March, 1921. 



SCIENTIFIC AGRICULTURE. 



123 



Figure 2. The condition of the roots of four varieties of 

 beans differing in their resistr...ce to the dry rot organism. 



The process of producing disease resistant types by " 

 hybridization is necessarily slow because of the length 

 of time that is required to isolate and fix the desired 

 type from the miseellanecus population that will appear 

 as the result of crossing. This diversity of the residting 

 population, while it constitutes a hindrance to rapid pro- 

 gress, is one of the factors in favor of the liybridization 

 method of plant improvement in tliat the diverse popu- 

 lation obtained affords a good opportunity of isolating 

 a plant possessing the maximum of desirable qualities. 



A few examples will suffice to illustrate the work that 

 is being done, mainly by our plant pathologists, in the 

 production of resistant strains by tlie isolation of re- 

 sistant individuals from commercial varieties. Blinn 



(9) by selection within commercial varieties was able 

 to isolate a cantaloupe resLstant to rust. Hansen 



(10) has secured by .selection a type of sand cherry re- 

 sistant to mildew. Shamel and Cobey (11) obtained 

 a strain of tobacco resistant to wilt by one year's 

 selection. Bain et al. (12) selected a strain of 

 Red Clover apparently resistant to anthraenose. Nor- 

 ton (13) found that the tomato varieties Stone 

 and Stirling Castle possessed considerable resist- 

 ance to the causal organism of leaf mold. Jones 



(14) developed a strain of cabbage possessing resistance 

 to yellows. Barrus (1.5) rejiorts a sti'ain of beans, ob- 

 tained by a grower from an individual plant selection, 

 which is resistant to both strains of the bean anthraenose 

 fungus. 



The possibilities of the slower and more difficult 

 method of securing disease resistant plants by means of 

 hybridization, wjll be dealt with somewhat more fully. 

 Ill this connection the work done in securing beans re- 

 sistant to the causal organism of bean antliracnose will 

 be used to illustrate both the possibilities and the diffi- 

 culties of this method of immmiization of plants. A few 

 illu.strations from work done along this line with other 

 crops will be included for purpose of comparison and of 

 more fully demonstrating the possibilities of this pha.se 

 of investigation. Biffen (16) and Nilsson-Ehle (17) 

 were each able to secure a number of hybrid wheats re- 

 sistant to yellow rust from the segregates of cresses be- 

 tween resistant wheats and those susceptible to this 

 disea.se. Orton (18) obtained the wilt resistant water- 

 melon Conqueror by crossing the citron and watermelon. 

 Tisdale (19) was able to secure a hybrid strain of flax 

 resistant to flax wilt by crosses between resistant and 

 su.sceptil)le varieties. 



In the case of bean anthraenose two strains of the 

 causal organism have been isolated by Barrus, (20) and 

 the resistance and susceptibility to these strains of a 

 large number of bean vai-ieties dtMnonst rated. For pur- 



poses of discussion the strains will be alluded to as 

 Strain A. and Strain B. Unfortunately very few of our 

 common field or garden varieties of beans are resistant 

 to lioth of these strains. A considerable number, part- 

 icularly of our garden beans, are susceptible to both 

 strains of the pathogene. The remaining varieties are 

 either susceptible to Strain A. and resistant to Strain 

 B. or vice ver.sa. This condition of affairs has been both 

 a detriment and an aid in the securing of resistant 

 types. The fact of the existence of two strains of the 

 causal organism has decreased the chances of securing 

 by selection individuals resistant to both of these strains. 

 This same fact was, however, an advantage in that it 

 presented the possibility of hyliridizing a desirable var- 

 iety resistant to Strain A. with a desirable variety re- 

 .si.stant to Strain B., and isolating from the offspring 

 strains resistant to both of these strains. This latter 

 possibility was of particular advantage in that the two 

 types of field beans that were resistant to both Strain A. 

 and Strain B. each possessed the disadvantage of being a 

 late maturing variety. One of these varieties also had a 

 colored seed and was in addition very susceptible to 

 bacterial blight. 



A type of White ilarrow bean resistant to Strain A. 

 was crossed with a white pea bean t\-pe resistant to 

 Strain B. The finst generation plants of this ero.ss were 

 resistant to both strains of the fungus and the .second 

 generation, when inoculated with a mixtui-e of both 

 strains of the fungus, gave resistant and susceptible 

 plants in the proportion of nine resistant to seven sus- 

 ceptible. One ninth of these resistant plants bred true 

 for this character. The remaining eight-ninths when 



tf If 



Figure 3. Four varieties of beans that have been inocu- 

 lated with strain A. of the bean anthraenose fungus. 



Figure 4. The same four varieties (se« Fig. 3) inoculated 

 with strain B. of the bean anthraenose fungus. It is evident 

 that variety No. 1 is resistant to both strains, variety No. 2 

 resistant to the A. strain but susceptible to the B. strain, 

 variety No. 4 susceptible to the A. strain but resistant to 

 the B. strain, while variety No. 5 is susceptible to both 

 strains of the fungus. 



