PHYSIOLOGY AND BIOCHEMISTRY OF RESISTANCE TO PINE RUSTS 471 



At the two lower concentrations, the spore diffusate from slash 

 resulted in greater germination, compared to the water-agar control, 

 whereas that from shortleaf inhibited germination. At 1.0 percent con- 

 centration, slash spore diffusate gave 128 percent of the water diffusate 

 germination compared to 74 percent from shortleaf. Thus, at the 1.0 percent 

 concentration, phytoalexin production was correlated with resistance, and 

 infection apparently induced an "anti-phytoalexin" (germination promoter) 

 in susceptible slash pine. 



In addition, many of the germ tubes in plates containing 10 percent 

 shortleaf water or spore diffusates were very short and severely curved, 

 forked, and otherwise malformed. Shortleaf pine may, therefore, have a 

 resistance mechanism that inhibits penetration of leaf cells. 



FUSIFORM RUST RESISTANCE IN WESTERN LOBLOLLY PINE 



INTROGRESSION 



Western seed sources of loblolly pine have repeatedly been shown to 

 be more resistant to fusiform rust than sources east of the Mississippi 

 River (Wells, 1966; Wells and Wakely, 1966). Since loblolly-shortleaf 

 hybrids are resistant (Henry and Bercaw, 1956; Henry and Jewell, 1963), 

 and natural loblolly-shortleaf hybrids occur in Texas (Zobel, 1953), it 

 has been proposed that resistance of western loblolly arises from intro- 

 gression with shortleaf (Wells and Wakeley, 1966) . Hare and Switzer 

 (1969) showed similarities in electrophoretic patterns of seed proteins 

 between shortleaf and western loblolly, compared to eastern loblolly. 

 They also found morphological characteristics in western loblolly inter- 

 mediate between shortleaf and eastern loblolly. The results strongly 

 suggest introgression in the western part of the loblolly range. Fusiform 

 rust resistance of western loblolly pine may arise from such introgression, 



TERPEN ES 



A gas chromatographic analysis of terpenes in the stem oleoresin of 

 loblolly pines from eastern and western seed sources also turned up 

 differences associated with fusiform rust resistance (Hare, 1970) . 

 Samples were collected in August and October from canker-free trees 

 growing in the Harrison Experimental Forest near Gulfport. The more 

 resistant western seed sources were compared with the less resistant 

 eastern sources. No differences associated with resistance were found 

 in a-pinene, camphene, myrcene, or S-phellandrene . But, at both times of 

 sampling, 3-pinene and limonene were much higher in the western than in 

 the eastern sources. Whether these differences will prove to be a 

 genetic marker for resistance remains to be shown. 



NUTRITIONAL FACTORS 



According to the nutrition theory of resistance (Garber, 1961) , 

 parasites may lose virulence because the host provides inadequate amounts 

 of an essential nutrient at the infection site, or because nutrient up- 

 take is inhibited by other compounds. Burrows (1960) used a leaf-sandwich 

 technique to show that infection could pass through resistant wheat- leaf 

 mesophyll without infecting it and infect attached susceptible leaf 

 mesophyll. In that case, resistance was not nutritional, since nutrients 

 could diffuse freely, but was due to an inhibitor induced by the fungus. 



