Treating gummosis on fruit trees starts with figuring out what is causing it, because “gummosis” is not a single disease. It is a symptom: sticky, amber-colored gum oozing from the bark of a tree. The gum can be triggered by fungal infection, bacterial invasion, physical damage, insect boring, or environmental stress, and the right treatment depends entirely on which of these is responsible. A fungicide applied to a tree suffering from bacterial canker accomplishes nothing, and pruning out damaged wood does little if a soil-borne pathogen is rotting the crown. The first step in any treatment plan is diagnosis.
Why Gummosis Is a Symptom, Not a Disease
When a fruit tree oozes gum, it is essentially mounting a wound response. At the cellular level, enzymes that break down cell walls become highly active, and the tree simultaneously produces new polysaccharides that form the sticky exudate you see on the bark.1Journal of Plant Physiology. Molecular Biology Influence factors and gene expression patterns during MeJa-induced gummosis in peach Think of it as the tree’s version of bleeding: the gum seals damaged tissue, but the damage itself can come from many directions. This is why gummosis shows up on cherries, peaches, plums, apricots, citrus, and mangoes alike, triggered by entirely different organisms depending on the tree and the region.
The practical consequence is that you cannot buy a single product labeled “gummosis treatment” and expect it to work. You need to look at where on the tree the gum is appearing, what the surrounding tissue looks like, what season it started, and whether any other symptoms are present. Those clues point you toward the real problem.
Identifying the Cause
The three broad categories of pathogenic gummosis are fungal, bacterial, and oomycete (water-mold) infections. A fourth, non-pathogenic gummosis, is triggered by mechanical injury, sunburn, or environmental stress with no infectious agent involved at all.
Fungal Gummosis on Stone Fruits
On peaches, cherries, plums, and apricots, the most common infectious cause is fungal. Species in the Botryosphaeriaceae family are frequently isolated from gum-producing lesions on deciduous fruit trees, and the decay fungus Phellinus has been found in over half of sampled symptomatic trees in some surveys.2Plant Disease / APSNet. Insights Into the Etiology of Gummosis Syndrome of Deciduous Fruit Trees in Israel and its Impact on Tree Productivity Cytospora species are another major group, producing cankers, cracking, dieback, and gummosis on peach trees across broad geographic areas.3PubMed Central. Diversity of Cytospora Species Associated with Trunk Diseases of Prunus persica (Peach) in Northern China These fungi typically enter through wounds: pruning cuts, insect holes, frost cracks, or mechanical scrapes from equipment.
Signs that your gummosis is fungal include dark or sunken bark around the gum site, visible cankers that expand over time, and sometimes tiny black fruiting bodies embedded in the dead bark. The gum itself tends to appear on branches and the upper trunk rather than at the soil line.
Bacterial Canker
Bacterial canker caused by Pseudomonas syringae is a major source of gummosis on sweet cherries, plums, and other Prunus species. Cankers and heavy gum production typically show up in late winter and spring, and the disease can kill entire branches or even whole trees as it progresses.4European Journal of Plant Pathology. Review of Pseudomonas species causing bacterial canker of Prunus species with emphasis on sweet cherry (Prunus avium) in New Zealand Bacterial canker lesions often look water-soaked or slimy when fresh, and the gum may have a sour smell. The bacterium thrives in cool, wet conditions and commonly enters leaf scars in autumn.
Phytophthora Gummosis on Citrus
On citrus trees, the usual culprit is Phytophthora, a water mold that attacks the bark near the soil line. This form of gummosis is sometimes called foot rot, and it is one of the most economically damaging citrus diseases worldwide.5PubMed. Incidence, Severity, and Characterization of Phytophthora Foot Rot of Citrus in Texas and Implications for Disease Management Look for gum oozing from the lower trunk, often at or just above the graft union, with bark that peels away to reveal brown, water-soaked inner tissue. Poor drainage and soil that stays wet around the trunk are almost always part of the picture.
Non-Pathogenic Gummosis
Sometimes gummosis has no infectious cause at all. Sunscald on the south-facing side of the trunk, physical damage from lawn equipment, frost cracks, heavy fruit loads that stress the tree, or even certain insect borers can trigger gum production. If the gum appears at an obvious wound site and there are no expanding cankers or rotting tissue beneath it, the tree may simply be sealing a mechanical injury. Non-pathogenic gummosis generally does not require treatment beyond protecting the wound from further damage and letting the tree heal on its own.
Treating Fungal Gummosis on Stone Fruits
For Botryosphaeriaceae, Cytospora, and similar fungal infections, treatment relies on a combination of pruning, sanitation, and preventive fungicide use. There is no curative spray that reverses an active canker. Instead, the goal is to remove infected wood, protect new wounds, and reduce the conditions that let fungi establish.
Cut out cankers and dead wood, making your cuts well into healthy tissue. Sterilize pruning tools between cuts with rubbing alcohol or a dilute bleach solution to avoid spreading spores. Dispose of removed wood away from the orchard rather than chipping it into mulch nearby.
Timing your pruning matters more than most growers realize. Research on peach trees found that spring-pruned trees were susceptible to fungal invasion through wounds for about a week after cutting, and while summer-pruned trees were susceptible for a similar period, fungal colonization rates were much lower during summer.6HortScience. Susceptibility of Peach Tree Pruning Wounds to Fungal Gummosis as Affected by Time of Pruning The same study found that a benomyl-captan fungicide spray applied immediately after summer pruning reduced both gumming and fungal recovery from the wounds. While benomyl is no longer widely available, the principle holds: if you prune during summer and apply a protective fungicide promptly, wounds are less likely to become entry points.
Broadly, avoid pruning during prolonged wet weather, and if you must prune in spring, consider applying a copper-based fungicide or another labeled product to fresh cuts. Keeping trees healthy through proper fertilization, irrigation, and pest management also makes them less vulnerable. Stressed trees produce more gum and are more susceptible to opportunistic fungi.
Treating Phytophthora Gummosis on Citrus
Citrus gummosis caused by Phytophthora responds well to specific systemic fungicides applied directly to the trunk. Research testing several compounds found that metalaxyl, fosetyl-Al, and phosphorous acid (H3PO3) all inhibited the growth of Phytophthora species in treated bark tissue for at least 117 days after a single application.7Plant Disease. Persistence of systemic activity for fungicides applied to citrus trunks to control Phytophthora gummosis These compounds also moved downward through the bark at least 15 centimeters from the application site, providing protection below the treated zone. A trunk paint of fosetyl-Al outperformed a foliar spray for bark-level disease control in lemon trees.
The practical takeaway for citrus growers: trunk-applied treatments work better than foliar sprays for Phytophthora gummosis. Products containing phosphorous acid (sold as phosphonate or phosphite formulations) and metalaxyl-based products are the most common options available today. Apply them as a bark paint or drench to the lower trunk, focusing on the area from the soil line up to and past the graft union.
Beyond chemicals, cultural management is critical. Improve drainage around trees if water pools at the base. Keep mulch, soil, and debris from piling against the trunk. Some growers excavate soil away from the crown to expose the graft union and let it dry, which discourages Phytophthora. Avoid flood irrigation systems that keep the trunk wet for extended periods.
Managing Bacterial Canker
Bacterial canker is harder to treat than fungal gummosis because antibiotics are not widely available or practical for tree crops in most countries. Copper sprays applied in autumn, at leaf fall, can reduce bacterial populations on bark surfaces and limit infection through leaf scars. A second application in late winter before bud break is common in cherry-growing regions. Copper is not a cure for existing cankers, but it slows the spread.
Prune out severely affected branches during dry summer weather, when bacterial activity is lowest and wounds heal fastest. Avoid heavy pruning in late autumn or winter when the bacterium is most active. Selecting rootstocks and cultivars with some tolerance to Pseudomonas is an important long-term strategy, since bacterial canker tends to be worse on certain cherry and plum varieties than others.
How Much Damage Does Gummosis Actually Cause?
Growers sometimes dismiss gummosis as cosmetic because a bit of gum on a branch does not seem like an emergency. The reality is that severe gummosis has measurable effects on both tree health and fruit production. In deciduous fruit trees where the gummosis syndrome was active, yield dropped by roughly 12 to 23 percent compared to healthy trees in the same orchard, and the value of the harvested fruit declined by about 15 to 19 percent due to smaller or lower-quality fruit.2Plant Disease / APSNet. Insights Into the Etiology of Gummosis Syndrome of Deciduous Fruit Trees in Israel and its Impact on Tree Productivity
The mechanism behind those losses is straightforward: gum clogs the water-conducting vessels inside the wood. As gum content in shoots increases, the number of functional vessels drops, hydraulic conductance falls, and water potential in leaves and stems declines. In severe cases, that water stress leads to shoot dieback.8PubMed Central. Ethephon-Induced Gummosis in Sour Cherry (Prunus cerasus L.) : I. Effect on Xylem Function and Shoot Water Status The encouraging finding from the same research is that where gummosis is less severe, the tree’s water-transport system can recover. This reinforces the value of early intervention: catching and treating gummosis before it becomes extensive gives the tree its best chance of bouncing back.
Choosing Resistant Rootstocks
For new plantings, rootstock selection is one of the most effective long-term strategies against gummosis, particularly for citrus. Phytophthora resistance varies enormously among citrus rootstocks. Some standard rootstocks like ‘Rangpur’ lime are known for susceptibility, while various citrandarin hybrids and trifoliate orange crosses offer much better resistance. Research trials continue to evaluate newer hybrid rootstocks for their vegetative performance when grafted with commercial scion varieties, aiming to combine disease resistance with acceptable fruit yield and tree size.9Comunicata Scientiae. Performance of ‘Tahiti’ acid lime on new rootstocks resistant to citrus gummosis
For stone fruits, rootstock resistance to fungal gummosis is less well-defined than in citrus, but vigor and adaptability matter. Rootstocks that promote strong, healthy growth and tolerate local soil conditions tend to produce trees that handle wounds and stress better, making them less prone to opportunistic fungal infection. If you are replanting after losing trees to gummosis, discuss rootstock options with your local extension service.
Common Mistakes in Treating Gummosis
A few errors come up repeatedly among both backyard fruit growers and commercial orchardists:
- Scraping off gum: Removing visible gum does not treat the problem. The gum is the tree’s response to damage or infection underneath. Scraping it off can actually reopen sealed tissue and invite more pathogens in. If you need to inspect the tissue beneath the gum to assess the extent of a canker, scrape carefully and treat the exposed area afterward.
- Using wound sealants: Older advice recommended painting pruning cuts and scraped cankers with tar-based or latex wound sealants. Most current evidence suggests that wound paints do not speed healing and can sometimes trap moisture and pathogens against the wood. Let fresh pruning cuts dry and callus naturally unless you are applying a fungicide directly.
- Treating all gummosis the same: Applying a Phytophthora-targeted fungicide to a peach tree suffering from Cytospora canker, or vice versa, is a waste of money. Phytophthora products like metalaxyl and phosphonates are specific to oomycetes and have no activity against true fungi like Botryosphaeriaceae or Cytospora. Get the diagnosis right first.
- Ignoring drainage: For Phytophthora gummosis especially, no amount of fungicide compensates for a tree sitting in waterlogged soil. Fix the drainage problem or the disease will return.
A Step-by-Step Treatment Approach
If you have a fruit tree actively oozing gum, here is a practical sequence to follow. First, note where on the tree the gum appears. Lower trunk near the soil line on citrus suggests Phytophthora. Branch cankers on stone fruits suggest fungal infection. Cankers on cherry or plum that appeared in late winter with water-soaked bark suggest bacterial canker. Gum at an obvious wound with no canker or rot suggests non-pathogenic gummosis.
Second, carefully examine the bark around and beneath the gum. If the underlying tissue is dark brown, spongy, or water-soaked, infection is likely. If the tissue looks healthy and the gum sits atop a healing wound, you may not need to treat at all.
Third, if the cause is fungal or bacterial, prune out dead and cankered wood during dry weather, cutting into healthy tissue. Sterilize tools between cuts. For Phytophthora on citrus, scrape away diseased bark to expose healthy tissue and apply a phosphonate or metalaxyl-based product to the trunk. For bacterial canker, apply copper sprays at leaf fall and before bud break. For fungal canker on stone fruits, protect pruning wounds with a labeled fungicide and time your pruning to minimize risk.
Fourth, address the underlying conditions. Improve soil drainage. Avoid wounding bark with mowers and string trimmers. Paint trunks with white latex paint to prevent sunscald. Maintain good nutrition and irrigation to keep trees vigorous enough to resist opportunistic pathogens.
What Happens to the Gum Itself
An interesting footnote to gummosis treatment: the gum that oozes from fruit trees is not waste material. Tree gum exudates, particularly from apricot and other Prunus species, are high-molecular-weight polysaccharides with an arabinogalactan structure. Apricot gum has been studied for use as an emulsifying and stabilizing agent in food and pharmaceutical products, and it shows antioxidant properties as well.10PubMed Central. Prunus armeniaca gum exudates: An overview on purification, structure, physicochemical properties, and applications In some parts of the world, collection of tree gum is a small but real industry. None of this helps you save your ailing peach tree, but it does explain why the gum has that distinctive sticky, amber, resin-like quality: it is a complex sugar polymer the tree synthesizes on purpose, not simply dead tissue leaking out.