Feb., 1907.] Phytophthora and Plasmopora. 8i 



fungus is employed, and, contrariwise, a dry summer will insure 

 freedom from its ravages." 



Professor L. T. Jones * reports upon a favorable season for 

 potato late blight in Vermont: "In 1902, the weather was re- 

 markably cool and moist up to about August 1st, and it has con- 

 tinued cool and moist with more than the usual amount of rain- 

 fall since." That year we found the first Phytophthora July 13, 

 "the earliest date in a dozen years' observations." 



In 1903, Dr. Halsted f again reports serious outbreaks of 

 rot in New Jersey, and mentions moisture as essential to this 

 dexelopment of Phytophthora and states "A second favoring con- 

 dition is warm weather — not hot or cold, but a condition of the 

 atmosphere which obtains when there is a week or month of show- 

 ery summer weather, often spoken of as close or "muggy" — 

 just such as we have experienced throughout the state during 

 August. " 



In this same connection Dr. Halsted gives the mean monthly 

 temperatures and rainfall for New Jersey during the 15 years 

 previous to 1903. These show for 1903 a June mean of 64.0° 

 (-.5.5°), July 73.3° (-0.6°) and August 68.4° (-3.9°) with rainfall 

 about double the mean for June, normal for July and 2.32 inches 

 in excess of normal for August. These conditions of 1903 fol- 

 lowed a cool summer in 1902. Without closer analysis of these 

 data we may now turn to other sources. 



Scribner | summarized the conditions favoring the disease as 

 follows: 



"1. Humidity — The years of great outbreaks have always 

 been years of excessive humidity." 



"2. A temperature ranging from 65° to 75° F. — a few de- 

 grese above 74° will check the development entirely, and down 

 to 45° F. the fungus will continue to grow." 



"3. Moisture in the soil — hence a clayey soil or one that 

 will retain moisture is more favorable to rot." 



While we may always need to keep in mind the distinction 

 between Phytophthora rot, and certain other forms of rot, we 

 may pass to a statement of Dr. Galloway 'sjl "The rapid spread 

 of the disease is dependent in a large measure upon certain con- 

 ditions of moisture and heat. A daily mean, or normal temper- 

 ature of from 72° to 74° F. for any considerable time, accom- 

 panied by moist weather, furnisher the best conditions for the 

 spread of the disease"; on the other hand, if the daily mean or 

 normal temperature exceeds 77° F. for a few days the develop- 



* Report Vermont Exp. Station: 15:210: (1902). 



t Report New Jersey Exp. Station: 1903: 541-555. 



t Report Section of Veg. Path.. U. S. Dept. Agric. 1888: 338. 



II Galloway, B. T. — Some destructive potato diseases — Farmer's Bull. 15: 1894: U. S 

 Dept. Agric. 



