INSECTS AND PLANT DISEASES — CARTER 331 



would be very great that a new brood of beetles would feed on this 

 diseased corn and become infective. 



DISEASES CAUSED BY INSECT FEEDING 



The transmission of disease organisms is perhaps the simplest way 

 in which insects are concerned in the spread of plant diseases, but 

 there are more subtle and insidious ways that are even more devas- 

 tating in their consequences. One of the oldest known cases of an 

 insect producing a plant disease by merely feeding on the plant is 

 that of hopperburn of potatoes. Entomologists disagree as to just 

 how the potato leafhopper does it, but there is no difference of 

 opinion as to the damage done to potato plants. Badly affected 

 fields have a scorched appearance, which is due to the fact that the 

 leaf tips are shriveled and brown, and the yields of tubers from 

 such fields are very much reduced. 



Some workers believe that the disease is due to a chemical sub- 

 stance injected into the plant along with the insect's saliva, which 

 upsets the delicate machinery of food manufacture in the plant; 

 others consider the explanation a more simple one — merely that the 

 leafliopper, when it feeds, injects so much saliva into the plant 

 that its plumbing system is plugged. 



There are other cases, however, which are clearly due to the 

 injection by the insect of some substance which is actually toxic to 

 the plant. In 1927, in the State of Utah, huge losses to the potato 

 crop were sustained on account of a disease which affected the entire 

 potato plant — leaves, stems, and tubers — and since then the disease 

 has been recorded from a number of western States. This disease, 

 called psyllid yellows, which also affects tomato plants, is caused 

 by the feeding of the immature stages of a single species of insect, 

 the tomato psyllid (pi. 2). Just why up to 1,000 adults of the 

 insect can feed on a potato plant without causing the disease while 

 a few immature forms of the same insect can produce typical 

 symptoms and damage, can only be explained by presuming that 

 the adults' secretions when feeding are different chemically from 

 those of the immature forms. There seems to be no doubt that 

 in this case the disease is due to the injection by the insect of a 

 substance, extremely powerful chemically, wliich spreads through- 

 out the plant and upsets the whole living process. The leaves of 

 a badly affected plant look as though they had been curled with 

 a hot curling iron. Tlie length between the leaf stems becomes 

 shorter so that the plant has a badly bunched-up appearance, while 

 small green tubers are actually produced on the stems above the 

 ground. Underground, many tubers are produced but they are small 

 and commercially wortliless. 



