DISEASE RESISTANCE. 61 



A ^vilt in tobacco occurs in North Carolina which is evidently a 

 parallel case with the one found in Connecticut, and in all probability 

 could be controlled in the same way. 



One of the most serious diseases affecting tobacco at present is the 

 root-knot caused by nematode worms. Tobacco seems to be particu- 

 larly susceptible to the attacks of nematodes, and many crops are 

 more or less seriously affected by this enemy. There is no known 

 remedy for this pest that is applicable in a practical Avay to field con- 

 ditions. Soil-sterilization methods are used successfully in plant 

 beds or in greenhouses, but such treatment is rather too expensive 

 to be used on large fields. The most practicable method for the con- 

 trol of this disease seems to be in the way of securing immune strains 

 of tobacco by seed selection and breeding. The writers have se- 

 lected a large number of individual plants that showed immunity in 

 the field, and the seed of these selections Avill serve as a basis for ex- 

 periments in the production of nematode-resistant types. The com- 

 plete success of other workers in obtaining resistance to nematodes 

 in varieties of sugar beets and cowpeas is good evidence that similar 

 results may be obtained in their efforts to obtain resistance to this 

 enemy in tobacco. The Iron cowpea shows strong resistance to the 

 nematode when planted on badly infected tobacco fields, and for this 

 reason can be highly reconnnended to tobacco growers for use in 

 this connection. 



The mosaic disease causes very serious injury in tobacco fields in 

 many parts of this country. The writers believe, from indications 

 observed during the past two years, that it will be possible to develop 

 strong, vigorous strains of tobacco which will be largely resistant 

 to this disease. In the case of some Maryland selections, resistance 

 to the mosaic disease seems to have been transmitted in a large 

 degree to the progeny of certain vigorous strains. In two plats 

 grown side by side under uniform conditions, one from seed of a 

 parent plant affected with mosaic disease, the other from a perfectly 

 healthy one, the following results were obtained: Plat 1, grown from 

 the seed of the mosaic plant, showed 80 per cent of diseased plants 

 in the field ; plat 2, grown from the seed of a perfectly healthy plant, 

 showed less than 20 per cent of diseased plants. It may be impos- 

 sible to entirely eradicate this disease by the production of immune 

 varieties, owing to the peculiar nature of the malady, but these fig- 

 ures, which have been duplicated many times, show very conclusively 

 that by the development of stronger and hardier types of tobacco, 

 especially where heavy seed is used for sowing, it will be possible to 

 gradually reduce the percentage of mosaic plants in ordinary to- 

 bacco fields. 



96 



