NSW Agriculture, Agricultural Institute, Forest Rd, Orange 2800 Australia

Background and objectives
The rice growing regions of Australia are located in south-western New South Wales between the Murray and Murrumbidgee Rivers. Because of the areas isolation, it is free of the more serious diseases affecting other rice-producing countries. Stem rot disease, caused by Sclerotium oryzae was first observed in NSW in 1995. Overseas it is a serious disease which attacks the stem of rice plants at the water line and progresses into the inner leaf sheaths, causing major yield losses. Because the disease was new to our rice growing regions, the susceptibility of our commercial cultivars and advanced breeding lines was unknown.

Materials and methods
Three experiments were conducted to rank cultivars according to susceptibilty to infection by Sclerotium oryzae. Thirteen cultivars (Amaroo, Bogan, Doongara, Goolarah, Illabong, Jarrah, Kyeema, Langi, Millin, Pelde, Namage, YRL 101 and YRL 38) were grown in a glasshouse at 25C. In separate experiments, plants were inoculated in one of two ways:
a) Inoculum (a 6mm diam. disc of a 14 day-old agar culture containing sclerotia) was held in place against each of 4 tillers/plant at water level using grafting tape, 64 days after sowing (DAS); stems were not injured. This experiment was replicated 4 times in a split-block design conducted twice.
b) Sclerotial inoculum was produced in culture and sprinkled on the soil surface around seedlings immediately prior to flooding (21 DAS). There were ~ 3.75 x 104 sclerotia per pot, well in excess of what is likely to be present in an infected rice field. (Californian experiments have used 200 sclerotia/plant, compared with 7,500 in this experiment).
Plants were grown to maturity, harvested and infection measured as the length of sheath, outer culm and inner culm of each tiller colonised by the fungus and on which sclerotia were formed.
Results and conclusions
Infection rates within the same plant were highly variable, especially from sclerotia on the soil. Some tillers within a pot were colonised by the fungus for >200 mm whilst others were not colonised at all. The outer leaf sheath rarely bore sclerotia and there was considerable variation between tillers of the same plant in the length of outer and inner culm bearing sclerotia. Reasons for this are not known. Analysis of each stem portion individually showed no consistent trend so data were analysed on the sum of length of each stem section bearing sclerotia for all plants in each pot. Cultivars were not uniform in their reaction to stem inoculation. Amaroo, Kyeema, Millin and YRL38 yielded significantly less when inoculated, with yield reductions ranging from nearly 30% in Kyeema to 9.5% in YRL38. The length of stem colonised by S. oryzae was not reflected in yield. Lange and Pelde had the most stem tissue bearing sclerotia but their yield was not significantly affected by the disease. Amaroo had 45% less colonised stem tissue than Lange but incurred a yield reduction of 27%.

Challenging the stems directly with the fungus is probably more indicative of cultivar tolerance to the fungus, although placing the inoculum on the soil is more realistic of the field situation. It would appear that the strain of S. oryzae present in NSW is not as virulent as strains elsewhere and therefore, under present conditions, only poses a threat to four cultivars. However, it is important that growers are aware of the vulnerable nature of the industry to new diseases or more pathogenic strains of existing fungal pathogens. Farmers travelling to other rice-producing areas pose the greatest threat of introducing more virulent strains of this pathogen into Australia.