DISEASES OF CULTIVATED PLANTS 353 
ATMOSPHERIC CONDITIONS AS AFFECTING PLANT DISEASES 
The relation between weather and the prevalence of certain | 
plant diseases has been often recorded. The diseases which prevail 
are none the less parasitic, the difference exists solely in the 
temperature and moisture conditions of the atmosphere. Here we 
must distinguish clearly between the cause of the diseases and 
the conditions which favor the given diseases. 
Certain parasitic fungi develop more rapidly under cooler con- 
ditions than the normal or average while others are favored by higher 
temperatures; all fungi are favored by large amounts of moisture 
when these stop short of water immersion and shutting out the 
needed air. In temperature we have an optimum which usually lies 
within certain maximum and minimum limits for any given species, 
but this temperature optimum varies with the organism; it is a 
matter which admits of exact determination for any organism. As 
to moisture, an abundant supply of water is the optimum for most 
fungi with which we deal in plant disease investigations. 
In these atmospheric conditions of temperature and moisture 
the seasons of the year, in our climate, vary one with another. The 
seasons of heavy rainfall are commonly those of low temperatures by 
reason of the check on temperatures exerted by evaporation. 
Further, our weather service records show a tendency for our 
seasons to come in groups of cooler alternating with groups of 
warmer seasons; that is, we may have several years as with 1904 to 
1907 (excepting parts of 1906) in which the mean monthly tempera- 
tures of those months which affect crops were decidedly below the 
normal or average. Evidently this normal lying as it does between 
the extremes, is surpassed by the warmer seasons which are said to 
be above normal. We have likewise, other alternating groups of 
years in which the season’s temperatures are decidedly above the 
normal. 
The effects of these cool seasons upon diseases are most clearly 
shown in outbreaks of leaf-curl of the peach and plum bladders in 
early season, and of potato late blight and rot, Phytophthora infes- 
tans, upon the potato crop. It is understood that plenty of moisture 
is the usual accompaniment ofa cool season; from the combined 
effect of this supply of moisture and cool weather we have outbreaks 
of the potato disease even in northern Ohio where it does not appear 
certainly to survive from year to year. Such groups of cool seasons 
culminate as arule in particularly injurious outbreaks of the potato 
Phytophthora with us;in more northerly situations, the disease is 
present nearly every season, but the outbreaks culminate with 
favorable weather conditions of excessive rains and lowered 
temperatures. 
