POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 15 



rather than the result of definitely planned and carefully controlled 

 experiments. There is great need for such experiments. 



In the San Joaquin district of California the principle is found to 

 be established that potatoes yield better after rotation with barley. 

 In no case known to the writer has the Fusarium wilt been eliminated 

 by rotation, but it seems that the amount of infection diminishes 

 after a few years to a point where a potato crop can again be grown. 

 In Ohio a 3-year rotation was not sufficient to prevent a general 

 epidemic on the station plats. 



A rotation of five to eight years could, however, be readily prac- 

 ticed in all districts, except those where the potato is the sole money 

 crop, and it is behoved that such a rotation would make the losses 

 from wilt negligible. 



The infection of the ground through potatoes left in digging is a 

 factor to be considered, and in warm climates like California, where 

 such potatoes grow as volunteers for one or two seasons or longer, 

 the disease is steadily carried over. Some means of ridding the land 

 of such potatoes seems necessary. 



RESISTANCE OF VARIETIES TO WILT. 



The results of variety tests of potatoes to date offer hope that the 

 future may give sorts resistant to Fusarium wilt, but there are none 

 at present that can be recommended as adapted for commercial 

 cultivation. There are now under trial in the Bureau of Plant 

 Industry several thousand seedlings, the best of which will later be 

 tested for resistance to this disease. 



EFFECT OF FERTILIZERS ON WILT. 



Smith and Swingle made rather extended experiments on the 

 effect of fertihzers on wilt, with results that were entirely negative. 

 Nothing has since been observed that would materially support the 

 suggestion that the disease may be connected with a deficiency of 

 any element of plant food. It occurs in some of the richest western 

 soils, both irrigated and nonirrigated. In California the soils were 

 almost pure organic matter, and the reduction in yields that followed 

 the appearance of wilt was at first attributed to soil exhaustion, but 

 the fungus factor is fuUy sufficient to explain the results, and fertilizer 

 experiments that were made by the writer gave negative results. 

 Further work along similar lines has been reported by Irish (1913). 



QUARANTINE MEASURES. 



In connection with the seed problem, there comes into considera- 

 tion the desirability of keeping the disease out of those districts 

 where it does not yet occur. Does this warrant quarantine restric- 

 tions by State or Federal authorities ? Would such a quarantine be 

 effective ? 



