False Hope for Cold Damaged Citrus Trees

False Hope for Cold Damaged Citrus Trees

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.

Regrowth of cold injured citrus tree from rootstock

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.

Friendly Fungus: More than Meets the Eye!

Friendly Fungus: More than Meets the Eye!

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.

Gardening in the Panhandle LIVE! Program Summary: Citrus in the Home Landscape

Gardening in the Panhandle LIVE! Program Summary: Citrus in the Home Landscape

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.  

Link: Cold Hearty Citrus – https://nwdistrict.ifas.ufl.edu/phag/2017/02/17/ufifas-evaluating-cold-hardy-citrus-varieties-for-the-panhandle/

  • 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.  

Link: Citrus in the home landscape –  https://edis.ifas.ufl.edu/publication/HS132

  • 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.  

Link: Growing fruit crops in containers – https://edis.ifas.ufl.edu/publication/MG243

  • 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.

Link: Made in the Shade –  https://crec.ifas.ufl.edu/media/crecifasufledu/extension/extension-publications/2020/2020_aug_shade.pdf

  • 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.

Link: Meyer lemon –  Meyer Lemon – Gardening Solutions – University of Florida, Institute of Food and Agricultural Sciences (ufl.edu)

  • 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.

Link: Citrus cold protection – https://edis.ifas.ufl.edu/publication/CG095

  • 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.

Link: Growing fruit crops in containers –  https://edis.ifas.ufl.edu/publication/MG243

  • 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.

Link: Citrus rootstock –  https://edis.ifas.ufl.edu/publication/HS1260

  • Matt was then asked if Meyer lemon is grafted:

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.

Link: Citrus cold protection – https://edis.ifas.ufl.edu/publication/CG095

  • Beth was asked next about post-freeze pruning:

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.

Link: Post-freeze damage in citrus – https://nwdistrict.ifas.ufl.edu/hort/2023/02/16/post-freeze-damage-in-citrus-symptoms-and-recovery/

  • 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.

Link: Selecting Citrus – https://crec.ifas.ufl.edu/home-citrus/selecting-a-citrus-tree/

  • 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.

Link: Citrus in the Home Landscape – https://edis.ifas.ufl.edu/publication/HS132

  • Danielle then further described 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.

Link: Citrus in the Home Landscape – https://edis.ifas.ufl.edu/publication/HS132

  • 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.

Link: Citrus Pruning and Recovery – Citrus Recovery and Pruning – YouTube

  • 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.

Link: Citrus pruning – https://nwdistrict.ifas.ufl.edu/hort/2021/11/22/dooryard-citrus-to-prune-or-not-to-prune/

  • 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.

Link: Citrus Leafminer – https://edis.ifas.ufl.edu/publication/IN165

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.

Link: CREC Home Citrus – https://crec.ifas.ufl.edu/home-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.

This program has been recorded and is available on Facebook here: https://www.facebook.com/GardeningInThePanhandle/videos/586710180186708

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Video: Citrus Recovery and Pruning

Video: Citrus Recovery and Pruning

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.

Post Winter Storm Elliott: Ambrosia Beetles in Citrus

Post Winter Storm Elliott: Ambrosia Beetles in Citrus

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.

For more information, please visit:

Post-Freeze Damage in Citrus: Symptoms and Recovery

Post-Freeze Damage in Citrus: Symptoms and Recovery

Written by: Muhammad Adnan Shahid and Mujahid Hussain

What made this freeze event damaging?

The duration of temperatures below freezing determine the amount of damage to citrus plants. In January 2023 the temperature dropped down to 17 F and stayed at that temperature for a longer time than in recent years. Unlike 2022-23, during the winters of 2013-14, the recorded low temperature was 18 F for half an hour. Similarly, during 2014-15, the temperature touched 19 F and stayed only for 15 minutes. Except for these two winters, in the past 12 years, temperatures never dropped below 20 F. What made this freezing event of December 2022 devastating is the long freezing hours between 17-20 F for 11 hours and even more the temperature stayed for about an hour at 17 F (Fig. No.1).

Fig. No.1. Shows the temperatures (17-20 F) duration during the past winters of 2011-2023. The temperature data is retrieved from Florida Automated Weather Network, UF/IFAS Extension Service (https://fawn.ifas.ufl.edu/). During these years, according to this data, only in December 2022, the temperature fell to 17 F which caused severe damage to the citrus in North Florida.
Fig. No.1. Shows the temperatures (17-20 F) duration during the past winters of 2011-2023. The temperature data is retrieved from Florida Automated Weather Network, UF/IFAS Extension Service (https://fawn.ifas.ufl.edu/). During these years, according to this data, only in December 2022, the temperature fell to 17 F which caused severe damage to the citrus in North Florida.

Symptoms After Freeze

Early Symptoms

The duration of appearance of symptoms after freezing was different for different citrus varieties. Even within the same varieties, the age of the plant and the rootstock are also factors that determined the duration of the appearance of symptoms. For example, in the UF 950 rootstock evaluation block at North Florida Research and Education Center in Quincy, the symptoms appeared on young UF 950 plants on Sour orange rootstock after 24 hours and after three days the plant completely turned brown.

Fig. No. 2. Shows the effects of freezing on UF 950 on Sour orange rootstock after day 1 (a) and day 3 of freezing (b). The young plants completely turned brown after three days of freezing.
Fig. No. 2. Shows the effects of freezing on UF 950 on Sour orange rootstock after day 1 (a) and day 3 of freezing (b). The young plants completely turned brown after three days of freezing.

However, a few symptoms that appeared after about 24 hours of freezing were similar on all plants regardless of age, variety, and rootstock. These include the blackening of new leaves and branches, hardening and yellowing of leaves, and formation of greenish and brownish patches (Fig. No. 3). The leaves of old citrus trees also became curly, stunted, and yellow. During the freezing event, the fruits on the trees became hard due to the nucleation of water.

Fig No. 3. Symptoms after 24 hours of freezing.
Fig No. 3. Symptoms after 24 hours of freezing.

Low temperatures below freezing point cause the formation of ice crystals inside the plant, particularly in leaf tissue, causing a damaging effect of cell walls, and leaves appeared water-soaked upon thawing when temperature rose above the freezing point (Fig. 4).

Fig 4: Water-soaked leaves due to the ice crystals formation and their thawing inside the tissues.
Fig 4: Water-soaked leaves due to the ice crystals formation and their thawing inside the tissues.

Late Symptoms

One week after freezing, leaves started curling – turning brown and dry. At this stage, both leaves and fruits dropped very quickly, within a week giving plants a bush like appearance (Fig. No. 5 and 6). The fruits that remained on the tree developed multiple symptoms depending on the varieties. Fruits of some varieties formed micro-pits on the peel, however, the fruits of other varieties showed patterns of bright orange and yellow colors (Fig. No. 6a). The rotting leaves and fruits also attracted ladybug beetles, consequently this increased populations of other insects in freeze damaged citrus groves. You should keep in mind that the fruits that have been harvested after freezing need extra care as these fruits are more prone attacked by rotting fungi.

Fig. No. 5. Late symptoms of freeze damage on citrus trees.
Fig. No. 5. Late symptoms of freeze damage on citrus trees.
Fig. No. 6. Fruits turn black and fall from the tree after freezing.
Fig. No. 6. Fruits turn black and fall from the tree after freezing.
Fig. No. 7. The freezing caused the fruit’s peel to form patterns of bright orange and light-yellow color on the fruits (a). The rotting of leaves and fruits produces an odor after freezing that attracts ladybug beetles (b).
Fig. No. 7. The freezing caused the fruit’s peel to form patterns of bright orange and light-yellow color on the fruits (a). The rotting of leaves and fruits produces an odor after freezing that attracts ladybug beetles (b).

Bark splitting or cracking is a late symptom of freeze damage. Bark splitting or cracking will mostly be seen on stem and branches directly exposed to the sun. Bark splitting starts when sun light increases the temperature on the surface of the bark during daytime, but temperature drops after sun set resulting in unequal shrinkage and contraction of bark and inner plant tissues (Fig 8). Dropping of leaves and fruits is a good indication of plant survival, but if leaves dry while attached and don’t drop then it’s the sign of severe damage to internal tissue with a low chance of plant recovery (Fig 9)

Figure 8. Bark splitting and cracking as a consequence of freeze damage.
Figure 8. Bark splitting and cracking as a consequence of freeze damage.
Fig 9. Tree with dried leaves still on the branches – it is indication of sever freeze damage to internal tissues
Fig 9. Tree with dried leaves still on the branches – it is indication of sever freeze damage to internal tissues

Recovering Plant After Freeze Damage

The recovery of plants is entirely linked to the level of damage to the plant internal tissues during the freeze event. Freezing damage can exacerbate the plant’s health, thereby minimizing the recovery of the plant. However, plants can recover if the internal cells (cambium and xylem and phloem) are alive enough to regenerate the branches. After an extreme freeze event, like what happened in December 2022, some citrus plants will recover in spring due to the presence of living cells in the branches and stems, but if the internal transportation system is damaged, the plant again starts to die back for the next several months. You should not apply any recovery techniques immediately after the freeze event. The best strategy is to wait for the spring to see if plants are giving any indication of recovery and observe their growth at least till May-June.

Fertilization

It is true that after damage, plants require nutrients to regenerate and grow but the application of nutrients is critical as excessive fertilization can also increase the risk of damage. Do not apply any fertilizer until new growth starts in spring. Fertilizers should be applied frequently, but rates should be decided depending on the degree of damage. The plants that are severely damaged will not be able to produce fruits in the next season therefore, the rate of fertilizer should be lowered to promote a slow recovery, as the damaged plants will not be able to uptake all applied nutrients due to slow water flow caused by minimum transpiration rate in the absence of leaves. In this case, any excessive application of fertilizer can cause toxic effects that can further halt the recovery of plants. The application of micronutrients is also important for the recovery of plants.

Plant Protection

Care should be taken to protect the plants from pathogens, insects, and weeds to enhance recovery. Citrus plants regenerate in the spring and aphids and whiteflies like to attack the young leaves, branches, and shoots. Therefore, an effective plant protection plan should be devised that includes the application of insecticides and fungicides. In general, one to two applications of fungicide should be planned along with nutritional spray to prevent infection on new growth. Removal of all fruit from freeze damaged trees is also important for quick recovery (Fig 10)

Fig 10. Freeze damaged stems with living and non-living tissues. Stem with living tissues will recover easily as compared to those with dead tissues
Fig 10. Freeze damaged stems with living and non-living tissues. Stem with living tissues will recover easily as compared to those with dead tissues
Fig 11: Fruit are removed from freeze-damaged tree
Fig 11: Fruit are removed from freeze-damaged tree

Pruning

Pruning should be avoided because it is not clear how much damage has been done to the plant. Pruning should be delayed until spring or summer. Ideally, pruning should be done on living wood to make sure that all the damaged parts have been removed. If the citrus tree has been damaged below the rootstock/scion union, it is better to re-graft the new scion on the rootstock instead of pruning. Pruning of trees that have been damaged below the scion union will result in the generation of rootstock. Therefore, re-grafting the plant or replanting is the best option. The Fruit Physiology Lab at NFREC-Quincy is developing tissue imaging techniques for quick estimation of percentage of tissue damage in freeze affected citrus tree shoots and roots.

Irrigation

Irrigation after freeze damage should be reduced. Damaged plants do not uptake as much water as healthy plants, therefore, excessive water in roots can cause the nutrients to move away from the plants and even can cause suffocation for young emerging roots. It is not recommended to completely cut off the water supply, because this can further damage the plants.

Research

The Fruit Physiology Lab at NFREC-Quincy is developing tissue imaging techniques for quick estimation of percentage of tissue damage in freeze affected citrus tree shoots and roots. A series of preliminary studies on the use of growth hormones (brassinosteriods, abscisic acid and jasmonic acid) and nutrients (silicon, K and Zn) to improve the cold hardiness is in progress. Preliminary data indicates that the use of growth hormones and plant beneficial nutrients has the potential to reduce freeze damage (Fig 12), but more detailed research and validation in large scale field trials is required. Postdocs and graduate students in The Fruit Physiology Lab at NFREC-Quincy are fully engaged with different research projects to improve the cold hardiness in citrus by hormonal and nutritional therapies. Recently, the lab got a funding to study new scion and rootstock combinations with high cold tolerance.

Fig 12. Effect of Silicon on cold hardiness of citrus plants sprayed with 100ppm of silicon one week before exposed to freezing temperature (-6 C) in programed freezing chamber
Fig 12. Effect of Silicon on cold hardiness of citrus plants sprayed with 100ppm of silicon one week before exposed to freezing temperature (-6 C) in programed freezing chamber

Conclusion

Florida has faced record freezing this winter season with recent events that occurred in the last week of December 2022 causing severe damage to citrus trees in North Florida. Citrus growers in South Florida are already fighting with the HLB disease and freeze damage in the North has added to the damage to the citrus industry in Florida. Careful planning and proper care can help damaged citrus trees to recover and start fruiting again. Trees with little damage to their woody parts will recover soon and start fruiting in the next season. However, the trees with more damage to their woody parts will take two or more years to recover enough to start fruiting. Therefore, proper assessment of the damage is important and recovery strategies should be devised according to the degree of damage. Recent freeze events gave us a big message that microsprinklers are not enough freeze protection for established trees. There is also a need to identify growth hormones, nutrients, or combinations of which can improve the cold hardiness of upper plant parts. In addition, exact estimation of recent freeze damage won’t be accurate until April-June.