126 MASSACHUSETTS HORTICULTURAL SOCIETY. 



strain, resistant to the very serious leaf spot disease due to Macro- 

 sporium cucumerinum. 



Selection oe Varieties. 



In order to obtain results by this method, all the varieties of a 

 crop obtainable 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. Uually, however, all commercial varieties will 

 be more or less susceptible. If such is the case, then the varieties 

 least susceptible must be studied further and selection of individuals 

 proceed as in the first method. I 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 practice individual selection. 



Hybridization. 



In case no varieties give promise of resistance, hybridization, or 

 crossing, may give satisfactory results. It frequently ha])pens 

 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 worthless commercially, but which is known to be 

 strongly resistant. If such a resistant species or variety be crossed 

 with a profitable commercial variety, and a close study of the 

 resulting offspring be made, some desirable new variety thus \n-o- 

 duced may show marked resistance. A notable example of this 

 method of procedure is the work of Mr. W. A. Orton with water- 

 melons. Watermelons in the South are seriously attacked by a 

 soil disease (Xcocosmo.spora), which it has been im])ossible to 

 combat satisfactorily. INIr. Orton first matle a study of all varie- 

 ties of watermelons and relatetl j^lants obtainable, and found that 

 all commercial varieties were almost ecjually susce})tible. He 

 found, however, that a form of citron (hard fleshed melon) was very 



