18 JAMESTOWN CONGRESS OF HORTICULTURE 



winter injury in more southern latitudes. On the other hand, plants 

 of southern range planted north start later, are less subject to late 

 frosts, but may be injured by early frosts. These cold injuries are 

 often hardly noticeable, but they are sufficient to weaken the plant and 

 open the way for trunk cankers and numerous other parasitic diseases 

 which the trees could otherwise resist. 



A soil slightly too acid or alkaline for a particular variety, though 

 not enough to prevent growth, may nevertheless weaken the root 

 system and, in fact, the whole plant, making it subject to serious dis- 

 ease. So also the moisture or temperature fluctuations of the soil and 

 its aeration may be unfavorable to a particular variety, making it less 

 resistant to disease, if not actually causing a pathological condition in 

 itself. Too little attention has been given to these factors by the 

 farmers and horticulturists as well as by the pathologists. 



An important duty in this new century will be to develop a better 

 appreciation and more accurate understanding of the relation of these 

 factors to health and disease. The cropping system of a farm or 

 orchard, the planting of a nursery or a park to be satisfactory and 

 successful in securing healthy growth must be undertaken only after 

 a careful consideration of all these factors involved. Like the archi- 

 tect, the horticulturist and the farmer must have a carefully thought- 

 out plan and as nearly as possible see the end from the beginning. 



RESISTANCE AND IMMUNITY. 



Our ideal, of course, is to cultivate plants that can in the largest 

 measure consistent with other requirements fight their own battles. 

 Observation and experience have given us a large amount of informa- 

 tion on adaptability to conditions and resistance to disease, which 

 remains to be classified and digested in order to be made generally 

 available. We often neglect to reap the benefits of a destructive 

 drouth, a cold wave, an epidemic of disease, or the failure of a crop, 

 fry neglecting to study and save what is left. The few straggling plants 

 left do not appeal to the average man. He plows them up or turns 

 in the hogs. But the man familiar with Nature's methods sees in these 

 survivors resistant strains and saves the few straggling plants for seed, 

 with the hope that the few survivors may have some peculiarity trans- 

 mittable to progeny, making them resistant to the factor that caused 

 the general destruction of the crop. In this way originated the wilt- 

 resistant cotton, wilt-resistant cowpeas and flax, and cowpeas and to- 

 bacco resistant to nematode or root-knot. Strains of red clover resis- 

 tant to anthracnose (a disease which in many sections of the South 

 makes it impossible to grow ordinary non-resistant clover) were also 

 originated in this way. Strains of corn, oats, wheat, rye, clover, 

 alfalfa, sugar beets, and other grains, forage plants, and vegetables 

 resistant to cold, alkali and drouth have been developed from such 

 selections, in some cases made purposely by subjecting the crop to 

 these conditions, in others by simply taking advantage of what occurred 

 naturally. In some of the older and more thickly populated parts of 



