Forest and Shade Tree Pathology


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Classification: Basidiomycota, Heterobasidiomycetes, Uredinales.

Rusts are obligate parasites and biotrophs.

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.

Spore stages
Fruiting structure Spore type
spermogonium spermatia
aecium aeciospores
uredinium urediniospores
telium teliospores
basidium basidiospores

Most rusts have up to 5 spore stages. The most apparent spore stage for many rusts is the uredinium. The group Uredinales is named after this. They are usually numerous rusty-orange pustules. Makes a good memory device (red uredinium, Uredinales).

There are about 4000 species, far fewer on trees.

White Pine Blister Rust (WPBR)

We will focus on white pine blister rust first. This is among the most famous of forest diseases. A student of forest pathology should be intimately familiar with the life cycle and disease cycle.

WPBR causes fairly elongate, perennial cankers on stems and branches of 5-needle pines. Some swelling can take place. It is the only stem rust of white pine in North America.

Introduced on white pine seedlings from France into eastern North America around 1900 and Vancouver 1910, it has been invading white pine areas ever since, and is still making progress into the Southwest and into southern California.

Here is how the disease cycle developed, starting with those seedlings. They were already infected back in Europe, but there were probably no symptoms. The fungus would have been in the bark of the stem or branch. Eventually the infected area would be discolored and somewhat swollen.

In late summer or early fall, spermogonia appear near the margins of the canker. They produce spores called spermatia. These are like the spores produced by some of the foliage ascomycetes that you have probably learned about - they don't infect, they are only involved in fertilization. Spermatia are produced in tiny drops of sweet liquid which attract insects. The insects carry the spermatia to other spermogonia. Spermatia fuse with receptive hyphae on the spermogonia. They establish the dikaryon (n+n).

Now the fungus is ready to roll. The following spring, aecia appear about where the spermogonia were the year before. The aecia look like swollen white to yellow blisters surrounded by a membrane when young. The bright yellow aeciospores are produced within, eventually pop the blister, and are released into the air.

The aecia break down and the bark dies. The fungus meanwhile has spread into living bark around the margin. It can't survive on dead bark. Every year more aecia are produced in spring, then spermogonia farther out in late summer. It keeps spreading until the branch or stem is girdled and killed. Airborne dispersal of aeciospores is long-range, up to hundreds of miles. They may live for months under good conditions. Now comes the amazing part - these spores cannot infect pines, or anything else except Ribes. Ribes (gooseberry) is a genus of shrubs fairly common throughout the white pine areas. The new leaves are infected in spring.

Now mycelium grows in the Ribes leaves. Yellowish leafspots may appear, but they are often not conspicuous. Within 2 weeks, urediniospores are produced in uredinia on the undersides of leaves. They are orange.

The urediniospores are called the "repeating stage" because they reinfect Ribes, leading to production of more uredinia, in a cycle that could go on until the summer ends. Thus infection builds up on the Ribes. The urediniospores cannot infect pine.

In late summer/early fall, telia begin to be produced instead of uredinia. They are like orange-brown rough hairs, also on the underside. They are composed of rows of brick-like teliospores.

When conditions are cool and wet, each teliospore germinates in place to produce a basidium, which produces - can you guess? - basidiospores! The basidiospores are dispersed by air. They are small and quite short-lived, depending on conditions. It is generally considered that a mile is about the upper limit for basidiospore dispersal.

Basidiospores can only infect pine. They enter through stomata into the needles. Within a month or so a small yellowish spot can be seen where infection occurred. The fungus grows down the needle into the bark, and the cycle is complete. Here is a summary:

Life cycle of white pine blister rust.
Symptoms from a distance: chlorotic, stunted, or dead branches (flagging), dead tops. Up close: cankers, moderately swollen, with yellowish margin.

Some miscellaneous things to note:

Introduced pathogen.

This is a premier example. You may already have learned learned another one, a root disease: Port Orford cedar root disease caused by Phytophthora lateralis. You understand that pathogens evolve with their hosts to strike some kind of balance, especially obligate parasites. There is clearly selection for ability to infect and cause disease. But if they kill too many hosts, they are biting the hand that feeds them. So in natural systems these fungi tend not to wipe out their host from the stand.

When you bring a pathogen in from Asia to North America, the trees are similar enough to Asian white pines to become infected, but there is no sophisticated genetic balance. The host has evolved no good mechanisms of resistance because there was never selection for them. So the disease may be more serious than similar, native diseases. That is an important concept.

A few more notes on white pine blister rust:

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 have names for "imperfect" stages of rusts. The aecia of Cronartium species are called Peridermium, so this one is called P. harknessii. There is still some argument about details of nuclear behavior in this fungus. If you believe there is some weird stuff going on with the life cycle, you would use the name Endocronartium harknessii. I don't believe it, so we'll use the other name.

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. And despite its name, it does occur in the East too.

Eastern gall rust

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. So no chains of galls would be expected.

How-to logoIdentify and Control Stem Rusts of Jack Pine

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. 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. 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 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 concentrate 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, reduction of oaks, and other approaches are all being used.

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. Probably a vestigial stage in this rust.

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The Peridium, a site maintained by Brian Geils of the U.S. Forest Service, has information on Cronartium rusts in the Rocky Mountains.

Cedar-apple rust

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

Life cycle of Gymnosporangium species.
What stage is missing in this rust?

It is perennial on juniper.

In this disease, conifer host is reversed, and seasons are reversed (relative to WPBR).

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 disease 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.

Worried man. Man oh Man - I don't think I'll ever get this rust life cycle down!!

Some important genera
Genus Common Name Aecial Hosts Telial Hosts Comments
Cronartium blister rusts; stem, limb and cone rusts pines various dicotyledonous families  
Peridermium asexual rusts related to Cronartium or asexual stages of Cronartium spp.
Gymnosporangium apple tribe of Rosaceae (usually) Cupressaceae, mainly junipers usually no uredinial stage
Coleosporium pine needle rusts pines various herbaceous plants
Melampsora Douglas-fir, larches, hemlocks, others poplars, willows very diverse genus, infect foliage and sometimes shoots

Some important rust diseases
Pathogen Aecial Host Telial Host Comments
white pine blister rust Cronartium ribicola 5-needle pines Ribes spp. What can I say?
western gall rust, pine-pine gall rust Peridermium (or Endocronartium) harknessii 2/3-needle pines probably none Globose galls, spermogonia uncommon. Decimated plantations like "orange groves."
eastern gall rust, pine-oak gall rust Cronartium quercuum f. sp. banksianae (and other f. sp.) jack, shortleaf, Virginia and other pines Many oaks (dwarf chinkapin, bur, chestnut, pin, red) Galls like those of western gall rust
fusiform rust Cronartium quercuum f. sp. fusiforme loblolly, slash, and other pines Many oaks (water, willow, laurel, etc. Spindle shaped ("fusiform") galls
comandra rust Cronartium comandrae lodgepole, ponderosa, other 2/3- needle pines comandra (herbs) In east but not common. Canker l/w only 2-3FIDL icon
stalactiform rust Cronartium coleosporioides lodgepole, other 2/3- needle pines Scrophulariaceae e.g. Indian paint- brush (West), cow wheat (East) Cankers long, l/w > 3. In eastern Canada and West as far south as southern California
cedar-apple rust Gymnosporangium juniperi-virginianae apple junipers, especially eastern red cedar No uredinial stage. This one is unique - conifer is the telial host
pine needle rust Coleosporium solidaginis 2/3-needle pines (red, jack, Scots) goldenrod or aster Overwinters in needles, where it can survive 2-3 years
leaf rust of poplar and larch Melampsora medusae larches poplars Most serious on hybrid poplars; little damage to larch. Telia winter on poplar leaves.

Funny picture ← Student after learning too much forest pathology. This could happen to you.

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Last modified 27 May, 2007