10 MASS. EXPERIMENT STATION BULLETIN No. 283 



Zoospores emitted from germinating conidia are, as has been described by 

 Kurosawa (loc. cit.), usually active for about one hour, following which they 

 remain motionless for about one hour before germinating. Conidia may, there- 

 fore, under favorable conditions, emit zoospores which later initiate infection by 

 germ tubes all within four to five hours after the conidia have reached a leaf 

 which remains moist during that period. As is discussed below in connection 

 with infection, it was found that there is some infection even of leaves which 

 remain wet no longer than five hours. 



The length of life of conidia was found to depend on the moisture conditions 

 which follow sporulation. The younger the conidia, the more freely they ger- 

 minated. Conidia removed from cucumber leaves and placed (on glass) in moist 

 air at 20° to 25° C. remained viable for 50 hours, but not half so large a percent- 

 age of them germinated at the end as at the beginning of that period. In drier 

 air, the life of the conidia was relatively short. Conidia were removed, at hourly 

 intervals beginning at 6 a. m., from cucumber leaves wet with dew on which 

 the fungus had sporulated in the night. Conidia removed before 10 to 11 a. m. 

 germinated freely, but there was no germination of conidia removed later in the 

 day after the dew had dried from the leaves in the sunlight. Similarly, germinat- 

 ing conidia and motile zocjspores were usually found on leaves wet with dew up 

 to about 10 a. m., but not later. 



Infection of Cucumber Leaves 



Inoculum used in infection experiments was usually obtained by placing in 

 water for two to three hours leaves on which the fungus had recently sporulated, 

 for at the end of that time the conidia had germinated and zoospores were abun- 

 dantly present in the water. Of course this method of obtaining inoculum was not 

 used when the time necessary for infection was being investigated, since infection 

 involves and must be preceded by germination of conidia. The high relative 

 humidity necessary in order that inoculated plants may remain wet until infec- 

 tion has occurred was obtained by means of Wardian cases, or by covering plants 

 with cloths supported on frames and kept wet with running water, or by increas- 

 ing the humidity of one whole compartment of a greenhouse, which was easily 

 possible at night and on cloudy days. 



Cucumber plants growing in soil in two-gallon crocks, or shoots of plants 

 rooted and growing in Knop's solution, were sometimes found convenient for 

 inoculation purposes. In some cases detached cucumber leaves floated on sterile 

 water were inoculated, but this method was not very successful. Cotyledons of 

 cucumber seedlings were ftmnd to be easily infected on the living plants. In 

 other cases, detached cotyledons, after being washed in sterile water, were 

 placed in sterile Petri dishes with distilled water or a five per cent solution of 

 glucose. These detached cotyledons were usable in inoculation experiments, for 

 they remained alive two weeks in the sucrose solution and about three weeks 

 in distilled water. 



In order to determine how long leaves must remain wet in order to permit 

 infection, cucumber plants were inoculated by placing on each plant 100 drops 

 of a freshly made water-suspension of the conidia of P. cuhensis. These plants 

 were then placed in moisture-saturated air (at 18° C), and after the intervals of 

 time indicated in Table 3, some of the plants were removed to dry air for 12 hours, 

 at the end of which time, they were returned to moisture-saturated air. The 

 percentages of infection points which resulted, as compared with the 100 points 

 inoculated on each plant (means for triplicates) are recorded in Table 3. 



