330 ANNUAL REPORT SJVnTHSONIAN INSTITUTION, 194 



It is obvious that with such diverse possibilities the problems 

 encountered are extremely varied, but in the account which follows, 

 only examples of the principal types are described. 



FUNGUS AND BACTERIAL DISEASES 



A tiny bark beetle, only one-tenth of an inch long, emerges from 

 a dying elm tree and flies off to feed on the leaves and buds of 

 healthy elms. These beetles are the couriers of death, for on their 

 bodies are the spores of the fungus causing Dutch elm disease, and 

 the trees that these spores infect soon become sickly and diseased. 

 Tlieso sickly, dying trees offer a perfect breeding ground for the 

 beetles which burrow between the inner bark and the sapwood and 

 lay their eggs in long vertical galleries. A vicious cycle is thus 

 established wliich spreads over the forest like the ripples in a pool 

 into which a stone is thrown. 



Tlio story of how certain varieties of fig are fertilized through the 

 agency of tiny wasps called Blastophaga is well known. It is not 

 so well known that these same insects are the carriers of a serious 

 fungus rot of the fig. The body of the insect as well as its wings 

 are covered with short spines, and it is on these that the fungus 

 spores adhere when the insect pushes its way out through the cavity 

 of a diseased fig. When the insect enters another fig, the fungus 

 spores are carried in with it, and in this manner the disease is 

 spread. 



In some cases an insect's intestinal tract becomes full of disease 

 organisms when the insect feeds on the tissues of diseased plants. 

 In addition, therefore, to the infecting of healthy plants by the 

 insect as it moves around from plant to plant, there is the added 

 complication that the insect becomes a reservoir of the disease 

 organisms that are able to live through the winter in the insect, 

 which thus becomes a carrier again as soon as spring brings new 

 activity. Such a case is illustrated in plate 1, which shows corn 

 affected by what is known as Stewart's disease. This disease is 

 caused by a bacterium that is carried to corn by several species of 

 tiny flea beetles, but principally by one species. The disease-produc- 

 ing organism with which the beetle infects the plant overwinters 

 in the insect's body. 



The beetle becomes active in the spring when the temperature of 

 the ground reaches 70° F. and attacks young corn as soon as the 

 shoots appear above ground. All of the overwintering beetles do 

 not carry the infection, of course, but by actual test, about 14 percent 

 were found infective. Since an infective beetle remains so for prac- 

 tically its entire life, it is evident that the disease coidd be widely 

 scattered through a corn field early in the season so that the chance 



