New hosts of Turnip mosaic virus in Zimbabwe

Necrotic rings, lines and spots (Fig. 1) were observed on approximately 90% of cabbage (Brassica oleracea var. capitata) plants growing in a field in Harare. Inspection of cabbages in nearby fields and in retail outlets, showed that these symptoms were widespread. A virus isolated from an affected plant and established as a single lesion culture, by serial passage in Chenopodium quinoa (chlorotic/necrotic local lesions, no systemic infection), produced symptoms typical of Turnip mosaic virus (TuMV) in a range of indicator plants. TuMV was confirmed in cabbage and C. quinoa sap by PAS-ELISA, using antiserum to a UK isolate of the virus. All PAS-ELISAs included positive and negative (healthy) controls and test samples were considered positive if their means (A405) exceeded negative controls by more than 3 SD and were significantly different (P<0.05).

Rothwell (1983) lists Brassica oleracea as the only previously-known host for TuMV in Zimbabwe. In a survey of TuMV in crop and wild hosts in the Harare region within 1-2 kilometres of the present outbreak, sap samples from the following hosts tested positive in PAS-ELISA and induced diagnostic symptoms in C. quinoa: the dicotyledons Amaranthus hybridus, Brassica juncea, Conyza sumatrensis, Galinsoga parviflora, Portulaca oleracea, Sida alba, Sonchus oleraceus and Hibiscus esculentus; the monocotyledons Commelina bengalensis and Musa sapientum L. (banana). Bidens pilosa and Cyperus esculentus (monocotyledons) also tested positive for TuMV in PAS-ELISAs, although sap was not infectious. With the exception of B. juncea, and H. esculentus, the above species are apparently new hosts of TuMV (Broadbent, 1957; Stobbs & Stirling, 1990). Calanthe sp. (Orchidaceae) is the only other monocotyledonous host reported for TuMV (Brunt et al., 1990).

In tests, the aphids Myzus persicae, Brevicoryne brassicae and Aphis fabae, transmitted the virus non-persistently, between infected and healthy cabbage plants in Zimbabwe, and may have been responsible for local spread of the virus. Further work is needed to establish whether infection of Musa and other apparently new hosts, is associated with disease.