Several systemic diseases of some subtropical grasses were investigated in the period 1978-1981. Most of the work concerns diseases of Sorghum leiooladum, but the studies have extended to the same or similar diseases of other grasses.
Downy mildew of Sorghum leiooladum is widespread in southern Queensland. The identity of the parasite, its location in its host from time to time throughout the year, and the symptoms exhibited by the host have been studied. It has been confirmed that the parasite is Peronosolero spora noblei, that being a fungus described from New South Wales in earlier years by Weston as Solerospora noblei. The fungus is perennial in tiller bases. There it enters axillary buds from which tillers with well defined symptoms develop. For any given diseased tussock, tillers of three kinds are possible, namely solely vegetative tillers which remain short, tall tillers which exhibit downy mildew symptoms prominently and tall tillers which flower, the last mentioned being indistinguishable from the flowering tillers of healthy plants. Tillers with disease symptoms have chlorotic leaves. In comparison with flowering tillers, the tillers exhibiting symptoms of downy mildew differ significantly in the following respects:- (i) the culms are greater in diameter (due to an increase in the volume of ground tissue), (ii) the culms have more nodes, (iii) the leaves are wider, and have a greater number of vascular bundles. Leaves of diseased tillers are held more erect and are closer together than those of flowering tillers. The location of hyphae in developing tillers has been studied, and the relationship between the position of hyphae at any time and the concurrent symptoms has been established. The progress of sporulation, both asexual and sexual has been monitored. The visible effects and the effects on the internal structure of the leaves have been described. During three summers, the seasonal production of flowering tillers and tillers exhibiting symptoms of downy mildew were recorded. A correlation between environmental factors, particularly rainfall, and the onset of tiller production has been shown. .............................