470 



Yearbook^ of Agriculture 1949 



The Division of Forest Pathology 

 now is establishing the best strains of 

 Chinese chestnut on the best types of 

 sites, so that there will be permanent 

 sources of seed for dissemination and 

 distribution. Cooperators' plantings 

 already are furnishing valuable seed 

 for these plantings. 



OTHER BREEDING AND SELECTION 

 WORK: From 1930 to 1941 studies by 

 R. G. Hall and others of the Forest 

 Insect Laboratory at Columbus, Ohio, 

 revealed that two recognized varieties 

 of black locust were resistant to at- 

 tacks of the locust borer, Megacyllene 

 robiniae (Forst.). This borer has 

 caused widespread damage to black 

 locusts in the eastern part of the United 

 States. From eggs deposited in bark 

 crevices, the young larvae mine the in- 

 ner bark and cambium. Later the mines 

 are extended into the sapwood and 

 eventually into the heartwood. Tests 

 made on one of the resistant varieties, 

 the Higbee locust of southern Indiana, 

 showed that about 95 percent of 

 the larvae planted in the bark crevices 

 started mines in the inner bark, but 

 only about 20 percent reached the 

 wood and matured there. In suscepti- 

 ble locust varieties, practically all the 

 planted larvae mined the inner bark 

 and lived to the adult stage. 



In 1924 the Oxford Paper Com- 

 pany, in cooperation with the New 

 York Botanical Garden, began a pop- 

 lar-breeding project. Approximately 

 13,000 hybrids were produced by cross- 

 ing 34 different types of poplars. A 

 number of plantations have been es- 

 tablished in the eastern part of the 

 country and the poplars are being ob- 

 served for their reactions to various 

 diseases. A. J. Riker, of the University 

 of Wisconsin, is also testing hybrid 

 poplars and selections of the native 

 poplars for their qualities, including 

 resistance to various diseases. The Di- 

 vision of Forest Pathology is inocu- 

 lating various poplar hybrids in an 

 attempt to obtain one that is resistant 

 to Septoria canker and other diseases. 



The mimosa is an important shade 



and ornamental tree in the South. A 

 wilt disease was discovered on mimo- 

 sas in 1935; since then the disease 

 has killed entire plantings and threat- 

 ens many more. Search for resistant 

 trees started in 1939. Hundreds of 

 seedlings, grown from seed collected 

 from Maryland to Louisiana, were in- 

 oculated with the wilt fungus, with 

 the result that 20 seedlings remained 

 wilt-free. These will be crossed with 

 one another in an attempt to get bet- 

 ter and more resistant mimosas. 



Selections of elms are being investi- 

 gated by Roger U. Swingle, of the 

 Division of Forest Pathology at Colum- 

 bus, Ohio, for resistance to the virus 

 disease, phloem necrosis. From an area 

 where the disease has occurred for 

 more than 50 years, about 2,000 trees 

 that were selected from open-pollin- 

 ated stock have shown high resistance 

 to the virus. The more resistant trees 

 are being propagated by root cuttings. 



Workers in the Arnold Arboretum, 

 Jamaica Plain, Mass., are breeding 

 species of pines for timber purposes 

 and are selecting from first- and 

 second-generation hybrids for resist- 

 ance to insects and diseases. They are 

 also crossing two Oriental species of 

 elm, Ulmus japonica and U, wilsoni- 

 ana. Hybrids of those species are re- 

 sistant to the elm leaf beetle. 



FEDERAL,, STATE, AND PUBLIC PARKS 

 nurserymen each year plant millions 

 of tree seedlings. The seedlings are 

 derived from seed that came mostly 

 from trees that are susceptible to at- 

 tacks of various insects, fungi, and 

 viruses. The planted trees will likewise 

 be subject to attacks of these pests, 

 resulting in the partial or total loss of 

 time and effort of many years. One 

 prime objective of the tree breeder is 

 to develop forms resistant to pest at- 

 tacks and to multiply those forms so 

 that they will be available in quanti- 

 ties for distribution and planting. 

 Both phases of this objective usually 

 require many years of work. Although 

 nature successfully replants tree spe- 

 cies generation after generation, man 



