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Pruning wounds can lead to cankers, UC specialist warns

Dennis Pollock Brent Holtz
Brent Holtz, University of California farm advisor for San Joaquin County, left, and Florent Trouillas, assistant University of California Cooperative Extension specialist with Kearney Agricultural Research and Extension Service, appear at a recent grower workshop.
Florent Trouillas, assistant University of California Cooperative Extension specialist, offers tips on how to prevent cankers from developing.

To paraphrase Shakespeare, almond pruning can be the unkindest cuts of all. They can open the way to a range of almond canker damage, says Florent Trouillas, assistant University of California Cooperative Extension specialist at the Kearney Agricultural Research and Extension Center.

Speaking at a Central California Almond Day at Fresno, presented by West Coast Nut, he gave some practical suggestions on how to prevent cankers from developing.

Among his tips: (1) prevent disease establishment in the early years, (2) prevent wounds near the trunk, (3) use promising fungicides, including Topsin M and Trichoderma products, (4) don’t prune trees during rainy weather, (5) remove dead wood, stumps, and trees from the orchard, (6) avoid wetting trees with sprinklers, (7) use remedial surgery, and (8) cut into clean wood.

Air blast sprayer application of fungicides is being tested, Trouillas says. Pruning sealers and acrylic paint do not help greatly, and appropriate tree training and scaffold selection, along with minimal pruning, are important.

26 FUNGAL SPECIES

He described various trunk and scaffold canker diseases, including fungal canker diseases that include Ceratocystis, Botryosphaeria, Eutypa, Cytospora and Silver leaf; as well as aerial Phythophtora, bacterial canker, and foamy canker.

His research has focused on diagnosing and determining the incidence of cankers in California almonds, identifying the pathogens that cause them, determining which pathogens pose the greatest threat, and pruning wound protection. In three years, he has surveyed and sampled 100 orchards from two years to 25 years old in 13 counties, and has identified 26 fungal species.

The Ceratocystis canker is associated with mechanical harvest injuries, he says, and pruning wounds and cankers are most active during the growing season. Bark injuries and pruning wounds are susceptible for up to 14 days.

It’s best avoided by avoiding shaker injuries and intensive pruning. The wounded areas should be cleaned to promote healing and callusing. A copper oil treatment, Topsin M, can be used, but paint, sealer, or tape are not recommended. It’s best to do “surgery” in winter when insects are not active, Trouillas says.

One way to avoid Phytophthora cankers is avoid “scaffold crotch pockets” that hold water. These cankers are fast growing and trees may die over one or two growing seasons. Gum balls occur through the disease area, and inoculum is blown onto trees during harvest.

The bud union of almond trees should be above the soil surface, he says, and scaffolds should be chosen to avoid forming pockets. While phosphite drenches and foliar applications can help, it should be pointed out that the European Union recently decided that phosphites are pesticides and there are maximum residue limits.

SPRING TREATMENT

One way to address Phytophthora cankers is early spring application of mefenoxam.

Band canker, associated with Botryosphaeria canker diseases, occurs with vigorous cultivars that are two years to six years old, and in orchards receiving excess amounts of nitrogen and water. Trees afflected with band canker usually do not die.

Sprinklers wetting trees favor band canker, Trouillas says; he recommends using splitters or drip for irrigation.

Botryosphaeria is caused by fungal pathogens and has a broad host range of hosts, including walnuts, pistachios, almonds, and grape vines. It’s common in riparian areas.

Eutypa dieback is associated with scaffold crotch pockets and pruning wounds; it likes more humid regions.

Trunk and scaffold diseases reduce the orchard life span and may go unnoticed during early stages of infection. Drought may exacerbate problems. Common symptoms include discoloration of vascular tissues, wood necrosis, and extensive gumming.

Management strategies against trunk and scaffold canker diseases rely mostly on remedial surgery and removal of trees, as well as taking precautions against infection.

Trouillas warns against pruning trees during rainy weather.

He is studying application of various pruning wound protectants. Photos of various cankers and tips on managing them are available at http://ceyolo.ucanr.edu/files/232615.pdf

TAGS: Crop Disease
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