36 Nebraska Agricultural Exp. Station, Research Bui. 10 
Fig. 8 — A severe infection of scab on the fruit 
TIME OF INFECTION 
Apple scab is a cool weather disease and thrives best under 
conditions that allow shade and moisture. Under ordinary con- 
ditions the period of greatest infection is from the time the leaf 
buds begin to unfold until two or three weeks after the petals fall. 
That infection occurs before the blossoms open is shown by the 
fact that scab was found on a large number of leaves which had 
been enclosed in Manila paper bags before the blossoms were out. 
No record of the percentage of infection was secured, because the 
scab was not noticed until the bags were being exchanged for 
mosquito netting and by this time many of the clusters had been 
discarded. Wallace (1913) says ''the first infection usually 
occurs when the blossoms are about to open or as soon thereafter 
as favorable weather conditions arise." 
Where there is an abundance of dead leaves under the trees, 
the period of infection may be prolonged by the continued develop- 
ment of perithecia in the leaves during a continued period of wet 
weather. 
Regarding the secondary or conidial infection, Wallace says: 
'The period of incubation may vary from eight to fifteen days; 
so that after this length of time has elapsed subsequent to the 
date of the earliest ascospore infection a crop of conidia is produced 
from which a second, and usually more abundant, infection may 
appear eight to fifteen days following the first period of weather 
favorable to infection that occurs after the above crop of spores 
has ripened. This generation may in turn produce another, and 
so on throughout the season. However, the various infections do 
not always occur only in successive jumps at intervals of eight to 
fifteen days, as the above discussion might lead one to believe. | 
The crop of ascospores are not all matured and do not all dis- j 
charge at one time. They begin to ripen at about the time indi- | 
cated above and furnish a constant source of infection for a month 
or more. Thus the individual infections belonging to the first 
