450 BOHUN B. KINLOCH, JR. 



Another objection is that young succulent seedlings in the cotyledon and 

 primary needle stage—especially in nursery and greenhouse environments -- 

 may be intrinsically more susceptible than older ones grown naturally. 

 The much greater infection of shortleaf x slash hybrids inoculated 

 artificially (Jewell, 1961a, b) as opposed to naturally (Henry and Bercaw, 

 1956) and of juvenile compared to 1-year-old seedlings inoculated arti- 

 ficially (Jewell, 1960; Goddard and Arnold, 1966) suggest such a rela- 

 tionship. In one study that compared the amount of infection occurring 

 on 16 open-pollinated families by both natural and artificial inoculation, 

 a low (0.46) correlation in progeny performance was found between the two 

 methods (Kinloch, 1968). More such comparisons are needed to evaluate 

 the practical utility of artificial inoculation. 



Nevertheless, the method has been useful in detecting genetic dif- 

 ferences in susceptibility among progenies (Goddard and Arnold, 1966; 

 Kinloch, 1968; Davis and Goggans, 1968), and in studies on modes of 

 inheritance of resistance. The most intensive and reliable work has been 

 done at the Institute of Forest Genetics, Gulfport, Mississippi (Jewell 

 and Mallett, 1967). Inoculation of progenies from a few slash pine parents 

 phenotypically selected for resistance or susceptibility demonstrated 

 that certain rust-free parents consistently transmitted about 50% resist- 

 ance to either open-pollinated progenies or to those from crosses with 

 susceptible parents, and up to 90% resistance in specific combination with 

 other resistant parents. The nearly complete susceptibility of progenies 

 from susceptible x susceptible crosses and the segregation (1 resistant: 

 1 susceptible) of progenies from resistant x susceptible crosses suggested 

 that resistance was inherited by a single dominant gene that was hetero- 

 zygous in the resistant parents (Jewell, 1966). Although subsequent data 

 were not entirely consistent with this hypothesis (Jewell and Mallett, 

 1967) , the discontinuous distribution of progenies into groups that were 

 either highly resistant or highly susceptible still suggested a relatively 

 simple mode of inheritance. 



RESISTANCE TO OTHER PINE STEM RUSTS 



Evidence of genetic variation in host resistance to other pine stem 

 rusts is scanty, but similar in kind to that for fusiform rust. There 

 appear to be distinct differences in susceptibility to sweetfern rust 

 (C. eonrptoniae Arth.) among pine species both native (Kaufman and Mains, 

 1915; Spaulding, 1917) and exotic (Molnar, 1961) to the natural range of 

 the fungus. Two hybrid progenies from P. banksiana Lamb, x P. aontorta 

 Dougl . crosses differed from their parental species and each other in 

 susceptibility to this disease and also to eastern gall rust (C. quercuum 

 (Berk.) Miyabe ex Shir.) (Anderson and Anderson, 1965). Recently, highly 

 significant differences in susceptibility to eastern gall rust were found 

 among range-wide seed sources of jack pine planted in Wisconsin (McGrath, 

 1967) . As with fusiform rust on loblolly pine, the relative rankings in 

 susceptibility of the 12 sources used were fairly consistent over the 3 

 test plantations, though the intensity of infection in each plantation 

 varied considerably. In Europe, a few reports have indicated similar 

 variation in susceptibility to Peridermium pini (Pers.) Le*v. among 

 different seed sources of Scots pine (P. sylvestris L.) and also among 

 individual parent trees (van der Kamp, 1968; KlingstrOm, 1967) . 



Great variation in susceptibility to the western gall rust [Pevidermium 

 harknessii Moore) is common among individual trees in natural stands of « -*- 

 native pines and planted Scots pine. Trees with hundreds to thousands of 



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