22 BULLETIN 121, U. S. DEPARTMENT OE AGRICULTURE. 



27° C. and in hanging drops between 14° and 27°. In no case were 

 any formed at the high temperature of 31°. The point to be noted 

 here is limited appressorium formation in the hanging drops of the 

 nutrient medium. 



To account for certain of these phenomena it seems quite reason- 

 able to assume that a rather abundant oxygen supply is one of the 

 essentials for appressorium formation. It is likely that the spores 

 about the extreme edge of a drop would be better supplied with oxy- 

 gen than those in the interior. Exposed drops have an abundant 

 oxygen supply. In Van Tieghem cells, where the oxygen supply is 

 limited, it is quite likely that the demand for oxygen would be greater 

 and hence the supply more quickly used up in the nutrient medium 

 than in pure water, since more rapid growth occurs in the former. 



With this fungus, then, the presence of food material does not 

 seem to inhibit appressorium formation. The contact stimulus is 

 apparently necessary as a rule, and a liberal oxygen supply seems to 

 be favorable to appressorium formation. The function of appressoria 

 will be considered later in connection with the relation of the fungus 

 to the host tissue. 



PATHOGENICITY. 



Whether or not we accept Edgerton's (13) basis of species distinc- 

 tion by host range, it is essential from the phytopathological stand- 

 point to know the exact host range of this fungus. 



Under general field conditions the disease has been observed com- 

 monly on cucumbers, watermelons, muskmelons, and Lagenaria 

 gourds. Experimental evidence of the identity of the fungus in 

 these cases is furnished by the cross inoculations made by Sheldon 

 (46), who secured infection of cucumber, watermelon, muskmelon, 

 and gourd plants with a strain isolated from watermelon, and by 

 Carsner, 1 who secured infection of cucumber plants with a strain 

 from muskmelon. 



During the course of this work, successful cross inoculations have 

 been made from watermelon to cucumber, from cucumber to water- 

 melon and muskmelon, and from muskmelon to cucumber. Leaf 

 infection of vigorous plants was secured in all cases. Using the fun- 

 gus from cucumber, successful cross inoculations have been made in 

 the field upon eight varieties of muskmelon, upon Cucumis anguria, 

 C. melo Yax.jlexuosus, C. melo var. dudaim and G. dipsaceus, and upon 

 Trichosanthes colubrina and Benincasa cerifera. In these inoculations 

 a spore suspension was applied with an atomizer. Out of three series 

 made upon about 40 varieties on August 4, 10, and 13, 1916, suc- 

 cess was obtained only in the inoculations of August 13. 



Out of a large variety of cucurbits exposed to natural infection by 

 drainage water under conditions which practically precluded the pos- 



1 Carsner, E. Op. cit. 



