Virus diseases of Trifolium subterraneum and Vicia faba in Tasmania

Studies were conducted on six virus isolates recovered either from subterranean clover (Trifolium subterraneum) or broad bean (Vicia faba) infected in the field in Tasmania. Two were identified as isolates of bean yellow mosaic potyvirus (BYMV-F, BYMV-K), two as isolates of clover yellow vein potyvirus (CYW-B, CYW-D) and two as isolates of soybean dwarf luteovirus (SDV-Ap, SDV-As). These isolates were compared with respect to some of their biological and physical properties and representatives of the isolates were also studied in the field to compare their spread and effects in plots of broad bean and how that might be controlled with insecticides. The CYW isolates generally caused more severe symptoms than the BYMV isolates on those host plants that they had in common. These two groups could be distinguished on the basis of their host ranges but neither host range nor serological tests could differentiate the isolates within the same group. However isolates of the same group did differ with respect to their transmissibility by various aphid species. The differences in the efficiency with which the isolates were transmitted by various aphid species seemed to reflect differences in the helper factors encoded by them. For example Rhopalosiphum padi transmitted BYMV-F quite efficiently but it only transmitted CYW-D if it had prior access to plants infected with BYMV-F. The host ranges of the SDV isolates were broadly similar although SDV-Ap generally had a more restricted host range and caused mllder symptoms on most of the hosts that they had in common. An important difference in host range from an epidemiological viewpoint was that SDV-Ap infected lucerne (Medicago sativa) but not white clover (Trifolium repens) while SDV-As infected white clover but not lucerne. This finding was particularly relevant because lucerne is an important oversummering and overwintering host for Acyrthosiphon pisum, as white clover is for Aulacorthum solani. The two different vector specific isolates (SDV-Ap and SDVAs) could not be distinguished serologically, unlike the barley yellow dwarf virus where serological specificity is closely related to vector specificity. However the viral groups were similar in that transcapsidation was indicated when the two SDV isolates were present together as a mixed infection because then, although A.pisum never transmitted SDV-As when this isolate was present by itself, this species did transmit the SDV-As genotype when it was present in plants that were also infected with SDV-Ap. Field investigations were undertaken over three seasons at Cambridge in south-eastern Tasmania to study the epidemiology and control of some of the virus isolates referred to previously. Infection foci of different virus isolates were established at the centres of plots of broad beans and the subsequent development of virus epidemics was monitored and related to aphid infestation of the plants in the plots and aphid flight activity. Spread of all the viruses was least in plots sown in the late autumn (April or May) and greatest in those sown in the early spring (September). In all three seasons, BYMV spread more rapidly than any of the other viruses. The patterns of BYMV spread were always contagious, radiating from infection foci established in the centres of the plots. The non-colonising R.padi as alatae, was implicated as the prime vector of BYMV in the first two seasons while apterous A. pisum colonising the plants were responsible for most of the spread of this virus in the third season. The differences noted in patterns and modes of spread of BYMV between seasons was partly related to plant density in the plots and the nature of neighbouring crops. When compared, the rates of spread of BYMV-F and BYMV-K were similar. Spread of SDV-Ap in the plots was always greater than that of SDV-As and this was correlated with the relative intensities of aphid flight activity of A. pisum and A. solani in south-eastern Tasmania. The development of SDV infections in the field plots occurred independently of the infection foci established at the centres of the plots. The distributions of infections of these isolates was non-random, infected plants being concentrated near the boundaries of the experimental area indicating that most SDV infections were established by viruliferous alatae alighting on the experimental area after flying in from external reservoirs of infection, presumably mostly infected lucerne plants. Experiments were conducted to assess control of secondary spread of the viruses with demeton-S-methyl and of primary and secondary spread with deltamethrin. These experiments were largely unsuccessful because they only provided significant control when infection rates were low (i.e. in the plots sown in late autumn) and pesticide treatments were not warranted for control of virus spread in those sowings. The BYMV and SDV isolates reduced the yields of broad bean plants in the field. They decreased the numbers of pods per plant, the numbers of beans per pod and the individual bean weights. The effects of all the isolates on yield and its components were greater the earlier the infections were initiated. The detrimental effects on individual plants were greater for the SDV isolates than the BYMV isolates. However on a crop basis, BYMV was more important in causing yield losses due to its prevalence in the plots. Selected subterranean clover pastures in drier areas of the State were surveyed regularly for infection with viruses. In addition, some legume crops in the north-western region of Tasmania were examined on a few occasions. The incidence of infection with different viruses was extremely variable between seasons and the incidence of infection with different viruses at the same site and time was never correlated. There were even large differences between levels of infection with a particular virus in pastures that were situated close to one another. The one consistent feature arising from the survey was the relatively common occurrence of SDV-As in the north-west region where white clover, is widespread. Infection with SDV transmitted by A. pisum was not identified in that region and this contrasted with the situation in south-eastern Tasmania. The factors determining the occurrence of viruses in pasture legumes and legume crops in Tasmania were concluded to be very complex. A wide array of environmental factors have the potential to affect the interactions between the various viruses, their host plants and their aphid vectors. Many factors that can have a role in affecting virus incidence were elucidated during the course of the field surveys, the field experiments and the greenhouse studies. However it seems unlikely that it would be possible to develop models that could reliably predict either serious virus disease outbreaks or freedom from them. The cropping and farming systems of Tasmania are diverse and intermingled and the climate is mild, the occurrence of extreme conditions, leading to high or low aphid activity and virus incidence, do not occur.