PHYSIOLOGY AND BIOCHEMISTRY OF RESISTANCE TO PINE RUSTS 



Robert C. Hare 

 Southern Forest Experiment Station, U.S. Department of 

 Agriculture, Forest Service, Gulfport, Mississippi, U.S.A. 



ABSTRACT 



This paper reviews the literature on physiology of tree-rust 

 resistance, mostly that to Cronartiur. rusts on North American 

 pines. The principal resistance mechanisms against white 

 pine blister rust are hypersensitivity of foliage and stem 

 cells, and production of periderm barriers in the stem in 

 response to infection. After blister rust infection, phenolics 

 appear to increase more in resistant than in susceptible pines. 

 Resistance in southern pines to fusiform rust may be more 

 passive (preformed) ; hypersensitive reactions and periderm 

 barriers seem much less important with this disease than with 

 white pine blister rust. But resistance to fusiform rust is 

 physiological since hypodermic injections do not bypass resis- 

 tance mechanisms and resistant species show transient infection 

 symptoms. 



Although fusiform rust infection does not stimulate height 

 growth, it increases both auxin and gibberellin content as 

 well as IAA oxidase and PPO activity in pine seedlings. In a 

 resistant species, these effects are transient, coincident 

 with infection symptoms. Thus, resistance is not due to lack 

 of growth substances for gall development. A positive correla- 

 tion between resistance and PPO activity suggests a quinone 

 resistance mechanism. Evidence is presented for both phyto- 

 alexin and preformed fungitoxin production correlated with 

 fusiform rust resistance. Basidiospore germination and germ- 

 tube growth are frequently inhibited by diffusates and 

 extracts from uninfected resistant tissue and promoted by 

 similar preparations from susceptible tissue. 



Fusiform rust resistance in loblolly pine is apparently 

 associated with peculiarities in terpene composition and 

 electrophoretic protein patterns. Protein and morphological 

 similarities suggest introgression with shortleaf pine as a 

 source of loblolly resistance. Certain isozyme patterns of 

 a number of enzymes are associated with rust resistance and 

 may serve as markers in breeding for resistance. 



465 



