AJSTTHRACHOSE OF CUCURBITS. 63 



Fruiting bodies, or acervuli, are conspicuous on fruit lesions, occur 

 in abundance on stem and petiole lesions, and are less abundant on 

 leaf lesions. 



The fungus grows and sporulates readily in culture and is not 

 exacting as to nutrients. Under optimum conditions the life cycle 

 may be completed in four days. 



As to essential elements, iron and sulphur are not needed in de- 

 tectable concentrations, and only very minute amounts of magne- 

 sium are necessary. As sources of carbon the more complex carbo- 

 hydrates seem to be more suitable. Corn starch and xylan were used 

 to good advantage, and fair growth occurred on cellulose. 



The fungus is quite sensitive to copper sulphate. Growth was 

 prevented by a concentration of molecular -=- 2,000 and was retarded 

 in molecular -=-64, 000. 



Spore germination is favored by the presence of a nutrient and by 

 a plentiful supply of oxygen. The optimum temperature for spore 

 germination lies between 22° and 27° C; the minimum is about 

 4° C. Guttation water from cucumber leaves is apparently not 

 toxic to spores. There is evidence that spores may germinate in 

 moisture condensed on the lower sides of watermelon fruits in the 

 field. 



Thick-walled egg-shaped appressoria are normally formed by 

 germinating spores. These appressoria are usually in contact with 

 a solid substratum. An abundant oxygen supply favors their 

 formation. Appressoria were not formed at 7° C. and below. 



Spores germinating upon a host normally form appressoria closely 

 adherent to the cuticle. Host penetration occurs directly through 

 the leaf cuticle, not through a stoma. The penetration tube issues 

 from a small pore in the under side of the appressorium. 



The mycelium is intracellular. Invaded host cells become col- 

 lapsed and stain deeply. There seems to be a previous stimulation 

 of nuclear and cell division. 



In a field the disease usually appears first in isolated " original 

 centers" of one or two infected plants each. Marked spread of the 

 disease from these centers follows rainy periods, particularly, it 

 would seem, when the temperatures are not too far above or below 

 75° F./the optimum for this fungus. The disease ordinarily becomes 

 epiphytotic only late in the life of the host crop. 



The principal agencies of dissemination in the field are rain and 

 surface drainage water. The spore masses disperse readily in water. 

 During rains the spores are washed to the soil and thence splashed 

 upon the leaves. Centers of infection are thus enlarged. Extensive 

 spread from such centers is usually in the direction of the slope and 

 is accomplished by surface drainage during heavy rains. The fungus 



