Pathogens
Phomopsis (syn. Diaporthe) is a broad and taxonomically complex genus composed of numerous species that attack woody and some non-woody plants. Historically, mycologists and plant pathologists believed that Phomopsis and Diaporthe were distinct fungi based on their morphology and host preferences (Udayanga et al. 2011). Molecular data revealed Phomopsis and Diaporthe are the same fungus but represent different sexual stages (Phomopsis = asexual; Diaporthe = sexual). Both names have been used extensively throughout the literature, and Phomopsis is often better known to green industry professionals. While Diaporthe was selected for conservation over Phomopsis (Rossman et al. 2015), the sexual stage is rarely observed on diseased woody plant material (Sinclair and Lyon 2005). As such, the use of Phomopsis persists for pathogen identification and disease management.
Diaporthe eres is one of the best-known species, having been documented on hundreds of woody plants (Sinclair and Lyon 2005). Studies have classified D. eres as a complex of closely related but unique species (Udayanga et al. 2014) and as a single species composed of various populations that exhibit host specialization (Hilário et al. 2021; Dissanayake et al. 2024). One recently described population (or species) in the D. eres complex is D. brevicancria, which causes a destructive twig canker on ornamental spruce (Picea) (Skalidis et al. 2021). Another common and damaging species is P. juniperivora, which causes a serious blight of landscape junipers (Juniperus).
Hosts
Due to the tremendous diversity of species in Phomopsis / Diaporthe, hundreds of woody plants are susceptible to infection, especially when they are stressed (Sinclair and Lyon 2005). Some species, such as D. eres, have very large host ranges while others are relatively host specific. Yet, most Phomopsis / Diaporthe species that occur on woody plants are poorly studied and the full extent of their host ranges remains unknown.
Based on samples submitted to the UMass Plant Diagnostic Laboratory, some of the most frequently diseased trees and shrubs in managed landscapes of southern New England include Japanese maple (Acer palmatum), boxwood (Buxus), dogwood (Cornus), holly / inkberry (Ilex), eastern red-cedar / juniper (Juniperus), privet (Ligustrum), apple / crabapple (Malus), spruce (Picea), pine (Pinus), cherry (Prunus), rhododendron (Rhododendron), umbrella pine (Sciadopitys verticillata), eastern hemlock (Tsuga canadensis), and elm (Ulmus).
Symptoms & Signs
Cankering fungi like Phomopsis attack the phloem, cambium, and outer sapwood of woody plants (Sinclair and Lyon 2005). The subsequent damage disrupts or ceases transport of water and minerals beyond the infection site. Phomopsis can cause cankers of young shoots, small stems and branches, and occasionally primary branches and the main trunk of trees and shrubs (Sinclair and Lyon 2005). Depending on the host, the pathogen can also be responsible for a damaging fruit rot (Narouei-Khandan et al. 2017). Foliar infections most often take place on conifers (i.e. juniper) and broadleaf evergreens (i.e. rhododendron) (Bienapfl and Balci 2013; Linderman and Benson 2014). Phomopsis typically becomes established on current year's shoots, or it invades stressed and weakened plant parts through some type of wound (e.g. canker, pruning wound, insect feeding site, freeze injury, mechanical damage, etc.). However, Phomopsis can also gain entry through buds and progress down the stem.
Typical symptoms of infection include thinning canopies, browning and prematurely shedding foliage, scattered branch dieback, and eruptive cankers on the bark. There may also be sunken lesions on shoots and branches with sap-stained bark, along with cracking, splitting, and sloughing bark or oozing sap near the canker site (Sinclair and Lyon 2005). However, in many cases there are no symptoms of infection aside from leaf wilting and canopy dieback, making it difficult to identify Phomopsis as the causal agent.
Phomopsis will sometimes attack larger branches (>4” in diameter) and the main trunk of shrubs and small trees, but it most often infects small diameter shoots and twigs, resulting in sporadic crown dieback. The pathogen is more common on trees and shrubs with thin bark and dense canopies where shade and free moisture linger, such as Japanese maple. After Phomopsis has established within the canopy, it produces an abundance of small, black-colored pads of fungal tissue (pycnidia) that erupt through the bark to release cream-colored spore tendrils (cirrhi). The spores (conidia) are washed down or blown onto nearby stems, creating new infection centers. By themselves, the resulting cankers may do little damage but are produced in such large numbers they often girdle stems and branches. Many otherwise healthy trees and shrubs harbor minor infections in the canopy that can go undetected until the tree is stressed.
Management
Phomopsis species behave primarily as opportunistic pathogens, taking advantage of stressed and weakened hosts (Sinclair and Lyon 2005). Common predisposing stresses include drought, excessive pruning, transplant shock, insect damage, and various compounding stresses common in managed landscapes (e.g. poor quality and compacted soils, girdling roots, frequent mechanical damage from lawn equipment). As with any opportunistic pathogen, minimizing stresses that predispose woody plants to infection is critical. Maintaining high plant vigor is important since natural defenses are often sufficient to minimize damage by the pathogen. To that end, irrigate susceptible trees and shrubs on regular intervals during extended dry periods to minimize drought stress, fertilize if soil nutrients are lacking, ensure the root zone is mulched to help retain soil moisture, limit mechanical wounds (e.g. string trimmers), avoid excessive pruning, especially on recently transplanted trees and shrubs, and avoid needless damage to the canopy and roots. For certain trees and shrubs, especially those with thin and easily wounded bark (e.g. beech, Japanese maple, dogwood), regular sanitation pruning should take place to remove dead twigs from the canopy. If possible, avoid pruning during wet periods in the spring as this is the time when the fungus is most actively sporulating, and sanitize pruning tools after working with plants known or suspected of being infected by Phomopsis canker.
Pruning of cankered stems and branches, at least 6–12 inches away from the blighted tissue (if possible), and removal of the diseased material from the site is the best management practice. Repeated scouting and pruning are often required, and complete eradication is difficult to achieve (Moorman and Lease 1999). For large and mature trees and shrubs, fungicides may also be helpful in suppressing the pathogen once the disease has been identified. Keep in mind that it can be difficult to control cankering fungi like Phomopsis with fungicides because the fungus lives beneath the bark and may be present on branches that show no symptoms of disease. Fungicides registered for use against Phomopsis include: azoxystrobin captan, copper salts of fatty and rosin acids, copper hydroxide, mancozeb, metconazole, phosphites, propiconazole, tebuconazole, and thiophanate-methyl. Applications should be made in the spring to protect newly developing tissues from becoming infected before these tissues mature.
Keep in mind that spores can travel long distances and Phomopsis is very common in the environment, making eradication of the pathogen difficult to impossible. If pruning can effectively remove all cankered stems then fungicide application may be unnecessary. Avoid fungicide application on plants that are actively flowering as the chemicals are detrimental to pollinators.
Literature Cited
Bienapfl JC and Balci Y. 2013. Phomopsis blight: a new disease of Pieris japonica caused by Phomopsis amygdali in the United States. Plant Disease 97(11): 1403–1407. https://doi.org/10.1094/PDIS-03-13-0226-RE
Dissanayake AJ, Zhu JT, Chen YY, Maharachchikumbura SS, Hyde KD, and Liu JK. 2024. A re-evaluation of Diaporthe: refining the boundaries of species and species complexes. Fungal Diversity 126(1): 1–125. https://doi.org/10.1007/s13225-024-00538-7
Hilário S, Gonçalves MF, and Alves A. 2021. Using genealogical concordance and coalescent-based species delimitation to assess species boundaries in the Diaporthe eres complex. Journal of Fungi 7(7): 507. https://doi.org/10.3390/jof7070507
Linderman RG and Benson DM, eds. 2014. Compendium of Rhododendron and Azalea Diseases and Pests. The American Phytopathological Society, APS Press, St. Paul, MN. https://doi.org/10.1094/9780890544396.fm
Moorman GW and Lease RJ. 1999. Effects of pruning in the management of dogwood and pine branch dieback in the landscape. Journal of Arboriculture 25(5): 274–277. https://doi.org/10.48044/jauf.1999.037
Narouei-Khandan HA, Harmon CL, Harmon P, Olmstead J, Zelenev VV, Van der Werf W, Worner SP, Senay SD, and Van Bruggen AHC. 2017. Potential global and regional geographic distribution of Phomopsis vaccinii on Vaccinium species projected by two species distribution models. European Journal of Plant Pathology 148(4): 919–930. https://doi.org/10.1007/s10658-017-1146-4
Rossman AY, Adams GC, Cannon PF, Castlebury LA, Crous PW, Gryzenhout M, Jaklitsch WM, Mejia LC, Stoykov D, Udayanga D, and Voglmayr H. 2015. Recommendations of generic names in Diaporthales competing for protection or use. IMA fungus 6(1): 145–154. https://doi.org/10.5598/imafungus.2015.06.01.09
Sakalidis ML, Medina-Mora CM, Shin K, and Fulbright DW. 2021. Characterization of Diaporthe spp. associated with spruce decline on Colorado blue spruce in Michigan. Phytopathology 111(3): 509–520. https://doi.org/10.1094/PHYTO-08-19-0287-R
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
Udayanga D, Liu X, McKenzie EH, Chukeatirote E, Bahkali AH, and Hyde KD. 2011. The genus Phomopsis: biology, applications, species concepts and names of common phytopathogens. Fungal Diversity 50(1): 189–225. https://doi.org/10.1007/s13225-011-0126-9
Udayanga D, Castlebury LA, Rossman AY, Chukeatirote E, and Hyde KD. 2014. Insights into the genus Diaporthe: phylogenetic species delimitation in the D. eres species complex. Fungal Diversity 67(1): 203–229. https://doi.org/10.1007/s13225-014-0297-2