Studies on resistance to yellow rust of barley

Relatively little work has been done on the epidemiology of Puccinia striiformis'f.sp- hordei. The present study was undertaken to provide a better understanding of the epidemiology of yellow rust of barley. Yellow rust has numerous stages in its development; certain of these were looked at. Effects of factors such as host cultivar, leaf position, rust isolate and temperature on the rate of development of the rust were studied.Spore production was chosen as the main subject of resistance studies, other components of resistance like germination, latent period and growth of the mycelium inside the host leaves were also studied. Races 23, 24, 24cp and 24vv of barley yellow rust and cultivarsA6trix, Proctor, Senta, Sultan and Zephyr were studied.It was found that the races 24, 24cp and 24vv isolates studied have a better chance of surviving at high temperatures in the field especially temperatures higher than 25°C than the race 23 isolates studied. The level of uredospore germination on primary leaves of susceptible and resistant cultivars was similar. There were w inter-cultivar differences in latent period for all the races except race 24. Race 24vv had the shortest latent period, i.e. 12 days. Some significant differences were detected between cultivars for growth within leaves 1 and 2. The ranking order of the races was nearly the same on all cultivars 24cp, 24 and 24vv in ascending order.Significantly more spores of all races (except 23) were produced on Astrix and Senta than on Sultan and Zephyr on seedling leaves in the glasshouse. Both pre and post inoculation temperatures were found to affect the development of the rust. Under a temperature regime of 28°C (day) and 15°C (night) in the post-inoculation period, all the cultivars became resistant to all the races. Whereas preconditioning of barley cultivars at high temperatures conferred some resistance upon them, reflected in the significantly smaller number of spores produced on 3 them than on the same cultivars preconditioned at the normal temperatures. 4. Highest spore numbers were produced at 20/20°C in most race X cultivar combinations and in few cases were these significantly different than spores at 15/15°C. At 17/11°C significantly more spores were produced on Astrix and Senta by all races than on the other cultivars. Spore production was significantly less at 25/15°C than at the other three temperature regimes in all race X cultivar combinations. In 1 cultivar Sultan the rate of spore production on leaf two was much less than that of leaf one, and the rate on the flag leaf was significantly lower than that on leaf one or leaf two. At leaf three there was a sharp fall in spore numbers. In nearly all race X cultivar combinations the ranking of leaves as regard total spore production was close to a constant 1, 2, 3, 4, 5, 6, 7 and flag leaf in descending order.Proctor and Zephyr showed greater resistance at' the flag leaf stage than the other cultivars. The wider implications of these results are considered in the general discussion. A detached leaf culture technique was included to develop a rapid standard reliable technique for testing detached mature plant leaves, suitable for employment on a large scale routine basis. Some success was achieved.