SOIL TEMPERATURES AS A FACTOR IN 

 PHYTOPATHOLOGY 



L. R. JONES 

 University of Wisconsin, Madison, Wisconsin 



The correlation between climatic conditions and the occurrence 

 of plant diseases is obvious to the most casual observer. This 

 has always led the unscientific cultivator to blame the weather 

 solely for his rusts and mildews. On the other hand, the myco- 

 logically-minded phytopathologist, whose first thought is natur- 

 ally of the spore, needs frequent reminder of the fact, recently 

 emphasized by Smith (1), that the most important, and the most 

 complex problems for long-time research include the critical 

 study of the relation of environment to parasitism. 



The way the plant pathologist is forced to face these prob- 

 lems in practice was well illustrated in Wisconsin in 1915 and 

 1916. During these years we had under critical study two fungus 

 diseases of conmionest garden crops, viz., the late blight of the 

 potato {Phytophthora injestans) and the yellows disease of the 

 cabbage {Fusarium conglutinans) (2). Of these two summer 

 seasons, 1915 was cool and moist and 1916 exceptionally hot and 

 dry. In 1915 the late blight fungus, stimulated by the favoring 

 weather, destroyed some millions of dollars worth of potatoes 

 with the worst outbreak for at least a decade. As a result almost 

 every lot of seed potatoes in the state carried the infection to the 

 fields in 1916, yet the dry heat held the parasite so completely in 

 check that the expert mycologist had to search the potato fields 

 of the state with a magnifying glass to find a single incipient de- 

 velopment of the disease. By way of contrast in 1915, alongside 

 these sick potato fields in Wisconsin, the cabbage crop was every- 

 where vigorous, even on the worst Fusarium ''sick''^ soils whereas 



1 Fusarium conglutinans, the soil parasite causing the disease known as cab- 

 bage 3'ellows, persists indefinitely in infected fields so that ordinary crop rota- 

 tions do not eliminate it. The growers term such lands "cabbage sick." 



229 



THE PLANT WORLD, VOL. 20, NO. 8 

 AUCrST, 1917 





