There is no miracle management for canker diseases in almond trees, but a correct diagnosis is a good first approach.
“Growers often scratch their heads trying to figure out what’s killing their trees,” says plant pathologist Florent Trouillas of the University of California Cooperative Extension.
He notes, “It can be a fungal canker, bacterial, aerial phytophthora, foamy canker or an abiotic disorder or injury.”
Trouillas, based at the Kearney Research Center at Parlier, says an accurate diagnosis can prevent the expense of unnecessary and ineffective management practices, and actually prevent unintentional disease spread. Finding the true culprit can be an investigative challenge.
The plant pathologist recommends looking for patterns of symptoms, including low or high branches, down the row, or on the orchard edges. Are all plants impacted and are symptoms progressing, he asks.
Growers or farm managers should examine water sources and soil factors, including salinity, cultural practices, and environmental factors, to determine if these factors are have an adverse effect on trees.
The most common canker is Ceratocystis which can kill young trees. This diamond-shaped canker has amber gum at the canker margin, and can be surgically removed from older trees. It is associated with mechanical harvest injury and pruning wounds. Bark injury and pruning wounds are susceptible to the pathogen up to 14 days.
This fungal canker develops only in the cambium and xylem tissue of the current year growth. The infectious material is found under the bark, spread by sap-feeding beetles or drosophilid flies and moved to fresh wounds.
Ceratocystis canker can be managed by avoiding shaker injuries or intensive pruning. Affected trees can be treated with copper oil. Canker removal is best during the winter months when insects are inactive and rain is not in the forecast.
Perennial Phytophthora cankers are associated with scaffold crotch pockets. These fast-growing cankers can girdle the scion and kill a tree over one to two growing seasons. These cankers are not associated with pruning wounds.
Signs of Phytophthora cankers include gumballs throughout the diseased area. Infectious materials from this canker disease are blown into trees during harvest.
Preventative management should begin at planting. Make sure the bud union remains above the soil surface. Proper scaffold selection helps avoid pockets in the tree’s crotch.
Trouillas says phosphite products for canker suppression have been declared pesticides by the European Union, triggering a need for a maximum residue level (MRL). An extension on a temporary MRL has been obtained by Almond Board of California, the California Walnut Commission, and the Pistachio Research Board.
Bacterial canker is a complex disease that’s poorly understood. Trouillas says this canker impacts all Prunus species, is found in sandy soils, and is the most destructive in two-to-six-year-old trees.
Bacterial canker pathogens are present on plant surfaces and enter through tree openings, including lenticels, stomates, and leaf scars. The disease is associated with stress, particularly from the presence of ring nematodes.
This canker disease can be easily identified by its sour smell and the islands of necrotic tissue under the bark with a speckled appearance. The cankers remain above the soil line.
To improve tree vigor, Trouillas suggests a foliar application of nitrogen in the fall. A post-plant nematicide application and the removal of dead branches and trees are also good preventive measures.
Band canker, caused by the pathogens Botryosphaeria dothidea and other Botryosphaeria spp., primarily affects trees during the third-through-sixth growing seasons.
This canker begins to appear in the spring or fall when temperatures and moisture create ideal conditions for infection. Affected trees have amber-colored gum exuding from the cankers, forming a band of gumballs around the trunk of the affected tree.
Trunks and lower limbs are infected through growth cracks in the bark. If not girdled, the tree may recover.
Band canker can be found on all almond varieties, but the most affected vigorous cultivars are Nonpareil, Carmel, Padre, and Butte.
Trunk, scaffold cankers
Trunk and scaffold cankers, caused by fungal pathogens, require pruning wounds or cracks in the bark for tree infection to occur. These cankers are common in orchards located near riparian areas, and are moved into the trees with water.
Tree training is the best option for prevention for these cankers, Trouillas says.