Limb Rust Damage to Pine 



INTRODUCTION 



Limb rust is a pine -killing disease in the western United States and southward to Guatemala. It was 

 first recognized as a serious problem in 1913, when Hedgcock (8)^ reported it to be the most destructive 

 rust of western pines. In view of the later discovery of white pine blister rust in the West, Hedgcock's 

 statement cannot now stand. However, limb rust remains the most destructive rust attacking ponderosa 

 and Jeffrey pines throughout most of their geographic distributions. These pine species are extremely 

 important for timber, watershed, and aesthetic values. 



Mielke in 1952 reviewed and greatly extended our knowledge of limb rust of ponderosa pine (11). The 

 present paper is a further review and extension of knowledge, presenting additional host, geographic, and 

 biological data based on several years' observations. Included, too, are results from a sampling in 1965 

 of Umb rust in Bryce Canyon National Park, and comparisons with data taken in 1935 from the same 

 transect . 



EFFECTS ON PINES 



Unlike other pine rusts, limb rust is a systemic disease: infection can spread throughout a tree 

 rather than being restricted to a canker or gall. The principal symptom, at least for old infections, is the 

 pattern of branch death (fig. 1). Surrounding a zone of dead branches that have lost their needles and twigs 

 are branches more recently killed that bear red-brown or gray needles. Live branches bearing spore sacs 

 of one of the causal fungi (fig. 2) can usually be found near the zone (or zones) of recently killed limbs. 



There is only one final effect of limb rust on pine: death of infected trees. No way is known to stop 

 the killing process once the fungus grows into the bole from the branch originally infected (16). Death 

 appears to result from loss of photosynthetic tissue and from energy demands of the fungus. Mielke (11) 

 estimated that death comes after loss of about 80 to 90 percent of the crown, measured linearly along the 

 bole (fig. 3). This loss is very slow because Linear spread proceeds at the slow rate of about 1.5 feet per 

 year, or half that in each direction. 



Many trees die from secondary effects before 80 percent of the twigs are killed by the fungus . The 

 principal secondary effect is loss of rust -weakened pines to attack by bark beetles (3, 19). 



Height and diameter growth rates of infected trees become negligible after about 50 percent of their 

 crowns are dead (11). Thus, appreciable wood is lost long before the rust kills trees. 



PINE SPECIES ATTACKED 



Unlike the gall, canker, and cone rusts of hard pines, each of which attacks pine species in several 

 taxonomic groups, Umb rust appears to be restricted to species that are usually placed in a single group- - 

 Ponderosae . The known hosts are ponderosa pine (pinus ponderosa) including its southern variety, Arizona 

 pine (P. ponderosa var. arizonica ), Jeffrey pine (P. jeffreyi). Apache pine (P. engelmannii ), and ocote 

 bianco or pino de montezuma (P. montezumae). There are also unconfirmed reports from Mexico of 

 infection in other species of the group; for instance, pino amarillo (P. cooperi) in the State of Durango. 



Underlined numbers in parentheses refer to Literature Cited, pp. 9-10. 



