The present paper deals with the induction of lesion formation by heat or cold treatment on TMV- or CMV-infected systemic hosts and also TMV-infected lesion hosts predisposed to systemic infection. 1) TMV-inoculated leaves of systemic hosts, N. tabacum Xanthi, N. tabacum Samsun, and Petunia hybrida, developed local lesions if they were immersed 2 days later in hot water (50°C) for 2min. CMV-inoculated leaves of systemic hosts, N. glutinosa and Petunia hybrida, also developed local lesions by the heat treatment. 2) The similar heat treatment resulted in local lesion formation on TMV-inoculated leaves of local lesion hosts, N. glutinosa, Xanthi nc and Samsun NN, during incubation at 30°C, a temperature at which no lesions appear and TMV spreads systemically. 3) TMV-inoculated leaves of local lesion hosts, N. glutinosa, Xanthi nc, and Samsun NN, developed local lesions if they were immersed 2 days later in ice water for 5 sec to 2min during incubation at 30°C. 4) The similar cold treatment occasionally resulted in local lesion formation on TMV-inoculated leaves of systemic hosts, Xanthi, Samsun, and Petunia hybrida. 5) Usually, heat treatment applied 2 days after inoculation induced dark necrotic rings which appeared in 15 to 20 hours after treatment, whereas cold treatment resulted in white necrotic rings which appeared in 1 to 2 hours. When leaves of N. glutinosa bearing lesions induced by heat treatment 2 days after inoculation with TMV were immersed in ice water 4 days after inoculation, dark necrotic rings developed around each original lesion during incubation at 30°C.
Susceptibility of coriander (Coriandrum sativum L.) to stem gall disease incited by Protomyces macrosporus Unger is mainly influenced by the inoculum potential, microclimate comprising of soil and air around the host plant, and susceptible host period amongst other factors. Occurrence of concurrent influence of all the environmental factors for a severe disease outbreak is probably thwarted by the shortfall of some factor(s) for an early maturing crop escaping the pathogenic infection.
Factors affecting the sporulability (capacity for sporulation) of cucumber downy mildew fungus, Pseudoperonospora cubensis (Berk. et Curt.) Rostow., in leaf lesions were investigated. The sporulability was evaluated on the basis of the number of conidia formed on lesion of 1cm2 after being kept for 17hr in a moist chamber under darkness at 20°C. 1. Relationship between the age of lesion and sporulability is shown in Fig. 1. The sporulability attained maximun 7 days after inoculation. 2. The sporulability was affected by the weather condition of the day when infected leaf specimens were taken from the plant. Conidia were more heavily formed on leaf specimens taken on a fine day than those taken on a cloudy day (Table 1). 3. To know the influence of sunlight on the sporulability, a half part of diseased leaf was covered with a sheet of black paper and the other half was left uncovered and exposed to direct sunlight from sunrise to 5p.m., being about 1hr before sunset. The sporulation in the lesion on the uncovered half, which was exposed to sunlight, was always greater than that in the lesion on the covered half, as shown in Table 2. The ratio of the number of conidia formed on the covered half to the uncovered half was 40-64: 100 in the first, 54-60: 100 in the second, and 30-71: 100 in the third experiment. 4. The sporulability in lesions either with or without healthy part was compared by using lesions 6 days after inoculation. Conidia were more abundantly formed on lesions with some healthy part attached than on lesions without healthy part. It was considered from these results that the sporulability of cucumber downy mildew fungus closely depends on the photosynthesis of host cucumber leaves.