Grapefruit or Citrus paradisi trees are one of the most popular subtropical citrus varieties in the world. Nevertheless, like all plants, grapefruits are susceptible to serious diseases. You’ll need to watch out for signs of disease to keep your grapefruit healthy.
If you’re new to raising grapefruit trees or just looking for a reminder, here are seven grapefruit tree diseases and how to treat them.
Melanose is a fungal disease caused by Phomopsis citri, a fungus that grows on dead wood. This fungus then spreads to live trees through water. Spores can infect any citrus tree but both lemons and grapefruits are especially vulnerable.
You’ll know your grapefruit tree has melanose by examining leaves and fruit. Leaves begin to show small reddish-brown spots where fungi are damaging them. Spots are often surrounded by yellow rings in the early stages which disappear gradually.
Leaves eventually become rough to the touch and the tree itself will begin to have unsightly brown spots or even pustules. These growths will begin to combine, crack, and split, creating “mudcakes.” They also form “tear stains” when fungal spots travel down fruit in sliding droplets of water.
Once your trees are infected, you will need to use fungicides to eliminate Phomopsis citri. This is necessary if infection has spread and is becoming severe. It’s true that fruit tends to be unaffected and remains edible, but that’s only mildly good news.
Trees can become damaged and lose leaves and branches. This can destroy a harvest and eventually your tree itself. Copper fungicide is a common option but be sure to ask your local nursery for options. Different fungicides have special application directions and requirements that can differ widely.
One of the most important ways to prevent citrus melanose is to regularly clean up dead twigs and other plant matter around your tree. Trim away dead twigs, branches, and leaves. Be sure not to leave any on the ground near your citruses, grapefruit or otherwise. An infection from one type of citrus can unfortunately easily spread to others, too.
Avoid overhead watering if possible, since this makes it easier for the fungus to spread between branches. Before and after pruning your tree, sterilize your shears and any other equipment that will come into contact with live wood. This helps prevent any fungal contamination between branches and plants.
2. Citrus Scab
Citrus scab or Elsinoe fawcettii, also known as sour orange scab, is a fungal disease. Scab pustules begin to develop on the tree, eventually producing conidia. These conidia function as spores that spread to other parts of the tree.
Spores can be either colored and spindle shaped or clear and shaped like ovals. Clear spores are spread through rainfall and die once they dry out. Spindle-shaped spores last longer and can be spread along wind.
You can find symptoms of citrus scab on the fruit, stems, leaves, pedicels, and buttons. Younger trees are the most vulnerable. When early scab pustules form, they contain both tree and fungal tissue. They have slightly raised edges that are colored light brown or even pink. They look similar to citrus canker when young.
Fruit and leaf pustules begin to develop and grow defined spots with nearby conical depressions. Pustules eventually harden and split as they grow. Eventually, colors may range from a gross yellowish brown or even gray.
Grapefruits display flatter pustules than other citrus types. Rinds can become scabby looking and blistered.
Managing a citrus scab infection is vital. Apply fungicide to your tree when it is in early bloom or shortly after, or during late dormancy. Single applications may be sufficient for light infections but anything more should be treated more than once. Groves that have been hit by serious citrus scab infections in the past are especially vulnerable.
Minimize overhead irrigation which naturally spreads the infection. This is especially important during the first few weeks of spring shoot growth. Leaf tissue is only vulnerable to this fungus while 1/4 or less of their final width. Once they grow beyond this early stage, citrus scab is no longer a looming threat.
Anthracnose, known more commonly as plant cankers, are spots of dead tissue that slowly grow over a number of years. These cankers are caused by various fungal infections, most from a type of Gloeosporium. Most plant cankers aren’t serious and don’t affect much as far as growth or harvest. However, some plant cankers are fatal to citruses. Woody trees, like grapefruits, are especially vulnerable. Young leaf tissue is most at risk.
Different hosts show symptoms of anthracnose. Nevertheless, they all share some commonalities. Irregular spots and dead spots on leaves are telltale signs. These usually follow leaf veins closely. Leaf tissue may turn tan or brown and begin to curl up, even falling off.
Thankfully, many anthracnose infections are mostly or entirely cosmetic. This means your tree should be treatable with fungicides. Treat three times: once at budbreak, once as leaves are halfway grown, and then a final time once leaves have finished growing.
Anthracnose spores thrive in dead leaves littered on the ground. This means that it’s easy for spores to travel from host to host and cause widespread damage. Cleaning up leaf litter is essential to preventing these infections.
While different anthracnose spores infect different tree species, they can all cross over once they have begun spreading on one tree. If other trees suffer, make sure to treat them to keep your grapefruit safe.
4. Citrus Black Spot
Citrus black spot occurs when a tree is infected with the fungus Phyllosticta citricarpa. This happens most commonly in subtropical regions, spreading thanks to high heat and humidity. Citrus black spot can cause fruit to deteriorate in quality as well as reduce the amount of fruit the tree produces.
This fungus infects the tree through fallen leaves in the vicinity. When these leaves become infected (and then heavily moistened by rain, humidity, or snow), the fungus spreads to the tree and takes hold.
Citrus black spot gets its name from the characteristic small black spots that cover the outside of the fruit. These spots can vary slightly in color, size, and other characteristics. Some lesions cause cracks or depressions in the peel. Serious infections can spread to the inner flesh of the fruit and cause it to fall prematurely.
Lesions only appear on leaves when other factors increase the severity of the infection. Stressed trees that are in drought or have sun damage may get citrus black spot on the leaves as well.
If you notice signs of citrus black spot on your grapefruit tree, it is vital to act quickly. Keep the tree pruned and the surrounding area free of leaf litter. In the meantime, apply fungicide every month during the growing season, starting in late spring and ending in the early fall.
Because citrus black spot spreads through fallen leaf litter, the single most important thing you can do is keep your yard clean. Rake up fallen leaves in the autumn and remove them from the area to avoid spreading fungal spores to the trees.
Pruning your tree at the appropriate time of year can also reduce the likelihood of spreading citrus black spot. Make sure to remove pruned branches and foliage after removing.
5. Citrus Blast
Citrus blast or bacterial blast is a disease causing fluid-filled lesions on fruit and foliage. This can ruin the fruit and cause early dropping. Because citrus blast is a bacterial infection, treatment usually involves sanitizing your tree with chemical agents.
Citrus blast appears as dark gray or rusty red lesions on leaves. Though it spreads throughout the tree, the bacteria generally can’t penetrate into the branches and is restricted to the outer tissue layers. In severe cases, the bacteria spreads to the fruit, causing ugly blotches that eventually eat through the peel and destroy the fruit.
If you notice signs of citrus blast on your grapefruit tree, the general recommendation is to start application of antibacterial copper spray. Additionally, you will need to prune the diseased leaves and fruit and remove the cuttings from the area. Between each cut, stop to sanitize your pruning shears in a bleach solution to avoid spreading bacteria throughout the tree.
There is no surefire way to prevent citrus blast, especially if you live in a cool, wet climate. Some measures can reduce your risk of developing the disease. Practice good garden hygiene by keeping tools clean and the ground tidy. Avoid using high-nitrogen fertilizers, especially in climates that are prone to citrus blast.
6. Brown Rot
Brown rot is caused by phytophthora fungus. It results in the destruction of leaves and fruit and can also attract other diseases, further sickening your grapefruit tree. Brown rot is usually transferred to citrus trees through infected leaves or soil coming in contact with the tree, often from splashing during rainstorms.
The first signs of brown rot are generally a grayish-green layer on the outside of the fruit. These are actually large lesions. They may have a rotten smell and cause fruit to become excessively soft, eventually penetrating through to the inner flesh. With time, brown rot will cause both fruit and leaves to fall prematurely.
Managing brown rot can be extremely difficult, with the emphasis on prevention rather than treatment. If you notice early signs of brown rot on your grapefruit tree, remove all affected branches and fruit. Clean the area and remove all cuttings, applying a copper fungicide.
The risk of brown rot is particularly high during wet seasons. Prevention of brown rot generally follows the same concepts as management, with the focus on pruning and keeping your garden in order to avoid spreading disease. Many gardeners preemptively apply fungicides in the late fall to the early winter when gardens tend to be wet.
7. Citrus Greening
Citrus greening is an extraordinarily destructive disease that can ruin fruit trees entirely. Prevention is the only way to avoid it. Once a grapefruit tree is infected, there is no cure.
Citrus greening is spread either through other infected plants or by an insect called the Asian citrus psyllid. Once infected, the tree loses its ability to take in nutrients. This will cause fruit to shrink in size and quantity, or become misshapen and acrid in taste. Other early signs include leaves turning yellow and mottled as they are sapped of nutrients.
Unfortunately, there is no way to manage citrus greening; once a tree is infected, it will eventually die. For this reason, farmers in areas where the disease is common must be diligent in prevention measures.
To prevent citrus greening infecting your grapefruit trees, keep an eye out for Asian citrus psyllids and take measures to remove them immediately if they appear. Tend your tree carefully, paying particular attention to its roots. If you notice a tree in your garden has been infected, remove it immediately to avoid spreading the disease to other trees.
What Diseases Do Citrus Trees Get?
In addition to the diseases listed above, grapefruit trees may be susceptible to armillaria root rot, blight, sooty mold, and phytophthora root rot.
How Do You Treat Grapefruit Tree Diseases?
While there are degrees of overlap, each citrus tree disease is treated in unique ways. Treatment usually involves diligent pruning, removing diseased fruit and foliage, keeping the area clean, and application of fungicide.
Familiarizing yourself with signs of disease and pests in grapefruit is key to keeping healthy plants. Monitoring your grapefruit for any signs of illness is vital to catching symptoms early and eliminating problems before damage is irreversible. Prevention is always better than treatment, so keep due diligence and save yourself time, money, and stress.
Have you had to treat your grapefruit tree for disease before? Let us know in the comments!
Growing other kinds of fruit in your home garden? Check out this link for articles about common diseases of other types of fruit trees and shrubs.