OBSERVATIONS ON SOME VINE DISEASES IN SONOMA COUNTY. 



15 



manner, which depends largely on the nature of the parasite and the 

 surrounding conditions. If the parasite lives on the roots of the plant, 

 it will spread, as it were, in a circle; but if it is a leaf parasite, the wind, 

 all other things being equal, will cause the disease to spread with greater 

 rapidity in the direction in which it blows. Furthermore, when a plant 

 is preyed upon by some parasite, it shows all the symptoms character- 

 istic of the specific disease, from the minor to the major in sequence. 

 Now if we apply these principles to the study of the Red-leaf disease, 



Legend: Iron sulfate spray. 



Witnesses. 



Bordeaux mixture. 



Iron sulfate and Bordeaux mixture. 



12 



11 !. 



July. 



.August 



September. October, 



FIG. 2. Progress of the Red-leaf disease (in per cent) on the South Sonoma 

 Experiment Plot. 



we find that it does not spread from a center of infection. A vine is 

 often diseased on a single shoot; it may show the first symptoms of 

 disease in the early summer, and never at any other time. Some 

 healthy vines may in seven days be affected in the worst form, while 

 others have only a cane or two, or even only the apical leaves of a few 

 shoots, affected. These observations tend to show that the Red-leaf 

 disease is not caused by a root parasite. 



It remains to be seen whether or not the Red-leaf disease can be 

 caused by a leaf parasite. The disease progressed, the reader will remem- 

 ber, from section 2 toward section 12 (Fig. 1). This progression was still 



