RESISTANCE TO RUSTS IN HARD PINES 



457 



Number of galls 



Figure 1 . --Frequency distribution of individual loblolly pines 

 into categories according to the number of separate infections 

 (galls) sustained by each. (A) 7-year-old trees (total: 3,918) 

 from 48 open-pollinated families; (B) 4-year-old trees (total: 

 1,376) from 54 controlled-pollinated families; (C) 4-year-old 

 trees (total: 318) from bulk seedlot , unknown parentage. 

 (Data for C courtesty of the U.S. Forest Service, Macon, Ga.) 



than their counterparts in susceptible families--see Fig. 2). Since 

 these extremely susceptible individuals were randomly distributed 

 geographically, the assumption of approximately equal and abundant 

 distribution of spores over the test sites seems valid. 



It then seems intuitively evident that the variation in number of 

 galls on individual trees is inversely related to the number of genes 

 for resistance the trees inherited. An interesting and perhaps signifi- 

 cant feature of the test populations is that the distribution patterns 

 of trees into "gall categories" appear to fall into a geometrical series, 

 wherein the number of trees in each succeeding category is roughly half 

 of the preceding one, up to 5 or 6 categories (Fig. 1). As already 

 discussed, geometrical series are properties of gene-for-gene relation- 

 ships (cf. Table 1 and Person, 1959). The same kind of series theoreti- 

 cally would be expected in the coffee-leaf rust system if host genotypes 

 were simultaneously inoculated with an equal mixture of each pathogenic 

 race: the number of pustules counted per leaf should decrease by multiples 

 of one-half as the number of genes for resistance in individual plants 

 increased from to 4 . 



