Figure 219. Branch flagging and top-kill are common signs of stem rust infection.

There are several species of stem rust fungi on conifers in the Southwest. The rust fungi are unique in many respects. Some species of rust fungi are autoecious, which means they complete their life cycle on one host. Other species are heteroecious, which means they complete their life cycles on two entirely unrelated hosts. The most important rust species of trees are those that invade the main stem and cause mortality. The most destructive stem rust listed below is white pine blister rust, a nonnative pathogen found in New Mexico.

There is one species of cone rust in the Southwest, and it infects Chihuahua pines in the southeast corner of Arizona.