A recurring theme this year has been; “What should we do with these old fruit trees?”
Two instances were established home orchards that had fallen into neglect and new owners were looking for advice.
Some of the concerns included:
— White fungus, dead bark and decay in apple limbs and trunks.
— Amber colored pitch globs on apricot stems.
— Cherry, peach, and olive trees failing to produce fruit.
Those are just a few examples from a host of fruit tree problems.
As an almost ideal area for growing maritime temperate climate fruit and nut trees, Napa home orchards host a wide array. Popular species—with their endless cultivated varieties—include apples, apricots, cherries, Citrus, figs, nectarines, olives, peaches, pears, persimmons, plums, pluots, pomegranates, prunes, and quince. Nut trees that can do well here include almond, pistachio, pecan and walnuts.
Avocados are a sort of Holy Grail for many local home orchardists. In my experience here, avocado trees sometimes grow fairly well but rarely produce good crops.
On the other hand, old figs are almost indestructible, and persimmons seem to have no problems other than bearing too much fruit and breaking themselves apart.
As an arborist, I won’t even get into the topic of other fruits and berries, but I have heard that grapes do pretty well here.
By far, irrigation is the most prominent limiting factor and the cause of many subsequent problems. Many folks seem to be under the misapprehension that established fruit trees do not need irrigation since they typically blossom, leaf out and survive without it.
When we have a close look at such trees, we often see effects of summer drought stress.
Apples, in particular, show areas of dead bark on trunks and limbs where the surface faces the afternoon sun. As sun-injured trees grow older, dead areas progress into decay.
I have seen old apple trees that are, literally, shells of their former selves. Because apple trees are weak compartmentalizers i.e. inherently unable to wall off decay—the trunk becomes a half-cylinder. Yet the tree stubbornly persists.
This often develops, not from a single event, but from a progression of injuries: The foliar canopy thins out under summer drought stress. Exposed limbs gradually die. Dead and dying branches allow hot sun to impact the trunk. Excessive pruning further opens the canopy and makes large wounds. Split gill fungus infects the dying and dead sun-injured stems. Other fungi and organisms take advantage of the exposed inner wood. The tree becomes a hollow relic.
Arboriculture has a term for this: the mortality spiral. The tree goes from “vigorous” to “stressed”, “injured”, “predisposed”, then declining and dead.
Going back to that short list above, consider the amber gobs of pitch often seen on apricots trees, peaches, nectarines and other stone fruits. Commonly referred to as “gummosis”, the condition represents the self-defense effort of the tree. A wound induces a response, where the tree pushes sap into the wound.
In some cases, the wound is from borers invading sun-injured or drought stressed tissue. In most cases I have seen, especially in apricots, the common culprit is a fungal infection, probably Monilinia, sometimes called brown rot canker or apricot blight. Scattered dieback of twigs progresses into severe dieback, with masses of amber gum on scaffold limbs.
This seems to be a losing battle for apricot lovers. Aside from good cultural practices and disease control sprays, planting disease resistant varieties is a good bet. Dave Wilson Nursery offers ‘Harcot’ and ‘Harglow’ apricots, which they say are inherently resistant to this disease.
Olive trees I saw on one property were not producing. The spring shoot growth was pretty good but by midsummer the leaves were dull and folding, typical of summer drought stress.
Lack of pollination is a common cause of stone fruits not producing. That can result from rainy weather during the blossoming period, when honeybees are inactive and pollen is ruined by rain. In some cases a pollinizing companion has died, leaving its partner unable to bear fruit.
In older, neglected trees, the lack of fruit is sometimes the result of rootstock suckers growing up from below the bud union. The tree is no longer the variety that was planted.
For a wealth of knowledge on this deep subject, look up: http://ipm.ucanr.edu/PMG/GARDEN/fruit.html.
While warm, dry weather continues, get out there and take a close look at the fruit trees under your care!