DOWNY MILDEW OF CUCUMBERS 9 



INFECTION AND SPORULATION AND THEIR RELATION 

 TO ENVIRONMENT 



A plant disease such as downy mildew of cucumber appears and spreads only 

 when the inoculum, in this case conidia, comes in contact with susceptible host 

 plants under conditions, as of temperature, humidity, and absence of fungicide, 

 which are favorable to spore germination, infection, and sporulation. Spore 

 germination, which necessarily precedes infection, will be first considered. 



Germination of the Conidia 



All germination of the conidia of this fungus observed irl this investigation 

 was by zoospores, the method of germination which has been seen and described 

 by others (21, 38, 10, 25). Rostowzew (loc. cit.) described the germination of 

 these conidia by germ tubes, but none were seen by the writer. 



Conidia germinated freely in water, including dew and water of guttation, 

 but not in merely moist air. Unsuccessful attempts were made to germinate 

 conidia in the diluted juices of crushed leaves of cucumber. Conidia have been 

 seen to germinate at temperatures between 8° and 29° C, with the best 

 germination at 16° to 19° C. 



According to both Kurosawa (25) and Hiura (19), the maximum temperature 

 for their germination is 30° C. The optimum temperature for their germination 

 as determined by Kurosawa was 20° to 22 °C., but Hiura placed it somewhat 

 lower, at 15° to 19° C. Kurosawa concluded that the minimum temperature for 

 their germination was 9° C, but Hiura observed some germination at 4° C. 

 Accepting even the more narrow limits (9° to 30° C. or about 46° to 86° F.), 

 it is evident that there is very rarely a night or early morning here in August or 

 the first part of September (when most of the conidial germination occurs) 

 which is too warm or too cool for some germination of these spores. 



The length of time required for the germination of conidia is of interest, for 

 the shorter this time, the less are the conidia at the mercy of a changing environ- 

 ment and the greater is the chance for infection. In the investigations of Clinton 

 (loc. cit.), the conidia germinated within two to four hours after being placed 

 in water, and as observed by Kurosawa (loc. cit.), most of the conidia germinated 

 within one to one and one-half hours. 



It was found by the writer that the time required for their germination is 

 affected somewhat by temperature. At temperatures between 8° and 24° C, 

 all germination was completed within three hours, but conidia germinated within 

 two hours and thirty minutes at 12° C. and within one hour and forty minutes 

 atl8°C. 



It is concluded that two to three hours on a leaf wet with rain or dew is enough 

 for the germination of most of the conidia of this fungus, although even conidia 

 from the same source, and not visibly different, do not all germinate at the same 

 time. 



Conidia in the preliminary stages of germination in water were much more 

 readily killed by subsequent exposure to dry air than were conidia which had not 

 been previously wetted. Cucumber leaves bearing conidiophores and conidia 

 were wetted and kept wet for fifty minutes, at the end of which time they were 

 quickly dried for twenty minutes. Conidia thus treated were killed, while conidia 

 which had not been previously wetted were uninjured by this short exposure to 

 dry air (at the same temperature, 20° C). In this way, some conidia may be 

 killed in nature if they are on a leaf which remains wet for a short time, but not 

 long enough to permit germination to be completed. 



