ANTHRACNOSE OF CUCURBITS. 37 



SURFACE DRAINAGE "WATER. 



But the mere spattering of rain and blowing of the droplets by 

 wind can not account for the extensive epiphytotics of this disease 

 which suddenly appear in certain fields. A considerable mass of 

 observational evidence has been accumulated relative to this point. 

 It has been generally noted that extensive spread of this disease 

 does not follow every rain, but only the very heavy rains during 

 which considerable surface run-off takes place. The general trend 

 of the evidence seems to indicate that epiphytotics tend to occur 

 more commonly where the topography of the field is sloping or 

 rolling. 



As to the effect of heavy rains, it will be recalled that the first 

 general spread of the disease in the Princeton fields in 1915 followed 

 the rains of the first week in August. In the Madison fields in 1916, 

 two periods of extensive spread of the disease were clearly recog- 

 nized, one after the heavy rains of August 3 to 5 and the other, which 

 resulted in a serious epiphytotic, after the heavy rains of September 

 4 to 7. Intervening lighter rains of 0.28 inch on August 10 and 0.31 

 inch on August 26 were not followed by an extensive spread of the 

 disease. It is realized that here there is a difference in duration as 

 well as in amount, and the longer rainy periods no doubt afforded 

 conditions more conducive to infection. Furthermore, reference has 

 been made to the significance of the lower temperatures of the periods 

 following the light rains. 



Additional observational evidence relative to the agency of surface 

 drainage during heavy rains in spreading this disease is furnished 

 by the direction of this spread from the old centers. This is revealed 

 by the location of even-aged new infection following such rains. 



Field 2 at Madison in 1916 afforded a good opportunity to observe 

 this phenomenon. This field occupied a decided south slope and 

 the rows extended across the slope. The new infection resulting 

 from the rains of the first week in August was very evident by 

 August 12 and a careful inspection of the field showed the manner 

 of spread from the old centers of infection to be somewhat as illus- 

 trated in figure 13. There was a decided tendency for the greatest 

 spread from the old centers to be across the rows and distinctly in 

 the direction of the slope. At least nine of the areas of infection 

 show elongations extending downhill; in fact, in four of these cases 

 the new infection in the row below the old center was along small 

 gullies or drainage channels leading directly from the old center in 

 the row above. 



Field 3 occupied a gentle southeast slope, and here again the rows 

 crossed the slope. One original center occurred in a row along the 

 upper edge, and from this center a drainage channel led down across 

 the entire field to the southeast corner, where it formed a delta 



