ANTHRACNOSE 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 
