224 AGRICULTURAL EXPERIMENT STATION. [Jan. 



way, the disease spreads rapidly, or may be artificially com- 

 municated to healthy plants. 



Under favoring conditions of heat and moisture the spots 

 increase very rapidly in size and in numbers. Those upon 

 a leaf may become confluent and involve the whole leaf. 

 The attacked tissues soon become yellow and then brown 

 and dry, and the plants are rendered worthless, if not 

 utterly killed. The parasite is not limited in its attacks to 

 weak or poorly nourished plants ; but on strong and vigor- 

 ous ones it often progresses more slowly, and less completely 

 overcomes its host than when the latter is enfeebled from 

 some other cause. This can readily be seen in plants grown 

 respectively on poor and rich soils, but otherwise under 

 similar conditions. 



The structure of the summer-spore stage described shows 

 plainly that this parasite is one of the Powdery Mildews, and 

 heretofore its perfect or winter form has been unknown. It 

 has therefore been impossible to say to what particular 

 species of the group it should be referred. It has been 

 known as the variety Cucurbilarum of Oidium erysijphoides 

 Fries, which embraces various undetermined summer-spore 

 stages of this group. But during last December, on several 

 of the leaves of cucumber plants on which the disease had 

 been allowed for six weeks to run its course, and which were 

 covered by the summer spores of the fungus, there appeared 

 smoky spots perhaps half an inch in diameter. On these 

 spots were seen the young yellow and brown spore-fruits or 

 perithecia of the winter stage. These soon reached maturity, 

 and furnished the means for specific identification of the 

 parasite. The dark-brown ripe perithecia are provided with 

 irregular brownish appendages around their bases, and con- 

 tain several spore-sacs each. Each spore-sac contains typi- 

 cally and most commonly two spores ; but this number is 

 often reduced to one, and less often rises to three or even 

 four. A careful comparison of this fungus with the described 

 species of the genus JErysiphe, to which it plainly belongs, 

 shows it to agree in all essential details of structure, peri- 

 thecia, haustoria, etc., with E. cichoracearum DC. The 

 appendages of the perithecia are distinctly brown in mature 

 specimens, but less deeply colored than is usually the case 



