Biscogniauxia canker or dieback (formerly called Hypoxylon canker or dieback) is a common contributor to poor health and decay in a wide range of tree species (Balbalian and Henn 2014). This disease is caused by several species of fungi in the genus Biscogniauxia (formerly Hypoxylon). B. atropunctata or B. mediterranea are usually the species found on Quercus spp. and other hosts in Florida, affecting trees growing in many different habitats, such as forests, parks, green spaces, and urban areas (McBride and Appel 2009).
Typically, species of Biscogniauxia are opportunistic pathogens that do not affect healthy and vigorous trees; some species are more virulent than others. However, once they infect trees under stress (water stress, root disease, soil compaction, construction damage etc.) they can quickly colonize the host. Once a tree is infected and fruiting structures of the fungus are evident, the tree is not likely to survive, especially if the infection is in the tree's trunk (Anderson et al. 1995).
The pathogen produces powdery masses of asexual spores called conidia that are released from infected trees in the spring or early summer. Conidia are produced on the surface of a mat of fungal tissue called a stroma (plural stromata). Later in the summer or fall, after the conidia are gone, small gray, pink, or black raised dots called ostioles may be observed on the surfaces of the stroma. Ostioles are the tips of perithecia (sexual fruiting structures), where ascospores (sexual spores) are produced and released. Stromata are the most readily recognizable sign of Biscogniauxia (Hypoxylon) infections (Florida Department of Agriculture and Consumer Services 1983) (Figure 1). The ascospores of Biscogniauxia are spread by the wind and deposited on wounded stems or branches, where they infect fresh tissue. The mycelium (vegetative growth) of the fungus develops as microscopic threads called hyphae in the phloem and in the outer layers of the xylem. Sporulation (the production and dissemination of spores) may continue over several years. Even when the tissue of the tree is dead, the asexual phase of the fungus can persist for several years (Cibrian et al. 2007).
Biscogniauxia spp. are ascomycete (Phylum Ascomycota) fungi that not only colonize live tissue and cause cankers (by killing live cambium), but also cause white rot decay of sapwood by breaking down the cellulose and lignin in the wood. Infected wood first turns white and spongy, and then strands (hyphae) of the fungus stain the wood as they grow, causing black lines in the sapwood, referred to as zone lines (Balbalian and Henn 2014). In healthy trees, the fungus survives in small colonies in the bark and sapwood and is kept under control by the tree's natural defenses. However, when trees are stressed, the fungus is able to grow rapidly and release spores that start new colonies (Sinclair and Lyon 2005).
Damage by construction is one of the biggest stress factors causing wounds to trees' root systems that contribute to the trees succumbing to Biscogniauxia canker (Figure 2). Root system damage impairs trees' ability to take up water and nutrients. Soil compaction from heavy equipment or construction materials stored around trees can also suffocate roots. These conditions cause trees to decline and become susceptible to Biscogniauxia canker disease. Additionally, fire, heat, drought, ice storms, hail damage, lightning, and flooding can also contribute to tree stress. Insect attacks and other diseases such as oak wilt and root rots induce stress and increase trees' susceptibility to infection by Biscogniauxia spp. (McBride and Appel 2009).
Signs and Symptoms
Biscogniauxia cankers appear as diffuse lesions in limbs and branches of infected trees. The fungi often go unnoticed until they produce fruiting structures and the bark falls away. By this time there is already extensive white rot decay of the tree's sapwood. The decay of the sapwood may make trees hazardous and cause failure and breakage.
Over time, dead and/or dying trees or dead portions of live trees exhibit a light to dark gray or almost black stromatic layer (Figure 1) that typically covers large surfaces of the phloem and can be seen lifting and eventually replacing the bark (Cibrian et al. 2007). Sometimes lichens are confused for Biscogniauxia spp., but lichens do not replace the bark in the same manner and are usually harmless to trees.
There are no management options for Biscogniauxia canker once infection has begun on the trunk. In some cases, infected branches can be pruned off and trees will recover; however, this is not always successful and depends on the health and vigor of the tree (Balbalian and Henn 2014). The presence of this disease is usually an indicator of severe stress in trees. The best way to prevent this disease is to reduce or eliminate the stress that makes trees susceptible to it. There are numerous ways to reduce stress factors such as:
prevent or reduce compacted soil near trees;
maintain good cultural practices, such as proper nutrition; and
avoid wounding trees' trunks and roots.
Sanitation is also important in situations where inoculum levels are high and numerous potential hosts exist nearby. In those situations, remove infected trees or branches and burn (or completely remove) stumps of severely infected trees.
Anderson, L. A., T. Price, and F. H. Tainier. 1995. How to Identify and Control Hypoxylon canker of Oaks and Other Hardwoods. US Department of Agriculture Service Southern Region.
Balbalian, C., and A. Henn. 2014. The Plant Doctor: Biscogniauxia (Hypoxylon) Dieback of Oaks. Mississippi State University Extension Service. Available at: https://extension.msstate.edu/publications/information-sheets/the-plant-doctor-biscogniauxia-hypoxylon-dieback-oaks
Cibrian, T. D., D. R. Alvarado, and S. E. D. Garcia. 2007. Enfermedades Forestales en Mexico/Forest Disease in Mexico. Universidad Autonoma Chapingo.
Florida Department of Agriculture and Consumer Services. 1983. Hypoxylon Cankers. Bulletin No. 196-A | Printed October 1983. Available at: http://www.freshfromflorida.com/Divisions-Offices/Florida-Forest-Service/Our-Forests/Forest-Health/Forest-Health-Publications/Insects-and-Diseases/Hypoxylon-Cankers
McBride, S., and D. Appel. 2009. Understanding, Recognizing and Keeping Hypoxylon Canker of Oaks at Bay. Texas AgriLife Extension Service; The Texas A&M University System. Available at: http://plantclinic.tamu.edu/files/2010/09/Hypoxylon-Fact-Sheet1-1.pdf
Sinclair, W. A., and H. H. Lyon. 2005. Diseases of trees and shrubs. Biscogniauxia Cankers and Dieback. Second edition. Ithaca. Comstock publishing associates.