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 believed 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 fertilizers 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 fully 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 ? 
