1 6 BULLETIN 45, HAWAII, EXPERIMENT STATION. 
locally called " poko," and mites (unidentified form of Tetrany- 
chida?). 
The nonparasitic- diseases of the potato may be occasioned by un- 
favorable environment such as unsatisfactory soil composition, un- 
even growth due to prevailing weather conditions, or mechanical or 
chemical injury from injudicious applications of arsenical sprays. 
Bright, hot sunlight after certain kinds of weather frequently causes 
sunscald and tipburn of the leaves. It may be noted that many of 
the diseases usually classed as nonparasitic, though imperfectly un- 
derstood, can be controlled through seed selection and elimination of 
undesirable strains. Such are the inherited diseases, leaf roll, curly 
dwarf, and mosaic, which probably will not be serious factors in the 
Hawaiian potato industry. 
IDENTIFICATION. 
Many times a knowledge on the part of the grower of the different 
diseases which attack the potato would save crops now needlessly 
lost through failure at the critical time to apply the proper treatment 
to hold the disease in check. 
Plants which appear unhealthy for any reason should be examined 
with the following points in mind; and with the accompanying key 
as a guide, a working knowledge of epidemic diseases in Hawaii is 
soon acquired. All growers should learn at least to recognize late 
blight, early blight, mite disease, Fusarium wilt, and rosette. 
Points to be Noted in Examining a Plant. 
Note how the plant differs from the normal. If the leaves have 
diseased spots, note their character. If the leaves wilt, note which 
ones do so, the new small leaves or the lower older leaves. The 
presence of worms in the leaves and stems indicates tuber moth. 
If the new top growth curls, twists, becomes fuzzy, and dries up, 
while at the same time the leaves become bronzed on the lower side, 
it is the work of the potato mite. If the lower leaves wilt and drop 
off more than normally or some' leaves roll up in tubular form and 
no diseased spots are present, examine the underground stem near 
the soil level. If it is girdled with a cankerous, dry, brown lesion, 
Ehizoctonia may be present, causing rosette disease, or if the stem 
is here a shell with the center rotted out, the plant is perhaps at- 
tacked by Sclerotium wilt. Sometimes the stem borer does similar 
damage, in which case the burrow is readily traced and the worm 
found. If the stem appears normal externally, split it with a 
knife and examine the inside at the ground level and below. If 
the vascular tissue — that is, the woody portion a short distance in 
from the surface — shows a 'brown discoloration which is traceable 
toward the root system, the plant is suffering from lack of water 
because its water-conducting apparatus is plugged by fungus threads 
(Fusarium wilt). 
