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 wUt 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. (PI. 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. 



