DOWNY MILDEW OF CUCUMBERS 13 



Sporulation 



Conidiophores and conidia of this fungus wero foiuid growing only on tlic 

 true under side of leaves. This was the case even when leaves of cucumber were 

 inoculated only on the upper surface, or when infected leaves were kept wet only 

 on the upper surface, or when petioles of inoculated leaves were so twisted as to 

 hold the leaves inverted. According to Kostowzew (loc. cit.), however, P. 

 cubensis may sporulate, although rarely, on the upper surface of leaves. In tlie 

 course of this investigation, cotyledons of cucumber seedlings were often inocu- 

 lated and on them the fungus sporulated on both upper and lower surfaces, 

 although the cotyledons, like leaves, have typical palisade cells above with 

 spongy mesophyll below. In this, the behavior of the fungus is like that of 

 Peronospora schactii, for Leach (loc. cit.) observed that the latter produced 

 conidiophores only on the lower surface of the leaves of beets, but on both the 

 upper and lower sui'faces of cotyledons. 



Good and equally complete sporulation was obtained at air temperatures 

 between 16° and 22° C. The cardinal temperatures for the production of the 

 conidia of this fungus as determined by Hiura (loc. cit.) are: maximum, 27° C; 

 optimum, 15° to 19° C; and minimum, below 10° C. This is the same as his 

 optimum temperature for the germination of conidia, but his maximum for spor- 

 ulation is a little lower than for germination. In either case the range is great 

 enough so that there is little likelihood of the disease being much checked by 

 temperature in the field here. 



Sporulation, as observed in the field, occurred most freely in the early morn- 

 ing on leaves then wet with dew. Rain is not necessary for sporulation. The mois- 

 ture of dew permits and induces sporulation, germination, and infection, all with- 

 in a few hours, ending with the drying of the dew by the sun in the course of the 

 forenoon. In this, P. cubensis resembles Sclerospora graminicola (Sacc.) Schroet., 

 for Weston (50) found that dew provides enough moisture for the production of 

 conidia by that fungus. Van Everdingen (46) has similarly emphasized the im- 

 portance of dew in favoring outbreaks of potato late blight. 



As has been mentioned in connection with the incubation period, P. cubensis 

 may live in inoculated leaves in dry air for several days or weeks without sporu- 

 lating. Leaf spots appear, often rectangular and at first limited by small veins, 

 but the violet or purpHsh-gray mats of conidiophores and conidia do not develop 

 on them nor do the spots increase in size until the relative humidity of the air is 

 raised almost to saturation. In long-continued dry weather, the tissue in these 

 leaf spots and the fungus with them may die without the whole leaf being killed.^ 

 However, if the air becomes sufficiently moist before this occurs, sporulation 

 follows very promptly. Cucumber plants inoculated w-ith P. cubensis were 

 allowed to remain in moist chambers, at 20° C, for two days following inocula- 

 tion, at the end of which time they were placed in the dry air of a laboratorj^ 

 where they remained for three weeks. Leaf spots indicative of infection appeared 

 on them six days after inoculation, but no conidiophores were produced in the 

 course of three weeks in the dry air. At the end of this time, the foHage was 

 wetted, the plants were placed in the moisture-saturated air of Wardian cases, 

 and an abundant gro^vth of conidiophores resulted within twelve hours. 



It is nut unlikely that the drouth of 1930 was in this way the cause of the limited inoculum in 

 the South the following year, to which reference has already been made. 



