October, ’20] 
DUDLEY: POTATO LEAFHOPPER 
409 
and browning. This curling and browning preceded by a diseased, 
yellow area may spread from the tip or margin inward, eventually 
reaching the midrib when the leaf dies. The lower two-thirds of the 
midrib with a narrow green area on either side is the last to be killed, 
often remaining green for weeks after the spread of the disease to the 
entire plant. 
In hot dry weather the disease usually spreads rapidly and whole 
fields of early potatoes have been killed in a week’s time. On the other 
hand, if the weather is cool and moist, or if protective sprays have been 
applied, the disease may be checked and throughout the summer show 
nothing more than its early symptoms. 
The potato leafhopper visits many plants but appears to reproduce 
on but few; potato, bean (string, pole, navy), hemp, apple, dahlia and 
hollyhock. A diseased condition with similar symptoms to the dis¬ 
ease on potato has been found on the above plants, and on raspberry 
and box elder. 
Laboratory Experiments 
Laboratory experiments were conducted to study the effect of leaf¬ 
hopper adults and nymphs upon their several hosts, in relation to 
transmission of disease. It was found that in nearly all tests adults 
readily transmitted a disease. Nymphs did not transmit it in as 
large a proportion of tests as did adults. One adult or one nymph 
often sufficed to cause a plant to show decided symptoms of disease. 
In one test four newly-hatched nymphs were placed on one of two 
healthy apple seedlings growing in a flower pot. In ten days the in¬ 
fested apple had become badly diseased, the uninfested one remaining 
green. In another test twenty newly-hatched nymphs were placed on 
a large healthy dahlia plant. Disease appeared in seven days, the 
plant dying in twenty days. A check plant remained healthy. These 
two are typical of many other tests. 
Field Observations 
During visits to potato growing sections of Wisconsin it was ob¬ 
served that, without exception when the disease was present the potato 
leafhopper was found; that when the disease was not present no leaf- 
hoppers were found. The extent of the disease as affecting both 
individual plants and whole fields was in close proportion to the number 
of leafhoppers present. An example is given: A field of Green Moun¬ 
tain potatoes on new land surrounded by woods had never been sprayed 
and, at first glance appeared uninfested by insects. Upon examining 
the field in detail, however, a leaf here and there was found showing a 
typically diseased tip. Without exception a leafhopper or cast skin 
could be found on or near every leaf thus affected. 
