University of California Cooperative Extension (UCCE) tree nut farm advisor David Doll’s PowerPoint photos of almond trees with severe cases of wood canker-caused trunk diseases are enough to make plant pathologists gleam and growers snarl.

The canker photos are very colorful but signal financial losses, in some cases large amounts, for California almond growers.

“For the growers who lose 30 percent of their trees and income to wood canker diseases it is a severe problem. They are trying to find a way to manage it,” Doll said.

Based in Merced County, Doll has developed data which links an increase in canker-based almond trunk disease to today’s modern cultural practices in almond orchards.

Doll discussed trunk diseases of almond in Tucson, Ariz. in late June during the Caribbean and Pacific Divisions meeting of the American Phytopathological Society, attended by about 130 plant pathologists and industry representatives.

Doll, and a handful of other UCCE tree nut farm advisors, author of The Almond Doctor website on a wide range of almond, walnut, and pistachio-related issues.

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During the APS meeting, Doll discussed canker disease, its negative impact on trees and grower bottom lines, and cultural changes which can reduce losses.

For years, three primary pathogens have taken dead aim at almond tree scaffolds - ceratocystis canker, band canker, and aerial phytophthora.

These pathogens cause infection of the trunk or the primary scaffolds which can lead to scaffold decline, tree death, and crop loss. Growers have enacted management techniques based on weather conditions and reducing damage to infected trees during harvest.

New cankers

Over the last several years, two new types of cankers have appeared – cankers which infect the tree’s pruning wounds and cankers which target tree cracks, festered by the wind.

These cankers are caused by the highly destructive fungal pathogen Botryosphaeria spp.

These latest cankers typically grow throughout the summer and are more commonly found in the Padre and Fritz almond varieties, but are also found in Nonpareil, Avalon, Carmel, and Aldridge.

“Usually when a farmer first sees this disease it is not very noticeable,” Doll said. “Suddenly the canker enlarges and it’s very obvious. Then the tree starts falling apart and the crop yield eventually declines.”

Doll has studied cankers, including those with Botryosphaeriaceae fungi, in almond plantings across Merced County.  One photo he shared was of a tree with the bark removed which revealed a bright yellow-colored canker at the pruning wound site.

“You can see this nice beautiful canker on this tree. Don’t call it a beautiful canker to a farmer. They don’t like that very much,” Doll laughed.

Beauty is in the eye of the beholder.

The fungal infection causes a weakening of the tree scaffolds. Later, the tree splits from the crop weight and must be replaced.Damage is more common 3-6 years after planting.

“Essentially, the tree is worthless to the farmer,” Doll explained. “The farmer loses the crop on the tree, but also faces the added costs to replant and bring the tree back into production.”

Doll has conducted studies in six Merced County almond orchards, looking for Botryosphaeriaceae fungi-caused cankers, the amount of damage to trees, and possible management changes to help reduce the problem.

Of the six fields studied, tree losses varied. The lowest loss was 7 trees in a five-acre area planted in Nonpareil and Carmel in a 18X22 feet spacing (110 trees per acre). About 50 trees had canker damage.

The worst damage and loss was in a 15-acre Nonpareil-Butte-Padre orchard with the Buttes and Padres at the 5^thleaf stage where 116 trees were lost to canker disease. The financial loss to the grower was $16,112, using the UC Tree and Vine Loss Calculator.

Preliminary results

Doll’s preliminary results from the studies suggest that large cuts made on primary and secondary scaffolds are more detrimental to tree longevity. Aldrich and Padre appear to be more susceptible to canker damage than Carmel, Nonpareil, and Butte. More field data is needed to confirm this.

“The take home message is large cuts made on primary and secondary scaffolds can impact the long-term sustainability of the orchard,” Doll said.

Pruning cuts are necessary in almond production to train the tree for growth and production, plus to remove branches so equipment can pass through the orchard.

In recent years, the almond industry has shifted to less pruning to boost tree vigor and yields.

“I really believe we are seeing more canker disease due to shifting cultural practices,” Doll said. “We are growing larger trees which means farmers prune less, but when farmers do prune larger cuts are made. This creates a larger target for fungi and possible infection.”

Doll says the almond industry needs to change cultural management in the early parts of tree development for better canker control. He has several thoughts.

First, make maintenance pruning cuts further from the trunk of the tree.

“This will help keep infection from moving into the trunk and risking the structural integrity of the tree,” Doll explained. “If farmers can prevent infection of the trunk and the original scaffolds coming off the trunk, I think they can do a pretty good job managing cankers.”

Another point, Doll says, is perhaps farmers should re-think the tree vigor issue.

“Maybe farmers shouldn’t grow the trees so fast,” Doll said. “It is a very hard argument to make to an almond farmer but it is something to consider since it impacts the number and size of the cuts along the tree trunk.

Additional research is needed to study the impact of rain events when cuts are made. Instead of making cuts during the summer months, research needs to weigh the impact of cuts during the early spring and late fall.

University of California plant pathologist Themis Michailides of the Kearney Agricultural Research and Extension Center is currently examining the healing time of pruning cuts as it relates to infection susceptibility.

Overall, Doll considers trunk canker disease a low risk to the overall almond industry. Yet control strategies should be developed and implemented early on as fungi infection impacts orchard longevity.

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