We studied the behaviour of tomato mosaic tobamovirus (ToMV) in a recirculating nutrient solution. Particles of tomato mosaic virus were found in the nutrient solution 3 days after leaf inoculation of plants and we demonstrated that the level could increase to a concentration readily detectable by electron microscopy. Virions remained infective in nutrient solution for at least 6 months, could infect bait plants and induced systemic symptoms within 10 days. This is the first report of measurement of infective virions in recirculating nutrient solutions. Hydroponic producers should be aware of the potential for rapid spread of virus diseases in soilless growing systems as one infected plant can serve as a source of inoculum that can ultimately result in an epidemic.

Mycelial extracts of Pythium irregulare were processed using CF-11 chromatography and analysed by gel electrophoresis for double-stranded RNA (dsRNA) content. DsRNA molecules of between 1 and 6 kb were found in 33 of 39 isolates tested. The dsRNA profiles of individual isolates remained unchanged after repeated subculturing of hyphal tips, after storage under water for 2 years, or when single zoospores were used to generate subcultures. However, dsRNA profiles varied between isolates, and even within individual populations; in the one case eight different dsRNA types were recovered upon testing 17 isolates from a single wheatfield. Isometric virus-like particles (VLPs) of approximately 45 nm were found in isolates containing dsRNA, while no such particles could be found in isolates in which dsRNA was not detected. Electrophoretic profiles of dsRNA extracted from partial purifications of VLPs were identical to those of dsRNA extracted from whole mycelium, suggesting that the dsRNA is at least partially encapsidated. Attempts to transmit dsRNA between isolates by hyphal anastomosis or by coinfection in wheat plants were unsuccessful, with only parental types being recovered. Analysis of 29 isolates representing 14 other Pythium species failed to detect dsRNA, even when co-isolated with P. irregulare known lo contain dsRNA

Only a few plants in a crop are generally thought to become infected by abiotic soil transmission. In glasshouse experiments we have induced almost total infection in tomatoes growing in soil with infective debris, but levels of infection are dependent upon certain conditions. We found that as the inoculum concentration decreased (i) a greater percentage of the infections were either latent (symptomless) or restricted to roots and (ii) infection levels decreased. Thus, apparent infection (based on symptoms) of 0. 10% was in reality 0 60–70% based on enzyme‐linked immunosorbent assay and immunoelectron microscopy testing of roots and leaves. This occurred whether seedlings or seed were planted into infected mix. Under conditions in which minimal root damage was caused (planting seed) or roots were mechanically inoculated only once, almost all systemically infected plants were symptomless.

Virus disease limit glasshouse production of capsicums in the Supraysia District of New South Wales. Symptoms develop on plants about 4 weeks after planting; and up to 83° of plants are infected. The major virus is cucumber mosaic (CMV) with some pepper mild niottle (PMMV) also present. Evidence is presented to show that the primary source of intection of both viruses is most likely to be debris from previous crops remaining in the soil. It demonstrates a previously unconsi‐dered mechanism tor survival and spread of labile viruses such as CMV and the ecological ecological consequence of these results are considerable.

This paper records the occurrence of an antigen produced in plants infected by potato leafroll virus (PLRV), which is copurified with the virus and induces an immune response in rabbits used for virus antiserum production. The antigen also appeared to be produced in uninfected but physiologically stressed potato plants. These plants reacted with antisera to PLRV in ELISA tests, thereby giving false positive results. We refer to the compound as virus‐stress antigen (VSA). The importance of this finding, not only to serological testing for PLRV, but also as a possible explanation for some false‐positive reactions that occur with other host‐virus combinations, is discussed. The necessity of having detailed information on plants from which samples are taken for testing is emphasized.