DISEASES OF CULTIVATED PLANTS 449 



tissue. There is no help for timber after it has once been attacked 

 by rot fungi. Whatever preventive measures are taken must 

 precede the attack. The most effective means of timber preservation 

 is to cause it to be injected or permeated with creosote or other anti- 

 septics. This is done by placing the timbers in vats containing the 

 solution and extracting the air from the timbers so far as possible. 

 The permanence of the effects of such timber treatment depends upon 

 the resistance offered by the material used to gradual solution by 

 water. In the case of creosote the results are quite satisfactory; 

 with chloride of zinc, subsequent solution takes place too readily, 

 while with crude petroleum there is a tendency toward the evapora- 

 tion of this substance when injected. The increasing cost of timber 

 will stimulate timber treatments by making treatment profitable. 

 One drawback at present is the necessity to import creosote for use 

 in such work; possibly refinery by-products from petroleum of a 

 character analogous to asphaltum may find application in timber 

 treatment. (Ohio Agr. Ex. Sta. Bui. 214.) 



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 distin- 

 guish 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 sea- 

 sons 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 temperatures of those months 

 which affect crops were decidedly below the normal or average. Evi- 

 dently this normal lying as it does between the extremes, is sur- 

 passed 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 infcs- 



