED IN 2 BRASSICA SPECIES

Infection characteristics and symptom pathology of 39 isolates of caul iflower mosaic virus (CaMV), obtained worldwide, were compared followi ng 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 turn ip plants. Local Lesions appeared between 4 and > 10 days post inocula tion (dpi) and systemic symptoms appeared from 10 to 20 dpi depending upon the CaMV isolate. Leaf coloration effects due to systemic infecti on ranged from generation of dark green tissue through various pattern s of vein clearing and vein banding to severe chlorosis. Plant distort ions in turnip varied from mild to severe stunting, and various types of leaf wrinkling and rugosity. In contrast, symptoms caused by the sa me CaMV isolates in kohlrabi were restricted to older leaves and were much milder, showing vein clearing, some chlorosis, but no plant disto rtions. Kohlrabi plants infected by all isolates tested eventually sho wed 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-bl ot analysis of CaMV DNA. The greatest difference in viral DNA amounts was observed between turnip and kohlrabi plants for all isolates. A si gnificant decline in viral DNA occurred in kohlrabi, concomitant with plant recovery from infection. Variations in virus titre observed earl y in infection (10 dpi) tended to become more uniform with time. We ob served no clear correlation between virus titre and symptom type and s everity in turnip, although plants infected with viral isolates produc ing 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 investigat ing mechanisms of plant viral pathogenesis.