Diseases, other than chestnut blight, have not played a major role in terms of chestnut tree growth and survival in Michigan. Chestnut blight is a very serious pathogen that needs to be taken seriously because it can infect and kill European X Japanese hybrids. Cultivars with Chinese chestnut backgrounds are generally tolerant or resistant. Grafting and pruning wounds are the primary entry ports for chestnut blight infection, although this pathogen has the ability to find natural wounds on the bark where it initiates the girdling blight cankers.
Chestnut blight is probably the single most important factor that dictates which chestnut species is planted in commercial orchards in North America. Generally, growers in western states (a chestnut blight-free environment) can plant chestnut blight–susceptible European chestnut (C. sativa) and their hybrids (C. sativa x C. crenata); and, eastern growers plant the naturally chestnut blight-resistant Chinese species (C. mollissima) due to the continued presence of the chestnut blight pathogen in eastern states. Michigan is one of the only states where growers routinely plant both Chinese and European hybrid chestnut species, sometimes side by side in commercial orchards. Research in Michigan has demonstrated that European hybrid chestnuts produce 2 to 5 times more nuts and go into production much sooner than Chinese chestnuts; however, Michigan growers need to constantly monitor for chestnut blight. This has also brought on the pollen-generated problem with internal kernel breakdown (IKB) where European X Japanese hybrid trees produce nuts with decaying kernels when pollinized by Chinse chestnut trees.
Another problem that is not recognized is that Chinese-like trees can become infected. These trees are probably not pure Chinese but would have originated from seed collected from a Chinese chestnut tree pollinized by an American chestnut tree. The tree that grew from this seed would be a hybrid tree where half the tree is Chinese and half the tree is American chestnut. These crosses (Chinese X American hybrids) are blight susceptible in the sense that they may not die from blight, but they will allow the pathogen to grow, colonize and sporulate all over the tree, spreading the chestnut blight fungus for many miles around the infected tree(s). As these blight “tolerant” trees get older, they get larger and the infections increase in size. These trees must be found and culled if you wish to plant ‘Colossal’ for example, within a few miles. This is a true example and it is currently damaging an orchard of ‘Colossal’ trees.
Sunscald or southwest disease. A problem that mimics chestnut blight in its appearance and damage to the tree is sunscald. Usually on the south side of the tree, the cankers that erupt due to bark death can look similar to chestnut blight. Sunscald can be managed by reflecting with latex paint. In Michigan, the sun reflects off the snow in the winter warming the bark on the south side of the tree. This action can “burn” the epidermis of a young tree or warm it sufficiently during the day that when night falls, the tissue is exposed to extremely cold and damaging temperatures. Weak, secondary pathogens can enter the bark at these wounds and the tree may suffer chronic infections and poor growth.
Root rot caused by various species of the fungal-like, water mold pathogen Phytophthora has been observed on dead and dying chestnuts in Michigan. Ironically, the trees that have been most susceptible are Chinese trees that are generally thought of as resistant to root rot disease. Michigan soils do not harbor the intensely aggressive pathogen Phytophthora cinnamomi that does kill American and European chestnuts. However, there are other species of Phytophthora in Michigan soils that can apparently kill Chinese chestnuts, and growers need to establish orchards in well-drained soils to escape root rots.
Nut rots. Michigan has not found Sclerotinia pseudotuberosa nut rot, a serious problem in Europe or Phomopsis castanea nut rot a serious problem in New Zeeland and Australian orchards. A nut decay has appeared in Michigan and primarily on the nuts from the cultivar ‘Colossal’ (and other European/Japanese hybrids, also) when they are pollinized by Chinese chestnut trees. The nut decay is called internal kernel breakdown (IKB) and 30 to 40 percent of the nuts from ‘Colossal’ trees when pollinized by Chinese trees can have IKB.
Chestnut blightAmerican chestnut is the most susceptible species to chestnut blight a fungus that was introduced to North America in the early 1900s. This fungus reduced the great American chestnut forest of the Appalachian Mountains to a simple sucker sprout population that rarely produces any nuts. Michigan has remnant populations of American chestnut and you can read about the research we have performed on surviving American trees, below. For orchard production, you need to understand the following:
- Chinese can be infected but is mostly tolerant. When infected, it acts like a carrier of the disease. The fungus may not kill the Chinese trees, but if it reproduces on them then it can act to promote the fungal pathogen into new areas.
- European chestnut is susceptible
- Japanese chestnut is tolerant, but again, it can carry the fungus
- Keep blight away by keeping American chestnut trees off the farm.
- Don’t bring in stock from areas where blight is found.
Controlling blight in chestnut orchards using hypoviruses
Andrew M. Jarosz, Associate Professor, Department of Plant Biology, Michigan State University
Michigan’s chestnut industry made significant strides in the last decade at expanding the market for chestnuts. A key component of this success is the variety ‘Colossal’, a Japanese-European hybrid, which has gained acceptance throughout the state because it survives well and produces an abundant crop of large, high-quality nuts that are sought after by the public. However, one looming problem with Colossal is its susceptibility to the chestnut blight pathogen, Cryphonectria parasitica. The pathogen was introduced into the United States in the early 1900s and has spread throughout the range of the American chestnut, including over 100 naturalized populations throughout Michigan. The disease causes a canker disease on stems and branches (Photo 1). On the highly susceptible American chestnut, cankers quickly girdle the stem and as a result the aerial portions of the tree die back. For some unknown reason, the pathogen cannot infect the roots, which retain the ability to produce stump sprouts that are often produced in abundance once the main stem dies. Unfortunately, stump sprouts are highly susceptible and they die from infection before reaching reproductive size. Since infected branches produce abundant spores that can be dispersed long distances on air currents, the presence of infected chestnut populations poses a significant threat to Michigan’s chestnut orchards. Indeed, at least one Michigan grower is dealing with a continuing problem where blight has entered his orchard on multiple occasions. At present, the only disease management option available is to destroy infected trees in an effort to eradicate the disease. Growers suffer significant costs with this management approach since replacing infected trees entails several years of lost nut production.
Photo 1. Canker caused by a highly virulent form of Cryphonectria parasitica on American chestnut. Note how the pathogen causes the collapse of stems along the outer surface of the stem. If the canker girdles the stem, then transport of nutrients is disrupted and all plant material beyond the canker dies back.
Cryphonectria parasitica was also introduced accidently to Europe where the European chestnut suffered a similar disease epidemic. However, about twenty years after epidemic spread, some European chestnut trees began to “recover” from infection. Recovery was not due to the spread of a few highly resistant chestnut trees, but instead was due to the spread of a virus that infects the pathogen. The virus resides in the pathogen’s cytoplasm, slowing growth and reducing spore production. In essence, it makes the pathogen sick in a manner that allows infected chestnut trees the chance to recover from the disease. Recovery occurs as the tree walls off the canker with wound tissue, which prevents the pathogen from girdling infected stems or branches. Over time the tree closes the canker and further pathogen spore production is prevented. These viruses are now called hypoviruses because they reduce pathogen virulence. Hypoviruses have been used extensively as a biological control agent in Europe where the chestnut industry has recovered from the severe epidemic that started in the 1950s. In addition, hypovirus mediated recovery has occurred naturally in at least four Michigan populations of American chestnut. These examples all indicate that hypoviruses may be a practical disease management option for controlling chestnut blight in Michigan’s chestnut orchards. If successful, hypovirus treatment would control the worst effects of the disease without the necessity of culling infected tress.
Photo 2. Canker with hypovirus. The tree has enclosed the canker with wound tissue that stops canker expansion. The canker in this picture has died (bare wood at the center of the stem, and the wound tissue is slowly covering the old infection site.
Our work on hypoviruses began in 2010 when we introduced hypovirus onto ten trees in an orchard of Japanese-European hybrids on the MSU campus that is infected with blight. One or two cankers per tree were inoculated with hypovirus. Our first goal is to determine if hypovirus treatments result in healing cankers. This will be accomplished by monitoring the treated cankers for three to five years to document that treatments halt canker expansion and lead to tree recovery. A second goal is to determine how well hypovirus spreads to untreated cankers on a tree. The extent of spread to new cankers on a treated tree will influence how hypoviruses are used. If the hypovirus spread quickly in orchard situations, then only a few cankers need to be treated on an infected tree, and subsequent hypovirus spread will provide long-term protection from blight. On the other hand, if the hypovirus does not spread then each canker will need to be treated with hypovirus making disease control much more time and labor intensive. Our work on American chestnuts suggests that hypoviruses can spread to new cankers on treated trees.
We are also evaluating treated trees for their rate of growth and nut production since hypovirus treatments can only be considered effective if the trees continue to grow and continue to produce large quantities of high quality nuts.
Within four to six years we plan to have a blight management scheme that will allow us to control blight within chestnut orchards. It is our hope that hypovirus treatments will replace eradication of infected trees. Eradication has large economic costs to the grower because production from infected trees is lost and makes it necessary to replant. While growers might be tempted to introduce hypovirus in advance of blight infections, we do not advocate such a plan because the hypovirus strain must match the blight strain in order to be effective. Initial testing is always needed to determine which blight strain has entered the orchard. Only then can we deploy the correct hypovirus treatment.
Phytophthora root rot
Phytophthora root rot of chestnut is primarily a disease of the root and lower trunk. While the top of the tree may be showing symptoms of wilting or death, the infection is occurring on the roots and at the crown of tree—that area between the root and trunk. Phytophthora root rot kills trees in forests and orchards of Europe, Australia, and North America. I have not heard about its presence in Chinese chestnut orchards in China, Japan or Korea.
Below is a photo of Phytophthora root rot in an Australian chestnut orchard. The dead trees are surrounded by trees with thinning tops indicating the infestation is spreading through the soil. Dry soils in summer only exasperate the condition of non-functioning infected roots. Obviously, this area of the flat orchard floods during the rainy season.
Root rot in Europe has been a major problem for chestnut production in forests and farms. Management in forests is different than on farms. In forest environments, managing natural water drainage is the primary focus. On farms, water drainage management and individual tree treatment is attempted.
Above, the top of a European chestnut tree in Italy is affected by Phytophthora root rot. The tree to your left is thinning and the tree next to it has yet to show obvious symptoms.
An injection site for Phytophthora root rot management in the trunk of a large European chestnut tree in Italy.
To diagnose root rot in the field, we peel back the outer bark until we see a discolored, black or brown coloration. This is dead tissue and it is referred to as the “flame”—as if a hot flame in the roots has scorched the trunk of the tree.
In the above photo, a non-grafted, seedling Chinese chestnut tree from an orchard in Michigan, which is suppose to be resistant to Phytophthora root rot (Chinese trees were reported to be resistant) is pulled and it shows the “flame” symptoms root rot. Later in the laboratory, a Phytophthora strain was isolated and it proved to be a unique strain.
Phytophthora can be carried from nursery to your plantation or it can already be at your orchard naturally or from other trees that came from other nurseries. It usually appears where roots are wet for extended periods of time. If trees die in a wet area, it is not always because of Phytophthora root rot, as chestnut roots do not do well in we soil, even if Phytophthora is not present. Stay away from wet soils that are low in oxygen. This is the primary reason we say to plant in well-drained soils.
The tree in the photo above was pulled a season after planting. After a series of trees died in an orchard, the grower ordered more trees and re-planted them in the same holes from which the dead tree was removed. Most of the replaced trees rapidly died of root rot and we assume the original trees died of root rot. Root rot will remain in the soil for several years. This root rot was assumed to be from unknown previous sources as a wooded area had been converted to a cherry orchard and then changed over to a chestnut orchard. The cherry orchard and the chestnut orchard did not have good production records.