Grosmannia clavigera is a widespread pathogen of North American pine trees and a common (although not obligate) symbiont of bark beetles (family: scolytidae) of the genus Dendroctonus. G. clavigera is known as a bluestain fungus because of the characteristic blue-gray stain that it gives to infected wood. Over the course of an infection, G. clavigera spreads throughout the wood of the host tree. If left unchecked, G. clavigera infections will eventually disrupt the transport of nutrients and water which will result in the death of the host tree. G. clavigera can also prevent the flow of sap, which drastically lowers the host tree’s resistance to infestation from bark beetles, who in turn, carry spores and help propagate the fungus.
G. clavigera is an ascomycete and as such, is characterized by having ascii that produce and discharge a special type of spore known as an ascospore. The assimilative phase of G. clavigera is spent in the phloem and sapwood of a host pine tree is and characterized by a large, blue mycellium. The cleistothecia are small (5.5-9.0 microns in diameter) but are grouped together in circular arrangements on sapwood with a diameter of 250-640 microns. The cleistothecia are globose, with a black and leathery exterior. Spore discharge occurs through an irregular rupture in the cliestothecium wall. After discharge, the cleistothecia are concave and saucer like. Each ascus produces eight ascospores, which are unicellular and semicircular in appearance. Since the primary mode of dispersal for G. clavigera is through spore adhesion to beetle symbionts, the ascospores are very sticky. This “stickyness” might also contribute to the fungus’ ability to stop nutrient transport in host trees by “plugging up” critical transport structures.
As a Pathogen
G. clavigera is an important pathogen of North American pine trees, specifically lodgepole pine (Pinus contorta), whitebark pine (P. albicaulis), ponderosa pine (P. ponderosa), western white pine (P. monticola), and limber pine (P. flexilis). A tree infected with G. clavigera shows relatively few signs or symptoms, although it is sometimes possible to notice large, blue-gray lesions underneath bark. The only sure-fire way to accurately identify whether or not a tree has been infected by a bluestain fungus is to examine the phloem and sapwood, which is typically only possible after the tree has already died.
Although G. clavigera can infect a wide variety of hosts, it is especially virulent in lodgepole pine. It is particularly lethal to seedlings although it has been known to kill mature lodgepole pines in less than a year after inoculation. The mechanism by which G. clavigera kills trees is thought to be a combination of its ability to prevent both nutrient transport in the phloem and the production of sap in the sapwood. The first mechanism is straight-forward in its lethality: if the tree can’t distribute nutrients, it will die. The latter mechanism, however, is usually only fatal when the fungal infection occurs simultaneously with a bark beetle infestation.
Another factor which adds to Clavigera’s virulence is the fact that it seems to be adept at colonizing otherwise healthy trees. This is in stark contrast with similar, closely-related fungi which are only found in trees that are already unhealthy.
As a Symbiont
G. clavigera can participate in several symbiotic relationships that aren’t directly pathogenic. The most striking relationship that exists—the one that is responsible for the great success of __G. clavigera in North America—is between G. clavigera and the bark beetle, Dendroctonus ponderosae. Since the bark beetles are interested in colonizing and laying their eggs in the same trees that G. clavigera is, these two organisms have developed a mutualistic relationship where both organisms receive some sort of benefit. The fungus receives a benefit from the bark beetles in that its spores can be carried safely and reliably between trees by bark beetles. Also, once again, since the symbiotic relationship between G. clavigera and the bark beetles is mutualistic, there are several advantages that the fungus confers to its bark beetle symbionts. As has been mentioned earlier, G. clavigera can help to exhaust the defenses of the host trees, which makes it much easier for bark beetle infestations to take hold. In addition to this, however, it has also been shown that G. clavigera can be eaten by the bark beetles and contains essential nutrients which increases the brood size and viability of the bark beetles. In an interesting complication, it has also been shown that G. clavigera shares an additional beneficial relationship with symbiotic bacteria that live on the bark beetles. In the presence of these symbiotic bacteria, spore production was greatly stimulated in G. clavigera.
These relationships are important because they are apparently a contributing factor to the virulence of G. clavigera and the damage potential of D. ponderosae. When either the bark beetles or fungus colonizes a tree on its own, the host tree has a reasonable chance of survival. When G. clavigera and D. ponderosae colonize a tree at the same time, however, it is almost assured that the tree will die.
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