14 BULLETIN f)4, U. s. DEPARTMENT OF AGRICULTURE. 



That parasites. other than Fusarium oxysporum also produce stem- 

 end browning has already been mentioned. Verticillium alho-atrum 

 can often be differentiated by an experienced eye by the blacker and 

 deeper discoloration, though the plant symptoms give a better basis 

 for the diagnosis of this disease also. A stem-end bro^vning is found 

 in the late stage of blackleg, Bacillus pJiytopTiihorus Appel, etc., and 

 of bro^^^l rot, B. solanacearum Erw. Sm., diseases hardly less dan- 

 gerous than wilt. 



Mention should also be made of a form of internal bro'svning that 

 has been known to the writer for five or six years. It may be con- 

 fused with Fusarium ring discoloration, though it is probably more 

 closely related to the physiological trouble, internal brown-spot. The 

 term ''net necrosis" has been suggested for the disease by Dr. H. W. 

 Wollenweber. It is characterized by the occurrence of narrow 

 streaks or dots of browned tissue outside of the vascular ring and 

 extending from the stem end into the tuber for a considerable dis- 

 tance or entirely through it. (PI. II, fig. 2.) These brown tissues 

 are free from fungi or bacteria. The cause is unknown. A fuller 

 description will be published soon. 



The final conclusion of the writer on this point is that tuber-ring 

 discoloration, if clearly marked, should cause the rejection of potatoes 

 for seed purposes, but that negative results from cutting tubers are 

 less valuable as proof of freedom from wilt than a field inspection 

 in autumn. A system of official certification of freedom from wilt 

 and other diseases would be of great value, if based on such field 

 observation. 



SOURCE OF SEED POTATOES. 



In all probability, the best seed for areas possessing a climate 

 where potatoes retain their vigor without renewal for long periods, 

 is that of the home locahty. In many districts, however, and par- 

 ticularly those in the Central and Southern States, it is necessary to 

 bring in seed from northern sources. Many western districts seem 

 to have the same need, particularly those where Fusarium wilt 

 already prevails, and have not yet discovered a source of seed as 

 satisfactory for them as New England seed is to the South. There 

 must be developed somewhere in the West communities where grow- 

 ing seed potatoes will be a special industry and where every means 

 will be taken to produce a perfectly healthy article. The accom- 

 plishment of this aim well merits attention from a cooperative asso- 

 ciation of buyers and seed growers. 



CONTROL OF WILT THROUGH ROTATIONS. 



Rotation of crops appears to be the most effective means for lessen- 

 ing the injuries from Fusarium wilt. What is known regarding the 

 effect of rotation is from observation and general farm experience 



I 



