452 BOHUN B. KINLOCH, JR, 



The degree of compatibility between host and parasite was measured 

 by the type and rate of symptom development (Hutchinson, 1935; True, 

 1938). In the most compatible reactions, incipient swellings were the 

 first macroscopic symptom discernible. In the most resistant reactions, 

 spots with well defined margins and necrotic, tannin-filled epidermal 

 and subjacent cells developed within 6 weeks. Spots with diffuse margins 

 that appeared water soaked usually indicated successful infections, 

 especially when their appearance was delayed. In partially resistant 

 hosts, hypertrophic and hyperplastic reactions of affected tissues 

 impeded intercellular spread of mycelium by mechanically restricting 

 available growing space. Formation of wound periderm, often in successive 

 layers, also resisted spread, but was completely successful only when 

 invaded contiguous cells died rapidly. All invaded cortical cells even- 

 tually died, and normal gall formation ensued only if and when mycelium 

 could reach the cambium. Thus, the rate of response of host cells to the 

 biochemical and mechanical lesion imposed by the parasite determined the 

 degree of resistance. This condition appears no different in kind to 

 hypersensitive mechanisms of resistance in many other host-parasite 

 combinations . 



VARIATION WITHIN RUST POPULATIONS 



The taxonomic history of Cronartium and its imperfect stage, 

 Peridermium, has been very confused, with many synonymous and invalidly 

 named species. Peterson (1967) recently brought considerable order to 

 the genus in his treatment of the aecial {Peridermium) stages on pines, 

 but pointed to serious remaining gaps. Some of the named species, such 

 as C. flaccidum (Alb. £ Schw.) Wint . (including P. pini) and C. 

 coleosporioides Arth . (including P. stalaotiforme Arth. § Kern, P. 

 filamentosum Peck, and P. harknessii) are quite difficult and exist in 

 complexes of subspecific or racial varients; these vary in their alternate 

 host preference or lack alternate hosts entirely. Further work may even- 

 tually elevate some of them to specific status. Obviously, a clearer 

 definition of the various taxa is needed before host affinities can be 

 firmly established and definitive genetic studies started. 



Within species, racial variation has been demonstrated for albinism 

 in C. fusiforme (Kais and Walkinshaw, 1964), P. stalaotiforme (Powell, 

 1966), and P. harknessii (Mielke and Peterson, 1967); for phenology, 

 spore morphology, and conditions for germination in P. filamentosum 

 (Peterson, 1968); and for germ tube growth, life cycle, and nuclear 

 behavior in P. oini (Hiratsuka, 1968) and P. filamentosum (Christensen , 

 1968, 1969) . 



On the all important question of variation in pathogenicity on 

 pines, little is known. KlingstrOm (1967) reported variation in virulence 

 in 3 aeciospore sources of P. pini. One source was avirulent on Scots 

 pine progenies used in his experiments, but not on other material. True 

 (1938) lacked proof, but interpreted the variation in symptoms he observed 

 on Scots pine trees inoculated with P. harknessii to be indicative of 

 variation in virulence among individual spores. In loblolly pine, the 

 maintenance of the same relative rankings in susceptibility of different 

 seed sources planted throughout the southern U.S.A. was taken as negative 

 evidence for the existence of pathogenic races of C. fusiforme (Henry, 

 1959). Snow, Powers, and Kais (1969), however, found distinct variation 

 in pathogenicity among isolates from different areas on open-pollinated 

 seedlings of a single slash pine parent. 



