16 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



quite noticeable in July. The interesting feature of this progression is 

 the fact that it was contrary to the direction of the wind, which blew 

 daily across the plot in the direction shown by the arrow. The wind 

 blew from right to left; the disease progressed from left to right; a leaf 

 parasite would, other things being equal, follow the direction of the pre- 

 vailing wind. Furthermore, the vines were, in at least seven tenths of 

 the cases, more diseased to windward, notwithstanding the Red-leaf 

 disease progressed against the wind. Again, the vines that were some- 

 what weakened by their proximity to the eucalypti (see shaded part of 

 map) and were shaded by them in the afternoon, were barely touched 

 by the Red-leaf disease, and that only after the terrific hot weather in 

 September.* The leaves of these vines were thinner and less consistent 

 than those of the vines in constant sunlight. They were decidedly 

 wanting in vigor, and yet remained practically untouched by the 

 disease. 



It seems difficult to reconcile the above facts with the development 

 of a leaf parasite. We are more inclined to believe that the Red-leaf 

 disease is due to the same causes as the Folletage and the Rougeot, 

 namely, a disturbance of the equilibrium between the absorption of 

 water by the roots and its transpiration by the leaves. 



The soil in which the vineyard is planted is a clay loam underlaid, 

 at a depth of from three to four feet, by an impermeable clay subsoil, 

 at the left of the plot; in the middle, however, the soil is deeper and 

 the subsoil more permeable; toward the right the soil becomes deeper 

 still and more gravelly as one goes down. At the end of July there 

 was from 2 to 3 per cent of free moisture in the soil, which is enough, 

 as the vines showed, to support a good growth and crop of fruit, but 

 which might easily become inadequate to supply the demands of the 

 leaves in moments of great transpiration activity. 



Wind, as is well known, activates transpiration considerably and., 

 under such conditions, might well become the inciting cause. If we 

 take into consideration that the vines were more affected with the Red- 

 leaf disease to windward, and that the disease progressed, in the early 

 part of the season at least, from that part of the plot underlaid by the 

 impermeable clay to that with the freer subsoil, we have two facts 

 which bear out our hypothesis very well. 



This hypothesis, however, has against it the results shown by section 3, which was 

 sprayed with the iron sulfate and the Bordeaux mixture. In this section, at the end of 

 the season, there was not more than ten per cent of the vines diseased, whereas in 

 the other sections the percentage of diseased vines was almost double. From the 

 production of the vines in the different sections, we are unable to gather any evidence 

 for or against our hypothesis. (See Fig. 3.) 



*This hot weather occurred on the 6th, 7th, and Sth of September, when the ther- 

 mometer stood at 90 and 105 F. in the shade. 



