Is defoliating for rust management in almond leaves a good idea?

David Doll, a University of California (UC) Cooperative Extension farm advisor in Merced County, stops short of an absolute no, but says, “I don’t think it’s as effective as we thought it was.”

Not only that but it can cost a lot of money, he notes.

In an internet posting, Doll says authors of UC guidelines on the notion of foliar fertilization to cause phytoxicity and defoliation are rethinking the idea and considering it ineffective, and no longer suggested as a general strategy for rust management.

The focus of the posting is the use of zinc sulfite in an attempt to trigger defoliation to combat the foliar disease rust caused by the fungus Tranzschelia discolor, which Doll said has been a major problem this year.

Doll said the material is expensive at $50 to $60 an acre, “and it is not doing what is supposed to be doing.” He added, “If it’s done effectively, it’s probably worth considering, but it’s not being done effectively.”

He said application would be needed in early October at the latest, and at a rate – at 40 pounds – than currently applied. Doll said zinc applications are often made too late to cause effective defoliation, and warmer fall temperatures or increased tree vigor has made applications less effective.

The farm advisor said increasing almond acreage and the lack of consistent area-wide sanitation practices reduce the efficacy of treatments since residual pathogen populations move rapidly from orchards which are not sanitized.

And, he adds, defoliating trees too early may reduce carbohydrate reserves, impacting next year’s crop.

“In contrast,” Doll said, “the money used for this application should be budgeted for an additional late spring or early summer spray targeting this disease. This most likely will be more effective in reducing pathogen populations, leading to a healthier orchard.”

On the leaf, the disease is characterized by rust-brown or black-colored spores which form on the side plus angular chlorotic (yellow) lesions on the upper side of the leaf. Rust can spread easily from orchard to orchard via the wind. Minimal periods of leaf wetness are required for the pathogen to infect.

This past year, Doll said long periods of conducive conditions, which included warmer temperatures, late spring rains, and increased canopy humidity making rust management a challenge.

The fungal pathogen attacks plants of the genus Prunus, including almond, apricot, cherry, peach, nectarine, plum, and prune.

Doll said rust is easily controlled by properly timed fungicide applications and that several modes of action provide protection. Those with the highest activity are FRAC Groups 3, 11, and 19 or mixtures of these groups.

Several broad-spectrum fungicides including micronized sulfur (M2, microthiol) and chlorothalonil (M5, Bravo) have very good efficacy with short- and long-term residual activity, respectively.

Doll said, “Since rust can infect from spring to summer, sprays need to be timed accordingly.”

He said spring sprays – two to five weeks after petal fall, based on monitoring for rust symptoms - are often used to initiate rust control practices at the beginning of a potential epidemic.

“One fungicide application, however, may not provide effective control if favorable conditions persist,” Doll said, adding that later sprays may be needed.

“Typically, spring time disease management programs for scab and Alternaria leaf spot will also provide management for rust,” he added.

High populations of rust can prematurely defoliate trees. As the fungus spreads, it ruptures leaf tissues, reducing the photosynthetic potential of the leaf and eventually causing leaves to fall. If too many leaves fall from the tree, the tree will re-leaf, reducing the cropping potential for next year.

“Although this re-leafing is concerning,” Doll said, “it is better (for) the tree to re-leaf than not to grow at all.”

Rust overwinters on leaf tissue, infecting leaves on the trees or debris stuck in the crotches of the trees or on the ground.

Compounding the challenge are numerous varieties of almond, many of which hold their leaves until late fall and even until winter, while others leaf out early, providing susceptible tissue during early spring rains.