28o . Journal of Agricultural Research voi. xvi. no. h 



PREVALENCE AND LOSS 



Fusarium-blight has not appeared to persist in any one locality. 

 Its visitations are sporadic. The losses are, therefore, not to be esti- 

 mated in concrete terms. In Colorado in 191 7, a favorable season for 

 the crop, about 46,000 acres of potatoes were grown. It is estimated 

 that, owing to disease, 10 per cent of the acreage planted gave either 

 reduced yield or no yield. In a good field most of the diseased plants 

 soon pass out of sight and make no impression on the casual observer. 

 Yet these occasional diseased plants probably resulted in a loss of 500,000 

 bushels in Colorado alone. At other times the disease spreads more 

 generally, and whole fields go down and are lost. Since the part of 

 Fusarium spp. in the creation of disease depends largely on environmental 

 factors, it is important to note that the conditions which prevail in 

 Colorado do not exist in the same way in other places. The descriptions 

 of diseases caused by species of Fusarium already published do not, 

 therefore, closely apply to Colorado, because the climate, coupled with 

 the soil conditions and irrigation practice existing there, creates a condi- 

 tion unknown in the East, where most of the work on Fusarium spp. 

 has been done. 



DESCRIPTION OF DISEASED PLANTS 



The common manifestations of the disease are a wilting and rolling 

 of a few leaves, followed quickly, slowly, or intermittently by the wilting 

 of the remainder and usually the premature death of the foliage. Occa- 

 sionally a single leaf will wilt, turn yellow, and die, whereas the remainder 

 of the plant may continue healthy throughout thes eason. Frequently 

 one of the two or more stems of a hill wilts and dies, while the others re- 

 main turgid and healthy. The time of appearance varies greatly. It may 

 be noticeable when the first leaves appear or at any time throughout the 

 season up to maturity. A faint lightening in the color of the plant may 

 indicate a gradual attack, and when the attack is severe, a large plant may 

 pass from health to complete collapse in two days. Late in the season, 

 three or four weeks before frost, plants turgid and unwilted, are found 

 upon which all the upper leaves are rolled. They are a little lighter 

 in color than normal plants, and have often been designated by the 

 uninitiated as being diseased by leaf roll. Plate 41,6, shows a plant of 

 this type; the leaves are rolled, but show no wilting. These plants 

 may continue to lighten in color, the leaves to roll, and gradually to 

 die. These conditions are due to the presence of Fusarium mycelium 

 in the vascular tissues of the stem. Plate 40,D, shows a plant on which 

 the leaves have rolled and are gradually dying. What has actually 

 happened in this plant is that the fungus has created a physiological 

 drouth which has extended over a long period. Upon examining 

 plants in the earlier stages of wilt, the stem and roots are usually found 

 clean and apparently healthy, but the seed piece has rotted, and remains 



