RESISTANCE TO RUSTS IN HARD PINES 447 



logical approach to breeding for resistance was through hybridizing 

 the susceptible but commercially preferred species with the resistant 

 ones . 



This approach has proved successful in numerous hybrid and backcross 

 combinations among these species tested at the Institute of Forest 

 Genetics, Gulfport, Mississippi, and elsewhere. Although few parents 

 were used, trends have emerged clearly. Hybrids with shortleaf are very 

 resistant to fusiform rust, and those with longleaf only slighly less so. 

 Progenies from two loblolly x slash crosses, on the other hand, were 

 much more susceptible than any other hybrid or their parental species 

 (Henry and Bercaw, 1956; Henry and Jewell, 1963; Jewell, 1959; Derr, 

 1966; Schmitt, 1968) . 



Early reports (Jewell, 1959; Jewell and Henry, 1961) suggested that 

 resistance was inherited from shortleaf in a simple, dominant manner. 

 Later analyses showed that some progenies with half to one-quarter of 

 their genes from shortleaf developed galls in varying amounts but were 

 still relatively highly resistant. Backcross progenies between moderately 

 susceptible Sonderegger pine (the natural longleaf x loblolly hybrid) and 

 its resistant parent species (longleaf) were much more resistant than 

 progenies from its susceptible parent (loblolly) . On the basis of this 

 gradient in susceptibility among hybrids, Schmitt (1968) proposed that 

 resistance was inherited polygenically and perhaps at different loci from 

 different species. 



Artificial inoculation of hybrids with shortleaf pine also pointed 

 to a more complex mode of inheritance. Although hybrids were generally 

 intermediate between parent species in susceptibility, individual progeny 

 means ranged from 4 to 92% infection, depending on the particular combi- 

 nation of parents used. These tests showed that the parent of the 

 susceptible species was as influential as that of the resistant species; 

 in other words, susceptibility appeared to be as strongly inherited as 

 resistance (Jewell, 1961a, 1965). 



RACIAL VARIATION WITHIN SPECIES 



Provenance tests provided the earliest and most extensive evidence 

 for genetic variation in susceptibility to fusiform rust. In no other 

 trait have genetic differences among seed sources been so strongly and 

 consistently expressed. In the Southwide Seed Source Study (Wells and 

 Wakeley, 1966) , seed lots from 15 sources throughout the geographic range 

 of loblolly pine were tested in plantations near each source. Severity 

 of infection varied greatly among test sites. But in those sustaining 

 heavy infection, the range between the most and least susceptible sources 

 was consistently on the order of 50 to 75%, and always in the same pattern 

 the most resistant sources were from near the extremities of the species 

 range--west of the Mississippi River and eastern Maryland (Wells and 

 Wakeley, 1966). With the exception of the Maryland source, resistance 

 tended to increase with the western longitude of the seed source. These 

 results are consistent with those of similar studies of more limited 

 scope (Kraus, 1967; Wells, 1966; Wakeley, 1969). 



A surprising degree of variation in susceptibility also exists among 

 stands within a local region. Significant differences were found among 

 seed sources from proximate stands in Louisiana (Crow, 1964), Texas 

 (Wells, 1966), and Georgia (Barber, 1966; Kraus, 1967). Two of the 



