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FOCUS TOPIC: SOME MAJOR PATHOGENS OF TREES

Several fungi in the Basidiomycota and Ascomycota are major pathogens of trees. These fungi can cause serious economic losses in commercial forestry. Several of them also cause significant damage to natural and amenity woodlands, with huge impacts on the landscape and environment. In this and the following pages we focus on some of these major tree pathogens.

The diseases we will consider include:

• Heterobasidion annosum (Basidiomycota), the fungus that causes "Fomes root rot", or "Annosus root rot" in conifer forests.

• Armillariella (or Armillaria) mellea (Basidiomycota), one of the most aggressive tree pathogens worldwide.

• Sudden oak death caused by the fungus-like organism Phytophthora ramorum. This organism is related to the potato blight pathogen, Phytophthora infestans, which is notorious for having devastated the Irish potato crops in the period 1845 to 1847, when perhaps more than half a million people died of starvation and a similar number emigrated to the rest of Europe or the USA, to escape starvation.

• Dutch elm disease, caused by a member of the Ascomycota, Ophiostoma ulmi (and the related fungus O. novo-ulmi).

• Chestnut blight in Europe and North America, caused by a member of the Ascomycota, Cryphonectria parasitica.

To put these diseases into perspective, it will be helpful to note a few basic points.

1. Many of the diseases that sweep across continents and cause major damage are the result of organisms that have been introduced from other countries. For example, this is true of chestnut blight in North America (thought to have been introduced on ornamental chestnut trees in the New York Zoological Garden in 1904). Similarly, the first report of Dutch elm disease in the USA was in Ohio in 1930. Since that time, it has caused major damage to the native American elm, Ulmus americana.

2. Most healthy, vigorously-growing trees show a significant degree of resistance to fungal attack, but their resistance can be compromised by wounds that provide portals of entry for pathogens. Quite often these pathogens are host-restricted. For example, Heterobasidion annosum (perhaps the most serious pathogen of conifers in the Northern hemisphere) is mainly confined to pines, including Scots pine (Pinus sylvestris). In contrast, the "beef-steak polypore" Fistulina hepatica, is most commonly found on oaks, and to a lesser extent on chestnuts (Castanea spp.).

3. Several fungi that cause serious damage to trees can spread by root-to-root contact, when the major roots of adjacent trees fuse with one another. This can create an underground network for spread of pathogens. Additionally, some tree pathogens, such as Armillariella mellea, can grow extensively below ground by producing bootlace-like hyphal aggregates termed rhizomorphs, which can initiate attack of adjacent healthy trees.

THE PATHOGENS:

1. Phaeolus schweinitsii

Fig 1. A young fruitbody of Phaeolus schweinitsii, about 15 cm wide and 3 cm thick. The young fruitbody is covered with a soft downy coating and shows clear concentric zones. In Britain this fungus is considered to be one of the most important pathogens of conifers. The fruitbodies often arise from the major roots below ground - one of the first indications that the tree is infected. This specimen was photographed under a larch tree (Larix europea) about 30 meters high, in an ornamental parkland. Larch is a native of the mountains of Central Europe, but is known to have been introduced into Britain in the 16th Century.[© Jim Deacon]

Fig 2. Underside of the fruitbody shown in Figure 1, showing that the fruitbody tapers like a cone towards a thick base. The basidiospores are released from small pores on the underside. [© Jim Deacon]

Fig 3. As the fruitbody of P. schweinitsii matures it turns brown and dries, as shown in this image. [© Jim Deacon]

2. Fistulina hepatica, the "Beef-steak polypore"

The fruibodies of Fistulina hepatica are usually seen towards the base of oak or chestnut trees (Fig. 4). The appearance of these fruitbodies in late autumn or early winter can sometimes be the only sign of infection, but is indicative of internal decay that leads to rotting of the tree base - a condition termed butt rot, which can significantly weaken the base of the tree and cause it to break in high winds.

Fig 4. Fruitbodies of Fistulina hepatica growing from the base of a dead oak tree that was about 100 years old. [© Jim Deacon]

This fungus is a member of the Basidiomycota, and is highly distinctive because of its red-coloured, bracket-shaped fruitbodies that exude a blood-red liquid when damaged. This gives rise to the common name for this fungus - the Beef-steak polypore. In fact, almost every feature of these fruitbodies resembles raw meat, including the spongy-fibrous texture. The fruitbodies can be stewed and eaten, but the taste is disappointing!

Fig 5. Two basidiocarps of Fistulina hepatica (the "Beef-steak Fungus") growing from the base of a dead oak tree. The larger of the two fruitbodies (left) was 12 cm wide and 22 cm long, including its stalked base (see the images below). [Note:this image had to be taken by flash and does not capture the true colour, which can be seen in the images below] [© Jim Deacon]

Fig 6. A true-colour image. One of the basidiocarps of F. hepatica, showing the upper surface and the long stipe (stalk) that was 'rooted' in the decaying stem base. Total length 22 cm. [© Jim Deacon]

Fig 7. The same fruitbody seen from below, showing the yellow-coloured, spongy layer of tiny pores from which the basidiospores are released.
[© Jim Deacon]

Fistulina hepatica is a significant pathogen of oak trees and also of chestnut trees (Castanea sativa). It invades the tree and causes a progressive brown rot of the internal woody tissues (termed a butt rot). The photograph was taken in October, but a few weeks earlier the base of this tree was surrounded by a ring of toadstools of Armillaria mellea (CLICK HERE) earlier in the growing season.

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