Abstract
1. Keeping the seedlings of wheat, inoculated with the uredicspores of Puccinia rubigo-vera tritici in moist chamber at various constant temperatures, the developmental features of the fungus presented for infection were observed at intervals, and the critical stage of development at which the infection is warranted was determined.
2. The final results of the inoculation experiments under variousconditions show that the formation of the penetration tube from the appressorium marks the period of infection. Therefore, to obtain infection the inoculated seedlings are required to be kept under a moist condition favourable for the fungus development at least until this stage is attained.
3. At a temperature of about 23°C the urediospores germinated within 1 hour, and appressoria were formed 3 hours after the inoculation. About one third to one half of the germinated spores produced appressoria after the lapse of 7 to 9 hours. Penetration tubes were found occasionally 4 hours after and very frequently from 6 to 9 hours after, whereas 24 hours later a great majority of the appressoria were provided with the tubes entering stomata. Infecting hyphae were produced 9 hours after, developing haustoria about 20 hours after the inoculation.
4. At a temperature of 28°C, the spores were inhibited in germination as well as in further development. At 15 to 25°C appressoria were formed 3 or more hours after, and at 8 to 13°C 6 or more hours after the inoculation. Above 15°C stomatal entry took place 3 to 6 hours after, and below 13°C 9 hours after the inoculation. Above 18°C the haustorium formation proceeded 14 to 24 hours after, but at 8°C the development of infecting hyphae was proceeding even 24 hours after the inoculation.
5. In urediospores, which had been preserved in refrigerator for 158 days, the germination and subsequent development were remarkably delayed in contrast to the fresh ones. In consequence, the stomatal entry took place later than usual.
6. The shortest period for infection was found to be 9 hours at 8 to 13°C, 6 hours at 15 to 20°C, and 3 hours at 23°C. This period coincides with that of the formation of penetration tubes. An abundant infection was obtained when the inoculated surface was kept moist for 14 hours at 8 to 13°C, and for 6 hours above 15°C.
7. Generally speaking, the optimum temperature for the infection of wheat by leaf rust is regarded to range from 18 to 25°C.
