Leigh J. Pilkington

Reducing the immigration of suspected leafhopper vectors and severity of Australian lucerne yellows disease

Three newly-sown lucerne stands in the mid Lachlan Valley region of New South Wales, Australia, were sampled, over 50 weeks, for Australian lucerne yellows disease symptom distribution and severity. Leafhopper populations were also monitored. Symptoms developed in all 3 stands within 32 weeks of sowing. There were statistically significant spatial differences in the density of symptomatic plants for 2 dates at this and another site. Two possible insect vectors, Austroagallia torrida and Batracomorphus angustatus were more numerous in some sections of crop margins at 2 sites. These 2 species and a third possible insect vector Orosius argentatus each had a statistically significant spatial and temporal correlation with symptomatic plant numbers for at least 1 site date. Two subsequent border treatment experiments evaluated the effect of crop-margin treatments on leafhopper movement into and from the stand. The second border treatment experiment examined also the treatment effect on Australian lucerne yellows disease symptomatic plant numbers. Treatment with insecticide or herbicide significantly reduced the overall movement of leafhoppers. In addition, the insecticide treatment lowered the incidence of disease expression in adjacent lucerne. Results suggest that there is scope for management of this plant disease by reducing immigration of leafhopper vectors into lucerne from non-crop vegetation.

Occurrence and severity of lucerne yellows disease in Australian lucerne seed crops.

Production of lucerne seed is one of the most important seed industries in Australia. Anecdotal reports suggested that the disease lucerne yellows was having a marked impact on the industry, but no firm data existed on its occurrence and severity. An interview-based survey obtained detailed information from eight seedhouse ofices and a telephone-based survey equivalent data from 62 individual growers. Only two of the 36 growers surveyed in New South Wales (NSW) and four of the 26 growers from South Australia (SA) had not observed the disease in their crops. In one instance the incidence of lucerne yellows was reported to be as high as 50–75 percent of plants but the majority of interviewees estimated the incidence to be less than five percent. Responses from farmers in NSW did not indicate a significantly greater severity for the disease than was apparent in SA. The results from the seedhouses support those from individual growers in showing that lucerne yellows disease occurs widely in both States and that its severity is such that significant economic losses are likely.