In his experiments Yamada (1955) found an incubation period of about 10 days for Puccinia horiana P. Henn. At a temperature of 15 ~ ~ Stahl (1964) found a period of 12 days, while Stark and Stach (1965) concluded that this time can vary from at least 10 days to 8 weeks 1. Little being known about the time of incubation in various conditions, the authors decided to do the pilot experiments reported below. Chrysanthemum plants of cv. 'Indianapolis White G 4' were inoculated on July 13, 1966, by spraying with a mixer-made suspension of sporulating lesions (Zandvoort et al., 1968). Immediately following inoculation the plants were placed in a glasshouse with a minimum temperature of 12 ~ ~ and a maximum temperature of 25 ~ ~ During the first 24 h after inoculation the plants were placed under a black polythene plastic foil. The sixth day after the inoculation the first symptoms of P. horiana became visible. In another experiment, plants of cv. 'Indianapolis White G 4' were inoculated on 27 June by spraying with a mixer-made suspension of spores. The treated plants were placed in glasshouses at four temperatures ranging from 10~ to 30~ For each temperature treatment 20 plants were used. Fig. 1 indicates that the time of incubation was shorter than 12 days at 17~ and 21 ~ 13 days at 10~ while at 30~ no symptoms became visible during 29 days. The development of the fungus at 10~ is definitely slower than 17~ or 21 ~ The number of infected leaves after 29 days suggests an optimal infection at 17 ~ At the field inspection done by the Plant Protection Service we commonly found new infections about 2-4 weeks after the destruction of infected plants, which is an indication for an incubation period of at least the mentioned time. An interesting indicacation of a very prolonged incubation period was found on one well isolated plot. Cuttings, rooted at an infected nursery where the temperature was over 30~ for several hours during some days of the rooting period, had been planted there about 17 June, 1966. After planting in the field there were several days with a high relative humidity and a temperature of 10~176On 5 August many necrotic spots and only one withered teleutosorus were found. One week later, fresh teleutosori were observed on ca. 2 ~ of the plants.
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