Abstract
This paper presents the results of the experiments on the diurnal cycle of cucumber downy mildew fungus, Pseudoperonospora cubensis (B. et C.) Rostow. on the effect of light upon its sporulation.
Mildewed leaves were detached from greenhouse plants every 3 hours from 6 a.m. to 3 a.m. next day. Immediately the leaves were washed to remove conidiophores, dried, and then cut into pieces, 1×1cm in size involving a downy spot. Ten pieces were then transferred to each of three moist chambers (fig. 1) which were placed under different conditions as follows:
a) In total darkness: The temperature within the moist chamber was 23°C, as measured with a thermojunction.
b) Exposed to fluorescent light (using two Mazda FL 20D lamps) at an intensity of 300Lux at the level of the leaf surface: The temperature in the moist chamber was 23°C.
c) Exposed to tungsten light (using a 300-Watt West reflector photo flood lamp) at an intensity of 4000Lux: The temperature in the moist chamber was maintained at 23.6∼24.2°C by interposing a 10cm layer of water between the light and moist chamber.
Cut leaf pieces were removed from moist chambers at 3 hour intervals, and examined for details of the development of conidiophores and the formation of conidia. The tests were repeated three times, from June 18 to 30, 1957, with similar results. The results of a test on June 29∼30 are represented in table 1, 2 and 3.
1) When the infected leaf pieces were placed in the moist chamber under total darkness, they began within 3∼12 hours to produce a fair amount of conidia, regardless of the time of detaching the leaf. However, length of time for sporulation varied with the time of leaf detaching: namely, shortest, 3 hours, for the leaf sampled at 12 p.m., and longest, 12 hours, for the one detached at 9 a.m. and 12 a.m.
2) When the infected leaf pieces were placed in the moist chamber, exposing to fluorescent light at 300Lux, those detached at 12 p.m. formed abundant conidia after 3 hours. But the leaves sampled at 3 p.m., 6 p.m. and 3 a.m. produced conidiophores after 9∼12 hours, and a few conidia in further 6 hours. Those detached in the morning, viz. at 6 a.m., 9 a.m. and 12 a.m. gave rise to long slender conidiophores after 15∼18 hours, without forming conidia in further 6 hours.
3) When the test materials were exposed to tungsten light, almost the same results were obtained with those under fluorescent light. It only differed in the conidium formation on the leaves detached at 9 p.m. and 12 p.m., being less than those under fluorescent light.
4) Although the daily rhythm of sporulation under natural conditions was not examined, diurnal cycle of sporulability does seem to exist in cucumber downy mildew. The infected leaves sampled at 12 p.m. showed the highest sporulability. Darkness is favorable but not indispensable for sporulation, as seen in the tables. Light apparently suppresses sporulation, but not always. It is suggested that alternating light and darkness may be responsible for the cyclic manifestation of sporulablility in the cucumber downy mildew, by indirect effect through host plant.
