Dormant season pruning is a traditional tree and garden chore, and it certainly has a rightful place in the annual cycle of garden activities.

Here in Napa Valley, it’s a relatively brief time when many trees and their associated pests and diseases are dormant. But the idea can also apply to summertime.

A basic idea behind the tradition is to do the least amount of harm possible while achieving a desired result. A good example would be pruning a large Monterey pine to reduce the risk of limb failures. In a tree showing warning signs that large branches are going to break, it’s better to intervene and do some thinning to reduce the risk. To prune away live branches is to wound the tree, but if it’s done properly, the benefits will outweigh the harm. The pruning produces a benefit, but if it’s done while the tree is actively growing in mid-spring it interrupts a growth cycle that the tree has just spent much of its stored energy to begin, the result could be a seriously weakened tree.

Seasonal pruning is not only about the tree itself. Monterey pines, especially older trees, are subject to bark beetle infestation, especially when they are stressed and wounded during the active flying season of the bark beetles, spring through autumn.  Therefore, pruning Monterey pines in winter takes advantage of the pest’s dormant season and accomplishes the desired result while the tree is at a maximum of stored energy.

The principle applies, at least in part, to summer pruning for olive trees, primarily for the management of one particular disease: olive knot. As olives have become more and more popular around the county, trees are commonly brought in from old groves in other parts of the state, where the bacterial disease has had many years to get a foothold. Heavily infected trees may look pretty good at first glance, but if you are familiar with the disease, you soon notice the telltale rough galls scattered through the twigs, branches and limbs.

Though the disease tends to weaken and disfigure trees, and it can affect the flavor of olive oil obtained from infected trees, it’s classified only as a “C” pest under California regulations, I learned in a conversation with Joel King of the Napa County Agricultural Commissioner’s Office. It’s so common that efforts to regulate it are pointless. Infected trees are not rejected, confiscated or destroyed by the county agriculture inspectors.  Efforts for control or rejection of infected trees fall to the buyer. Caveat emptor! 

In contrast, “A” pests, like the medfly, gypsy moth and light brown apple moth get highest priority to prevent them from becoming established in the state.  “B” pests, like glassy winged sharpshooter and sudden oak death disease, have next priority in attempts to limit them to existing infested counties.

Olive knot, which appears as rough bumps on branches and limbs, is caused by the bacterium Pseudomonas syringae savastanoi. It may enter any opening in trees, such as pruning wounds, freezing cracks, and even the tiny scars where old leaves fall from the tree. 

Key factors for seasonal pruning include that the bacteria spread readily in water, and infection tends to occur at low temperatures. So, pruning in warm, dry weather — July and August are pretty reliably warm and dry in Napa — is a simple management practice that can help interrupt the disease cycle. 

 Summer pruning should include eradicative pruning to remove infected branches. In some trees I’ve seen dozens of small outer branches festooned with pea-sized galls. If infection is not too extensive, these can be removed back to healthy lateral branches without significant structural damage.  Galls may form on larger diameter branches as well, so eradicative pruning could be severely disfiguring to the tree. That’s where other modes of control come in to play.

The UC IPM Online Pest Management Guidelines recommend Gallex, for direct treatment of galls on larger branches. The material, which is pink and smells something like creosote, is painted full-strength on the galls. UC IPM also recommends fixed copper sprays for protection against new infections in fall and spring.

Other fruit trees such as apricots, cherries, peaches and nectarines often have fungal or bacterial infections with similar modes of transmission.  Summer pruning for disease control can be beneficial for them as well. It’s also been shown that  summer pruning can have a somewhat dwarfing effect on trees.

At this time of year, the trees have generally finished a flush of growth, which is beginning to store energy. Pruning to remove terminal shoots will redirect growth to lateral buds further down the shoot. Some of these may sprout, helping to generate a compact branch structure, which is desirable for most home-orchard fruit trees. Other lateral buds may not sprout, but instead form flower buds or fruiting spurs, increasing the amount of fruit you can reach in the lower canopy.

Fruit tree pruning is not just for winter. Not by a long shot!

Bill Pramuk is a registered consulting arborist. Contact him through his Web site www.billpramuk.com, info@billpramuk.com, or call 226-2884.