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16 Citrus Tree Diseases and How to Treat Them

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Being able to grow your own lemon trees can be a great experience. But no matter how rigorously you care for them, sometimes problems still appear. There are lots of diseases that affect citrus fruit trees. All kinds of bacteria or fungi can affect your lemon or orange trees. Other dangers like nutritional deficiencies or insect pests can also cause damage to various tree parts. Understanding all these diseases as well as following the correct treatment procedures represents an essential aspect of caring for any kind of citrus tree.

While some diseases affect certain species such as lemon trees more compared to others, it’s important to be aware of all the citrus fruit tree diseases out there. The article will primarily focus on the most common diseases experienced by citrus tree owners. It’s safe to say that you will usually run into more problems when you’re growing your trees in areas of high humidity and rainfall. Regular pruning is often the preferred prevention method for most diseases. It’s time to figure out how to maintain your citrus trees healthy and keep strong control of various diseases.

1. Melanose

Melanose is a common citrus fruit tree disease that creates a visual impact instead of affecting fruit quality. Grapefruits are particularly susceptible to it but other young citrus fruits can be vulnerable as well. Melanose is characterized by the presence of small lesions and irregular spots on the fruit. The pustules can have a rough texture and they’re usually larger on grapefruits. Melanose is a fungal infection that can often impact older trees more harshly.

Due to the fact that spores develop on dead branches or twigs, it’s essential to remove any kind of dead tree material to fight against melanose. Another option to control this disease involves the use of a liquid copper fungicide like this reliable one from Amazon. Melanose can affect different parts of the citrus tree initially. The foliage, twigs, and fruits will eventually become resistant to the disease but complications can appear in young fruits or when wet weather conditions persist. Vigorous trees can usually fight off melanose but vulnerable ones need protection.

2. Citrus Scab

Citrus trees can be affected by a scab disease that produces spots on leaves and fruits. This is a wart-like infection that affects the appearance of the fruits with corky groupings of pustules. The outgrowths have a grey-pink look initially but they can become darker as the disease progresses. Citrus scab can affect the leaves and twigs but it’s usually more commonly found on the fruits of the tree. The look of the circular, elevated spots can look similar to the lumps from other diseases such as botrytis or wind rub abrasions.

The citrus scab disease is often more severe on certain types of fruits like lemons, grapefruits, and minneolas. Spores of the scab fungus can easily spread by wind and rain. It almost goes without saying that moist conditions are favorable for the development of citrus scab. One of the best solutions for controlling this disease is to rely on copper sprays but the timing is very important. Ideally, you should apply the spray at around half petal fall. An extra treatment a month later may be required.

3. Greasy Spot

Another damaging citrus tree disease that’s caused by a fungus is called greasy spot. This one creates blister spots on the underside of leaves. You can differentiate this disease from others by looking closer at the blisters. They have a yellow-brown color and later become oily-looking. The foliage of the tree can get swollen and starts to collapse as the disease advances. Greasy spot infections are very damaging for the citrus tree’s leaves causing them to fall prematurely.

Controlling for greasy spot requires the gardener to collect fallen leaves because they can help spread the disease. A spray of liquid copper can make a difference when applied in the summer. Orange, lemon, lime, and other citrus trees can be affected by this fungal disease. Even hardier citrus varieties can be susceptible once the right conditions appear. The leaf drop effect can be very severe for the health of your tree. If left unchecked, greasy spot disease can kill the infected citrus tree and run rampant among nearby trees.

4. Phytophthora Root Rot

This is a harmful citrus tree disease that can cause a slow decline of the tree’s health. Phytophthora root rot could be particularly damaging for new plantings. The disease is usually characterized by yellowish leaves which is a sign of feeder roots issues. There are many phytophthora pathogens but this infects the root cortex. That is why the uptake of water and nutrients can be severely impeded. Citrus trees affected by the disease will have stunted growth.

Most citrus fruit tree species can be damaged by phytophthora root rot. The spores come from the soil so the infective agents can handle more difficult conditions. When the weather conditions get really moist, this when the spread of phytophthora can accelerate and infect the roots of your citrus tree. Aside from sticking to resistant rootstocks, it’s a good idea to control this disease through more careful irrigation management and potent fungicides. Avoid overwatering and take a soil sample from the feeder roots to figure out the best treatment decision.

5. Anthracnose

If your citrus fruit tree is affected by dark spots on the fruit and twig dieback, chances are that it’s infected with anthracnose. There are other symptoms to watch out for before confirming the right diagnostic. This is a fungal disease that can have severe consequences like premature leaf drop and postharvest fruit decay. Anthracnose is particularly harmful to stressed citrus trees that have old wood. It can affect the rind tissue of many types of citrus fruits like lemons, oranges, and grapefruits.

Weakened twigs are often susceptible to this disease. When spring rains are more persistent, ideal conditions for the development of anthracnose spores can appear. Wet weather helps the infective agents to drip onto fruit where you will see distinctive streaks. Young fruits have reddish spots while mature ones are usually stained brown or black. Management of anthracnose involves fungicide application. It’s not always necessary to get a thorough treatment for this disease as careful pruning can prevent it and minimize its damaging effects.

6. Sooty Mold

As its name suggests, sooty mold has a blackening effect that will be noticeable on the leaves of your citrus tree. Because of honeydew secretions from various pests, the mold can develop and have an impact on the health of the tree. It’s important not to leave sooty mold to grow unchecked as it can become really dense and spread to other parts of the tree such as the stem and fruit. This fungal infection is best prevented by controlling insect infestation.

Keep the citrus tree from aphids and whiteflies and you won’t have to worry about sooty mold. For this reason, it’s recommended to try an insecticide such as neem oil that can protect the tree’s foliage from harmful pests that create the conditions for sooty mold. Some severe pest issues might require another treatment spray at a later date. If the mold growth has already progressed quite a lot, it’s best to use a potent fungicide such as products based on liquid copper.

7. Alternaria Brown Spot

Many citrus fruit tree varieties can be susceptible to the fungus causing Alternaria brown spot. The first sign of the disease is the emergence of small brown-black spots on the leaves and the fruit. They can grow bigger as the disease develops further. A notable difference compared to other diseases is the depressed look of the spots. Young fruits are vulnerable to Alternaria brown spot while mature ones can get immunity to the infection.

Some of the most affected types of citrus trees are tangerines and tangerine hybrids like tangelos. The fungus responsible for this disease produces a toxin that can have various harmful effects. Sometimes it creates lesions with necrosis of the veinlets on leaves. Another important sign to look for is the yellow halos created on mature leaves. Copper and strobilurin fungicides seem to work well against Alternaria brown spot. If you’re growing more citrus trees, it helps to keep wide spacings as a prevention method.

8. Citrus Canker

One of the most contagious bacterial diseases that can affect citrus trees is called citrus canker. Mild cases will show yellow lesions and scabs on various parts of the tree, including fruit. Severe cases of citrus canker can cause a lot of damage such as leaf loss and fruit drop. There are multiple transmission vectors used by this bacterial infection. Citrus canker spreads easily with the help of wind, pests, and even human clothing or tools. Moist weather seems to facilitate the spread making it more effective.

Citrus canker bacteria can penetrate leaf stomates and get onto fruits. There’s no reliable chemical control that you can try but the disease can be suppressed using liquid copper treatments. Such sprays work best as preventative measures because a tree that’s severely infected will most likely need to be destroyed. A good care procedure to consider is to remove any infected plant material. Copper spraying might be required at 3-week intervals to get the most benefits.

9. Armillaria Root Rot

Armillaria root rot is a tricky citrus fruit tree disease to watch out for. It’s potentially dangerous because symptoms don’t always appear before the disease gets firmly established. The damage can be already done before you’re even aware of any issues. Important signs of an Armillaria infection include small yellowing on the leaves and stunted tree growth. This disease is caused by a fungus that penetrates citrus roots from the soil. It can infect both the roots and crown as well as severely impact the whole root system of the tree.

Preventing infection in your citrus trees represents the best management method for Armillaria root rot. Infected trees are practically compromised. Make sure you’re careful when choosing planting sites because this disease is common for crops on hillsides, floodplains, and areas susceptible to overflow. An infected citrus tree needs to be removed from the orchard and properly disposed of before the disease can spread. Individual tree sites can benefit from pre-plant fumigants.

10. Citrus Black Spot

The citrus black spot disease is caused by a fungal pathogen and can damage all kinds of citrus varieties. However, some types of trees are more vulnerable compared to others. Examples include grapefruit and lemons. The fungus spreads from leaf litter. When the weather is warm and wet, the risk of infection can increase and spores travel by the help of air currents. Although the disease doesn’t damage the fruits internally, it makes them very unsightly due to cracked and hard black spots on their surface.

Fruits damaged by citrus black spot are not commercially viable. The symptoms will often appear a month before the citrus fruits are ready for harvest. The harmful effects of the disease seem to get more intense in high light conditions or times of drought. Declining trees are also more vulnerable to develop a severe infection. Aside from correct leaf litter management, there aren’t many reliable methods to treat this disease but you can try liquid copper spray formulations. Early harvest can help as the disease causes premature fruit drop.

11. Botrytis Rot

An occasional citrus fruit tree disease to be aware of is botrytis rot. It’s caused by a ubiquitous fungus that often infects the tree through injuries. Twigs, blossoms, leaves, and fruits can be affected. Due to the emergence of gray mats of sporulating tissues, botrytis rot causes increased fruit drop. Surviving citrus fruits will show injuries and gray mold decay can appear after the harvest. Lemons seem to be more susceptible to this disease compared to other citrus crops.

Controlling botrytis rot disease requires the use of preventive measures. Regular pruning is a recommended solution to ensure good airflow. It’s also important to protect the trees against mechanical injury and frost. Disease phases can be reduced with the help of copper-based fungicides. Make sure you opt for this kind of treatment before it rains. More frequent sprays could be needed when facing extended periods of cool and wet weather. Although it’s not a very economical solution, it may be important to rely on postharvest treatment for preventing premature fruit decay.

12. Citrus Blast

If you notice some black lesions in the leaf petiole of your citrus tree, that’s an early sign of the disease called bacterial blast. Twigs can eventually die back once the disease develops and affects the leaves. The citrus blast disease appears to affect parts of the tree differently depending on winter wind exposure. Once the disease impacts the fruit, you will see small black spots while other areas of the tree can show brown scabs. Bacterial blast is a common disease in the Sacramento Valley where conditions are more favorable for its spreading.

Orange and grapefruit trees can get their leaves and twigs damaged while lemon plants seem to have their fruits more vulnerable to infection. Managing citrus blast can be done using cultural control quite effectively as preventive sprays are not very economical. Adding plant windbreaks or relying on bushier varieties of citrus trees can help. An alternative solution to control the disease is spring pruning the infected twigs. Another pruning session in early summer can contribute to minimizing new fall growth that’s more susceptible to the disease.

13. Dothiorella Blight

Dothiorella blight is a disease whose symptoms can be easily spotted from a distance. It causes groups of leaves and twigs to decline in your citrus trees. The fungal pathogen responsible for this disease produces cankers that can be deadly for younger trees. The bark can get discolored and exude a protective gum that has a role in sealing off the infection. For this reason, you can often see that fruit and leaves tend to stay attached to dying branches.

Affected tree tissue has to be eliminated before dothiorella blight spreads to the entire tree areas. Infected limbs should be burned to destroy the fungus. Make sure you control the disease through pruning in dry conditions as wet weather can make it spread more effectively. This is usually the way mature citrus trees contract this fungal disease. Spores can spread easily when the tree has open wounds. Severe cases of dothiorella blight usually result from infected graft unions.

14. Citrus Greening

One dangerous bacterial disease to watch out for is the citrus greening. You can recognize it easily by the distinctive yellowing of leaf veins. Other parts of the tree are also affected so you can expect to see twig dieback and misshapen fruit that drops prematurely. Also called the yellow dragon disease, citrus greening has two main transmission methods. It spreads by grafting or through citrus psyllids which feed on the tree’s sap and leave honeydew. Infected insects will transmit the disease to healthy citrus trees.

Controlling citrus greening is pretty much all about preventing the infection in the first place. Once the tree gets the disease, there’s no workable cure that you can apply. Thankfully, this is a fairly exotic disease that you should normally not have to worry about. It’s recommended to stick to proper prevention procedures. If you have multiple citrus trees, it’s essential to detect any infection early so you can remove the diseased tree from the plantation. Managing citrus psyllid populations can be done with the help of chemical sprays.

15. Exocortis

Exocortis is the name of a disease that only impacts certain citrus fruit trees. You normally have to worry about it only if you’re using susceptible rootstock such as trifoliate. It’s very important to be careful about obtaining clean rootstock as exocortis is an incurable disease. The infection affects the citrus tree’s bark through a process called shelling. Pathogens of the disease can spread from an infected budwood to another. This is why it’s important to disinfect your pruning tools.

Aside from shelling of the bark, exocortis can also affect the citrus tree in other ways. Another common symptom is stunted growth. Some types of citrus trees can present other signs like yellowing leaves or spots on the twigs. The stunted growth will impact fruit yield though exocortis shouldn’t pose concerns in terms of fruit quality. As there’s no treatment for this disease, the best solution to try is to prevent it by choosing trusted nurseries that have clean rootstock.

16. Septoria Spot

Septoria spot can affect citrus fruit trees through sunken spots on the fruit rind that look very similar to frost damage. Lemons are especially vulnerable to this fungal infection. The disease can spread from infected tree materials such as leaf litter. Symptoms of Septoria spot can be seen more clearly on over-mature fruits. The dark spots can form irregular areas or they may be scattered on the body of the fruits.

Damp weather conditions favor the spread and development of a Septoria spot infection. You need to be extra careful if you live in areas with extended periods of cool, rainy, and foggy weather. The fungus is able to sit in a dormant state and can reactivate during cold conditions. A protectant copper spray is strongly recommended for managing a Septoria spot infection. Due to the way this disease prefers autumn rains, you will also need to make use of some reliable cold damage protection.

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