Plant Sanitation. 



166 



[February, 1909. 



best success knows that he must treat his 

 cabbage seed with formalin or corrosive 

 sublimate as a precaution against the 

 introduction of the bacterial rot. 



There are a host of diseases which 

 might be mentioned for which some 

 specific treatment is in common practice. 



Why then is it necessary for us to 

 have resistant varieties ? Spraying is a 

 nuisance at best, and any man who has 

 had any actual spraying to do would 

 gladly eliminate this not inconsiderable 

 item of labour and expense. The second 

 reason is that, while spraying may be 

 profitable for the large grower, parti- 

 cularly the man who makes a speciality 

 of some one crop, it frequently is not 

 profitable for the small grower. The 

 man who grows a few apple trees, a few 

 acres of potatoes, tomatoes, cucumbers 

 and other vegetables in proportion, 

 frequently has to give up the cultivation 

 of any crop that is repeatedly attacked 

 by a serious disease. Spraying frequent- 

 ly is not profitable for men who carry 

 on diversified farming. I do not wish to 

 be misunderstood in this connection. 

 When the mere item of added expense 

 of materials and labour necessary to the 

 spraying alone is considered, we will 

 find that spraying is profitable in most 

 cases even for the small grower. 



The labour question, however, adds 

 another factor. When it is possible for 

 the small grower or the man who carries 

 on diversified farming to obtain addi- 

 tional labour of the right sort at any 

 time, he can spray his various crops with 

 profit, but when labour is limited and the 

 grower must depend on his steady labour 

 to do the spraying, he will usually find 

 that to carry on the work properly other 

 important things must be neglected. 

 Spraying must be done at the right time, 

 or it is of no value. The third reason is 

 that there are many diseases which 

 experimentally can be controlled, but 

 not profitably. As an example, we may 

 mention the rust of wheat and many 

 other diseases of field crops. The fourth 

 reason is that certain very serious 

 diseases have never been satisfactorily 

 controlled even by experiments. To 

 this class belong such diseases as peach 

 yellows, crown gall, and various soil rots 

 and wilts. Most bacterial diseases of 

 plants belong to this class. 



Methods. 



In order to develop varieties resistant 

 to disease, several methods of work are 

 open to the investigator 



(a) Selection of individuals. 



\b) Selection of varieties. 



(c) Hybridization followed by selec- 

 tion of varieties and individuals. 



Selection op Individuals. 

 Every grower of plants has doubtless 

 observed that in a field of a crop suffer- 

 ing from an epidemic disease, certain 

 individual plants may show more or less 

 resistance to the disease. By selecting 

 seed from such plants we would expect, 

 if the resistance is an actual one, to have 

 offspring which would inherit the 

 character of resistance. By repeated 

 selection through several generations, 

 always saving seed from the most 

 healthy plants, we would expect to 

 gradually develop a variety or strain 

 much more resistant than ordinary one. 

 This has been practically tested in a 

 number of cases and found to be success- 

 ful. Mr. P. K. Blynn, of the Colorado 

 Experiment Station, has obtained most 

 satisfactory results by this method in 

 selecting a strain of cantaloupe known 

 as the pollock strain, resistant to the 

 very serious leaf spot disease due to 

 Macrosporiu m cucm nerinum. 



Selection of Varieties. 

 In order to obtain results by this 

 method, all the vatieties of a crop obtain- 

 able are grown on infested ground in 

 order to study their relative resistance 

 to disease. Experimenting in this way 

 some variety may prove exceedingly 

 resistant to the disease in question and 

 further work rendered unnecessary 

 except to keep the variety pure. Usu- 

 ally, however, all commercial varieties 

 will be more or less susceptible. If such 

 is the case, then the varieties least sus- 

 ceptible must be studied further and 

 selection of individuals proceed as in 

 the first method. 1 believe that the 

 most certain and best results are to be 

 obtained by combining the first method 

 with the second. That is, select the 

 most promising varieties with which to 

 practise individual selection. 



Hybridization. 

 In case no varieties give promise of 

 resistance, hybridization, or crossing, 

 may give satisfactory results. It fre- 

 quently happens that wild species which 

 may be worthless commercially, but 

 closely related to cultivated forms, 

 show marked resistance to diseases which 

 seriously attack the latter. Sometimes 

 we may have a variety which is worth- 

 less commercially, but which is known 

 to be strongly resistatit. If such a 

 resistant species or variety be crossed 

 with a profitable commercial variety, 

 and a close study of the resulting off- 

 spring be made, some desirable new 

 variety thus produced may show marked 

 resistance. A notable example of this 

 method of procedure is the work of 

 Mr. W. A. Orton with water melons, 



