24 BULLETIN 45, HAWAII EXPERIMENT STATION. 
Control. — The experience of investigators on the mainland has 
shown that dry blight can be controlled by timely and thorough 
spraying with Bordeaux mixture. Where the conditions are fa- 
vorable to the development and spread of this malady, the plants 
should be sprayed as recommended for the late or wet blight. As 
a general practice, potato plants in the islands should be sprayed 
from three to six times per crop with Bordeaux mixture, fewer- 
sprayings being required in dry seasons than in wet seasons. In the 
former case the spray will prevent the attacks of the early or dry 
blight and in the latter the attacks of the late or wet blight. 
Black Scurf (Russet Scab) axd Rosette (Rhizoctonia solani). 
The diseases of the potato commonly attributed to the parasitic 
fungus Ehizoctonia are quite diverse in their general aspects or 
signs, but they are conveniently grouped together for discussion. 
The Ehizoctonia fungus is a soil inhabitant which attacks the under- 
ground stem and roots of the plant (PL VII) with effects varying 
according to the portion injured. 
Black scurf is the name commonly applied to the black accumu- 
lated mycelial masses or sclerotia of the fungus which adhere closely 
to the skin of the tuber in such a way as to be readily mistaken for 
bits of soil. (See PI. VIII.) They do not wash off easily, although 
they are superficial and readily scraped off with the finger nail. 
This stage of the fungus does little damage other than to the appear- 
ance of the tuber, but it is hy this resisting stage of the parasite that 
the disease is largely spread to new land and to new localities. 
If tubers affected with the black scurf are planted, the sprouts 
may be attacked and girdled by the vegetative threads of the fungus. 
As a result of this action the following types of disease may occur : 
Weak plants : The fungus kills or injuries the sprouts and a poor stand 
and weak plants result (PI. VII, fig. 1). 
Rosette disease and leaf roll: The leaves of the plant become light green 
and roll upward on the midrib as an axis, becoming tubular (PI. IX). The 
plants may be more or less dwarfed. The few undersized tubers which form 
are set close to the stem, often pushing out of the ground (PI. VII, fig. 2). 
Aerial tubers : The stem below ground is girdled more or less completely. 
The food elaborated in the leaves and designed for subterranean tuber forma- 
tion is diverted, and tubers form above ground in the leaf axils (PI. X). 
Growth to top: The stolons or stems of the young tubers are attacked by 
the fungus and the young tubers being partially cut off from nourishment fail 
to develop normally. The plant nfay develop luxuriantly and yield only a few 
small tubers. 
Disfigured tubers: The tubers are frequently covered with the black scurf 
or sclerotial resting stage of the fungus. The same stage of the fungus is be- 
lieved commonly to be responsible for rough brownish and russet areas on the sur- 
face of the tubers. Throughout such areas the skin is finely and irregularly 
cracked. Large cracks in tubers are also attributed to the action of this organism 
as well as to uneven growth, and deep pit-like holes often occur which may he 
caused by this fungus (PI. VIII). 
