4 CIRCULAR 263, U. S. DEPARTMENT OF AGRICULTURE 
appears, and a new crop of spores is produced, which in turn may 
produce new lesions. This cycle can continue as long as the weather 
favors disease development and living tobacco leaves remain to 
become infected. Just what combination of weather conditions most 
favor the disease is not certain. The hot weather of summer, how- 
ever, definitely puts an end to its activities. The development of 
FIGURE 2.—Tobacco leaves five to seven days after the first appearance of the disease. 
The lesions are seen as numerous irregular dead areas scattered over the leaves. 
At this stage the fungus growth on the lower leaf surface has disappeared 
spores, which are requisite to the rapid spread of the disease, appears 
to be favored by low temperature (40°-50° F.) following a period 
of warm, humid weather. 
The manner in which the fungus is able to live over from late 
spring until the following season is not certain. Many similar fungi 
produce heavy-walled resting spores (odspores) which, like the seeds 
of higher plants, are able to remain alive a long time. These 
