Western gall rust, also known as pine-pine gall rust, is a fungal disease of pine trees. It is caused by Cronartium harknessii (formerly known as Endocronartium harknessii or Peridermium harknessii (describing the aecial phase under the now-superseded system of dual nomenclature), an autoecious, endocyclic, rust fungus that grows in the vascular cambium of the host. The disease is found on pine trees (Pinus spp.) with two or three needles, such as ponderosa pine, jack pine and scots pine. It is very similar to pine-oak gall rust, but its second host is another Pinus species. The fungal infection results in gall formation on branches or trunks of infected hosts. Gall formation is typically not detrimental to old trees, but has been known to kill younger, less stable saplings. Galls can vary from small growths on branch extremities to grapefruit-sized galls on trunks.
Hosts and symptoms The hosts of the aecial stage of the fungus includes two and three needled Pinus spp. The most important aecial hosts include jack pine (P. banksiana), lodgepole pine (P. contorta), western yellow pine (P. ponderosa), and the European Scots pine (P. sylvestris). A variety of other pines, such as Pinus nigra, P. mugo, P. palustris, P. banksiana, P. muricata, and P. radiata have also been reported as hosts to western gall rust (C. harknessii), but these pine species are considered less valuable. Because C. harknessii is an autoecious short-cycle rust lacking telial a host, there is no alternative hosts. The pathogen can infect actively growing shoots of any age very quickly without infecting an alternative host, making the disease cycle more destructive that typical rust species that switch between hosts. This also prevents control of the fungus by management of the alternative host species. Symptoms of western gall rust can be quite conspicuous and are useful for diagnostic identification. The most prominent symptoms are hip cankers and swollen, spherical to oblong growths (galls) on the branches, stems, or main trunk of the host. The galls on small twigs of 1–2 years of age are often pear-shaped. Gall formation results from the overproduction of xylem tissue stimulated by the fungus. Witches brooming occasionally occurs along with galling. The bark on large galls will slough off over time, exposing the smooth wood beneath. Signs of western gall rust include the pale yellow aecia (1–8 mm in diameter) formed on galls in spring as well as the yellow-orange aeciospores contained within. Immature galls are spindle-shaped. Infections usually occur on more succulent (thick) branches. When mature, galls can grow as large as a softball (30.5 cm circumference), but most are the size of a golf ball (4.2 cm). Before the production of spores, the needles on highly infected branches become chlorotic or red, eventually turning brown when the branch dies. Western gall rust can cause dwarfing to occur if leader stem of younger pines are infected. Early identification of the disease is important for treatment, but is often hard to diagnose due to the inconspicuous initial symptoms and a lack of galls until the second year. A red pigmentation may develop on the epidermal galls of P. contorta seedlings 14–28 days after infection, but this is not always a reliable indicator.
Disease cycle Western gall rust has characteristically brown to yellow-orange sori visible on large globular galls on pines. Gall formation on trunks occurs over 2–4 years and is stimulated by the pathogen, which causes cells to grow and divide quickly at the site of initial infection. When mature, the gall splits open and the yellow-orange aeciospores are dispersed and carried to new hosts by wind. Because P. harknessii does not require an alternate host, the aeciospores can infect another pine directly. This typically occurs during moist conditions and rarely infects older shoots. Infection continues on the host shoots and needles until they have reached 90% of their elongation. Gall mortality is associated with squirrel feeding or invasion and inactivation by secondary organisms. The life cycle of C. harkknessii is different from other pine stem rust in that it is autoecious, making large stands of pines ideal hosts for survival and reproduction of the fungus. Wind carries teliospores to pines shoots that then germinate under cool humid conditions, producing germ tubes with up to three side branches that act as basidia. The basidium directly penetrate the cuticle and epidermis. Other pines rusts like Cronartium ribicola and C. comandrae infect through the stomata. After penetration and establishment of a intracellular infection structure, primary hyphae are produced, infecting the epidermis and cortex intercellularly. Haustoria extend through neighboring cells and the cortex to reach the vascular cambium before the host becomes dormant follow the first infection season. The cambium is invade inwardly through the phloem and cortex, as opposed to a vertical or peripheral hyphael growth. Initiation of gall formation is through exogenous stimulation of the cambium and pith rays, causing an increased production of ray parenchyma. The host reacts by hyperplasia (increased division) providing the resources needed for further hyphae proliferation in the cortex, phloem, and cambium until the galls death. The gall will enlarge for 2 years and sporulate on the third. Spermogonia ooze from infected bark in early spring, but they are non-functional since they do not form aecia (vestigial). Dikaryotization of the haploid mycelium that produce the teliospores takes place in the outer cortex, just beneath the first periderm. They are surrounded by a membrane called a peridium that bursts, releasing the spores. Stimulation of dikaryotization is not understood, though it is likely a combination of host sap flow and environmental cues.
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