Colorado's peach industry is threatened by a fungal disease. Helen H. Richardson/The Denver Post via Getty ImagesIn western Colorado, home to the treasured Palisade peach, cytospora canker is one of the most economically consequential fungal diseases faced by growers. A recent survey conducted by Colorado State University in Orchard Mesa found that 100% of the orchards have trees infected with cytospora canker. In some orchards, you can smell the sweetness of gummosis, the sweet oozing of sap from a tree that occurs from injury, stress, pathogen infection or insect damage. We are part of a team of fruit tree growers, extension personnel and researchers who are developing tools for mitigating cytospora canker in fruit tree orchards in Colorado and Utah.In a study we published, we estimate this disease results in at least US$3 million in annual economic losses for growers in Colorado. In infected large branches, which are called scaffolds, the damage can result in a 50% loss of peaches per tree.Peaches were first planted in Palisade and Grand Junction in 1882 by one of the first white settlers to the area, John Harlow. Peaches and other fruit trees have been Colorado staples ever since. In 2024, Colorado farmers produced roughly 15,000 tons of peaches valued at $34 million. However, fruit tree production in the Intermountain West, which covers Colorado, Utah and Idaho, is threatened by diminishing water supplies, spring frosts, variable winter temperatures and soils that are above the ideal pH range for peach trees. Further exacerbating the environmental stresses are pest problems and the persistent cytospora canker disease. What is cytospora canker?Cytospora canker is caused by fungi within the genus Cytospora. These pathogens are found globally and affect more than 70 species of woody shrubs and trees. These fungi have been present on fruit trees in the U.S. since at least 1892 when cytospora canker was first discovered on peach, plum and almond trees in Pennsylvania and New Jersey. Cytospora canker was first described as only a disease of stressed trees, but now it is recognized as a destructive disease in tree fruit across the U.S. Plant Talk Colorado: What is cytospora canker? A video from Colorado State University Extension. Growers expect peach trees to live for 20 years. The first five of those years are initial growth. The next 10 years are full production. Then, the tree’s productivity tapers off in the last five years of its life. The disease has halved the life of an orchard in Colorado from 20 years to 10 years or fewer. Trees that get infected during the first or second year are typically dead by year four or five before they reach peak production. Cytopora canker typically enters through wounded and woody branches or twigs. Wounding occurs when branches are pruned to maintain tree vigor or through severe freezing or hail events. Freeze events are common in Colorado and are particularly harmful in the fall if temperatures drop abruptly without giving trees enough time to acclimate to the temperature shift. Ice formation within plants causes swelling and cracking in woody tissues, as well as the formation of ice crystals within plant cells that can puncture the cells, leaving them vulnerable to oxidative damage and infection. Small cracks enable cytospora spores, like the seeds of a plant, to enter and begin to cause infections.Cytospora canker and freezeIn 2020, a major freeze event damaged many trees throughout Colorado. Following a warm October, temperatures dropped from 65 degrees Fahrenheit (18 degrees Celsius) to below 10 F (-23 C) in a 48-hour time span in the fruit region around the town of Hotchkiss. Because the recent temperatures had been in the 70s, there was not an appropriate amount of acclimation in the trees to be prepared for this large temperature drop. Leaves were still green, and sap was still flowing through the woody tissues.The damage from this single freeze directly led to the death of tens of thousands of peach trees across the western slope of Colorado.The sudden freeze also allowed for a proliferation of new cytospora canker infections on peaches trees that were not killed outright by the freeze. The surviving trees were often more vulnerable because the cracked skin and bark of peach branches was now exposed to infection by the fungus. This correlation between cytospora infection and cold damage is thought to be a major reason why cytospora canker is a particularly significant disease in Colorado. To manage the pathogen, growers can remove trees that are infected, protect wounds with chemicals to prevent new infections and ensure that established trees are free of stress. However, management strategies have limited efficacy due to the growing conditions. While Palisade has the most ideal peach-growing microclimate in Colorado, the cold season is near the limits of what peaches can tolerate. In April 2026 there were several nights when the temperatures reached into the low 20s F (-7 degrees C) in different orchards in Delta County, Colorado. Fruit had already started to grow and was very susceptible to the cold temperatures. As a result, growers around Hotchkiss and Paonia lost their peach crop. Palisade orchards avoided that level of damage because on those same nights the temperatures dropped only to the upper 20s F (-2 degrees C), which damaged some fruit but left enough behind to have a full crop in most cases. Spring frosts like these reduce fruit production but generally aren’t going to contribute to increased proliferation of cytospora canker.Solutions in progressResearchers from Colorado State University are working toward developing strategies to combat this disease. Our team has developed chemical options for conventional and organic growers that have helped slow the spread. We are determining whether some peach cultivars are tolerant to the pathogens, and we are continuing to understand the population biology of cytospora to help us develop new management strategies. The pathogen can be spread through air, on insects, during irrigation and possibly with the movement of new peach trees into orchards. Many fungi that produce cankers in trees can move spores only short distances through rain splash. But spores of the fungus have been found in collection traps about 250 feet (76 meters) from a tree with canker that is making spores. We have established the cytospora working group as a collaborative research, extension and grower group to collectively develop solutions for cytospora canker. We are continuing to better understand factors involved in disease development and establish best management practices to help growers combat this disease and keep the Colorado peach industry vibrant. Read more of our stories about Colorado.Jane Stewart receives funding from USDA NIFA AFRI. David Sterle does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.