Development of stripe rust was observed on wheat cultivars that differed in reaction to the disease at the post booting stage of growth over 4 years (1984-1987) at Yanco and Wagga Wagga in southern New South Wales. In 1984, the epidemic began in August and the disease affected up to 20% of leaf area by the booting stage. The disease then ceased to develop in cultivars with moderately resistant or resistant adult plant reaction (APR) to stripe rust, but in susceptible wheats up to 82% of leaf area was affected by early milk stage of growth. The early onset in 1984 was associated with the highest rain in the previous summer-autumn (January-April) of the 4 years. In the other 3 years, the epidemics began laer. Stripe rust did not develop on cultivars with resistant APR, but it affected up to 97% of leaf area of the highly susceptible cultivar Avocet by early milk. The disease was more severe on later sown than early sown Avocet. The apparent rates of infection both before and after booting ranged from 0.02-0.41 per day. In each experiment, the rate was less on wheats with higher levels of APR while in 1984 the rate on all cultivars decreased from the pre-booting to the post-booting stage of crop growth. After booting, the apparent rates of infection on susceptible and moderately susceptible cultivars were postively correlated with the mean temperature during the period over which the rate was calculated, for the range 12.9-16.2�C. Over this range, the apparent rate of infection of susceptible wheats increased at 0.095 per day per �C while that of moderately susceptible wheats increased at 0.045 per day per �C. From 16.2-203�C the rate of susceptible wheats was negatively correlated with the mean temperature, and declined at 0.043 per day per �C. There was no significant relationship between apparent rate of infection and temperature for moderately resistant wheats after booting, or for rates before booting in 1984. Development of wheat, measured on the Zadoks scale, was linear from first appearance of the flag leaf (GS 37) to mid milk (GS 75) at both sites over the four years.

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Stripe (yellow) rust, caused by Puccinia striiformis f.sp. tritici. was associated with mean losses of up to 84% in the yield of wheat in southern New South Wales, Australia, between 1984 and 1987. Yield loss became smaller in cultivars with increasing levels of resistance in adult plant reaction and did not appear in cultivars with seedling susceptibility and moderate or better adult plant reaction levels in the 3 years when epidemics began after emergence of the flag leaf. However, in 1984 the stripe rust epidemic began during stem elongation, and yield losses of up to 21% occurred in cultivars with seedling susceptibility and a moderate or better aduh plant reaction.Yield was significantly negatively correlated with the proportion of leaf area affected by stripe rust at stages of crop development from the end of heading to late milk. The correlation was greatest at the early milk stage of growth where the relationship was logarithmic. Two factors significantly influenced this relationship. Yield loss increased as the length of the epidemic increased, and decreased as temperature increased during grain development. The relationship that was developed for predicting yield loss accounted for 80-5% of the variance across all experiments, and was:.1= 100(1 _e-IO •'WM*-I65Z.-1417-1) where>• = yield loss (%); X= stripe rust (leaf area affected, range 0-99%) at early milk; Z. = time from 1 % disease to early milk (range 0-70 days); and T= mean daily maximum temperature from 7 days before to 14 days after early milk (range 19 8-27 5 C).

The effect of light on in vitro germination of urediniospores of Tranzschelia discolor was studied over time at different intensitities (up to 400 8E m-2 s-l) within the temperature range 5�C to 20�C. A model was also developed from the data to predict germination at different combinations of light, temperature and times of leaf wetness. Light retarded the germination process, and its effect increased in direct proportion to intensity. At 20�C, for example, the time taken to exceed 80% germination increased from 2 h in the dark to 9 h at 200 8E m-2 s-l. The model showed that there was an interaction between light and temperature, with the effect of light becoming more pronounced as the temperature declined below 20�C. Germination percentages of the order of 90% were, however, recorded within 24 h at all combinations of light intensity and temperature studied. Light also influenced germ tube growth, causing a reduction in the rate of growth. As in germination, its effect increased with increasing light intensity. At 20�C, the average germ tube length at 9 h was 541 8m in the dark, compared with 227 8m at 200 8E m-2 s-1 and 148 8m at 400 8E m-2 s-l. A similar effect was observed at 5�C, where the average germ tube length at 24 h was 274 8m in the dark compared with 157 8m at 200 8E m-2 s-l. The effects of light on the germination and germ tube growth of urediniospores under field conditions are discussed.

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