POTATO DISEASES IN HAWAII. 19 
or vascular system and limits the amount of water available to the 
plant. Whether there are other injuries to the plant through se- 
cretions of harmful products by the fungus has not been demon- 
strated. 
The wilt disease results when seed carrying the disease is used or 
when healthy seed is planted upon soils already infected with the 
parasite. With continuous cropping the disease becomes more ser- 
ious from year to year until finally, unless seed selection and crop 
rotation are practiced, potatoes can no longer be profitably grown. 
Some of the fields in Maui appear to have reached this stage, partly 
from soil exhaustion. The plants die and dry up shortly after 
blossoming, and the tubers remain small and only partly developed. 
The fact that Fusarium wilt may be readily overlooked, the prema- 
ture drying of the plants being mistaken for the natural result of 
prevailing weather conditions, constitutes an insidious danger, since 
diseased plants yield a preponderance of small unmarketable tubers, 
which, according to the Hawaiian method of marketing, are kept 
for seed, the large and medium sized tubers being sold. The result 
of this practice and of continuous cropping is that only in an 
occasional season can anything like a satisfactory crop be raised in 
fields where this disease is established. 
Control. — The control measures which have been suggested for 
this disease consist chiefly of seed selection and crop rotation. 
Neither of these measures will entirely eliminate the wilt fungus, 
nor is it probable that a rotation of less than three years will ap- 
preciably improve badly infected fields, but it is expected that in 
the absence of susceptible crops for a three-year period the virulence 
of the disease will be reduced and one crop of potatoes can then be 
grown profitably provided healthy seed be used. 
Seed from disease-free fields should be obtained whenever possible. 
If imported seed is used, it should be secured from regions where 
this disease is not prevalent and certified seed should be insisted 
upon (see p. 4). 
Detection of wilt infection in seed. — The following is a useful 
though not absolutely sure means of detecting the presence of the 
wilt parasite in seed potatoes. The stem ends of a large number of 
the tubers to be tested are cut across with a knife, exposing the flesh 
within the tuber where the woody fibers from the stolon (rootlike 
stem to which the tuber is attached while growing) spread out to 
form the tuber ring. If a dark-colored or brown ring appears or 
brown fibers penetrate the flesh at this point (PL I, fig. 1), the tuber 
should be considered highly suspicious. If many tubers show such 
a discoloration, the whole lot should be rejected for seed purposes. 
Considering the prevalence of wilt infection in most available seed, 
it is advisable to cut off and discard the stem ends of all seed planted. 
