74 STAKMAN 



wilt-resistant varieties were produced about fifty years ago. But the varieties 

 apparently lost their resistance after a few years in farmers' fields. The dis- 

 covery that the flax wilt fungus comprises many physiologic races gave the 

 clue to a method of permanent control. Permanent flax wilt plots were estab- 

 lished in which the soil is inoculated with all known races of the wilt fungus 

 and with other infective materials from many sources. All potentially new 

 varieties must survive this severe test, and by such continual testing and 

 selecting, flax wilt has been kept under control by a continual succession of 

 varieties. There are several reasons why this procedure has been effective: 

 first, there are a few resistant plants in most varieties; second, the wilt fun- 

 gus selects rigidly, usually killing susceptible plants outright and leaving only 

 the most resistant; third, the fungus is not disseminated widely by the wind. 

 Fortunately there have been enough resistant individuals in flax to check- 

 mate the activities of the pathogen in producing new races. This continuous 

 effort has made it possible to save the flax crop of the United States and to 

 maintain its productivity during the past fifty years. 



A somewhat similar disease of cabbage has been kept under control for 

 the past thirty-five years by means of resistant varieties. The cabbage yel- 

 lows, also caused by a species of Fusarium which accumulates and persists 

 in the soil, was so destructive in many cabbage-growing areas about 1910 

 that cabbage growing had to be abandoned. Fortunately, however, there are 

 resistant strains within most of the commercial t3T3es of cabbage. By selecting 

 and crossing it has been possible to keep the disease under control. In this 

 case, physiologic races have not complicated the problem. 



The virus curly top of sugar beets, which is disseminated by leaf hoppers, 

 became epidemic so often as to threaten the sugar-beet industry in certain 

 areas of western United States a few years ago. Strains of sugar beet have 

 been selected that are sufficiently resistant to the disease to yield satisfactorily 

 when non-resistant selections are virtually ruined by the disease. Immunity 

 has not been found, however, in the cultivated sugar beet, but it is known that 

 certain wild beets are immune. As it has not yet been possible to obtain 

 satisfactory crosses between the wild and the cultivated beets, the problem is 

 to devise ways of obtaining fertile hybrids between the two. There often 

 are, of course, many barriers to the transfer of genes from one kind of plant 

 to another. If wide crosses could be made at will, many problems would be 

 simplified. Possibly ways may be devised to combine genes that now seem 

 to be non-combinable. 



Some epidemic diseases can be controlled by spraying or dusting with 

 fungicides, but usually at heavy expense. Thus, the Sigatoka disease of 

 banana, which became epidemic and ruined many banana plantations in cer- 

 tain banana-growing districts of tropical America about twenty-five years ago, 

 is kept in check by spraying the plants from seven to fifteen times during 

 the season with bordeaux mixture. The cost, however, is extremely high. It 



