Effect of temperature, light and host on prepenetration development of Puccinia graminis avenae and Puccinia coronata avenae

The influence of temperature and light on prepenetration development of single and mixed isolates of Puccinia graminis avenae and Puccinia coronata avenae was studied on 0–2% water agar and on leaves of three oat cultivars and on three non-cultivated species of Avena. Germination of uredospores of P. graminis avenae and P. coronata avenae occurred best at 10–30^oC and at 20^oC respectively. The optimum temperature for germ-tube growth and for appressorial formation was 20^oC for both rusts. An inverse relationship was observed between light intensity and prepenetration development with maximal germination of uredospores, germ-tube growth and appressorial formation occurring in darkness. Under optimum conditions maximum percentage germination and appressorium formation of both rusts was attained within 4 and 12 h after inoculation respectively. The proportion of germinated uredospores of crown rust which gave rise to appressoria was about twice that observed for stem rust. No significant differences were observed in prepenetration development between the single and mixed race inocula of the two rusts. Although germination of uredospores was significantly greater on water agar than on oat leaves, there were no significant differences in prepenetration development of the rusts on the various oat cultivars and species examined. Consequently, the data failed to indicate the presence of resistance mechanisms operating during the prepenetration phase of the infection process on the cultivars and species examined.