Citrus scab, caused by the fungal pathogen Elsinoë fawcettii, is a common disease affecting citrus trees in the home landscape such as satsuma mandarins, tangelos, grapefruit, and other tangerine hybrids. This disease affects the leaves, fruit, and twigs of citrus trees, creating unattractive, wart-like lesions. While citrus scab is rarely fatal to the tree, it can reduce the quality and marketability of the fruit.
Citrus scab on young satsuma mandarin fruit. Photo Credit: Danielle Williams
Identification and Symptoms
The most noticeable symptom of citrus scab is the development of corky, irregularly shaped lesions on the surfaces of leaves and fruit. These lesions start as small, pale, slightly raised areas that gradually enlarge and turn dark brown to light gray. On leaves, the lesions often appear on the lower surface, giving them a warty appearance. Fruit may exhibit scab lesions, which can cause deformities and discoloration, making them less appealing for consumption, however it does not make the fruit inedible.
Raised lesions of citrus scab on leaf. Photo Credit: Danielle Williams.
Understanding the Disease Cycle
Citrus scab thrives in warm, moist conditions and is primarily spread through wind-driven rain that carries the fungal spores to new infection sites. The spores can overwinter in infected plant debris and on asymptomatic parts of the tree, leading to new infections during the growing season. Recognizing this cycle can help residents take preventative measures during critical times of the year.
Management and Control
To keep citrus scab at bay, homeowners should employ a combination of cultural practices as well as careful chemical use when necessary.
Cultural Practices
Sanitation: Regularly remove and dispose of fallen leaves, fruit, and other plant debris to reduce the number of overwintering spores. This practice is particularly important in the fall and early spring.
Pruning: Prune the tree to improve air circulation within the canopy. This reduces humidity levels around the foliage, making the environment less conducive to fungal growth.
Irrigation Management: Avoid overhead irrigation. Overhead watering can splash spores onto healthy tissue, spreading the infection. Instead, water trees at the root zone or use microsprinkler irrigation.
Chemical Control:
Fungicides: Applying fungicides can help protect new growth from infection. Copper-based fungicides are commonly used and can be effective. It is important to read and follow the product label on application timing and directions for use. Typically, fungicides for controlling scab should be applied in early spring when new growth begins.
Conclusion
Regularly inspect your citrus trees for signs of citrus scab, especially after periods of rain and humidity. Early detection and treatment are crucial to prevent widespread infection. Maintaining a healthy tree through proper fertilization, watering, and pest management can also reduce the tree’s susceptibility to diseases like citrus scab.
The widespread multiple night hard freeze that occurred in North Florida near the end of December 2022 resulted in numerous citrus trees becoming severely damaged. The above ground portion of many of these trees died as a result of the extreme cold.
I talked with numerous homeowners who were concerned about their citrus trees following that weather event. Many of these homeowners earnestly and hopefully watched for any sign of new growth on their cold injured citrus trees the following spring. When new growth appeared from the lower portion of the trunk and from the roots, the homeowner became excited with a false sense of hope that their citrus tree had survived and would again produce an abundance of desirable fruit.
Cold injured citrus tree with new growth coming from rootstock. Credit: Larry Williams
Now in the spring of 2024, many of these citrus trees have somewhat regrown from those root shoots, not from the completely dead tops. In most cases, the freeze damaged, dead tops of the once large trees have now been pruned away to allow the multitude of small diameter vigorous green shoots to grow.
Most purchased citrus trees are grafted. So, what survived and is now regrowing is coming from the rootstock, not from the original, desirable, edible fruit producing top. That desirable top was completely killed.
To better understand this scenario, perhaps a basic definition of grafting will help. This definition was taken from a University of Missouri Extension publication on grafting. “Grafting is the act of joining two plants together. The upper part of the graft (the scion) becomes the top of the plant, the lower portion (the understock) becomes the root system or part of the trunk.” Understock is also known as rootstock.
Grafting involves joining two different individuals. These individuals have to be closely related. For example, citrus can be grafted to other types of citrus and peaches can be grafted to other types of peaches. But citrus cannot be grafted to peaches.
The rootstock was selected because of some beneficial trait(s): resistant to a root pest, superior cold hardiness, imparts a dwarf growth habit to the top, etc. But the same rootstock produces undesirable fruit: bitter, hard, extremely seedy, etc. The top (scion) was selected because of a superior fruit: sweeter, bigger, more disease resistant, etc. But the same scion produces an inferior/weak root system. So, grafting the two together allows for the “best of both worlds.”
When all that is left is originating at or below the graft union or rootstock, the eventual resulting fruit will usually be undesirable, sometimes not edible.
So unfortunately, the best option in this scenario is to start over with another healthy grafted tree that is well suited for the potential cold weather of extreme North Florida.
Figure 1. Aschersonia aleyrodis, entomopathogenic fungi feeding on immature whiteflies on a satsuma tree. Photo Credit: Doug Mayo, UF/IFAS Jackson County.
Proper plant disease and insect identification is essential, not just in agriculture production, but in the garden and landscape setting too! The presence of “friendly fungi” on a citrus tree is a prime example of the phrase, “there is more here than meets the eye”. Friendly fungi is an entomopathogenic fungi that attacks citrus whitefly and cloudywinged whitefly nymphs. At first glance though, it can be a scary sight and may look like your citrus tree is being plagued with a new citrus disease or a new species of scale, when in fact, the whitefly nymphs are being controlled by a beneficial and naturally occurring biological control agent!
Figure 2. Adult citrus whitefly feeding on the underside of leaf. Photo Credit: Lyle Buss, UF/IFAS Entomology.
The citrus whitefly (Dialeurodes citri) and the cloudywinged whitefly (Singhiella citrifolii) are two insect pest species of whiteflies that occasionally cause injury to citrus. The adults are small, white and resemble tiny moths (Figure 2). Adults lay eggs on the underside of leaves and eggs hatch into nymphs (Figure 3). The nymphs cause injury to the plant by feeding and consuming large quantities of sap. As a result of the large amount of sap consumed, nymphs excrete honeydew which causes growth of sooty mold fungi. Severe sooty mold infestations give plants an unhealthy appearance and can reduce plant photosynthesis.
Figure 3. Citrus whitefly nymph feeding on the underside of leaf. Photo credit: Lyle Buss, UF/IFAS Entomology.
Citrus whiteflies have historically been controlled by a suite of predators including two strains of the entomopathogenic fungi, Aschersonia aleyrodis, the red strain and Aschersonia goldiana, the yellow strain. The red strain infects the citrus whitefly and the yellow strain infects the cloudywinged whitefly. These fungi are commonly referred to as “friendly fungi”. Both strains are present in North Florida and are normally observed now, through mid-September, following the rainy season.
Figure 4. Friendly fungus attacking whitefly nymphs that are feeding on the underside of a satsuma leaf. The black growth is sooty mold, caused by the whitefly nymphs. While this leaf looks bad, the friendly fungus is helping to reduce the whitefly population. Photo Credit: Danielle Williams, UF/IFAS Gadsden County.
The friendly fungi can be clearly seen from a distance with their bright red and/or yellow spots. While it may be a scary sight to see, the entomopathogenic fungi does not harm the tree and is beneficial in helping control whitefly populations! For more information, please contact your local Extension Office.
This month’s program focused on Citrus for the Home Landscape. Citrus is a wonderful addition to your landscape. You may have tried it before and run into some issues. This episode of Gardening in the Panhandle seeks to demystify these trees. Below is a summary of the program with links and references.
Panelists introduction:
Daniel Leonard – County Extension Director/Agriculture, Horticulture, and Natural Resources Agent, Calhoun County (Moderator)
Beth Bolles – Horticulture agent, Escambia County
Matt Lollar – Commercial Horticulture agent, Santa Rosa County
Danielle Williams – Regional Commercial Horticulture agent, Gadsden County
Danielle was asked what the best cold hearty citrus is in the Panhandle:
She began by describing the conditions brought to the region by winter storm Elliot, and that one tree which performed well during it was Satsuma Mandarin (Citrus unshiu) which tends to be the most cold hearty citrus in our area that will grow. Some of these varieties are Brown Select, Owari, and Zaishan. Another variety of citrus that may be grown here is kumquat (Citrus japonica) and calamondin (Citrus x microcarpa) are other types that typically grow here. At the moment, there is evidence of damage to other types of citrus from the cold. Particularly with lemons, limes, and grapefruit.
Matt was then asked which limes grow best in N. Florida:
His top two recommendations are limequat (Citrofortunella x floridana) or if you’re interested in more juice or zest, a rangpur lime (Citrus x limona) which is a lemon crossed with a mandarin. They produce heavily and make an excellent key lime pie. They can be propagated from seed and be true to type. However, it was noted that it is illegal in Florida to propagate your own citrus tree and that they must be purchased from a certified citrus nursery.
The next question returned to Matt concerning growing key limes in containers:
There are a number of citrus recommended for containers as they are very adaptable to containers. Key lime (Citrus x aurantiifolia) should do well in containers though you’ll want a large container to accommodate root growth and drainage. The second of these is important as citrus does not like to sit in excessive moisture. Purchase a potting mix (one that does not specify “garden soil”) from the store preferably one without a moisture control element. They will need to be put inside for cold protection and will need supplemental lighting while indoors.
Beth was asked next about the viability of growing citrus under the canopy of other trees:
She said that citrus needs sunlight to produce well in general, but there is a possibility of growing citrus with a little shade. They will most likely not do well in excessive shade and if grown in a pine hammock you may need to amend soils. Recent research has shown promise in citrus production and potentially some greening protection under a 30% shaded environment.
The panel went back to Danielle about which lemons do well in the Panhandle:
She said that Meyer lemon (Citrus x meyeri) is one of our more cold-tolerant types, but that is not always the case. Harvey lemons (Citrus x pyriformis) may work as well though they may suffer the same as Meyer types.
The whole panel was asked to provide some varieties of grapefruit that may do well:
Danielle suggested ruby red grapefruit (Citrus paradisi ‘Ruby Red’) or marsh grapefruit (Citrus paradisi ‘Marsh’) are her favorites that may do well with cold depending on the microclimates where they are grown. Matt agreed with Danielle and added that a pomelo (Citrus maxima) could also be an option though they can get very large fruit and may need extra work to eat. Beth added that she agreed but to keep in mind that these may still suffer in colder years and emphasized microclimates and knowing your growth environment.
Matt was asked next about cold mitigation strategies for citrus:
He said you should plant new trees close to your house on the south side or use surrounding trees to create microclimates. Heavily wetting the soil will also provide some protection when cold is expected due to heat absorption with wet soil. You can also pile dirt or mulch around the graft union of the tree which will need to be removed when the cold is gone. He also pointed out that commercial growers often use microjet irrigation throughout the night in freeze which can release some heat.
Beth then answered a question about growing in containers:
She said you may have to accept the frangible nature of citrus. She went on to say you need to match the container to the tree and that will come with a large amount of work. So containers are not worth it in her opinion.
Danielle was asked if the tree will bear fruit from suckers:
All citrus is grafted, and where the scion wood and root stock meet is called a bud union. Above this union is the desired tree capable of producing the anticipated fruit, below is the rootstock that was chosen for its ability to survive and will not necessarily produce good fruit which is most likely this is a sour orange. The sprouts originating from the rootstock are known as suckers which may present differently than the rest of the trees (leaves in threes, excessive spines). Growth from above the graft union will be the desired tree capable of producing the desired fruit.
The answer is that there is a high likelihood that your tree was grafted if bought from the nursery. Citrus may be grown from seed but will take a long time to grow and produce fruit.
Danielle was then asked about letting citrus grow in a pot prior to planting into the environment and whether this will add cold protection:
She said that you can of course allow the tree to grow in a container prior to planting in your yard, but this is not a requirement. Established trees handle cold weather better than stressed trees which have recently been planted. To this end, make sure you plant the trees in spring or summer prior to August to allow them some growth time in your yard before the cold arrives.
She advised waiting on pruning after a freeze to allow the plant to come back naturally. Once the plant has come back with enough new growth to help it survive. At about 4 months post-freeze event you may begin to assume the plant has died if growth is not beginning. Danielle added that there is a chance the tree will begin to grow again, and the new growth will not survive. This is not unusual, and you should wait to prune till May or June to ensure you’re only removing dead tissue.
Matt next answered a question about cold damage in containerized citrus:
He pointed out that your tree in this situation may be more susceptible to damage as it did not have the surrounding soil as an insulator from the cold.
The whole panel was asked about replacing a tree in the same spot that has died:
Danielle answered that you should take into account that the tree was in the correct place to begin with and if the reason for death was from the cold or something else. If from cold there should be no issue with replacing that tree.
Danielle was then asked about citrus greening:
Citrus is a major crop in Florida and has been declining due in part to citrus greening. It is a bacteria transmitted by the Asian Citrus Psyllid. This disease plugs up the phloem or nutrient transmission tissue in the tree leading to poor quality fruit and causing decline in tree health. There is a research station dedicated to breeding and dealing with this disease. UF has developed varieties with greening tolerance, and the disease is not as widespread in the Panhandle. ‘Sugarbelle’ and ‘Bingo’ are two of these varieties and are a type of mandarin that did somewhat ok in the cold. Look for the adult insect in the new growth to monitor tree health along with yellowing, corky veins, and lopsided fruit. The difference between this and nutrient deficiency lies in the leaf which will lose symmetry with the disease. Contact your extension agent to identify the disease.
Beth then addressed when you should plant citrus trees:
She emphasized the need to buy registered citrus trees from a reputable nursery. Pick the correct location and plant spring to summer. The key to citrus is planting depth. They need to be shallow in the soil to allow proper water, nutrients, and air flow.
Matt was asked next about fertilization:
He said you can begin fertilization about a month after planting keeping in mind where the root zone of the tree. Trees that are a year old or more will benefit from multiple fertilizations throughout the year approximately 6 times in ¼ to ½ pound increments. The table at the end of the link below will guide you to correct fertilization.
For satsuma, the tree requires more fertilizer with age. Anything five years of older needs about 1-1.5 pounds of nitrogen per year, split into multiple applications. Reference the guide from Matt’s answer for the number of applications. The preblended fruit tree fertilizers sold in the store are great for use with citrus trees.
Beth was asked next about mulching:
Many documents don’t recommend mulch due to the disease potential. This is for commercial groves and does not necessarily relate to the home landscape. Be careful to keep the mulch away from the base of the tree to prevent water retention.
Matt was asked next about trees that flower but don’t produce fruit:
Two guesses to the reason are irrigation being off (too much or too little), the other is too much fertilizer at one time particularly when the tree is setting fruit. This is especially important when the tree is flowering as it can discourage reproductive growth. Danielle added that it is not uncommon for citrus to have a “June drop” of fruit.
The panel went back to Matt to discuss using permaculture concepts with citrus trees.
He said this will work with citrus though you’ll want to remember the tree may have a larger canopy so something like lettuce (Lactuca sativa) may be a better option to grow below the tree and to avoid vining plants in this scenario. Keep in mind the local environment and that we have a higher moisture level so spacing your crops apart is important to avoid disease pressure.
Danielle next answered a question about when to harvest the fruit of your citrus tree:
She said that the commercial growers use a Brix to acid ratio, but that requires special equipment which most homeowners don’t own. A better solution for homeowners is to watch for color break (this is when your fruit changes from green to its mature color) and begin tasting the fruit at this time. Things here tend to be ready Oct-Dec (may differ based on variety), and you’ll want to remove the fruit at that point. Leaving the fruit on the tree through winter may result in a loss in a hard freeze.
Beth was then asked about citrus pruning:
She said that in a home scenario, you’ll not need to prune much. It’s okay for your trees to look different. Pruning involves the removal of dead or diseased wood to a live bud or lateral shoot. You can also remove any branches which have become too large or out of bounds. As well any branches growing too close to the ground can be taken off. Also, any suckers originating from the root stack should be taken off. Finally, any aggressive growth through the center of the plant should be cut off. Branches that are growing downward don’t necessarily need to be removed completely unless appropriate to do so.
Next, Matt was asked about using kumquat as a hedge:
He said it is possible and has been done commercially to ease equipment use or promote health after the plant is harvested. He emphasized Beth’s thoughts on cutting back to a lateral bud to simplify making a hedge from citrus. One plant he said will do well in this scenario is a trifoliate orange (Poncirus trifoliata), but keep in mind their thorns.
The panel finished with a few questions for Danielle on insects:
The first was what is and what to do about wrinkled leaves on your citrus:
To this she said the damage is most likely old citrus leaf miner (Phyllocnistis citrella) insect injury. These are the larval form of a moth which lays eggs in new growth of citrus. The biggest signifier of this are lines or tunnels throughout the leaf. As the tree grows, the leaves are stunted and become curled. There is nothing that may be done about these as the larvae have grown and moved on. It will not harm the tree aside from destruction of photosynthetic materials but is generally not broad ranging. The effect is cosmetic and will not cause harm to the tree or yield.
The next question was about insect protection through the summer using natural products.
She began by emphasizing the FFL concept of right plant, right place. Essentially, make sure you’re setting your plant up to thrive. Once you’ve picked the proper spot, purchase a disease free tree from a certified nursery that has been inspected and is tagged. You’re likely to see leaf miners as discussed above or “orangedog” caterpillars. These are the larval form of the giant swallowtail butterfly (Papilio cresphontes) which are a pollinator. She pointed out that pest identification is crucial. One example is a friendly fungus that is entomopathogenic and attacks white fly nymphs. That said, there are natural solutions such as neem oil and Spinosad. You always want to know what you’re controlling and make sure you follow the label for all pesticides.
The program wrapped up with a question to Danielle about citrus resources for novice growers:
She began with our Gardening in the Panhandle newsletter. Another resource recently launched from the Citrus Research and Education Center has videos on everything from planting to insecticide use in citrus.
Citrus trees are a wonderful addition to your home landscape. As with all plants, there can be challenges growing them. A little patience and knowledge will go a long way to helping you grow a beautiful tree that can provide a bountiful harvest. Your local extension professional is always on hand to help. Reach out to us at this link: local extension agents.
Some citrus is recovering from our December 2022 hard freeze and pruning will be needed. Learn to identify your tree’s graft and how to prune away any rootstock material with UF IFAS Extension Escambia County.
Winter Storm Elliott brought freezing temperatures to the Panhandle on December 24th that lasted through December 28th, 2022. While we’ve seen freezing temperatures in years past, none remained below freezing for as long as Winter Storm Elliott did, resulting in significant injury to citrus in our region. Those trees that received significant freeze damage are more vulnerable to pests and diseases. One such pest we are seeing as a result of Winter Storm Elliott, are ambrosia beetles.
Granulate Ambrosia Beetle, Xylosandrus crassiusculus Photo credit: David Almquist.
Ambrosia beetles are a group of wood-boring insects that live in dead or severely stressed trees or dead wood. They are attracted to the odor that the dead/dying trees give off, which is why you may be seeing them now in freeze damaged citrus trees. Once they locate a sick tree, they bore into the lower part of the tree (about 2-3 feet from the ground), creating a tunnel or a gallery. You’ll likely notice sawdust from the galleries at the base of the tree or you may notice a toothpick like protrusion of sawdust at the base of the gallery.
Toothpick like protrusions from ambrosia beetles. Photo credit: Danielle Williams.
Several species of ambrosia beetles are considered true pests that attack living trees, but most species are secondary to another issue. Because ambrosia beetles generally prefer dead or dying trees, they are not typically a problem for citrus trees. If you are seeing signs of ambrosia beetles on your trees, the beetles are likely targeting trees that sustained major freeze damage from Winter Storm Elliott.
Unfortunately, there are no effective strategies to control ambrosia beetles once they attack a tree, so the best line of defense is to keep your trees healthy. Consider the first three UF/IFAS Florida-Friendly Landscape principles for maintaining tree health:
#1 Right Plant, Right Place: Select plants that match the site’s soil, light, water, and climatic conditions. Plants that are well adapted for our region will be lower maintenance and have a better chance of flourishing as opposed to those that aren’t. For information on citrus varieties adapted for our region, please visit: UF/IFAS Evaluating Cold-Hardy Citrus Varieties for the Panhandle | Panhandle Agriculture (ufl.edu)
#2 Water Efficiently and #3 Fertilize Appropriately: Proper irrigation and fertilization enhances plant growth. Over watering or over fertilizing can do more harm than good so it is best to follow UF/IFAS recommendation rates and application timing.
