Vergara - 1976 - Physiological and morphological adaptability of ri

417
Some topics in a disease cycle of
rice blast and climatic factors
H. Kato
1N'1'R()D1_JC'l'I(JN
he principle of the studies on southern corn leaf blight which Dr. Nelson
mentioned can be very applicable to rice blast since both diseases are airborne,
and lesions are produced on the terrestrial parts of a host plant. There
is no doubt that rice blast is one of the serious diseases of rice plants. especially
when application of nitrogenous fertilizer increases. Dr. Ou. the ehainnan,
asked me to make a thumbnail sketch of some segments ofa rice blast disease
cycle in relation to some climatic factors.
EXPANSION OF LESION
When inoculated plants are placed under different temperature regimes immediately
after blast lesions appear. differences are apparent in the rate of lesion
enlargement. Exposure of plants to 32°C caused lesions to expand rapidly
in the first S days and level off shortly thereafter (Fig. 1A). A swift cessation of
lesion enlargement then took place. On the rather hand. at 16°C the rate of
enlargement was slow and constant over the 20-day" period. Lesions expanded
slower at 20°C than at 25° or 32°C. Cessation of lesion enlargement also occurred
gradually at 20° and 25°C (Kato and Kozaka. 1974).
SPORIJLATTIJN
PCYIENYIAL
Alter a lesion is formed. spore production occurs on lesions. 1t is well known
that the rnycelia of the pathogen living in a lesion have different capacities to
produce conidia when the lesions have developed under different circumstances.
Sporulation potential is defined
under optimum conditions per unit of time. The leaves belonging to the same
leaf-order on main culms were inoculated immediately after expansion. Each
time lesions were incubated. they were wiped to remove the bearing eonidiophores
and conidia, and kept at 28°C at relative humidity (RH) of more than
95 %. and in darkness for l5 hrs. The maximum potential reached a peak value
earlier at higher temperatures than at median and 1OWCT temperatures (liig.
113). We can classify these patterns into three types. The maximum number
H. Karo. Division of Plant Pathology. National Institute of Agricultural Sciences. Nishigahara.
Kita-ku. Tokyo. Japan.