by secondary borers). Of these 12 "beetle" trees, all but the last three were also infected by limb rust. 

 No live trees were found that had been attacked by Dendroctonus . 



Rust fungi are primary pests, favored by vigorous host trees rather than weakened ones (1), but bark 

 beetles, at least in endemic conditions, are favored by weakened trees (6). It thus seems highly probable 

 that all or nearly all the endemic beetle population observed in the sample existed because of rust-weakened 

 or rust -killed pines. 



Although some pines with advanced rust infections can survive for many years , it was observed in the 

 Park that beetles tend to eliminate trees with limb rust even where infection is still in the "intermediate" 

 stage. For example, in the Yovimpa Pass area (not in the transect) only two of the many large trees were 

 rust-infected in 1965, both with "intermediate" infections. These were also the only two trees in the area 

 attacked by bark beetles (one in 1964 and one in 1965). 



DISCUSSION AND CONCLUSIONS 



It appears fairly clear that timber losses due to limb rust can be held to an acceptable minimum by 

 selective cutting and short cutting cycles where infection levels are normal. Cutting cycles up to 25 years 

 probably are short enough. This assumes that tree markers can and do spot rust infections at early stages 

 and that they eliminate the rusted trees wherever it is economic to do so. Cutting cycles shorter than 25 

 years would be needed where reduction of endemic bark beetle populations is critical. 



In some National Forests , rust trees are being eliminated by wise selective marking and cutting; in 

 others, the rust goes unrecognized. The Powell Ranger District of the Dixie National Forest provides the 

 only known example of careful cutting of rust trees over a long (20-year) period where rust has not yet been 

 reduced to an acceptable level. The problem is that there was too much rust at the outset. 



In some areas a shift is being made from individual tree selection to block cutting followed by planting. 

 If- -as seems likely- -plantations are as susceptible to limb rust damage as are natural stands, block sys- 

 tems will be in trouble where limb rust outbreaks occur. Whether block cutting could be continued without 

 losses to rust would depend upon the times of infection in relation to times of thinning and of final harvest. 



In stands managed to maintain or to regain the appearance of natural conditions rather than for timber, 

 limb rust continues to be a real problem. In such recreational and wilderness areas the knowledge that 

 good silvicultural practices can control limb rust damage is little comfort. These stands may appear 

 ragged even where less than 20 percent of trees are infected because it is the larger and more conspicuous 

 trees that are usually damaged (fig. 4). More serious than the ragged appearance --at least in Bryce Can- 

 yon National Park- -is the likelihood that a large part of the endemic bark beetle population exists in rusted 

 trees. Should conditions again favor a beetle outbreak, such as has occurred in the past at Bryce Canyon, 

 limb rust --by affecting the endemic population base- -will be partly responsible for the extent of damage. 

 Limb rust is undoubtedly one of the damaging agencies with which these pine stands have alwaj^ had to 

 contend; therefore elimination of the disease would be undesirable where one management objective is to 

 maintain the environment as a "museum" of the living forest. However, a new factor has been introduced 

 where heavy use by people precludes normal regeneration of pine. Trees killed by disease are not 

 replaced as they are in the pristine forest, and continued infection leads to unnatural depletion of the desired 

 stand. An even greater source of worry is that diseased, heavily trampled stands may be susceptible to 

 catastrophic outbreaks of bark beetles. 



We do not understand the patterns in time and space of limb rust outbreaks . There are analogies 

 with outbreaks of the better known comandra blister rust. For instance, at Bryce Canyon, present damage 

 appears to be the result of a wave or waves of infection three or more decades old that are not being re- 

 peated now. This situation parallels that of comandra rust in lodgepole pine further north (10). Outbreaks 

 of limb rust in southern Colorado that probably occurred in the nineteenth century (7) have not recurred to 

 a serious extent, just as the early comandra rust outbreaks in ponderosa pine in that area have not 

 recurred (14). 



8 



