442 ROBERT F. PATTON 



variability of spore germination and the influence of subtle effects of 

 the microclimate at the site of basidiospore germination. The infection 

 capability of a spore may be determined by interaction of an inherited 

 tendency toward a specific type of germination with a given set of micro- 

 climatic influences on the host substrate. 



Infection in the needle results from the formation of infection 

 structures in the substomatal chamber. The production of infection 

 structures is influenced by fluctuating temperatures, a stimulus provided 

 by an artificial membrane, and a contact or other stimulus provided by 

 the guard cells of a stoma. These are factors which apparently also 

 influence infection of the pine needle. 



In evaluating results of natural and artificial inoculations on 

 clones and progenies, factors influencing host susceptibility, especially 

 age of stock, inherent susceptibility or resistance of the stock, vigor 

 of the trees, and maturation of tissues at infection courts are all of 

 importance . 



Both foliar resistance to initial infection and bark resistance to 

 invasion and establishment of the fungus are known, but these have not 

 yet been correlated with discrete resistance genes. 



Little detailed information is available on the nature and mechanisms 

 of resistance. Bark resistance is expressed by wound periderm formation 

 or a hypersensitivity reaction. In the needle, stomatal influences play 

 a role. Wax plugs appear to reduce the number of opportunities for 

 infection, especially of secondary needles, and may be one influence of 

 age but not a major mechanism of resistance. Inhibition of vesicle 

 formation, even after growth of a germ tube into the outer stomatal pit, 

 seems to be another aspect of stomatal influence on infection and resis- 

 tance . 



Inheritance of resistance seems to be largely through polygenes with 

 additive effects. Information is just beginning to accrue on the numbers 

 and kinds of genes associated with specific resistance reactions. 



The possibility of pathogenic races on pine, of which there is 

 still no evidence, emphasizes the importance of continued investigation 

 of the sexuality of C. ribicola. There are now some indications that 

 the fungus may be homothallic, but a more complete understanding of its 

 sexuality and variability in pathogenicity for pines is necessary to 

 assure continued success of our efforts to develop resistant planting 

 stock. 



LITERATURE CITED 



Anderson, R. L. , and D. W. French. 1955. Evidence of races of Cronartium 



ribicola on Ribes . Forest Sci. 1: 38-39. 

 Bega, R. V. 1959. The capacity and period of maximum production of 



sporidia in Cvonartiwn ribicola. Phytopathology 49: 54-57. 

 Bega, R. V. 1960. The effect of environment on germination of sporidia 



in Cronartium ribicola. Phytopathology 50: 61-69. 

 Bingham, R. T. 1966. Breeding blister rust resistant western white 



pine. III. Comparative performance of clonal and seedling lines 



from rust-free selections. Silvae Genet. 15: 160-164. 



