By Dr. Thomas T. Yamashita

Over the years, I have often been called out to examine orchards in which light to severe gum exudation is emerging from the trunk and main scaffolds of almond and peach trees. The cause of this gummosis varies. Below is an overview of some of the most common causes.

Almond trees varieties that are faster growing form pockets of weak tissue that are vulnerable to gummosis.

Fast growing varieties, such as the Padre and Aldrich almonds, are known for their rapid rate of growth. If a grower is conscientious and brings them up rapidly, the rate of tissue development literally exceeds the rate at which the root system can harvest key elements. Foremost among these elements are calcium, boron, phosphorus, and potassium. These elements, especially calcium and boron, are instrumental in stabilizing the integrity of tissues.

Gummosis occurs when the rate of root harvest of these elements lags behind demand. The plant does not hold back growth due to a deficiency, instead continuing to form tissues. The tissues formed during these periods of rapid growth are consequently quite weak.

As the girth of the trunk and limbs expands, these weak points are the first to split. The tree responds to the splitting with a healing response, whereby profuse quantities of gums are formed to seal the cracks and prevent the entrance of pathogens. A similar phenomenon occurs in slower growing, weakened trees which are suddenly spurred into rapid growth by external stimuli, such as wet weather. Pockets of weak tissue are formed, undergo natural attrition, and eventually deteriorate and become necrotic. The tree will again respond to this injury with the formation of gums, which are exuded to the outer bark.

Gummosis is common in part because calcium—and boron to a lesser degree—is readily locked into the soil. For instance, calcium is commonly locked in the soil as calcium carbonate and calcium phosphate, both of which are relatively insoluble. Their availability is further reduced by the absence of soil microbial activity, a condition highly characteristic of most agronomic soils. Many organic acids and related byproducts of microbial metabolism are known to act as chelating agents, thereby enhancing the release of otherwise locked minerals.

Gummosis is further induced when secondary pathogens colonize the open wounds. Byproducts and direct enzyme activity of the intruding organism elicit further gumming. Various organisms can be isolated from these wounds, but none of them are original cause. If there is abundant moisture as provided by sprinkler irrigation, rains or heavy dew, the most common organism found is Phytophthora. Drier conditions are associated with Botryosphaeria, Ceratocystis, and Penicillium.

Pruning and wounds left by wood borers can instigate gummosis.

Younger trees that receive multiple pruning cuts are often subject to gummosis. Poor nutrition and incomplete healing of the pruning wounds invites many of the same secondary pathogens mentioned above.

In addition, callus tissue formation and bark convolutions resulting from attempts at wound healing invite various wood­boring species. The most commonly isolated wood-boring species are the American plum borer, prune limb borer, and the peach twig borer. The folds and convolutions in the bark serve as desirable hiding spots for pests. However, these convolutions are also the site of natural weak points where the gumming response fails to occur in a timely manner. Because the bark has already been distorted and folded naturally, the entrance of a wood borer does not elicit the normal, copious exudation of protective gums that would occur on smoother bark. By the time the gums finally push forth, the borer will have rendered considerable damage.

A similar entourage of secondary fungi are also often seen colonizing these wounds. In many cases the problem can become severe enough to cause limb breakage (especially during wind storms), severe secondary infections that kill the limb or entire tree (e.g. aerial Phytophthora), and complete girdling of the limb or trunk by the wood borer attack.

Infections of aerial Phytophthora following freeze damage often result in serious cases of gummosis.

Extensive rainfall or irrigation wetting of the wood oftentimes creates conditions conducive to aerial Phytophthora infections. In many cases, this infection is associated with cold damage. The first freeze, especially following a previous span of mild weather, can especially affect weak points in the bark or wood. The weak tissue pockets formed due to the lack of calcium and boron are oftentimes the first to succumb to this freeze.

Following a freeze, minute to macroscopic fissures can be seen on the bark. If rains follow this damaging freeze, cold-adapted species of Phytophthora (e.g. P. syringae) can invade the wounds and colonize aerially. The freeze-damaged cells typically exude or leak cell sap, a food base conducive to the rapid spread of the fungal pathogen.

Some years ago, we had such a scenario play out, where in mid-January temperatures suddenly dipped to about 25 degrees Fahrenheit. This freezing weather was preceded by mild temperatures. Following the two days of freezing temperatures, there was a day of rain. The result was an epidemic of aerial Phytophthora and subsequent gummosis. Many orchards still showed the aftermath of these events nearly a year later.

The splitting produced by crown rot and collar rot is a common cause of gummosis.

Orchards sometimes manifest trunk gummosis that originates from the crown. Upon excavating the area, it becomes clear that these infections originate from Phytophthora crown rot. Many species of Phytophthora can cause this syndrome. For example, P. cactorum and P. cambivora can be isolated from apricot, peach, plum, prune, and cherry trees. Phytophthora cactorum, P. citricola, P. megasperma, and P. cinnamomi are often found associated with walnut trees. Phytophthora cactorum, P. syringae, P. megasperma, and P. citricola are often isolated from infected apple trees.

Many times, the fungus will move upwards towards the trunk and continue to incite trunk canker, thereby eliciting above-ground gummosis symptomatology. This behavior is characteristic of P. cactorum, P. cambivora, P. citricola, and P. cinnamomi, but not P. megasperma, a water mold that primarily invades roots.

Treating injured and vulnerable bark can help to mitigate and prevent gummosis.

Gummosis of the bark should always be approached in the following order.

First, clean up the existing infection, and prevent secondary infection of the wound with a deep, penetrating fungicidal treatment (use amounts listed below per 100 gallons of water):

• Captan WP – 4-6 pounds
• Liquid Calcium – 1 gallon
• Liquid 10% Boron – 1 pint
• Silicone Surfactant – 6-8 ounces

Thoroughly coat the affected tissues with the mixture. If gum balls are hardened, wait until early morning dew or a shower softens the gum. This allows the material to creep behind the gum and coat the affected wood. Apply with a handheld spray gun. Be sure to check for proper registration of the fungicide for the crop treated. Other broad-spectrum fungicides that may work are Bravo and Dithane. If borers are present, include an insecticide.

Then, provide for more long-term protection of the wound to allow time for the formation of callose tissue, and healing of the wounds:

• Captan WP – 4-6 pounds
• White Latex – 33 gallons
• Tap Water – 66 gallons
• Silicone Surfactant – 6-8 ounces

Before applying, allow the above spray to soak into tissues for at least 24 hours before coating with this mixture. The white latex extends the longevity of the fungicide. If borers are present, include a recommended rate of insecticide. Be sure to check for proper registration of the fungicide and insecticide.

The spray and paint treatments should be completed before extensive rainfall commences. If irrigation waters (e.g. sprinklers) are striking the area, alter the irrigation delivery to minimize wetting of the wounds.

If the gummosis originates from the crown, excavate representative trees to determine if the crown area is girdled. If crown damage is less than 25 % of the circumference, you have a chance of saving the tree:

• If viable foliage remains on the tree, treat foliarly with a systemic fungicide (e.g. Aliette).
• Dowse the infected crown and surrounding areas with a systemic fungicide (e.g. Ridomil) or a broad-spectrum contact fungicide (e.g. Bravo, Captan, Dithane, etc.).

Then, determine the soil mineral content and correct deficiencies in calcium, boron, phosphorus, and potassium, as shown in the below chart.

After supplementing minerals as needed, escalate the microbial activity of the soil to maximize release rates of the supplemented elements. Lastly, follow up the next spring with topical fungicide treatments of wounds or gumming sites, using the latex-based treatment above.