Breeding and Selecting Pest-Resistant Trees 



469 



Central States, were grown in test 

 nurseries. The seedlings were inocu- 

 lated with the Dutch elm disease fun- 

 gus, with the result that only two 

 seedlings withstood inoculations in 

 three consecutive seasons. 



The American, Siberian, and rock 

 elms bloom in the early spring, and low 

 temperatures and strong winds are not 

 conducive to delicate manipulations of 

 the flowers or favorable to pollination 

 and seed setting. A difference in 

 chromosome numbers leads to further 

 difficulties in obtaining progeny in 

 large numbers. From about 20,000 con- 

 trolled American- Siberian elm crosses, 

 fewer than 100 seed were obtained, and 

 only a fraction of those germinated. 

 Of the hybrids obtained, one has re- 

 sisted repeated inoculations with the 

 Dutch elm disease fungus. 



In 1939 some specimens of a Eu- 

 ropean elm, selected for their resistance 

 to the disease, were imported. They 

 have retained their high degree of re- 

 sistance but have failed to grow as rap- 

 idly as American elms nearby, and they 

 do not have the characteristic shape of 

 the American. 



CHESTNUT TREES RESISTANT TO 

 BLIGHT: The chestnut blight was dis- 

 covered in New York City in 1904, 

 and within 40 years all American 

 chestnut stands from Maine to north- 

 ern Georgia and westward to Ohio, 

 Kentucky, and Tennessee were killed. 



So far as the American chestnut is 

 concerned, there is no control for the 

 blight. This chestnut apparently is com- 

 pletely susceptible to the blight fungus. 

 Even today there are few seedlings or 

 sprouts that appear to be resistant. 



Large-scale introductions of blight- 

 resistant species of chestnuts from the 

 Orient were necessary for an effective 

 breeding program. The early breeding 

 work of Walter Van Fleet was limited 

 to a few introductions of the Chinese 

 and Japanese chestnuts. Hybrids of 

 these and the American chestnut 

 usually died from the blight a few 

 years after bearing. 



In the breeding program, continued 



since 1925 by Russell B. Clapper, the 

 present objective is to obtain the maxi- 

 mum vigor and resistance to blight in 

 the first-generation trees derived from 

 crossing the American chestnut with 

 proved selections of Chinese chestnut. 

 One lot of first-generation trees in 

 Maryland grow an average of 2% feet 

 a year and have considerable blight 

 resistance. New combinations of Amer- 

 ican and Chinese chestnut are being 

 produced for testing. Natural-crossing 

 plots, where the American will cross 

 naturally with a proved Chinese chest- 

 nut, will be established for the produc- 

 tion of hybrid seed in quantity. 



A number of the Chinese-American 

 chestnut hybrids have been backcrossed 

 to the resistant Chinese parent trees. 

 The resulting backcross generation 

 shows somewhat less vigor and, in some 

 instances, poor stem form, when com- 

 pared with the first-generation trees. 

 They have practically the same degree 

 of blight resistance, however, as the 

 Chinese parent trees. 



The Brooklyn Botanic Garden, in 

 cooperation with the Department of 

 Agriculture, began breeding chestnuts 

 in 1930. Arthur H. Graves has headed 

 the project. His objective also is to ob- 

 tain a superior blight-resistant forest 

 tree to replace the American chestnut. 

 Promising hybrids, some with genes 

 from the Japanese chestnut, some with 

 genes from the Chinese chestnut, and 

 others with genes from both species, 

 are being tested on the same forest 

 sites along with hybrids produced by 

 the Division of Forest Pathology. 



From about 1930, seedlings of the 

 Chinese and Japanese chestnuts were 

 available in large numbers, and experi- 

 mental forest plantings were estab- 

 lished under many varied site, soil, and 

 climatic conditions, with varying de- 

 grees of success. With few exceptions, 

 the Chinese chestnut appears to be 

 superior to the Japanese chestnut in 

 blight resistance, rate of growth, and 

 stem form. Planted in the most favor- 

 able environments, the Chinese chest- 

 nut shows promise of making a fair 

 timber tree. 



