7 Lemon Tree Diseases: Causes and Cures

Have you looked forward to a bountiful lemon harvest only to find rotten, splotched, and diseased fruit? Citrus trees, like lemons, are wonderful to grow, but they can fall victim to several diseases. 

Some diseases harm the appearance of fruit and leave the interior quality undisturbed. Others can wipe out your entire crop and possibly kill your tree. To prevent this, you have to be aware of what to look for and what to do to treat it. 

Citrus Black Spot

Citrus black spot is a common lemon tree disease that causes tiny black spots on the leaves and fruit. It leaves lesions that harm your lemons’ aesthetic value. Farmers first identified it in Florida in 2010. 

Due to this unsightly disease, citrus farmers throw away tons of crops each year. This is a shame because citrus black spot doesn’t affect the juicy interior of the fruit. 

The fungus Phyllosticta citricarpa causes the disease. It occurs in all citrus-growing regions of the US, including California and Florida. Severe infections can cause your tree’s fruit to drop prematurely. 

How to Prevent Citrus Black Spot

Citrus black spot grows on the fruit and leaves of infected lemon trees. The fungus that causes it can get on your trees through infected leaf litter, rain splash, or contact with diseased plants. 

Unfortunately, one of the most common ways of spreading the disease is using unsanitized pruners while pruning. 

Prevent citrus black spot by disinfecting your pruners or sheers before and after every use. Monitor quarantine areas by checking the USDA website. 

Keep all leaf litter, plant debris, or green waste from other locations away from your tree. Examine your tree frequently to catch the disease before it becomes a problem. 

How to Treat Citrus Black Spot

Start citrus black spot treatment by sterilizing all your pruners, gloves, or equipment that contacts your infected tree. If it’s a light infection, you can harvest your lemons early. 

You can treat severe infections by spraying an organic copper fungicide on the infected areas. Repeat every 3 to 4 weeks until harvest. Wash your lemons thoroughly before using them to remove any residue from the copper fungicide. 

Citrus Canker

Citrus canker is caused by the bacteria Xanthomonas citri pv. citri. It is most common in Florida but can appear in Texas, Alabama, and Louisiana. It needs high humidity and temperatures to grow. This keeps it away from Californian gardens and orchards. 

Infected lemons will appear on the fruit as brown lesions and yellow halos. The infections are only superficial and won’t affect the inner fruit. However, if the infection is severe, it will cause the tree to drop its fruit and leaves. 

Wind, rain splatter, infected leaf litter, and unclean garden equipment spreads the disease. It can move through orchards and home gardens quickly through various insect pests as well. 

How to Prevent Citrus Canker

As always, keep leaf debris and plant litter away from your tree. Always disinfect your gloves, pruners, and tools after every use. 

Check the USDA quarantine maps regularly to see if other growers in your area have experienced problems. Check the fruit frequently to see signs of the disease. 

How to Treat Citrus Canker

Unfortunately, there is no treatment or cure for citrus canker. Your infected tree will stop producing fruit over time. 

If you choose to dispose of your infected tree, keep the infected parts of the tree away from other citrus trees in your garden or area. Don’t add any of it to your compost, either. This prevents the disease from infecting new hosts. 

Citrus Greening

Citrus Greening is one of the most destructive diseases you can encounter on your lemon tree. In other parts of the world, it’s called Huanglongbing (HLB). There is no cure or treatment, and once your tree is infected, it will probably die in a few years. In orchards across the U.S., citrus greening has killed millions of acres of citrus crops. 

This disease spreads by the small Asian citrus psyllid. This insect is around 3 to 4 mm long and lives on the undersides of leaves. It produces 9 to 10 generations in a single year. 

When the insects draw sap, it leaves the phloem-inhabiting bacteria Candidatus liberibacter asiaticus on the foliage. After infection, your lemon may remain asymptomatic for a long time while infecting other trees.

Symptoms that do present themselves include blotchy, asymmetrical leaves, and lower yields that are misshapen or smaller. Eventually, it will produce fewer and fewer fruit until it stops fruit production entirely and the twigs of the tree die back. 

How to Prevent Citrus Greening

The best way to prevent citrus greening is to stop the Asian citrus psyllid from infecting your tree. Check the tips of your lemon’s foliage often to see signs of the pest. The psyllids and their nymphs are incredibly tiny, so try using a magnifying glass to spot them. 

Try using organic insecticides such as neem oil and horticultural oil if you spot infections. Spray every week directly on the psyllids. Try to avoid spraying when other helpful pollinators and insects are around. Because these oils can cause sensitivity to leaves and twigs, spray only on cloudy days out of direct sunlight.

The best prevention is by creating a healthy ecosystem of natural predators around your lemon tree. This includes beneficial bugs like ladybeetles, spiders, small birds, and parasitic wasps. 

How to Treat Citrus Greening

There is no effective treatment for this harmful lemon tree disease. Once you realize your lemon is infected, contact your State Plant Health Director to report your issue. 

Citrus Stubborn Disease

Citrus stubborn disease is a lemon tree infection that resembles citrus greening (HLB). The bacteria Spiroplasma citri causes the infection, which can devastate your lemon tree. 

The most common vector for the bacteria is leafhopper insects. Once your tree is infected, you’ll see symptoms that resemble freeze-damaged leaves and poor nutrition. The leaves will start to curl upright, and the flowers can appear out of season. 

Your lemons will be small and shaped like acorns. They’ll taste bitter and have almost no aroma. 

How to Prevent Stubborn Disease

The best way to prevent stubborn disease is to eliminate its vectors. Leafhopper insects, such as beat leafhoppers, are the primary insects that you can treat through a variety of methods. 

Once you see leafhoppers in your garden or lemon tree, treat them immediately. Organic insecticides such as neem oil sprays are a great first defense. They kill juvenile leafhoppers, preventing them from reaching adulthood. 

The best prevention is encouraging helpful insects in your garden. Lady beetles, lacewings, and spiders will feast on leafhoppers before they even become a problem. Planting a wide array of nectar plants and flower shapes in your nearby garden will support a healthy population of beneficial insects.

How to Treat Stubborn Disease

Unfortunately, there is no treatment for stubborn disease on lemon trees. Once your citrus is infected, you’ll have to replace them with healthy saplings.

When you dispose of your infected tree, keep all its foliage away from other plants and your compost. This will stop leafhoppers from infecting other plants. 

Armillaria Root Rot

Armillaria root rot is a disease caused by the Armillaria family of fungus. Often known as honey fungus, it infects lemon trees at the roots. 

Root rot occurs when the roots become soggy after receiving too much moisture. Sometimes heavy rain or too much watering can cause this. The fungi thrive in moist environments and quickly infect your lemon tree’s roots. 

You’ll see symptoms including yellow or brown leaves and stunted growth. Your plant can eventually die from the infection. 

How to Prevent Armillaria Root Rot

The best way to prevent this infection is by planting your lemon away from an infected area. Remove any infected trees in your garden or yard. Watch the area for signs of the fungus, which include large clusters of honey mushrooms in summer or fall. Water appropriately, cutting back on irrigation in particularly rainy seasons.

How to Treat Armillaria Root Rot

There is no effective cure for Armillaria root rot. Once your tree becomes infected, dig it up and dry it out. Some gardeners create a small bonfire on the site to kill the spores and infected roots. 

Phytophthora Gummosis

Phytophthora gummosis is a common citrus bark disease that causes oozing from your lemon. Eventually, the bark will crack off, leading to branch and foliage decay. 

Phytophthora fungus spreads the disease. Cool and damp weather creates favorable growing conditions for this harmful fungus. Areas such as Northern California can be especially susceptible. 

In many cases, the disease can kill your lemon within a year. It’s important to treat it immediately before the lesions become worse. 

How to Prevent Phytophthora Gummosis

The best prevention is growing your plant in well-drained soil. This prevents moisture buildup and allows the soil to dry between rain events. 

If it’s cold and rainy, check the bark and trunk frequently for signs of infections. Avoid over-watering, especially if the weather is cold and moist. 

How to Treat Phytophthora Gummosis

Once you spot the oozing canker on the trunk, cut it away as fast as you can. Remove the outer layer with a disinfected blade. Keep removing the layers of bark until you see green bark underneath. 

Let the cut area dry, and apply an organic copper fungicide. Monitor the infected area to make sure the infection doesn’t come back. If you act early, you can stop this harmful disease in its tracks. 

Bacterial Blast

Bacterial blast is a disease that creates black lesions on the leaves. The bacteria Pseudomonas syringae causes it by entering the plant tissue through openings and abrasions. It’s common in the winter and spring in many areas of California. 

If left untreated, your tree’s leaves will wither and fall off. Sometimes the bacteria will infect the fruit, causing dark lesions on the skin. As alarming as the disease is, it won’t kill your tree. 

How to Prevent Bacterial Blast

There are several ways to prevent bacterial blasts from infecting your lemon tree. The first line of defense is pruning and clearing dead plant debris frequently. This prevents intense fall flushes of leaves which bacterial blast feeds on. 

Wind abrasion is often the reason openings form in your lemon trees in winter. Planting wind-blocking plants around your lemon tree could prevent bacterial blast infections. 

How to Treat Bacterial Blast

Pruning infected parts of the tree after the winter rains will help treat the infection. Try spraying the foliage with an organic copper fungicide before the first heavy rains in fall. 

Final Thoughts

No matter if you’re growing Meyer lemons or heirloom varieties, you’ll have to stay on top of disease prevention. Prune and remove dead debris regularly. Keep dead plant debris away from your tree. 

There is no cure for some diseases, such as root rot or stubborn disease. That’s why prevention is your best defense.