PHYSIOLOGY AND BIOCHEMISTRY OF RESISTANCE TO PINE RUSTS 467 



sylvestris L.) to C. flaecidwn (A. § S.) Wint.is greatest in small 

 seedlings and mature trees and least in developing trees (BjOrkman, 1966) . 

 Foliar resistance to blister rust is important in western white pine, on 

 which it may prevent stem invasion from the needles (Hoff, 1966) . Similar 

 effects have been noted for resistant slash pine seedlings inoculated with 

 fusiform rust (Jewell and Mallett, 1967). 



Alternate hosts also have foliar resistance which appears to be 

 physiological, at least in young leaves (Anderson, 1939). Only young, 

 soft water oak {Quercus nigra L.) leaves are susceptible to C. fusi forme 

 aeciospores or uredospores which produce uredia in very young leaves and 

 telia in slightly older leaves (Snow and Roncadori, 1965). But soft 

 leaves of some water oak seedlings show only a hypersensitive flecking in 

 response to infection (Eleuterius, 1968). Dwinell (1969) reported a 

 similar hypersensitivity of black oak (Q. velutina Lam.) to C. fusiforme 

 but not to C. quereuum (Berk.) Miyabe ex Shirai. 



WOUND PERIDERM BARRIERS 



Bark resistance to blister rust may involve both hypersensitivity 

 and "corking out," the formation of a wound periderm in response to 

 infection. The hypersensitive reaction has been reported on P. montioola 

 (Bingham, Squillace, and Wright, 1960), P. peuoe Griseb. (Boyer, 1962), 

 and P. armandii Franchet (Hoff, 1966) . Cortical cells are killed, 

 stopping the fungus and causing a sunken area at the base of infected 

 needles. Patton (1966) observed hybrids which responded similarly 

 unless the needle was killed before the fungus reached the stem. Inheri- 

 tance of bark resistance is independent of that of foliar resistance, 

 indicating polygenic control (Hoff, 1966) . 



Corking out was first reported as a response of P. sylvestris to 

 a Peridernriym gall rust (Hutchinson, 1935; True, 1938) , but resistance 

 was ascribed mainly to hypersensitive accumulation of tannins. McKenzie 

 (1942) reported that resistant northern pines formed similar cork 

 barriers and accumulated tannin in response to a Peridermivm infection. 

 The resistance mechanism was similar to that of wound healing. This 

 form of bark resistance has since been described for blister rust in 

 P. strobus (Struckmeyer and Riker, 1951; Boyer, 1966). In loblolly 

 pine, fusiform rust may induce formation of perderm immediately ahead 

 of the advancing hyphae, but in this species the barrier can do no more 

 than retard the growth of the fungus (Jackson and Parker, 1958). 



OTHER PHYSIOLOGICAL MECHANISMS 



In addition to hypersensitivity and corking out, physiological resis- 

 tance can involve selective toxins of host or pathogen origin, lack of an 

 essential nutrient for the parasite, enzyme activity or inhibition, 

 inhibition of spore germination or germ-tube growth by preformed fungi - 

 toxins, failure of the host to produce a gall or other symptoms, phyto- 

 alexin production (related to hypersensitivity) , or differences in water 

 content or mineral uptake of host tissue (Schreiner, 1966) . Growth- 

 habit resistance .occurs in P. griffithii McClell. (syn. P. wallichiana 

 A.B. Jacks.), which sheds needles before blister rust infections on 

 them can reach the stem (Heimburger, 1962) . Active resistance induced 

 by the invader seems much more prevalent in plants than preformed or 

 passive resistance (Allen, 1959). 



