Anthracnose and seedling blight of Bathurst Burr caused by Colletotrichum xanthii Halst

Abstract

In the summer rainfall regions of New South Wales during the past three seasons, a disease caused by the fungus, Colletotrichum xanthii, has caused widespread destruction of Bathurst burr, which ranks as one of the most serious weeds of pasture land in these areas. The disease, previously unrecorded in this State, was first observed on the Central Tableland but has since become established throughout the North-western and Central Western Slopes and Plains. It appears to be restricted to the genus Xanthium and is characterized by anthracnose-type stem lesions which tend to produce a 'ring-bark' effect. A seedling blight condition may also occur. The disease carries over from season to season on old burr remains. Since 1947-48 it has occurred naturally in 50 different districts of the State. During 1949-50 it was artificially established in 16 districts where it had not been observed previously. Disease development is favoured by prolonged periods of wet weather in the summer and autumn. It is difficult on the basis of three seasons' observations to assess the economic value of the disease as a means of biologically controlling Bathurst burr. During these three seasons, however, it has largely removed the necessity for weedicide spraying and hand cutting of burrs on many properties in northern New South Wales and thereby proved to be the most useful parasitic disease of a weed host ever encountered in this State.