New York Agricultural Experiment Station. 353 



the disease spreads from plant to plant with great rapidity. In 

 what form the fungus i)asses the winter is not known. The ordi- 

 nary conidal spores, described aboAa^ retain their germinating 

 cai)acity for a short time only. Some species of closely related 

 fungi produce, in addition to these ordinary spores, thick- walled 

 resting spores, which retain their power of germination for a long 

 time and serve to carry the fungus over the winter oir other un- 

 favorable period. No such spores liave been found in connection 

 with Plasmopara cuhensis. 



The large, short-stalked spores* shown in Figures 4, 5 and G 

 are a modified form of the ordinary conidia. Spores of this 

 character are known to occur occasionally in a few other species 

 of Peronosporae, the family to which Plasmopora cuhensis be- 

 longs; but it is unusual for them to occur in such large numbers 

 as we have found them in this species. 



The downy mildew fungus likes hot weather and a moderate 

 rainfall. The time of worst attack is generally in August. Dur- 

 ing the first half of last August there was a period of ten days of 

 excessively hot weather. The disease spread with such alarming 

 rapidity that by August 20 the majority of cucumber fields were 

 ruined. The influence of moisture is seen when diseased cucum- 

 ber leaves are placed for about twenty-four hours in a tight tin 

 box containing blotting paper saturated with water. In this 

 moist atmosphere the sporophores attain a greater length and 

 produce myriads of spores which give to the diseased spots a 

 decided violet tinge. The spores are so numerous that when a 

 leaf is suddenly jarred they fall like a cloud of violet-colored dust. 

 Under the microscope the fungus is seen to be in a state of active 



* These peculiar spores do not seem to have been previously observed as they are not 

 mentioned in the literature of this species; The writer found them in abundance dur- 

 ing August on field-grown cucumbers and muskmelons. On both of these plants one 

 or more such spores might bo found on nearly every surface section taken from the 

 under surface of the leaves. They closely resemble the ordinary conidia except that 

 they are considerably larger. The sessile form (Fig. 5) is much more common than 

 the stalked form (Fig. 4). On page 311 of the Botanical Gazette for 1883, Dr. Farlow 

 mentions having seen similar spores in Peronospora cranii, Pk. and Prronospora 

 violae, DBy; and Dr. Max Cornu has described and figured such spores found in con- 

 nection with Plasmopara viticola. See his article, Lc Peronospora dcs Vignes. 



In the opinion of the writer these spores should be considered abnormal and due to 

 some unfavorable condition, probably insufficient moisture. As previously stated, they 

 were found in abundance on leaves collected in dry weather; but they were rarely 

 found on leaves collected on damp days or on leaves kept in a moist chamber 



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