This is the first part of a series on summer tree diseases. This article examines annosum root rot, and American chestnut blight.
During the summer months, plants are in the midst of their development. While many plants flourish due to the warmer climate, others can be subject to infections from a slew of disease pathogens. The following describes some of the most common diseases to affect plants in summer, and how they impact their hosts.
Annosum Root Rot (Heterobasidion annosum)
Annosum root rot, also referred to as annosus root rot, is a fungal disease of pine, spruce, and fir trees. The causal agent of the disease is the fungus, Heterobasidion annosum. Annosum root rot is capable of infecting a wide range of host species in forested and landscape settings. It is most problematic in areas that feature an abundance of pines. Annosum root rot is considered one of the most destructive diseases of conifers in the northern hemisphere.
Annosum root rot generally targets pines, but it may also affect spruce and fir trees. It is particularly common on white fir. Infections occur on subalpine fir, Douglas-fir, blue spruce, and Engelmann spruce, albeit with less frequency.
Symptoms of Infection
The first symptoms of infection are often most apparent beneath the soil. Once the fungus has infiltrated a tree’s root system, it causes abnormalities to develop in the root structure. These abnormalities inhibit the tree’s vascular system, preventing it from absorbing water and nutrients. More than half of the tree may be killed before infection symptoms become visible. As the infection progresses, the tree’s bark turns pale yellow. Eventually, the bark coloration deepens to a light brown before transitioning to white, with a speckling of irregular black dots. Resin flow may be evident at the root collar, an indication of the tree’s attempts to fend off the pathogen. Infected trees will gradually wither, before finally succumbing to the disease. In landscape settings, a ring of dead trees will gradually develop, with the oldest tree rooted in the center.
In pines, the disease is most active in the sapwood, where it eradicates healthy tissue. While decay may occur in pines, the rapid tissue death often precludes the decay process from becoming too extensive. In spruce and fir trees, the fungus grows in the root collar, often resulting in butt rot. Decay in any host may be preceded by a pink staining of the wood. Later, this decay is defined by pockets, or pits, that form in the wood. Small black flecks can often be observed in the more enlarged pockets. The pockets tend to be lost as the decay process renders the wood spongy, or stringy.
Conks are frequently found in moist sites around white fir. The conks may measure up a foot in diameter. The conks produced by annosum root rot may have a prominent brown cap or bracket. They may also be completely flat on the substrate. The pore surface of the conks is white, with small pores. The flesh is tan. Developing conks often emit a fresh mushroom aroma.
- When planting, select trees that are resistant to annosum root rot.
- A prophylactic stump treatment can be applied immediately after infection to stifle disease progression. Prophylactic stump treatments increase the pH level of the stump to the point where the fungus is unable to survive.
- Several benign fungi are aggressive stump colonizers, and can be administered as stump top treatments to prevent the annosum root rot pathogen from becoming established in susceptible wood.
American Chestnut Blight (Cryphonectria parasitica)
American chestnut blight, also called chestnut blight and chestnut bark disease, is a fungal infection that has significantly reduced the chestnut population in North America. It is caused by the pathogen Cryphonectria parasitica. The fungus is spread by wind-borne ascospores and conidia disseminated by wind and rainstorms. In the first half of the twentieth century, American chestnut blight is estimated to have killed around four billion chestnut trees.
American chestnut blight primarily infects American chestnut. Infections also occur on numerous other species of chestnut, albeit with less frequency. Some of the more commonly infected species include American chinquapin, European chestnut, and West Asian chestnut.
Symptoms of Infection
The first symptom of chestnut blight is a discoloration of the tree’s bark. Infected areas turn orange and brown. As the mycelial fan spreads under the bark, it creates a sunken canker. As the hyphae expand, they produce several toxic compounds, the most notable of which is oxalic acid. The oxalic acid lowers the pH level of the infected tissue from around 5.5 to approximately 2.8. This increases the acidity of the tree, which is toxic to the plant’s cells. While this transpires, the canker enlargens, girdling the tree. This prevents water and nutrients from being properly distributed throughout the tree, effectively killing the foliage above it.
Distinctive yellow tendrils often referred to as cirrhi can be seen extruding from the bark during periods of heavy rainfall. Small fruiting bodies called perithecia also appear on the tree, expelling spores once they mature in fall.
- American chestnut blight is highly infectious. As such, conventional methods of treatment are not effective.
- The root collar and root system of the chestnut tree exhibit some resistance to blight infection. The soil organisms react adversely to the fungus, effectively repelling it. Consequently, a large number of small American chestnut trees remain as shoots growing from existing root bases. These regrown shoots seldom reach maturity before being killed by the fungus. Instead, they survive as living stumps, with only a few developing enough shoots to produce seeds.
- Surviving chestnut trees have been selected for breeding programs, notably by the American Chestnut Foundation. The intention of these breeding programs is to replenish American chestnut populations by introducing blight-resistant chestnuts to their original forest range. Resistant species, particularly Japanese chestnut, Chinese chestnut, Seguin’s chestnut, and Henry’s chestnut, are being hybridized with American chestnut to create new disease-resistant species. Chinese chestnuts vary considerably in their resistance to blight; some individuals are susceptible, while others are essentially immune to the disease.
- Environmental stressors may reduce a tree’s resistance to blight. At higher elevations in areas exposed to harsh weather, and extreme temperatures, normally resistant Oriental chestnuts can be weakened, or killed by blight.
- The Forest Health Initiative, along with contributions from the New York College of Environmental Science and Forestry, Penn State, the University of George, and the US Forest Service, is currently attempting to use modern breeding techniques, and genetic engineering to create disease resistant tree strains. Their strategy is to select blight-resistant genes during back-crossing, while preserving the traits inherent to the American chestnut. The newly bred hybrid chestnuts are anticipated to reach the same heights as the American chestnut. Mature chestnuts often grew up to 100 feet tall, with a spread of 50 feet.
- At the New York College of Environmental Science and Forestry, plant pathologists William Powell and Charles Maynard have developed blight resistant American chestnuts. Full resistance was attained by introducing a wheat gene coding for the enzyme oxalate oxidase into the American chestnut genome. The enzyme converts the oxalic acid secreted by the blight into carbon dioxide and hydrogen peroxide. Transgenic trees produced through this method have been shown to exhibit blight resistance equal to, or surpassing that of the Chinese chestnut. The New York Botanical Garden has planted several of the transgenic trees for public display.
Photo courtesy of Jason Hollinger CC-by-2.0