| Behind the Blight |
The Story of Cryphonectria parasitica-The chestnut blight fungus
Though the story of the American chestnut is well known, the blight itself is more of a mystery. Below, the nature of the chestnut blight fungus, Cryphonectria parasitica, is explored in depth, with help from Mark Double, of West Virginia University.
"It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles; if you do not know your enemies but do know yourself, you will win one and lose one; if you do not know your enemies nor yourself, you will be imperiled in every single battle."
-Sun Tzu, The Art of War
One of the main difficulties in understanding chestnut blight is visualizing the organism itself.The fungal strands, called hyphae, grow in the vascular cambium of the tree, killing tissues as they grow. While microscopic, the hyphae have a noticeable impact on the tree. Symptoms of infection by the chestnut blight fungus are generally flagging or yellowing of branches.
The hyphae of the chestnut blight fungus grow together in the tree to form a mass; called mycelium. The mycelium of the fungus can grow in fan-like waves under the bark (see photo left). Mycelial fans produce organic acids that kill tissues in the cambium of the tree. Cambial tissue is the living part of the trunk, and it is a highway where sugars travel from the leaves to the roots. (think maple syrup.)the blight fungus is feeding on the tree's photosynthetic food products, mainly glucose. in other words, it has an insatiable sweet tooth.
There are many more varieties of the blight in Asia, so scientists attribute its origin to that region of the world. . It probably developed as a parasite in the cambium of the Oriental (Chinese or Japanese) chestnut. Because the fungus and the Oriental chestnuts evolved together, they are equally matched. The Oriental chestnuts have developed resistance to the fungus, but the fungus is able to survive in the tree's bark so that it acts as a carrier.
The structures normally associated with fungi, such as toadstools and mushrooms, are the reproductive and not the vegetative structures of the organism. Their function is to disperse spores. In some fungi, this is quite spectacular. For example, the puffball mushroom explodes, catapulting thousands of spores into the wind with an audible pop.
Cryphonectria doesn't make button caps, but its reproductive structures are familiar to anyone who studies chestnuts. They are remarkably complex, and come in two basic styles.
First, there are asexual spores, called conidia. They are sometimes extruded in a sticky matrix called cirrhi when humidity levels are high. Cirrhi contain hundreds of thousands of conidia that can be picked up by animals. They may stick to birds' feet, or be carried by, insects and mammals. They are also washed down the tree by rain and can start new infections on the tree. Because they are asexual, they don't need to combine with another spore. They can land in any open wound and begin growing.
Cryphonectria also makes sexual spores, or ascospores. Below is a photo to compare the two. The larger two-celled ascospores are sexual spores and the smaller, one-celled conidia, are asexual spores. Ascospores are produced when an asexual conidium lands on the surface of a canker and enters a structure of the fungus called a trichogyne, a receptive filament of the female organ (think pollen grains and pistils in flowers). The conidium follows the trichogyne filament and fertilization is complete--a diploid cell is produced. This fertilization produces a perithecium, a pear-shaped structure in which the ascospores are produced. Upon maturation, ascospores (produced in sacs or asci, eight spores to a sac) are expelled from the perithecium and picked up by wind currents. For this reason, ascospores are thought to be long-range propagules of the fungus. both the asexual conidia and the sexual ascospores are capable of germinating--given the correct conditions--and creating a canker.
The orange bumps sometimes visible on the infected bark of a chestnut tree are called stroma. The stroma are the eruptions that release both kinds of spores.
<--This is what stroma look like up close.
The pen drawing to the right depicts a cross-sectional view of the stroma, which has an asexual fruiting structure, or pycnidium (top) and a sexual fruiting structure, or perithecia (bottom).
All Photos (and most of the good science) courtesy of Dr Mark Double, West Virginia University.
