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.
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.
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.
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.
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. 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).
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.
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 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
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.
Being a gardener in Florida is exciting. We have many plants to choose from and the weather is mostly pleasant but always seeming to surprise us. One disadvantage of living in such a place, in addition to having so many growing zones, landscape nurseries, and major shipping ports, is that invasive species that land in Florida don’t like to leave. One newcomer in the lineup of invasive pests is the crape myrtle bark scale (Acanthococcuslagerstroemiae). This invasive insect pest made its way from Texas and, as of a few weeks ago, the only Florida sighting had been in Santa Rosa County. Unfortunately, this pest has now been confirmed in Leon County. Landscapers and gardeners in north Florida should learn how to identify this pest and what options are available for control.
The crape myrtle bark scale, as the name suggests, has a pretty specific host – the crape myrtle (Lagerstroemia spp.). However, in the United States, this scale has also been found feeding on our native beautyberry (Callicarpa americana). Due to the widespread planting of crape myrtles, these should be the gardeners primary focus when looking for this new pest. If present on the tree, it is hard to miss. These scales feed on the bark and are a snow-white color. Being a scale, they produce honeydew that is then covered in black sooty mold. So, if you see a crape myrtle with black branches, look closely and you may see the white scales. If you crush these scales and they leave a pink goo on your fingers, then you very likely have crape myrtle bark scale. Of course, you can send a picture or sample to your local county extension office for confirmation.
Black sooty mold and white scales along the trunk and branches are a tell-tale sign of crape myrtle bark scale. Credit: Mark Tancig, UF/IFAS.
If you do happen to find crape myrtle bark scale in your own landscape, or one that you manage, proper control is important to prevent it from moving along any further. Since scales suck on sap and are protected by an outer shell, systemic insecticides are the preferred product for effective control. These include the neonicotinoids, such as imidacloprid (Merit) and dinotefuran (Safari). The larval stage can be controlled with a horticultural oil, sometimes mixed with another insecticide like bifenthrin (Talstar), however, this will not control the adults. Another option is to completely remove the tree and burn all of the plant material.
Excessive sooty mold has turned these branches black. Be on the lookout for crape myrtles looking like this to help identify crape myrtle bark scale. Credit: Jim Robbins, Univ. of Ark. CES. Retrieved from bugwood.
The crape myrtle bark scale can significantly reduce the aesthetic value of crape myrtles due to the black sooty mold that covers the bark. It is also known to reduce flowering and can lead to thinning of leaves. Since crape myrtles make up a big part of our managed landscapes, let’s all work together to scout for this pest and control it when found. If you have questions, please contact your local county extension office.
Difference in flowering due to crape myrtle bark scale damage. Credit: Jim Robbins, Univ. of Ark. CES. Retrieved from bugwood.
Having trouble with flea beetles? Tired of them showing up unannounced? Do not be alarmed here are a few tips to get rid of the unwanted guest in your garden.
Description: Flea beetles vary in appearance, where colors range from black to tan, with other, brighter colors mixed. They may also have a solid, striped, or spotted pattern depending on the species. Beetles are tiny with large hind legs which allow them to jump like fleas when disturbed.
Lifecycle: These unwanted guests will overwinter as adults in the soil or beneath plant debris and become active in early spring when temperatures reach 50°F, and begin feeding on weeds or early-planted crops. Eggs are laid by adult flea beetles normally around May in the soil or at the base of host plants. After 7-14 days eggs will hatch and larvae will feed and develop on various plant parts. They pupate in the soil for 11-13 days before emerging as adults.
Host plants: Some species attack a wide range of plants, while others target only certain plant families. (Table 1). In the garden, several vegetable crops are eaten by these pests, particularly those in the Brassica family.
Table 1: Common flea beetles and host plants.
Scouting: Adult flea beetles are particularly active on warm, sunny days. To identify damages, scout every 1-2 days in newly planted fields, since it is easier to identify the damages than to see the beetles themselves. Flea beetle populations can be monitored with yellow sticky traps.
Damage: Adult beetles feed on foliage, producing shot holes in the leaves, especially new leaves which will have a lacy appearance. Additionally, in leafy crops like lettuce or spinach, the holes can reduce the quality of the leaves.
Photo Credits: Jeffery Hahn University of Minnesota
Photo Credit : Jeffery Hahn University of Minnesota.
Management / Control strategies:
In the spring delay transplanting or planting by a couple weeks if possible.
In the fall, till the garden to uncover any hiding flea beetles.
Plant “push” or repellant crops such as catnip, sage, mint, hyssop, nasturtium, and basil.
Use a “trap crop” such as radishes, taking the pest’ focus off more valuable plants.
Dusting leaves with plain talcum powder repels flea beetles on tomatoes, potatoes, peppers, and other plants.
Insecticides may be used early in the season.
Water deters adult flea beetles. Any watering should be done in mid-day.
Planting after adults have emerged or crop rotation can help minimize flea beetle damage.
Apply commercially available nematodes that feed on flea beetle eggs, larvae, and pupae.
Recently, I’ve been asked about a deer fly trapping method that I wrote about a number of years ago. So, here it is. This aggravating insect is active now.
Deer flies, which are in the horsefly family, are annoying as they repeatedly and persistently dive for their victims until they inflict a painful bite.
Dr. Russ Mizell, now retired UF/IFAS Extension entomologist, experimented with a method to trap this insect. Mizell wanted to identify the optimum shape, size, color and speed to attract deer flies. If successful, he could temporarily remove a deer fly population long enough to enjoy an outdoor gathering without being bothered by deer flies.
Mizell said he started the research as a high school science project with his son but “it got so interesting, I just kept doing it.”
Deer flies wait for prey to walk before attacking. So, they are highly attracted to movement.
With this in mind, Mizell and his son decided the best way to snare deer flies was to “troll” for them from a slow-moving vehicle. Working in spring and summer when deer flies are most prominent, they set out to discover what kind of trap worked best.
They built a test platform on the hood of their vehicle that could troll seven different shapes at once. They ambled along in deer fly-infested countryside for set periods of one to five minutes, testing pyramids, squares, balloons, plant containers and other shapes, all coated with Tanglefoot (commercially available sticky spray for insects), then counting immobilized prey. They tried black, tan, blue and shapes of other colors suspended from various heights.
The trap that enticed the most deer flies proved to be a 6-inch flowerpot painted bright blue and coated with Tanglefoot. This trap captured as many as 30 deer flies in a one-minute test. It worked best when suspended three to six feet above the ground and trolled no faster than 10 feet per second or about 7 miles per hour.
The traps are remarkably effective, Mizell said. “Many times, after running the traps through an area, we found there were no deer flies left,” he said. “You trap them out for a short period until they repopulate the area.”
The traps also work when attached to a baseball cap and trolled by the hat’s wearer. But instead of attaching a flowerpot to your cap, you could attach a blue drink cup painted with Tanglefoot.
Despite its effectiveness, its aesthetic appeal leaves something to be desired.
Mosquitoes are a pesky nuisance we’re all too familiar with, especially as we move into the summer months. Their presence can certainly make being outdoors undesirable. Not only are they a blood-sucking nuisance but they also carry and can transmit many diseases to humans. Mosquito-borne diseases of public health concern in Florida include St. Louis encephalitis, eastern equine encephalitis, West Nile virus encephalitis, dengue, and Zika. While it can be difficult to eliminate mosquito populations completely, there are steps we can all take to protect ourselves and to prevent our landscape from becoming a major breeding ground for mosquitos.
Asian Tiger Mosquito. Photo Credit: James Newman. UF/IFAS Photo.
Source Reduction
One way to keep mosquito populations down is to prevent the landscape from being a breeding ground for them. Many species of mosquitoes require standing water to lay their eggs; therefore, eliminating standing water can help keep populations low. Mosquitoes can develop in a variety of water-holding containers such as flowerpots, birdbaths, pet dishes, tree holes, bamboo trunks, and many others. It is important to:
Drain water from garbage cans, gutters, buckets, coolers, or any other containers where water is collected
Discard any old tires, bottles, broken appliances, or items not being used that could potentially hold water
Change water in birdbaths and/or outdoor pet dishes once or twice a week
Mosquito larvae. UF/IFAS File Photo
For areas such as ornamental ponds or water gardens, aeration or stocking them with mosquitofish (Gambusia species) can also help to keep mosquito populations down. The small fish will feed on the mosquito larvae and add movement to the water. They are most effective in small ponds with no other fish present.
For other areas with standing water that cannot be drained, such as rain barrels or ornamental ponds, products containing Bacillus thuringiensisisraelensis (Bti) are effective in controlling mosquito larvae. Products containing Bti come in the form of granules or “dunks”, which look like miniature donuts. These Bti products are considered a form of biological control as it is a naturally occurring bacteria that is specific to mosquito, blackfly, and fungus gnat larvae. Therefore, Bti products are not harmful to fish, waterfowl, pets or humans when used according to label directions.
Protective Clothing and Repellents
The most effective way to protect yourself from mosquito bites is to avoid infested areas, wear protective clothing, and wear insect repellent when outdoors. There are several repellents that are currently available such as DEET, picaridin, and IR3535, or plant derived chemicals such as citronella and oil of lemon eucalyptus.
It is important to read the label before applying mosquito repellent and to remember that there are different recommendations for frequency of application for different products. Below is a table comparison of products based upon University of Florida research.
