20 BULLETIN 45, HAWAII EXPERIMENT STATION. 
This test for wilt is not absolutely sure, as not all tubers carrying 
the wilt disease show the discoloration sufficiently to indicate defi- 
nitely the presence of the fungus. Certain other diseases cause a 
somewhat similar discoloration, and in some cases tubers show a very 
slight yellowing of the vascular ring which appears to follow a long 
period of dormancy and to be unassociated with disease. In any 
event these doubtful tubers are not desirable types to propagate. 
Late Blight (PJiytophtlwra infestans). 
Xo disease of the Irish potato is more destructive than the late 
blight when the conditions favor its development. Late blight is 
caused by the fungus Phytophthora infestaiis. The name late blight 
was probably given to this disease to indicate that it occurs most 
commonly late in the year and to contrast it with the early blight 
(Altemaria solani) which is prevalent in the dry weather of mid- 
summer. It is apparent that the common names of these two dis- 
eases are without significance in Hawaii. The names dry blight and 
wet blight for early blight and late blight, respectively, are thought 
to be locally more appropriate common names. 
Wet blight, or late blight, is widely prevalent and destructive in 
the Hawaiian Islands. It seems probable that this disease and the 
wilt disease {Fusarium oxysporum) are the most potent factors in 
lessening yields and discouraging potato growing in these islands. 
When the late blight appeared in Ireland in the middle of the last 
century it so devastated the potato crops, upon which the people 
largely depended for food, that famine resulted. Since that time the 
disease has become prevalent in many of the potato sections of the 
world, and it has been the object of special investigation wherever it 
occurs. Various means have been devised for its control, but there 
is no ground for hope that it can be exterminated, and potato growers 
will probably always have to contend with it. 
The disease is manifested in the leaves by dark, more or less 
rounded, water-soaked spots or areas (PL II), which may or may 
not increase rapidly in size and number, according to weather con- 
ditions. Upon the lower surface of the leaves of affected plants 
growing in damp situations a characteristic delicate frostlike mil- 
dew appears. This is the parasitic fungus bearing its microscopic 
spores. The latter are minute seedlike bodies which, spattered about 
by rain or transferred by contact of the leaves with adjacent moist 
foliage, carry the disease from plant to plant. In this way a dis- 
eased spot upon one leaf may serve to infect a whole field in an in- 
credibly short time. Similarly the spores falling and being washed 
upon the ground find their way to the developing tubers which they 
may infect, causing them to rot in the ground or subsequently in 
storage (PI. VIII). With continued dry weather the spots upon the 
