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Pest, Weed, and Disease & Frost Control

 Olive trees are native to some parts of Africa, tropical and central Asia and the Mediterranean. Several things are required in order to ensure that the olive trees produce quality fruits. They need a winter chill, a frost free environment and a long, hot growing season to set the fruit. The process of growing olive trees can be quite challenging. Below are some of the challenges and how to manage them.

Pest Control

  1. Olive Fruit Fly

The olive fruit fly is probably the most dangerous threat for olive growers. It’s about 5 mm long, has small antennae, large reddish eyes and is brown in color. Their larvae are yellowish-whitish maggots with a pointed head.

It’s believed to have originated in east Africa but now it can now be found throughout the state of California. They like infesting olive trees in landscape and urban locations that are not actively managed; causing significant damage to the olives.

There is a lot of research being done on the olive fruit fly including possible control measures. The olive fly infests and starts to damage the fruit when it starts to turn color and soften…usually from September to November.

The fly can lay up to 400 eggs in one fruit. Unfortunately the eggs are difficult to see and are usually embedded under the surface of the fruit so they can’t be manually removed. The larvae that hatch from the eggs are also difficult to see during the initial stages but when they feed for a while they get larger and more visible.

The first sign that an olive has been damaged by the olive fruit fly is an oviposition sting on the surface of the fruit and sometimes there is discoloration. The larvae tunnel through the fruit while feeding, allowing bacterial and fungal infection.

This considerably drops the quality of olive oil because the free fatty acid level of the fruit increases significantly when infected. Sometimes the damage to the fruit can be so severe that it causes it to drop prematurely.

It has been known to destroy 100% of the crop in places where it is not controlled. There are areas where growing olive oil is not possible because controlling the pest is not economical.

The fruit can be visually inspected to check for damage. It can be inspected for decay, tunneling, maggots or oviposition stings. Adult flies can be caught earlier in the season using yellow sticky, Olipe or McPhail traps. Comparing the number of flies caught before and after treatment will help determine if the treatment method being used is working.

Controlling olive fly infestation should be a systematic and vigilant process. The fly could already be in the orchard whether it has not been seen or there is no evidence of fruit damage. The best way to manage the olive fly is to monitor population with traps and apply bait sprays when the fly population seems to be increasing in early summer. You should also get rid of all the fruits during harvest including the one on the ground.

There are four ways that you can control olive fruit flies:

  1. Chemical control

Chemical control involves a bait and insecticide mixture. Aerial application is not recommended for bait sprays because it results in very tiny droplets which evaporate very fast and end up being ineffective; ground application is the best option.

The flies will feed on the bait and die so there is no need to cover the whole tree. The bait spray should not be applied when the weather is too hot because it will evaporate too fast. The best time to apply is in the early morning or late afternoon, on every other tree each week.

  1. Barrier Control

Kaolin clay is mixed with water and applied using a high pressure sprayer. Once the solution dries up, it forms a white powder which repels the flies. It’s not clear why it repels them given that it is not toxic to them. This mixture should be applied every six weeks and the first application should be a week before pit hardening.

  1. Cultural Control

All fruit on the ground should be destroyed by taking it to the landfill or burying it in the ground at least 4 inches deep. All fruit remaining on the trees after a harvest should be removed. Fruiting on landscape trees can be prevented by destroying the fruit on the ground to reduce the invasion of commercial grooves.

  1. Biological Control

A number of other parasitoid species that are not harmful to the olive fruit can be introduced in the orchard to help get rid of the pest.

Weed Management

If weeds are not properly managed in olive orchards, they can create a number of problems. First, weeds are known to compete with plants for nutrients, water and even sunlight. Second, they lower productivity of other plants and thirdly, they create a breeding ground for pest infestation.

Weed management depends on the appearance of the orchard, terrain, irrigation method, soil type and weed species. Generally, weed management should start before planting. It is critical that weed management be done around young trees to prevent them from dying or becoming susceptible to infection.

There are many options for weed control. You can hand weed around the trees or use herbicides. Apart from being an environmental hazard, herbicides have several disadvantages. 

There are many ways you can manage weeds in your olive oil orchard:

  1. Mulches

Mulches prevent weed germination and growth by blocking sunlight. You can use organic or synthetic mulches. Organic mulches have to be replaced annually because they degrade with time. As the organic mulch degrades, it becomes a new breeding ground for weeds. Organic mulches should never be applied when weeds are already growing. Organic mulches can be made from shredded tree prunings, sawdust, composted wood chips, green waste and cereal straw. Examples of synthetic mulches include polyester, polypropylene and polyethylene.

  1. Cover Crops

These can be planted to replace the weed vegetation but it is critical that the cover crop chosen should not compete with the olive trees. Cover crops should be changed to reduce the potential for insect pests, rodents and weeds. 

  1. Herbicides

Herbicides registered for use on olive trees should be safe to use and they control most weed species. It’s advisable to first of all identify the type of weed species that is in the orchard before using any type of herbicide

  1. Cultivation

Some olive tree growers prefer to use cultivation or mowing between trees especially for the first year or two after planting. This method only works best when the weeds are still in their seedling stage. You can also use the cultivation method in established orchards to control perennial weeds. Trying to cultivate established perennial weeds in an irrigated orchard only increases the weed problem. In addition, it might result in damages and cuts on the roots of the olive trees.

  1. Flame Weeding

Propane fuel flamers can be used to control very young weed seedlings. Repeated flaming is a great way of controlling perennial weeds but it doesn’t work well on grasses because their burning point is below the ground. This method of weed management should also not be used around young trees.

Flaming should never be used around dry vegetation because it might start a fire. This method might also ignite mulches.

  1. Animals

Animals have been used for years to control weeds. An example of an animal that has been used in olive orchards is the weeder geese, which prefer to eat grass species. Be careful not to place them where there are grass crops like sorghum. Goats and sheep can also be used to manage weeds but they will only eat the weeds to the ground level. This will only reduce weed competition but will not completely eliminate it. Goats need to be carefully managed because they are browsers and could eat the trees.

The animals should be protected from the sun and supplied with water. You can keep them where you want them to be by using movable fencing.

Disease Control

It is very important to identify some of the diseases that affect olive trees and how to manage them to avoid losing 100% of the crop. Some of the most common diseases that affect olive trees include:

  1. Oak Root Fungus

This is not a serious disease but over time it can cause serious damage to the tree and is more common in wet winters. Infected trees appear weak with thinning canopies. The fungus attacks one side of the tree and then slowly involves the entire tree over several years. 

  1. Diplodia Canker

This disease does not affect the tree but it does worsen the damage caused by olive knot disease. 

  1. Olive Knot Disease

This is a detrimental disease to the olive trees and is caused by bacteria. Olive knot looks like swellings of about 1.3-5cm. Bacteria manifests itself where there are openings on the tree which result from orchard operations or leaf and blossom scars.

The swellings interfere with transportation of sugar and water resulting in death of branches and twigs. Bacteria survive in the knots and are transported to other parts of the tree by water. Fortunately, the disease does not affect the roots. Infections usually start during the early stages when the olive tree is still a seedling. So infections may not be noticeable immediately.

It is difficult to control olive knot disease because there is no cure but it can be prevented. It’s advisable to protect the leaves by applying copper containing bactericides on the scars. This will help to minimize the disease provided the copper treatment is applied repeatedly when new wounds occur. Copper treatments should never be applied before a harvest because they are toxic.

Pruning is also a reliable method of controlling the disease because it helps to remove galls on branches and twigs. Pruning shears must be disinfected every time after pruning to avoid re-infecting the trees.

  1. Olive Leaf Spot

This is another common disease although it takes years for it to become serious enough. New infections typically occur during winter and fall. The disease affects the fruit stems, the fruit and the leaves. During summer, the disease is inactive provided the conditions remain hot and dry. The disease is spread by water which explains why it affects the lower part of the tree.

A common symptom of the disease is sooty blotches on the leaves which later develop into circular spots with a yellow halo around them. If you notice this symptom, you should treat the trees annually. 

  1. Verticillium Wilt

This is a serious incurable disease in olive trees. The first symptom of this disease is sudden wilting of leaves or branches early in the growing season. As the tree grows, the rate of infections also increases.

Since there is no cure for this disease, the best management strategy would be to prevent it in the first place. One way of doing this is by planting in areas where crops that are highly susceptible to verticillium wilt haven’t been planted. Plants that are highly susceptible to the disease include potatoes, peppers, cotton and tomatoes.


Frost Prevention

Temperatures below 29F can damage the olive fruit and the branches. This depends on the duration of the cold temperatures, the age of the tree and the olive variety. Here are some of the ways to reduce frost:

  1. Spray the trees with products containing copper compounds because they have been proven to be the most effective for reducing frost activity. They should be applied in advance of the frost and not when the frost is already taking effect.
  2. As winter approaches, the amount of nitrogen fertilization should be reduced and deficit irrigation should be introduced. This will ensure that the olive trees will not be flush with moist growth as winter approaches.
  3. Make sure there are no weeds in the orchard. Hand weeding, herbicides, cultivation methods or any other weed management method should be used to remove weeds around trees.
  4. Plant new olive trees as soon as possible after the last frost date so as to give them enough time to adapt before the next winter.
  5. Only the most suitable variety of olive trees should be planted to suit the conditions of a particular area. A local farm adviser can recommend the most suitable varieties.


In order to produce high quality fruits, it is very important to manage diseases, weeds pests and prevent frost. If any of the olive trees are exposed to any of these four elements, the quality of the fruit will be compromised. From the information given here, it is evident that prevention is the best way to manage pests, weeds, diseases and frost. Before using and chemicals to control diseases, weeds and pests, ensure they are safe to use and won't affect the olive trees.


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