POTATO DISEASES IX HAWAII. 21 
leaves remain about the same size or spread slowly, but with heavy 
dews or rainy weather the disease progresses rapidly. In a few days 
the foliage may be all rotted away, the layman perhaps having over- 
looked the first few spots and being inclined to attribute the trouble 
to the rain or the hot sun following the rain. etc. In Hawaii, pro- 
tracted hot weather is sometimes observed to control the disease 
completely. 
"Weather conditions which favor this blight are as follows: 
High relative humidity, low soil temperature, and a high moisture 
content of the soil. Since such general conditions frequently prevail 
in the main potato sections of Hawaii, this disease may be expected 
to appear on almost every crop planted in these localities. Such 
is the rule. In some places one crop a year may suffer less than the 
others. The damage from this disease is popularly supposed to 
be due to the hot sun shining upon wet foliage following light 
sprinkling rains. There is no question that the major portion of 
the trouble thus described is the wet or late blight, and not sunscald 
or tipburn. 
In the islands the wet blight is probably carried from crop to 
crop, as these follow each other almost continuously in the same or 
adjacent fields, largely by volunteer plants. Where any sort of 
rotation is practiced, volunteer potato plants are allowed to mature 
and are dug. This custom will have to be discouraged in any at- 
tempt to control diseases and insect pests. 
Control. — The control measures worked out for this disease con- 
sist entirely of methods of preventing infection. Healthy and 
vigorous seed stock and rotation of crops are important. Fortu- 
nately, Bordeaux mixture, applied thoroughly and in time to the 
foliage, has proved beneficial in controlling this disease (Pis. Ill 
and IV). The insoluble copper in the mixture gradually becomes 
soluble and prevents the germination of the spores of the fungus 
which fall on the moist leaves. To be effective the spray must cover 
the entire surface of the foliage, and it must be there before the 
spores find lodgment. After infection has taken place, the parasite 
is out of reach of the spray. 
The practical arjplication of Bordeaux mixture to many of the 
larger fields is being rapidly worked out. In some sections where 
the blight is most destructive and where the potato would otherwise 
be at it> best, as in the Glenwood section of Hawaii, the frequent 
and excessive rains make thorough and timely -praying an uncer- 
tain and, for the most part, an impossible matter. Even under these 
conditions, spraying may afford some relief when the weather con- 
ditions are such as to permit the -pray to dry. The Hamakua 
Hybrid potato promises to resist the blight under the conditions at 
