Young olive trees are very sensitive to weed competition. For the first few years of a young tree’s life, its growing capacity can be greatly increased by eliminating weed competition. Hand hoeing is the best organic method for doing so. There are tractor-mounted implements that can help, with care taken not to damage the tender trunks of the small trees. Herbicides can also be utilized, though there are no effective organic options and there is risk of poisoning the trees, too.
Once a reasonable radius (one to 2 feet) around the trunk has been cleared, the soil can be mulched with any type of compost, mulch, wood chips, or straw. Leaving a small ring of unmulched area around the base of the trunk is important for avoiding problems like crown rot.
A vigorous, mature olive tree will no longer need a weed-free radius. Simply mowing the cover crop or weeds around the base is sufficient, though some farmers prefer to disk the rows to keep the orchard floor clean. At McEvoy Ranch we use a flail mower down the middle of the rows and a rotary sidekick to cut the grass underneath the canopy. The irrigation lines must be suspended above ground for a sidekick mower to work.
There are many reasons that an olive tree may have yellow leaves and the course of action you take will depend upon the root cause.
If there are just a few yellow leaves scattered throughout the canopy, it may be natural senescence. Since olive trees are not deciduous, they have continual leaf loss through the season. Most olive leaves have a lifespan of two to three years, at which point they will yellow and fall off naturally. There is often a flush of leaf loss post-bloom.
If the majority of the canopy is yellow, it may be a lack of nitrogen that is causing the yellow leaves. If nitrogen deficiency is the cause, then a few good doses of fish emulsion (or any nitrogen-heavy fertilizer) could help. Use the label on the package to determine dilution and rate for the fish emulsion. Wet, cold soils in winter can also cause yellowing leaves. In this case, creating better drainage for your tree will help.
If your tree is in the shade or inside and does not have enough sunlight it will not photosynthesize properly, resulting in many problems including yellow leaves. If potted, this tree should be moved into an outdoor area with full sun and good air circulation. Potted trees in general need more frequent fertilizing because they have such a limited supply of soil to draw from.
If the leaves are yellow with brown spots, your tree may be infected with Olive Peacock Spot. See below for Peacock Spot for treatment suggestions.
If the leaves are turning brown and staying on the branches, your tree may be suffering from Verticillium wilt, which frequently results from overwatering or poor drainage. Oxygen-deprived roots are more susceptible to verticillium infection. See below for more information or try your local Master Gardener Program.
Olive Fruit Fly (OLFF)
Bactrocera olea is a Mediterranean fruit fly unintentionally imported to California in 1998. The fly does not affect the health of the olive tree but can ruin a crop. The female fly has an ovipositor that she uses to deposit her eggs into the olive fruit. The young larvae then eat their way out of the fruit, leaving a tunneled mess of desiccated fruit behind. There are many options for control from trapping (ball traps, olipe traps, McPhail traps, sticky traps) to spraying (spinosad, kaolin clay). See the >UC Davis IPM website, as well as these articles and the >Sonoma County Extension website for further reading.
Peacock Spot (Spilocaea oleaginea)
Spilocaea oleaginea is aleaf fungus that appears as small brown or silver spots on the upper side of olive leaves, often coupled with a yellowish halo around the dark blemish. Peacock spot, left unchecked, can defoliate a tree; areas with high rainfall in particular are susceptible to infection and must be treated annually. A fixed copper, full canopy spray immediately post-harvest is advised. A second, mid-winter application can also help in overly wet areas. Cercospora Leaf Spot is often found in tandem with Peacock spot and manifests as a grey, ashy fungus on the bottom of the leaf. It should be treated the same as Peacock spot. Annual pruning will help reduce Peacock Spot pressure by opening up the canopy to ample air circulation. See the UC Davis IPM website, as well as these articles published by Paul Vossen for more information.
Over-watered trees or those with poor drainage are more susceptible to infection of the fungus verticillium dahliae. Branch dieback is a typical symptom when the roots become infected; it is also possible for the entire tree to succumb to the disease. The leaves will turn brown but will remain hanging on the branches. It is not recommended to plant olive trees in a field previously planted to other verticillium-susceptible crops such as cotton or any plant in the nightshade family. See the UC Davis IPM website for further information.