No abstract

This investigation was undertaken to answer the question: "does a plant grown under irrigated conditions really need an extensive root system?" Wiersum (1967) is of the opinion that plants do not need the extensive systems they could grow. The question was of great relevance to the irrigated areas in the Riverina of Australia where root penetration is a problem on dense clay soils.

Alfalfa mosaic virus (AMV) infection of the annual barrel medic, Medicago truncatula, has been shown to cause significant reductions in growth and productivity in field and pot trials. The degree of reduction was dependent on the medic cultivar and the virus isolate. In some experiments herbage production was reduced by more than 50%. Although infection did not appear to be associated with significant changes in root growth, root nodulation of infected plants was about one-third less than that of healthy plants. Two AMV isolates showed different effects on seed production. Infection with one virus isolate was associated with a 15-30% decrease in seed production, and the virus was detected in more than 2% of seed from the infected plants, whereas the other isolate failed to reduce seed yield and did not appear to infect any seeds.

A polyhedral virus was isolated from commercial broad bean seedlings in South Australia and identified as broad bean stain virus on the basis of host range, symptom expression, morphology and serology. The virus is shown to have a divided genome and the data available confirm that it should be included in the comovirus group.

SummaryTwo distinct viruses have been isolated from cultivated Oymbidium plants in Australia. Each virus isolate has been transmitted to herbaceous plants, has been purified, and high-titre antiserum has been produced. One isolate has been identified as a serotype of tobacco mosaic virus; the other is suspected of being Oymbidium mosaic virus and has flexuous rods about 13 by 475 mIL. Electron micrographs of particles of the latter show that they have hollow central canals and consist of helical structures with a pitch of approximately 2· 8 mIL' Results indicate that inoculation to herbaceous plants and electron-microscopic examination and serological testing of leaf extracts can be used as methods for the routine indexing of Oymbidium plants.