446 BOHUN B. KINLOCH. JR. 



Members of the genus Cronartium and Peridermium that attack pine 

 stems are among the most destructive pests of forest trees. Of the 15 

 taxa recognized by Peterson (1967) , several are currently epidemic in 

 different parts of the world, and others are undoubtedly capable of 

 becoming so. An obvious lesson from cultivated plants that is increasingly 

 apparent in forestry is that disease problems amplify when a crop becomes 

 domesticated. Endemic diseases of formerly minor importance can become 

 major threats to their natural hosts and disastrous to introduced ones. 



Fusiform rust was little more than a mycological curiosity until 

 major reforestation programs started in the southern U.S.A. Now, in 

 many areas of the South, it is the factor most severely limiting planta- 

 tion management. Plantations in the Mediterranean area have been 

 similarly affected by Scots pine blister rust (Biraghi, 1963), as have 

 those in India by chir pine blister rust (Bakshi, 1963). Comandra rust 

 was never a problem on loblolly pine until it was planted outside its 

 natural range on the Cumberland Plateau in Tennessee, where it became 

 victim to a serious epidemic (Powers, Hepting, and Stegall, 1967). 



Sweetfern rust has ravaged exotic plantations of shortleaf and 

 lodgepole pine in eastern Canada (Van Sickel, 1969), and Monterey pine 

 in British Columbia (Molnar, 1961) . It is morbidly fascinating to con- 

 template the fate of the latter species over millions of acres of exotic 

 plantations in Australia, New Zealand, and other countries if its endemic 

 and autoecious rust, P. harknessii Moore, ever became established. 

 Although widespread epidemics of indigenous rust diseases in natural 

 stands are unusual (Peterson and Jewell, 1968), a report of heavy out- 

 breaks of cone rust in Guatemala suggests that they occasionally do occur 

 (Schieber, 1967). 



The need for sources of resistance to these rusts is obvious and 

 urgent. But we know from experience with other rust diseases that resis- 

 tance is often labile. Sustained progress must depend on a thorough 

 understanding of the genetic architecture of host-parasite populations 

 and the specific genetic interactions involved among different host geno- 

 types and pathogenic races. Presently, our knowledge of these matters 

 in the pine rusts is limited. Little is known about mechanisms and modes 

 of inheritance of resistance in hosts, or even some basic aspects of the 

 taxonomy, life cycles, and sexual behavior of the fungi. 



The intrinsic difficulties of working with wild, heterozygous, 

 and long-lived forest trees simply do not make traditional breeding 

 methods feasible for answering some of tnese questions. However, 

 promising new approaches to analyzing genetic interactions of host- 

 parasite combinations now exist and will be discussed later in this 

 paper. 



RESISTANCE TO FUSIFORM RUST OF SOUTHERN PINES 



VARIATION AMONG SPECIES AND HYBRIDS 



Among the four major (and closely related) southern pines, slash 

 (Pinus elliottii Engelm.) and loblolly (P. taeda L.) are highly suscep- 

 tible, longleaf (P. palustris Mill.) highly resistant, and shortleaf 

 (P. echinata Mill.) nearly immune to fusiform rust (caused by Cronartium 

 fusi forme Hedge, and Hunt ex Cumm.) under natural conditions (Siggers, 

 1955; Schmitt, 1968; Wakeley, 1969; Derr, 1966). Thus, an early and 



