Sometimes called “incense cedar broom rust”, this disease is also known as “Pacific coast pear rust” when the focus is on the alternate (aecial) host, especially commercial pear.
As described on the main Rusts page, most rusts are heteroecious, alternating between two unrelated hosts in completing their life cycle. This disease is no exception. Calocedrus decurrens (incense cedar) hosts infections on its scale-like leaves that can last for multiple years, producing telia and basidia.
The basidiospores cannot reinfect C. decurrens; they can only infect a number of deciduous flowering shrubs and trees in the family Rosaceae. These include species in the genera Amelanchier (serviceberry), Crataegus (hawthorn), Pyrus (pear), Malus (apple), Sorbus (mountain ash), Cydonia (quince), and Chaenomeles (“flowering quince”, native to Asia) . Aecia are produced there, usually on leaves, and aeciospores can infect only C. decurrens.
The two life stages were initially described as two different species. However, in 1909, Arthur  proved the connection by culturing from both Calocedrus and Crataegus. Later, by culturing and by inoculations from branches bearing telia, the connections were made to the aecial stage on Amelanchier , Malus, Pyrus, Cydonia, and Sorbus .
The pathogen is Gymnosporangium blasdaleanum (until recently called G. libocedri). A well-known relative is G. juniperi-virginianae, which causes cedar-apple rust. These fungi have all the spore stages of rusts except uredinia.
Mildly amusing but very esoteric nomenclatural trivia
For much of the 20th century, this fungus was known as Gymnosporangium blasdaleanum, then until 2011 it was G. libocedri, and now it is G. blasdaleanum again! Consider the following synonymy, essentially as presented by Frank Dunn Kern in 1911  (some synonyms left out for simplicity):
Gymnosporangium blasdaleanum (Dietel & Holw.) F. Kern, Bull. NY Bot. Gard. 7:437. 1911
- Aecidium blasdaleanum Dietel & Holw., Erythea 3:77. 1895.
- Phragmidium libocedri Henn., Hedw. 37:271. 1898.
- Gymnosporangium libocedri (Henn.) F. Kern, Bull. Torrey Club 35: 509. 1908
This stuff can be mind-boggling, so if you want to understand it, I’ll deconstruct it for you. Dietel and Holway in 1895 were the first to describe this fungus and named it Aecidium blasdaleanum. The genus is essentially a form-genus for cup-shaped aecia, considered the asexual or “imperfect” stage. They did not find and connect it with the telial stage on incense cedar.
A few years later, Hennings described the telial or sexual (“perfect”) stage on incense cedar, naming it Phragmidium libocedri (incense cedar was then in the genus Libocedrus). Kern in 1908 transferred it to the genus Gymnosporangium because that’s where it’s close relatives are.
In 1909 and later, the connection was made between the aecial stage on various hosts and the telial stage [2, 3, 6]. So when Kern wrote his monograph , he took the earliest name for the fungus, Aecidium blasdaleanum, and moved it into Gymnosporangium, making G. libocedri a later synonym. It makes sense that the earliest name has priority, right?
The very next year, the great and mighty German father-son powerhouse mycologists, Paul and Hans Sydow, published a monograph of the family . They rejected Kern’s combination. Shortly thereafter, Kern had the opportunity, which he apparently relished, to review their monograph in the journal Science . In his scathing review, he argued against the rejection. He surmised they had rejected it because it was based on the imperfect (aecial) stage, and argued that they didn’t follow that logic in many other cases.
However, there was an article in the Botanical Code of the time that gave favor to names based on the perfect stage, so the Sydows were right. By the time he revised his treatment of Gymnosporangium in 1973 (I know, what a long career!!), the botanical lawyers had apparently gotten to him, and he rejected G. blasdaleanum in favor of G. libocedri .
Fast-forward to 2011, when mycologists did away with the separate names and initiated the One Fungus, One Name policy. Part of this said that the earliest specific epithet, whether for the perfect or imperfect stage, has priority. So Kern sort of had it right the first time!
[Thanks to Dr. Konstanze Bensch, Westerdijk Fungal Biodiversity Institute, Utrecht, Netherlands, for helping me understand the current status.]
Moisture is needed in spring for telia to expand and ultimately for basidiospores to be formed, then again for basidiospores to infect Rosaceous hosts. It is needed later in the spring/summer for aeciospores to infect if they have returned to incense cedar. But since the fungus is perennial in incense cedar, it can survive many years that are unfavorable for alternating through the Rosaceous host.
Because the rust appears to be progressively less common toward the southern limit of C. decurrens , it is likely that climatic factors limit it there.
Aeciospores are windborne and infect the scale-like leaves of C. decurrens in late spring or summer. The next spring, telia form and mature into gelatinous lobes. Telia are largely composed of teliospores. Teliospores are not dispersed, but produce basidia that in turn produce basidiospores.
The infection overwinters and survives on incense cedar as long as the twig lives, producing telia each spring. If it penetrates to infect the twig itself, it may result in a witches’ broom, which can produce many telia [4, 6]. Infection of the main stem when young may result in formation of woody burls as the stem matures .
Basidiospores are windborne and can infect only the Rosaceous hosts. Usually they infect leaves, but may infect fruits and tender, current-year shoots. Infected tissue becomes more or less yellow-orange and may be quite distorted and swollen. Small spermogonia form, producing spermatia that can fertilize other spermogonia, forming a dikaryon. Very soon, aecia form densely on infected tissue. Aecia are about 0.2 – 0.3 mm in diameter. At first they are covered with a short peridium, but that soon deteriorates, leaving a white fringe on the cup-like aecium. Aeciospores are produced in chains in the aecium, completing the cycle.
Symptoms and Signs
On C. decurrens, infection results in small, brown to red-brown cushions on the leaves. These develop into telia in the spring. When wet, telia absorb water, swell, and become light orange, so signs of the rust are most obvious during moist periods in the spring. Symptoms include leaf discoloration; spindle-shaped, hypertrophic, woody swellings on twigs and branches; death of twigs and small branches; large woody burls; and witches’ brooms, which are tight, erect clusters of abnormal branching up to about 60 cm in diameter [4, 5, 12]. Large trunk swellings can develop over as much as 200 yr .
For those who use the name “incense cedar broom rust”, it is important to note that, as common as brooms may be, only a minute proportion of infections result in brooms . The great majority are leaf infections without fasciation or hypertrophy.
The sapwood and heartwood in the swellings, burls, and brooms are marked with numerous, dark brown flecks . In cross-section they have various shapes; in longitudinal section they appear as short streaks following the grain of the wood. Mycelium is abundant in these browned areas. Mycelium in swellings and burls remains alive as long as the sapwood lives, but it can’t reach the surface to form telia . However, the perennial mycelium in witches’ brooms does grow in the twigs and foliage and produce telia.
On the Rosaceous host, infected leaves, fruit, and shoots are yellow-orange, distorted and conspicuous. The tiny cupulate aecia can be seen with the naked eye, but a hand lens is helpful.
Distribution and Damage
Because the aecial hosts are common and widespread, distribution of the disease is limited by the distribution of Calocedrus decurrens, as well as by climate in the southern end of its distribution. The disease is common through most of the native range of incense cedar, from northern Oregon, on the eastern slope of the Cascade Mountains, south through California. In northern California it is the most common Gymnosporangium sp. However, it is quite uncommon south of Yosemite National Park and was not found in the isolated populations of incense cedar in northern Baja California . It may be found where C. decurrens is planted out of its native range, such as in Washington.
On C. decurrens, damage is generally minor. Numerous witches’ brooms and killing of twigs and small branches may result in deformed crowns and growth loss when infection is severe, especially on small trees , but trees are rarely killed by the disease . Hypertrophic burls on the stem reduce timber value . On the deciduous hosts, damage is often minor, but when severe, spotting and deformation of leaves, fruits, and tender shoots likely causes growth loss. The disease can cause losses in commercial pear orchards, where infected fruit become distorted and drop prematurely .
In commercial pear orchards, managers are advised to remove nearby incense cedar if possible and to use relatively resistant varieties when damage is frequent [1, 11]. Chemical protectants can be applied during the spring when telia are active on incense cedar . Some sources recommend pruning and removing infected tissue as well as fallen, infected pears, but since aeciospores cannot infect pear, that is not likely to accomplish much.
On Calocedrus decurrens, damage rarely warrants consideration of management, nor are there feasible options for doing so.
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