How to Identify & Manage Chestnut Risks: Pathogens
Since the first agricultural revolution nearly 10,000 years ago, plant pests and pathogens have driven societal change in strange and tragic ways. From the Irish potato famine to the ergot-tinged madness of the Salem witch trials, plant diseases are inextricably woven into human history. These outbreaks are products not simply of plant-parasite interactions and evolutionary biology, but also of human decisions: the farm-level management, technological advancement, and socioeconomic arrangements of time and place.
Every year, the consequences of plant pathogens are profound: up to 40% yield losses for important staple crops like rice, corn and soy; $220 billion lost across the global economy; and a human toll that is difficult to quantify (He, 2020). Invasive species like Cryphnoectria parasitica, or Chestnut Blight, have altered ecosystems at massive scales across countries and continents. Though every species has its own risk profile, certain crops appear better adapted to threats than others. In this article we’ll examine the pathogenic risks associated with Chinese Chestnut, dive into specific diseases that impact chestnuts, discuss methods of prevention to ensure success, and highlight lessons learned from our work across over 1,850 acres of chestnuts.
In the next installment, we will cover pest risks associated with the Chinese chestnut.
Understanding Risk
Chestnut evolutionary biology
Chestnuts (Castanea spp.) are an emerging perennial staple crop for the American farmers, who are well-poised to meet growing global and domestic demand for this versatile crop. Chestnuts are among the most ancient tree crops cultivated by humans, with selection and grafting of improved genetics beginning 3,000+ years ago in Europe and 2,000+ years ago in China. (Rutter et al 1991)
More ancient still are the progenitors of today’s modern chestnuts, with the Castanea genus emerging nearly 60 million years ago in Asia, and the 9 chestnut species known today branching off nearly 20 million years later within distinct biogeographies, resulting in 3 North American species (C. dentata, C. ozarkensis, and C. pumila), 4 Asian species (C. mollissima, C. henryi, C. seguinii, and C. crenata), and one European/Turkish species (C. sativa) (American Chestnut Foundation). The distinct clades that emerged co-evolved with different pests and pathogens, resulting in variation in climatic preferences and susceptibility to diseases and pests with a now global reach.
Species-level impacts
Castanea dentata (American Chestnut)
The decline of the American chestnut – once colloquially referred to as the “redwood of the east” — is a well-known story here in the United States. The beginning of the end for Castanea dentata was instigated by Cryphonectria parasitica, or Chestnut Blight, an introduced fungal pathogen from Asia that was first observed in New York in 1904. But the American chestnut’s demise began even earlier, some researchers speculate, with the introduction of Phytopthera cinnamoni – the pathogen behind “Ink Disease” – from Southeast Asia, which killed large numbers of low-lying American chestnuts in the Southern US during the 1800s (Gustafson et al 2022). The loss of this keystone species and staple food source had far-reaching impacts both culturally and ecologically.
Castanea sativa (European Chestnut)
Like American chestnuts, European chestnuts are susceptible to Ink Disease and Chestnut Blight (but to a lesser extent than American chestnuts), and the introduction of these diseases to Europe and Anatolia in the 19th and early 20th centuries caused the large-scale decline of chestnut forests throughout the tree’s range. Known as the “bread tree” in Italy for its importance as a staple crop, Castanea sativa’s decline at the hand of blight, Ink Disease, and the Asian Chestnut Gall Wasp drove not only economic losses and cultural erosion, but also rural exodus from chestnut producing regions (Wall et al 2021). The increased incidence of drought in these regions due to climate change will exacerbate chestnut forest decline (Frietas et al 2018).
Castanea mollisma (Chinese Chestnut)
Because Chinese chestnuts co-evolved with the major pests and pathogens that impact European and American chestnuts, they are relatively resistant to these threats – but not immune. Chestnut orchardists in Michigan, for example, report that other species of Phytophthora beyond P. cinnamoni and P. cambivora have killed Chinese chestnut trees in their orchards (University of Michigan Extension). Similarly, North Carolina State Extension reports that Chinese and Japanese chestnuts can succumb to the Chestnut blight, but that these trees “are typically unhealthy and already stressed from other environmental factors.”
Chestnut Diseases
There are four primary pathogens impacting Chinese chestnuts, including those typically assumed to be innocuous. Generally, most trees are susceptible to 50+ pathogens, therefore this summary represents a non-exhaustive list of potential chestnut pathogens.
Oak Wilt (Bretziella fagacearum)
The first reported chestnut death from Oak Wilt was reported in 1950’s Missouri, but since then the pathogen’s range has expanded to 25 states (Sakalidis 2023). As its name suggests, Oak Wilt primarily impacts species in the Fagaceae or oak family, colonizing the vascular tissue of trees and disrupting water and nutrient flow. When fungal hyphae colonize the xylem, trees produce defensive structures called tyloses that clog the plant’s vascular system, causing rapid death in as little as 2 weeks.
Risk Factors:
Wounding of trees, be it through storm damage, pruning, etc.
Temperatures in excess of 67 degrees F (when most picnic beetles are active)
A new predictive model developed by the University of Wisconsin can help understand the precise risk for your location.
While some sources suggest a majority of infections occur via root graft (Bonelo, 2016), others suggest this type of transmission is limited by soil texture (e.g. clay pan and other coarse soils), which limits the ability of trees to form root grafts. In places where root grafts are limited by soil, whether due to compaction or texture, a majority of infections may be vectored by picnic beetles (Hayslett, 2008).
Symptoms & Signs:
Chlorosis & necrosis at leaf margins; moves inward
Yellowing, browning & wilting of outer canopy leaves, followed by wilting of interior
Leaf drop & rapid tree decline
Management:
There is no widely-accepted strategy for oak wilt management in chestnuts; however, lessons learned from oaks can be applied. Key to this strategy is careful monitoring to enable quick intervention, as oak wilt can spread rapidly throughout the orchard.
Trenching around infected trees can prevent disease spread via root graft. A trenched buffer around uninfected trees can further prevent spread.
Insecticides are not recommended for management of picnic beetle vectors. Instead, growers can avoid injuring trees during picnic beetle activity, which can be gleaned via this predictive model.
Ink Disease (Phytopthera spp.)
A soil borne oomycete that can live in moist soil for up to 6 years, the primary drivers of Ink Disease globally – Phyopthera cinnamoni and P. cambivora – are transmitted via infected nursery stock or foot traffic.
Chinese chestnuts possess the greatest resistance to these species, but not phytophthora generally (Revord et al 2022).
Other endemic and introduced Phytopthera species – including over 210 identified species, and up to 500 undiscovered species – may be able to kill Chinese chestnut if conducive conditions and pathogen virulence align with host susceptibility.
Risk factors:
Saturated soils
Optimal temperature ranges vary by species, but can occur between 68-90 degrees F (Syngenta)
Symptoms & Signs:
Chlorosis (leaf yellowing)
Wilting
Necrosis (tissue death)
Root rot
“Flame rot” visible when collar bark peeled back
Can result in tree death
Chestnut blight (Cryphonectria parasitica)
Chestnut blight (Cryphonectria parasitica)
Chinese chestnut’s resistance to this infamous ascomycete varies between cultivars. Infected trees develop cankers that girdle whole stems, branches, and the trunk, preventing nutrient and water flow. Transmission occurs via spores that come into contact with wounds in a tree’s bark. The spores are dispersed via wind, water, or wildlife – birds and insects. Once the fungus has colonized the tree, it can be neutralized via a naturalized “hypovirus” that parasitizes the pathogen and reduces its virulence. In this way, blight can become “endophytic,” meaning it colonizes the tissues of chestnut but does not cause overt damages.
Risk factors
Wounding of bark (e.g. through pruning)
Most susceptible 0-1 days after wounding
At day 4 after wounding, disease cannot establish
Time of year: Peak sporulation in autumn in the US
Moisture & Humidity: humidity, rain splash, dew induces sporulation
Stressed trees - drought stress and cold stress found to exacerbate severity of outbreaks and canker production.
Optimal temperature for canker growth: 80 F
Symptoms & Signs:
Flagging- Dead branches with yellow, wilting, or dead leaves
Most common early symptom
Newly infected bark turns red
Cankers (sunken, reddish brown lesions)
Orange Stroma (reproductive structures)
Water sprouts (new vigorous shoots growing from the trunk or large branches; often upright)
Mycelial “fans” beneath bark
Disease management:
Selection of resistant cultivars and maintenance of tree health is the most important strategy to prevent infection. Because drought worsens disease, irrigation may be a preventative measure.
“Mudpacking” is a biocontrol strategy whereby mud is applied to the cankers of infected trees, with high rates of success observed by growers (American Chestnut Foundation).
Hypovirulence is another biocontrol strategy whereby slurries or plugs of hypovirulent strains of CHV-1 are applied to trees. Unfortunately, there currently are no commercially available strains available; moreover, the efficacy of this strategy depends on the “vegetative compatibility group” of the host-parasite and thus varies widely (American Chestnut Foundation).
Though generally not recommended, several chemical controls can be used to treat Chestnut Blight. A walkthrough of these controls can be found here.
Chestnut Anthracnose (Colletotrichum henanense)
Formerly known as “blossom-end rot,” this disease caused up to 80% yield losses for chestnut growers in the 2018 season. Research is ongoing to dissect the life cycle of the pathogen responsible, risk factors that might impact prevalence, and disease management strategies (Miller and Ivey, 2022)
Chestnut anthracnose becomes apparent post-harvest.
Symptoms & Signs
Symptoms include black spots on the nut shell that are not always visible, but often seen when kernel blackening exceeds 10%
Kernel blackening (ranging from 1 to 100% of the kernel
1 cm cankers on seedlings (likely infected via affected lots); and black, spore-producing acervuli on cankers. Sometimes orange spore masses are visible on nuts, but rarely (Miller, 2022)
Disease Management
Genetics may affect disease incidence, with Qing, Liu, and AU Homestead appearing to resist infection better than other cultivars.
Healthy trees appear to better heal around cankers and maintain shoot growth (Miller 2022).
Disease Prevention
The best way to manage chestnut diseases is by maintaining high levels of tree health across the orchard. Ensuring your chestnuts are planted in suitable ground is the first and most important preventative action you can take, but regular scouting and monitoring not only for pests and disease, but also for nutrient deficiencies, is non-negotiable.
Try out our free suitability tool to learn more about your land.
Symptoms of nutrient deficiency can co-occur with and resemble disease symptoms, and mineral deficiencies generally contribute to lower immune function and higher susceptibility in host plants (University of Tennessee). Sap and soil sampling is a proactive measure that can help identify nutrient deficiencies before they occur, but is not a replacement for proper soil management. Managing soils to align with the quality preferences of different species – including attributes like pH, OM% and nutrient balance– will influence nutrient availability and uptake while ultimately helping you prevent infection. Understanding the seasonality and environmental risk factors associated with pest and pathogen virulence in your region is likewise an important step, as is ensuring your nursery stock is sourced from reliable, quality-assured vendors. Many pests and pathogens, including Chestnut Gall Wasp, Chestnut Blight, and Ink Disease, can be transmitted via contaminated seedlings or clones, though nurseries can prevent contamination through testing, sanitation, and overall hygiene.
Interested in planting chestnuts, but concerned about risk?
Propagate helps growers manage pest and disease risk by adhering to the Chestnut Gold Standard – a comprehensive installation program that ensures proper site selection and site preparation, soil management, curation of disease-resistant and regionally appropriate genetics, procurement, and planting procedures. Contact us to learn more.
Sources:
American Chestnut Foundation (2024) Chestnut Chat: Evolution of Castanea and Cryphonectria, YouTube. Available at: https://www.youtube.com/watch?v=gMt46pq7bNs (Accessed: 10 June 2024).Bonello, P. (2016). Oak wilt. Ohio State University Extension.Freitas, T.R., Santos, J.A., Silva, A.P., Fraga, H. Influence of Climate Change on Chestnut Trees: A Review. Plants (Basel). 2021 Jul 16;10(7):1463. doi: 10.3390/plants10071463. Erratum in: Plants (Basel). 2022 Jun 06;11(11):1518. doi: 10.3390/plants11111518. PMID: 34371666; PMCID: PMC8309319.Gustafson, E. J., Miranda, B. R., Dreaden, T. J., Pinchot, C. C., & Jacobs, D. F. (2022). Beyond blight: Phytophthora root rot under climate change limits populations of reintroduced American chestnut. Ecosphere, 13(2), e03917. https://doi.org/10.1002/ecs2.3917Hayslett, M., Juzwik, J., and Moltzan, B. 2008. Three Colopterus beetle species carry the oak wilt fungus to fresh wounds on red oak in Missouri. Plant Dis. 92:270-275.He, S., Creasey Krainer, K.M. Pandemics of People and Plants: Which Is the Greater Threat to Food Security? Mol Plant. 2020 Jul 6;13(7):933-934. doi: 10.1016/j.molp.2020.06.007. Epub 2020 Jun 17. PMID: 32562879; PMCID: PMC7298473.
Miller, A. and Ivey, M. (2022). Chestnut Growers of America. (2022). The Chestnut Grower [PDF]. Retrieved from http://www.chestnutgrowers.org/TheChestnutGrower_Fall2022.pdfRevord, R., Gold, M., Meier, N., Webber, J. B., Hunt, K., & Warmund, M. (2022). Growing and marketing Chinese chestnuts. University of Missouri Center for Agroforestry.
Rutter, Miller, Payne. (1991). CHESTNUTS (CASTANEA). Acta Hortic. 290, 761-790
DOI: 10.17660/ActaHortic.1991.290.17
Sakalidis, M., Lizotte, E., McCullough, D., & Guyer, D. (2023, March 8). MSU investigating oak wilt as the cause of sudden chestnut tree decline. Michigan State University Extension.
Syngenta. (2022). "Phytophthora Solutions Guide." https://www.greencastonline.com/programs/pdf/phytophthora-solutions-guide.pdf
Wall, Jeffrey & Köse, Nesibe & Aksoy, Elif & Köse, coşkun & Okan, Taner & Allred, Shorna. (2021). ‘We live and die in chestnut’: remaining and adapting in the face of pest and disease outbreak in Turkey. Landscape Research. 46. 1-12. 10.1080/01426397.2021.1929888. MSU Extension. “Chestnuts: Diseases.” Michigan State University. www.canr.msu.edu/chestnuts/pest_management/diseases. NC State Extension. “Chestnut Blight.” North Carolina State University. https://content.ces.ncsu.edu/chestnut-blight Image Sources:
Anthracnose: Amy Miller; Blight: Missouri Dept. of Conservation; Victoria Dept. of Energy; Ink Disease: American Chestnut Foundation; Washington State University Oak Wilt: Michigan State Extension