14 BULLETIN 64, U. S. DEPARTMENT OF AGRICULTURE. 
That parasites other than Fusarium oxysporum also produce stem- 
end browning has already been mentioned. Verticillium albo-atrum 
can often be differentiated by an experienced eye by the blacker and 
•deeper discoloration, though the plant symptoms give a better basis 
lor the diagnosis of this disease also. A stem-end browning is found 
In the late stage of blackleg, Bacillus pliytopJithorus Appel, etc., and 
of brown rot, B. solanacearum Erw. Sm., diseases hardly less dan- 
gerous than wilt. 
Mention should also be made of a form of internal browning thai 
has been known to the writer for five or six yp^rs. 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. (PL 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 locality. 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 
