Interactions:
My worst enemy
Chestnut Blight, Cryphonectria parasitica
History:
In 1904, a forester working at the Bronx Zoo in New York noticed a strange new disease infecting chestnut tree bark on the outside ridges of the zoo. This moment in time is regarded as the first known case of Cryphonectria parasitica, commonly referred to as chestnut blight, and infamously known otherwise as perhaps the most destructive forest disease on record. C. parasitica is a highly virulent fungal pathogen that caused almost complete destruction of Castanea dentata in the Eastern United States between 1904 and 1950. Chestnut blight was inadvertently introduced into the United States via a shipment of trees from Asia that were intended to be used to produce larger chestnuts by cross-breeding. What was unknown at the time, however, was the fact that Asian chestnut trees were infected but resistant to C. parasitica yet the American chestnut was highly susceptible. Beginning in the early 1900's, virtually all trees were killed or reduced to stump sprouts within the next few decades, with a fear of extinction of the legendary American tree.
Life of Chestnut Blight:
C. parasitica is classified in the phylum Ascomycota of the kingdom Fungi, which reproduces sexually and asexually by ascospores and conidia, respectively. Both forms are spread by means of wind, rain, and possibly beetles (Agrilus) and birds. The blight produces 2 forms of spores that can be transmitted: a dry disk carried by wind and a smaller stickier form that clings to the feet of birds and beetles. These spores can be transported for miles and eventually come into contact with its host. Entry is completed through breaks in the bark, scrapes, woodpecker holes or wounds produced by insects.
Those trees that are unfortunate enough to be succumbed to
C. parasitica are rapidly overtaken by the parasitic
fungus.
Fungal hyphae spread through the phloem and xylem tissues and
produce fungal cankers (below, left) that girdle and eventually
kill the tissues. The distal portions of the lesion will
then die because of lack of transport of food and water.
Noticeable presence of the blight is indicated by a definite
canker or dead patch on the stem or trunk. When the
cambium is killed, the a sunken area results. Most recently
infected layers swell and subsequently crack the outer bark.
Yellow-orange to reddish-brown pustules develop and leak out of
infected bark, shown above. To the right, a chestnut fallen
victim to blight is still standing but dead. Young trees will
die within a year, but more mature trees may take several years
to fully surrender to the disease.
The interesting aspect of Chestnut blight is that it does not affect the root system. The reason for this is because C. parasitica can not act while in contact with the soil because of an antagonistic interaction with the soil bacterium, Bacillus megaterium. Therefore, roots are able to resprout and survive even after extensive infection. Here lies the reason why Castanea dentata is still not extinct but nevertheless is no longer ecologically significant. What used to once be ranked amongst our largest forest trees is now only known as an understory component.
Research & Control Efforts:
During the time of rapid spread of chestnut blight back in 1911-1913, Congress approved a $165,000 to be geared towards the research and control of tree devastation. Pennsylvania's legislature also promoted strong efforts by establishing a Chestnut Blight Commission in 1913 which spent $275,000, a large amount of money at the time, for chemical spraying in hopes of halting the disease. No such efforts were successful in slowing the progression of C. parasitica and C. dentata showed no natural immunity to the foreign pathogen.
We have far more advanced research methods in present day that are showing to be quite promising in the success of someday resurrecting the American Chestnut. Two major organizations have become devoted to extensive research of finding a treatment for blight and also a means of producing a more resistant form of the American chestnut, similar to that of the Chinese chestnut.
The
American
Chestnut Cooperators' Foundation in Virginia is in the
process of developing a economical biological
control measure. Such a method is described as using a
hypovirulent form of blight, or weakened form, to infect the
normal robust form of C. parasitica with a virus. In
other words, the disease in itself would be infected. The
goal is to weaken the virulent fungi so that the tree is able to
withstand its effects and build up a wall behind new bark
tissue. Eventually, if the pathogen is weakened enough by the
introduction of a virus, the tree will be able to build up the
proper defenses to heal itself, such as is shown to the right.
In addition to finding a biological control measure, The American Chestnut Cooperators' Foundation and The American Chestnut Foundation, separately affiliated, are also conducting resistance breeding programs by utilizing controlled genetic crosses. Successive generations of backcrossing are being conducted with the Asian chestnut genes to eventually introduce disease resistance into our native trees. The crosses are proving to be more and more promising with the ability to regain 98% of the native genes with the resistance. To learn more about their efforts of a biological war against blight, check out the videos below.
American Chestnut Restoration Program, VA (1:48)
Giving Back Series: The American Chestnut Foundation (6:29)
Other parasitic relationships with Castanea dentata:
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Phytophthora cinnamoni: 80 years prior to blight, killed trees in low, wet ground
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Dryocosmus kuriphilus: 1974, Chestnut gall wasp
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Melittoma sericeum: Chestnut timber worms
