The overwintering ofExserohilum turcicum in Israel
Exserohilum turcicum (Pass.) Leonard and Suggs, the causal agent of northern leaf blight of corn, overwinters onSorghum halepense L. plants and on corn debris (dead leaves). Spqrulating lesions ofE. turcicum were observed on sorghum plants in the winter (February). Spores from these lesions were pathogenic to susceptible sweet corn plants cv. ‘Jubilee’. Infected sporulating leaves of corn were stored for 1 year at 20°C (40-60% relative humidity), at 5°C (60% relative humidity), or buried 5 cm underground. During the storage period, 32% and 22% of the spores formed chlamydospores, at 20° and 5°C, respectively. Leaves buried 5 cm underground were totally decomposed after 6 months. After 4 months, 25% of the spores in the buried leaves had formed chlamydospores. Spores with chlamydospores were pathogenic to corn plants. The viability of spores without chlamydospores stored at 20°, 5°C or buried underground was 0, 60 and 0%, respectively. In a parallel experiment infected leaves were stored under similar conditions and allowed to sporulate. No sporulation occurred on infected leaves buried in soil. Spores produced on infected leaves stored at 20° and 5°C were highly pathogenic to corn plants. In leaves treated with 0.1N glucose, chlamy dospore formation was significantly inhibited.
Sporulation of Helminthosporium turcicum on sweet corn: effects of light and sugars
Conidial formation, but not conidiophore growth, at Helminthosporium turcicum Pass, on detached blighted leaves of corn was greatly inhibited by blue light. Percentage inhibition of conidial production increased with increasing leaf temperature and light level. With 8 μE m−2 s−1 of blue light, spore production was reduced by 30, 87 and 96% at 15, 20 and 25°C, respectively, compared to the respective dark controls. Light exerted no inhibitory effect on spore formation if conidiophore-bearing leaves were first kept for 2 and 3 h in the dark at 15 and 20°C, respectively, or for 1 h at 10°C in either dark or light conditions. Sucrose had a great enhancing effect on conidial production in the dark, but could not counteract the inhibitory effect of light. Prolonged incubation of intact infected plants in darkness favoured subsequent sporulation. A suggestion is made that a single photoreceptor pigment system operates the synthesis or activation of antisporulants which may act directly or may counteract the activity of chemicals essential for spore production.
INTERACTION OF NEAR-ULTRAVIOLET LIGHT AND TEMPERATURE ON SPORULATION OF THE FUNGI ALTERNARIA, CERCOSPORELLA, FUSARIUM, HELMINTHOSPORIUM, AND STEMPHYLIUM
Alternaria dauci, A. tomato, Cercosporella herpotrichoides, Fusarium nivale, Helminthosporium catenarium, and Stemphylium botryosum, all Fungi Imperfecti, were grown on temperature gradient plates (5–40 °C) under different regimes of near-ultraviolet radiation (320–420 nm) and darkness. None of the fungi normally sporulated in darkness on the media used. The effects of interaction of temperature and near-ultraviolet radiation (NUV) on sporulation were significant for all six fungi. The fungi were grouped into two categories, "Diurnal Sporulators" (A. dauci, A. tomato, and S. botryosum) and "Constant Temperature Sporulators" (F. nivale, H. catenarium, and C. herpotrichoides). The "Diurnal Sporulators" have two distinct phases of photosporogenesis, an "inductive phase" leading to the formation of conidiophores, and a "terminal phase" resulting in the formation of conidia. The "inductive phase" is stimulated by NUV and operative at relatively high temperatures, while the "terminal phase" is strongly inhibite...