Mar. 17, 1919 Fusarium-Blight of Potatoes under Irrigation 299 



San Luis Valley, where so many unfavorable conditions are at work, 

 owing to subirrigation and a high water table, the large majority of the 

 potatoes show signs of this infection. Other investigators have found 

 vascular infection of the seed to be the cause of much trouble, and the 

 seriousness of that manner of infection elsewhere can not be judged from 

 these experiments. In the Greeley District, where the Fusarium-blight 

 has been so serious for many years, a fortunate change has taken place. 

 This is regarded as being due to the introduction of other crops, potatoes 

 being brought into the crop rotation only once in four years or more. 

 The use of seed beans, sugar beets, grain, and alfalfa in tlje definite rota- 

 tion is extending the time between the same crops with corresponding 

 advantage to each. The potato was desirable as a high-priced crop, and 

 still is, and the percentage of loss is less with rotation. 



Alkaline soils are a favorable medium for Fusarium spp. Pratt (12) 

 found them to be abundant in virgin desert soils. The prolific and lux- 

 uriant growth of species of Fusarium on alkaline media in pure cultures 

 is an indication of what may be expected in part in alkaline soils where 

 humus is abundant. In disease investigations of this kind it was found 

 desirable to conduct the experiments as much as possible in the field, for 

 the reason that conditions there came about naturally, and the response 

 was immediate and proportionate. Greater care must be taken to note 

 and record every conceivable change of condition. In the gross the 

 changes from day to day are observable and are recorded by suitable 

 instruments; but the changes that occur in the plant are more delicate 

 and rapid than gross observations indicate. Each square foot of soil 

 has its own conditions, not distinguishable from the adjoining square 

 foot perhaps, but of sufiflcient difference to be felt by the plant. 



The plant feels these things and responds. If resistant stock is to be 

 selected, these changes and conditions should be known, and the finer 

 symptoms indicated by the plant must be recognized for the purpose 

 of anal3'sis. 



Temperatures of the soil are vital as regards infection. The critical 

 temperature for infection has not been determined and it varies for the 

 manner of infection. Seed-piece infection will occur at a considerably 

 lower temperature than root infection. In the Carbondale District, at a 

 higher altitude and in cooler soil than the Greeley District, only those 

 plants show Fusarium-wilt symptom.s which have decayed seed pieces. 

 Usually the seed piece remains sound throughout the season there, and 

 the plants are free from blight. In the Greeley District the soil temper- 

 atures are higher, and the seed pieces are generally attacked. Root 

 infection occurs with temperatures higher than the average. As the 

 plants get larger and shade the ground, and the roots penetrate deeper 

 the danger from root infection is lessened. 



There has been a belief that less blight occurs when the potatoes 

 follow alfalfa than otherwise, and that the older the alfalfa was the 



