350 Journal of Agricultural Research voi. xv, no. 6 



is carbonaceous to coal-black, is covered with small elevations, a milli- 

 meter or so in diameter lying close together, in which the pycnidia are 



embedded. 



FOOTROT 



Footrot, a disease caused by Plenodomus destruens Harter, is one of 

 the most serious diseases of the sweet-potato crop, once it has become 

 established. It also occurs in storage and is carried over mostly on 

 the potatoes held for seed. The results obtained by the writers con- 

 firm those of McClintock (26), who found that field infection, while not 

 entirely lacking, is relatively small, the disease being carried to the field 

 mostly on the slips. During the early part of the season the fungus 

 grows slowly, but in July and August, when warm weather comes on, it 

 progresses rapidly. It has been shown (77) that the footrot organ- 

 ism invades the stem of the plant near the ground and grows down 

 into the potatoes, causing a decay beginning at the attached ends (Pi. 

 26, A). This decayed portion is generally slight at digging time and, 

 therefore, easily overlooked. Occasionally it may involve an inch or 

 more of the end of the potato, in which case it would probably be thrown 

 out. Potatoes but slightly decayed may find their way into storage, 

 where the development of the disease would be continued. By bedding 

 time a considerable percentage of potatoes are thrown out in a badly 

 decayed condition. Those slightly decayed, however, escape detection 

 and, therefore, may be used for seed. The causal fungus may also enter 

 through wounds, or invade surface lesions made by other fungi. 



DISTRIBUTION 



In 1 91 3 (77) footrot was known to occur only in Virginia. By 191 6 

 Iowa, Ohio, Missouri, and Kansas (19) were added to the list. In 191 7 

 it was found in New Jersey, Maryland, and California. In New Jersey 

 it is doing very little damage as yet, but in California and Maryland con- 

 siderable loss resulted from it in 1917 and 1918, respectively. Just how 

 long it has been present in California has not been learned. The organ- 

 ism isolated from material collected in California, when inoculated into 

 healthy plants on the Potomac Flats, produced typical symptoms of the 

 disease. In fact, it was found to be identical morphologically and para- 

 sitically with the strains isolated from material collected in Eastern 

 States. The following varieties of sweet potatoes have been inoculated 

 with Plenodomus destruens and the disease produced: Yellow Jersey, 

 Big Stem Jersey, Pierson, Miles Yam, Early Carolina, Yellow Strasburg, 

 Red Jersey, Red Bermuda, Extra Red Carolina, Southern Queen, Yellow 

 Yam, Pumpkin Yam, Vineless Yam, Dooley, Triumph, Vineless Pump- 

 kin Yam, Nancy Hall, Florida, General Grant Vineless Yam, White 

 Yam, Red Brazilian, and Dahomey. 



