286 Journal of Agricultural Research voi. xvi. no. h 



the stem. Many plants are to be found which show the violent symp- 

 toms, wilting, drooping, and death, within a few days. 



The great majority of plants in a field may advance to late maturity 

 with no visible signs of Fusarium- blight. Entire fields have been ob- 

 served which showed natural wilting caused by delayed irrigation; wet 

 periods may occur, owing to excessive rain following an irrigation; and 

 either of these conditions are conducive to the increased activity of the 

 fungus, though recovery is possible and often occurs. In an entirely 

 healthy field at any period of the season conditions may arise in which the 

 blight gains the ascendancy, the plants wilting and dying in the course of 

 a week. This may happen as late as September, yet infection did not 

 occur immediately before the appearance of wilt, as the fungus had been 

 present since the time of planting. 



Whole seed is protected by a sound epidermis underlain by an active 

 vascular tissue, the best protection the seed may have. Whole seed 

 germinates quickly and estabUshes a sound, vigorous plant weeks before 

 the seed piece has been destroyed by fungi, and the plant becomes hable 

 to attack. It is not unusual for whole seed to remain sound through the 

 growing season, though the ultimate death of the nongerminated eyes, 

 the wom-o^t vitality of the vascular region, and the dead epidermis make 

 infection possible. Injuries in handling or planting, such as are received 

 from picker planters, render infection comparatively easy. Clipping the 

 stem end to inspect for vascular infection is a most reprehensible practice, 

 as it breaks the epidermis and makes a wound in that part of the tube, 

 tissue which is lowest in vitality and least in the power of seh-protection. 



OCCURRENCE OF THE CAUSAL FUNGUS 

 In the case of seed pieces which obtained a favorable start and sent up 

 Sprouts the decay is slow. In individuals where it takes weeks to decay, 

 the decayed watery portion leaches away and a callus forms when the 

 growing tissue is reached. Where field conditions are right, the fungus 

 will continue its slow advance into the foot of the stem, causing no decay 

 and shght or no discoloration. As conditions unfavorable to the plant 

 arise, the fungus grows in the vascular bundles and causes discoloration. 

 Plate 39, B, shows a 6-weeks'-old stem to the left portion of which the 

 decomposed and dried piece clings. Discoloration of the pith is found 

 only at the very foot of the stem, and the upper vascular bundles are free 

 from any trace of the fungus. In the field this plant would be regarded 

 as healthy. In plants of this type the lowest roots on the stem are cut 

 off from supplying water,' and thereby cause some of the temporary 

 queer symptoms to be noted in the foliage. The plant may recover, 

 draw on the roots above more heavily, and continue growth. Nothing 

 more may happen throughout the season; harvest may arrive, and the 

 plant yield normally. In other cases where the soil is wet and compact 

 the fungus is more active; it decays the foot of the stem, cuts off stolons, 

 decays new potatoes, and finally kills the plant. Plate 39, A, pictures a 



