BREEDING DISEASE-RESISTANT PLANTS 415 



the various citron characters appearing to be dominant in the majority 

 of plants. From among 3000 or 4000 plants ten fruits were selected on 

 the basis of resistance and quality and the seeds were planted the follow- 

 ing year, 1904, in isolated, infected plots. Of these ten plots two were 

 found to bear melons of uniform appearance and quality one of which 

 resembled the Eden parent. These were sesquihybrids from the F\ 

 pollinated by Eden. Again all the best melons were selected and planted 

 separately the following year and further variations were found. After 

 five more years of selection a variety was obtained which had great uni- 

 formity and disease-resistance while the fruit had a thin, tough rind which 

 enables it to endure long railway shipments. The flesh is so juicy 

 that the melons are heavier than Eden melons of the same size; the 

 quality and flavor are good although not equal to the finest (Fig. 168). 

 These qualities have been preserved and resistance maintained at a dis- 

 tance of 740 miles from the place of origin, but on the Pacific Coast the 

 resistance was not maintained. 



This failure of the supposedly resistant variety when grown in a far 

 distant locality is not strange when we remember that the wilt fungi are 

 highly specialized in their adaptation to hosts. According to Orton 

 Fusarium niveum attacks no other living plant than the watermelon and 

 "in this respect, coupled with their close morphological resemblance and 

 their common geographical distribution, they seem to be analogous to the 

 biological strains of Puccinia and Erisyphe." In combating all such 

 diseases the importance of developing locally adapted varieties must not 

 be overlooked. 



The specific nature and heritability of disease resistance is also evi- 

 denced by the results of numerous other experiments among which may 

 be cited the following. In the tomato wilt resistance was found by 

 Norton to be recessive to susceptibility and varieties of Fusarium-resis- 

 tant tomatoes from Tennessee were found to be susceptible in Maryland. 

 Stuckey found that cherry, pear and currant tomatoes were immune to 

 the blossom-end rot, a functional disease and, when crossed with com- 

 mercial varieties, they transmitted resistance as a dominant character. 

 Resistance to leaf blight in the cantaloupe was found by Blinn to be 

 inherited as a dominant character. Jesse B. Norton when breeding for 

 resistant varieties in combating the asparagus rust, Puccinia asparagi DC., 

 found resistance dominant in all the FI offspring in his crosses between 

 the female plants of the rust-susceptible American varieties and a rust- 

 resistant European asparagus. The resistance was somewhat variable 

 but was fixed by selection in succeeding generations. These few cases, 

 taken almost at random, together with the typical illustrations already 

 discussed, amply justify the recommendation that the breeding of disease- 

 resistant varieties of economic plants by hybridization and subsequent 



