POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 13 
TESTS FOR FUSARIUM INFECTION OF SEED POTATOES. 
The most effective method of selecting seed stock free from wilt 
is to examine the fields where it is being grown as late as possible in 
the autumn, but before the foliage has died down or been frosted. 
The wilt is more readily detected at this time than at any other 
through the premature ripening or actual wilting which it causes, 
coupled with the characteristic brown discoloration of the woody 
part of the lower stem. 
Another indication much relied on is the browning of the vascular 
ring shown when the stem ends of the tubers of diseased plants are 
cut off. (PL II, fig. 1.) This is an important test to apply, and it 
is an excellent rule to reject for planting purposes all lots of potatoes 
any considerable number of which show such a ring discoloration, as 
some other diseases produce a similar effect. 
It is desired to emphasize here, however, that not all tubers from 
infected fields show stem-end browning. The writer has hundreds 
of times observed tubers from wilted hills which showed no discolor- 
ation or only a very slight one. The fungus had apparently not 
gone far in its usual course down the stolons and into the tubers, yet 
the fields were thoroughly infected, and the circumstances warranted 
grave doubts as to the value of such tubers for planting. It is 
probably inadvisable to endeavor to select, for seed purposes, from 
stock containing a large percentage of infected potatoes, any fungus- 
free tubers on the basis of this stem-end test when it is at all feasible 
to secure for planting potatoes entirely free from suspicion. As a 
practical farm procedure, however, growers should be urged to dis- 
card all stem-end pieces which show any brown stain, and it is likely 
that the greater part of the infection would be avoided if the stem 
ends of the seed tubers were cut off and not planted. 
Some confusion may result in the application of this stem-end 
browning test by those unfamiliar with the subject, on account of 
difficulty in distinguishing in certain cases a natural browning in 
many potato varieties, like Irish Cobbler, for example, which have a 
deep depression at the stem end into which the stolon fits. The cork 
layer in these varieties may be bared by a shallow section through 
the stem end and show a brown color quite natural to the variety. 
It is necessary to cut deep enough to reach below the point where 
the vascular bundles diverge from the stolon to form the tuber ring. 
Any browning at this point is highly suspicious, but not positive 
proof. The weakening of the plant by leaf -roll or other diseases may 
hinder the formation of a cork layer at the stem and permit the en- 
trance of saprophytic fungi which produce a discoloration. The dis- 
cussion of this point in the European literature on leaf-roll will be of 
interest. 
