466 ROBERT C. HARE 



Although Cronartium, Peridermium, and Melampsora fungi cause rusts 

 on numerous tree hosts, by far the greatest economic loss is to Cronartium 

 rusts on pine. Most of the literature reviewed here concerns the two 

 most important, white pine blister rust, caused by C. ribicola J. C. 

 Fisch. ex Rabenh., and southern fusiform rust, caused by C. fusiforme 

 Hedge. § Hunt ex Cumm. Included in this review is a summary of my work 

 with C. fusiforme at Gulfport, Mississippi, and the University of 

 Florida. The primary goal of my research was to identify resistance 

 mechanisms that might be exploited to develop resistant trees. 



GENERAL PHYSIOLOGICAL MECHANISMS OF RESISTANCE 



HYPERSENSITIVITY 



Although morphological barriers may also be significant, foliar 

 resistance to tree rusts appears to be primarily physiological, like 

 most resistance mechanisms in plants (Rubin and Artsikovskaya, 1963). 

 Fusiform rust seems able to infect pines only through cotyledons, primary 

 needles, and stems of seedlings, and through the succulent stems of older 

 plants. Hardened stem tissue and even young secondary needles seem 

 immune (Jewell, 1960). When he injected basidiospores into slash pines 

 (Pinus elliottii Engelm. var. elliottii) , Powers (1968) found that infec- 

 tions too far out on the cotyledon did not reach the stem before the 

 cotyledon was shed. He suggested that secondary needles may be suscep- 

 tible, but that the hard needle-bundle sheath prevents close infection 

 and that rapid growth of young infected needles may not allow the mycelium 

 to reach the stem. I have failed to produce infection symptoms by 

 injecting basidiospores close to the base of very young and older secon- 

 dary slash pine needles or into hardened stems. Injection of the same 

 material into susceptible tissue caused galling in nearly 100 percent of 

 the trials (Hare, 1970). I have obtained similar negative results with 

 shortleaf pine (P. echinata Mill.) seedlings. Therefore, resistance in 

 secondary needles and hardened stems of susceptible pines, and in seedlings 

 of a resistant species, involves physiological or biochemical factors in 

 addition to possible external morphological barriers to penetration. 



In resistant shortleaf pine, injection (Hare, 1970) or external 

 inoculation (Jewell, 1966)of seedlings frequently induces needle 

 symptoms and even temporary stem swelling, but a sporulating gall is not 

 produced with C. fusiforme. Presumably, the incipient infection is 

 repulsed by some physiological or biochemical means. 



Although hypodermic injection does not usually bypass resistance 

 mechanisms, there may be exceptions. Western sources of loblolly pine 

 (P. taeda L.), which are more resistant than eastern sources in the field 

 (Wells and Wakeley, 1966), all produced galls when injected in the 

 epicotyl (Hare, 1970). Galls frequently showed up later and developed 

 slower in western than in eastern sources, however. When outplanted, 

 more of the inoculated western sources were severely stunted and killed 

 during the first year in what may have been a massive hypersensitive 

 reaction to fungus injection. Water-injected controls grew normally. 



t 



According to Pierson and Buchanan (1938), current-season needles of 

 western white pine (P. montioota Dougl.) are more resistant to blister 

 rust than older needles. With eastern white pine (P. strobus L.), 

 seedling resistance increases with age (Patton, 1961; Van Arsdel, 1968) 

 and secondary needles are susceptible. Resistance of Scots pine (P. 



