POTATO DISEASES IN HAWAII. 17 
Key to Potato Diseases. 
Foliage diseases : 
Leaf spots. 
Spots small, one-sixteenth to one-fourth inch in diameter, 
rounded to angular, brown, and becoming dry and falling 
out, leaving shothole effect. Spots often marked with con- 
centric lines or successive borders of growth. Prevalent in 
dry seasons. 
Early blight (PI. VI and p. 23). 
Spots one-fourth to one-half inch or more in diameter, rounded, 
spreading rapidly, and bordered by lighter yellow-green 
zone ; on underside of leaf in damp weather a delicate frost- 
like mildew may be seen ; in wet weather the plants entirely 
rot in a few days. Preventive treatment must be given 
before disease starts or as soon thereafter as possible; if 
plants are badly attacked, spraying will not be worth 
while. 
Late blight (PI. II and p. 20 L 
Wilting and rolling of leaves, etc. 
Young leaves and new growth first affected. Leaves bronzed 
on underside, twisted, and curled up, becoming fuzzy and 
drying up, the plant dying from the top downward. Mites 
can be found with hand lens. Prevalent in the dry season. 
Potato mite disease (Pis. XII and XIII and p. 31). 
Plant appears to suffer from lack of water and dies prema- 
turely ; lower leaves wilt and drop off ; interior of lower 
stem at ground level shows browning of vascular or woody 
portion. 
Fusarium wilt (PI. I and p. 18). 
Leaves more or less rolled and tubular ; plant spreads out and 
grows all to top ; tubers few, small, and set close to stem 
in a bunch or forced out of the soil; tubers forming in 
axils or leaves in advanced cases; stem girdled below 
ground with brown cankerous lesion. 
(Rosette (Rhizoctonia) (Pis. VII, VIII, and IX and p. 24). 
Lower leaves wilt ; stem at soil level rotted through or hollow, 
with white fungus growth producing mustard-seed-like 
sclerotia. 
Sclerotium wilt (p. 25). 
Leaves roll up in tubular form, or plant wilts and generally 
appears as if the stem were broken off; interior of stem 
may show burrow of the stem borer, or possibly the plant 
is cut at soil level by cutworms. 
Borer, cutworms, etc. (Text fig. 5 and p. 30). 
Tuber diseases. 
Tuber rots. 
Field rots. 
Burrows with worms inside and soft ill-smelling rot, 
following foliage infestation by leaf-mining worm. 
Tuber moth (PI. XI and p. 29). 
Brown dry rot running just under surface of tuber, 
which often has a purple tinge. This is the late 
blight rot, usually followed by various soft rot 
organisms; it occurs in heavy, cold, wet soils 
when late blight has attacked the tops. 
Late blight rot (PI. VIII and p. 28). 
132864°— 19 2 
