Fruiting structureSpore type
  • Classification: phylum Basidiomycota, class Pucciniomycetes, order Pucciniales.
  • Rusts are obligate parasites and biotrophs (i.e., in nature they require living hosts to grow).
  • They are often host specific, but many tree rusts just go to a genus or part of one.
  • They may use only one host during their life cycle or may alternate between two unrelated hosts in completing life cycle (which is odd considering they are so specific!).
  • Basidia have 4 cells, but some don’t seem to produce basidia and are apparently asexual.
  • Rusts have up to 5 spore stages. Students love to learn this!

The most apparent spore stage for many rusts is the uredinium. They are usually numerous rusty-orange pustules. Makes a good memory device (red uredinium).

There are about 4000 species, far fewer on trees.

White Pine Blister Rust (WPBR)

White pine blister rust is one of the most important forest diseases and certainly deserves its own page!

Other Cronartium rusts

Like WPBR, other Cronartium rusts are often called blister rusts because of the prominent aecia. They infect limbs and stems of pines, and a few are specialized as cone rusts, only occurring on cones.

Here are a few other members of this group:

Western gall rust

Also called pine-pine gall rust. Although some people have claimed they have found some, there is probably no alternate host for this one. The aeciospores clearly can reinfect pine. There are no telia or basidia. Because of rusts like this and rusts for which full life cycle is not known, we actually had names for “imperfect” stages of rusts. The aecia of Cronartium species were placed in the form-genus Peridermium, and this one was called P. harknessii. Of course now, with one fungus, one name, and getting rid of form-genera for asexual states, Peridermium is deprecated, and Cronartium will be the name for the group, whether they have the sexual stage or not.

The disease is characterized by globose galls on hard pines. Sometimes when a branch infection gets into a stem that is too large to be girdled, a canker will form with swollen sides; this is called a hip canker.

This disease can really become abundant in some stands. Probably because it can easily go from pine-pine. Let’s hope other rusts don’t figure out how to do that! Chains of galls may be seen because of local source of inoculum. There are wave years of infection, due to suitable weather at the right time. If one or more wave years occur in a stand of seedlings or saplings, it can be decimated. And despite its name, it does occur in eastern North America too.

Eastern gall rust

Identify and Control Stem Rusts of Jack Pine

The species name here is helpful, Cronartium quercuum, indicating that the alternate hosts are oaks. Damage to oaks by the pine-oak rusts is usually slight.

This disease is actually caused by three different races, or special forms, of C. quercuum. Special form in latin is forma specialis. One of the pathogens, for instance, is C. quercuum f. sp. banksianae. Galls are a lot like those of western, but no pine-pine infection.

Fusiform rust

This may be the most important disease in southeastern forestry. Before 1930, fusiform rust was one of those rusts noone ever heard of. It now has tremendous impact, and can devastate a plantation. The pathogen is C. quercuum f. sp. fusiforme. Note it’s in same species as eastern gall rust, but a different special form.

This is a native disease, not introduced. Why then is it so devastating? Aren’t these obligate parasites supposed to work out a balance where they don’t cause too much damage to their host?

Well, this is a case where they did have it worked out, and then we upset the apple cart:

  1. Fire suppression increased the abundance of oaks, the alternate hosts. Oaks are more sensitive to fire than pines. Apparently the disease was only common naturally in swampy areas, which never burned and in which oaks were more dense.
  2. Slash and loblolly pine have been planted in areas where they didn’t grow naturally but oaks did in abundance. These are areas where longleaf pine was natural. Not surprisingly, longleaf pine is more resistant.
  3. Intensive management maximizes growth rate, which makes a better host for an obligate parasite. Studies have shown that cultivation and fertilization clearly increase rust infection.
  4. Selection of superior trees was first done without regard to rust resistance.
  5. The first seeds of loblolly and slash collected often came from diseased trees, as they had the most cones (stress crop).
  6. Nurseries concentrated highly susceptible seedlings, then disease was shipped all over in the seedlings.

For management, serious breeding for resistance has been a major effort. Control in nursery with fungicides, reduction of oaks, and other approaches are also being used.

An interesting thing about this rust is that telia are produced on oak leaves in Spring not long after infection. They produce basidiospores and go back to pine before June. So the uredinia don’t seem to play a role in life cycle. There is no significant buildup on oak before telia and uredinia are produced. Uredinia may be a vestigial stage in this rust.

Cedar-apple rust

This is different from the others. There are probably 20 or so species of Gymnosporangium in North America. They mostly follow this cycle, though there are a few oddballs:

Look over the disease cycle. What stage is missing in this rust?

It is perennial on juniper. Telia appear during in spring as striking, bright orange horns (see featured image above). They dry and shrink during dry weather and can swell and produce more basidia during wet weather.

In this disease, telia occur on the conifer host and in the spring, very different from white pine blister rust and other Cronartium species), where the conifer bears aecia and telia appear in late summer and fall.

There are a dozen or more species of Gymnosporangium in North America. In the East, G. globosum commonly alternates between prostrate juniper and hawthorn. If you find one of these diseases on both hosts it is quite interesting to follow it through the summer and see all the stages.

Other rusts

Pine needle rust and other Coleosporium species infect pine needles, and each has different alternate hosts. The aecia are pretty neat to see sticking out of the pine needles.

There are various other genera of rusts (Melampsora, Chrysomyxa, Pucciniastrum, etc.) that infect trees, mostly on foliage, sometimes including twigs. On both hardwoods and conifers, some even alternate between foliage of the two.

Rust Genera

Essential facts of the major rust genera on trees.
GenusCommon NamesAecial HostTelial HostsComments
Cronartiumblister rusts; stem, limb and cone rustsPinus spp.various dicotyledons
Peridermiumdeprecated genus for asexual rusts related to Cronartium or asexual stages of Cronartium spp. (see One Fungus, One Name)
Gymnosporangiumcedar-apple rustsapple tribe of Rosaceae (usually)Cupressaceae, mainly Juniperus spp.usually no uredinial stage
Coleosporiumpine needle rustsPinus spp.various herbaceous plants
MelampsoraPseudotsuga, Larix, Tsuga spp., otherspoplars, willowsvery diverse genus, infect foliage and sometimes shoots
Chrysomyxa, Melampsorellabroom rustsAbies, Picea spp.various dicotyledons

Man oh Man – I don’t think I’ll ever get this rust life cycle down!!

Rust Diseases

Some important/interesting rust diseases of trees in North America
DiseasePathogenAecial HostTelial HostsComments
white pine blister rustCronartium ribicola5-needle Pinus spp.primarily Ribes spp, some othersOne of the most studied forest diseases
western gall rustCronartium harknessii2/3-needle Pinus spp.NonePine-to-pine. Globose galls when fruiting make decimated plantations like “orange groves.”
eastern gall rustCronartium quercuum f. sp. banksianae (and other f. sp.)jack, shortleaf, Virginia and other pinesMany oaks (dwarf chinkapin, bur, chestnut, pin, red)Globose galls
fusiform rustCronartium quercuum f. sp. fusiformeloblolly, slash, and other pinesMany oaks (water, willow, laurel, etc.Spindle-shaped (“fusiform”) galls. YouTube video
comandra rustCronartium comandraelodgepole, ponderosa, other 2/3- needle pinescomandra (herbs)Canker L/W ratio only 2-3
stalactiform rustCronartium coleosporoideslodgepole, other 2/3- needle pinesScrophulariaceae e.g. Indian paint- brush (West), cow wheat (East)Cankers long, L/W ratio > 3. In eastern Canada and West as far south as southern California
cedar-apple rustGymnosporangium juniperi-virginianaeapplejunipers, especially eastern red cedarNo uredinial stage. Unusual: conifer is the telial host
horny rust of serviceberryGymnosporangium inconspicuumAmelanchier sppJuniperus section Sabinahorny rust on serviceberry fruit
Long, stiff horns on serviceberry fruit. "Inconspicuous" only on juniper!
pine needle rustColeosporium solidaginis2/3-needle pines (red, jack, Scots)goldenrod or asterOverwinters in needles, where it can survive 2-3 years
leaf rust of poplar and larchMelampsora medusaelarchespoplarsLittle damage to larch; Most serious on hybrid poplars. Telia winter on poplar leaves.
spruce broom rustChrysomyxa arctostaphyliPicea spp. (spruce)Arctostaphylos spp. (bearberry)Dense brooms become large, reduce growth, cause tops to die.
fir broom rustMelampsorella caryophyllacearumAbies spp. (fir)Stellaria, Cerastium, Arenaria spp. (chickweed, starwort, sandwort)Can grow into stem and cause swelling and canker.