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.
Please join us for the Persimmon Field Day on Friday, October 20th, from 8:30 – 11:30AM at the UF/IFAS North Florida Research & Education Center (NFREC), located at 155 Research Road, Quincy, FL.
This is a free field day on growing persimmons in North Florida! Attendees will be able to visit the persimmon grove to see how trees are grown, maintained, and harvested as well as sample the different persimmon varieties grown at the UF/IFAS North Florida Research & Education Center in Quincy. Light refreshments will be provided. Space is limited, so please register using the link below or by calling 850-875-7255 to reserve your spot!
Tentative Agenda:
(All Times Eastern Standard)
8:30-8:45 AM – Registration
8:45-9:00 AM – Welcome and Introduction, Dr. Muhammad Shahid, Fruit Physiologist, UF/IFAS North Florida Research and Education Center
9:00-9:05 AM – Opening Remarks, Dr. Dean Pringle, Center Director, UF/IFAS North Florida Research and Education Center
9:00-9:35 AM – Introduction to Persimmon Fruit, Dr. Muhammad Shahid, Fruit Physiologist, UF/IFAS North Florida Research and Education Center and Dr. Ali Sarkhosh, Associate Professor, UF/IFAS Horticultural Sciences
9:45-10:00 AM – Load Trolley and Travel to Persimmon Grove at UF/IFAS NFREC
10:00-11:00 AM – Persimmon Grove Walk and Talk (Persimmon Fruit Tasting and Open Discussion in the Field)
11:00-11:15 AM – Load Trolley and Travel Back to NFREC Conference Room
11:15 AM – Adjourn
For any questions, please contact Danielle Williams (dsprague@ufl.edu or 850-875-7255) or KeAndre Leaks at leaks.k@ufl.edu or 850-875-7150.
The University of Florida is committed to providing universal access to all of our events. For disability accommodations such as sign language interpreters and listening devices, please contact KeAndre Leaks, (leaks.k@ufl.edu, 850-875- 7150) at least 2 weeks in advance. Advance notice is necessary to arrange for some accessibility needs.
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.
Pesticides can be helpful tools in the home landscape and garden, helping to protect fruits, vegetables, and landscape plants from pests. While pesticides can be valuable tools in controlling pests, their improper use can pose risks to human health, beneficial organisms, and the environment. One crucial aspect of responsible pesticide use is understanding and interpreting the pesticide label. The label provides information on how to mix, apply, store, and dispose of a pesticide product to ensure the safety of the user and the environment.
First, it’s important to recognize what a pesticide is. A pesticide is a chemical substance or mixture used to kill or repel pests. Pests can include insects, weeds, plant pathogens, nematodes, mold, nuisance animals, and other organisms that can injury crops, plants, structures, or cause harm to human health. If you are unsure whether a product is considered a pesticide or not, the label will have an Environmental Protection Agency (EPA) number listed. This number is the EPA’s stamp of approval for the product to be sold and used as a pesticide (Figure 1). In the example shown in Figure 1, the EPA registration number is found on the back page of the pesticide label.
Figure 1. Example of EPA Registration Number found on a pesticide label.
A pesticide label is the information on or attached to the pesticide container and is more than just a piece of paper. This document is a legal document. If you’ve contacted your local Extension office for pest control assistance, you’ve likely heard “follow the label – the label is the law” repeatedly. That is because the label is a contract between the product manufacturer, the Environmental Protection Agency (EPA), and the end user.
What information does the label contain?
Brand or Trade Name: The brand, trade, or product name is used to identify and market the product. It is found on the front panel of the pesticide label. For example, in Figure 2, the brand name of the pesticide is “Complete Insect Killer”. Different manufacturers may use different brand names to market products, even if the same pesticide active ingredient is used.
Active Ingredient: The active ingredient is the chemical that is responsible for killing, repelling, or controlling the target pest. The front panel of the label identifies the name and percentage weight of each active ingredient.
Child Hazard Warning: The front panel of every pesticide label must have the statement “KEEP OUT OF REACH OF CHILDREN” due to poisoning being a major cause of injury to children.
Signal Word: A pesticide label must also display a signal word on the front panel of the label to identify the relative toxicity of the product. Since all pesticides are poisons to some extent, it’s important to note that they do pose some risk. The signal word is based on the entire contents of the product, not just the active ingredients. Signal words are:
DANGER: Highly toxic
WARNING: Moderately toxic
CAUTION: Slightly toxic
Figure 2. An example of the front panel of a pesticide label.
Personal Protective Equipment (PPE): The type of clothing that must be worn during the handling mixing process to protect the applicator from harm.
Statement of Practical Treatment: The statement of practical treatment or first aid statement, describes what to do if a person is exposed to the pesticide.
Directions for Use: This section of the label is the bulk of a pesticide label. This section provides detailed instructions on how to correctly apply the pesticide, including the site where the product may be used, application method, amount of pesticide that should be applied, timing of application, and frequency. It will also contain information on restrictions (if any) on product use including weather, time of day, season of year, contamination of sensitive areas, etc. Applying a pesticide to a site not listed on its label is illegal.
Storage and Disposal: Each pesticide has general storage and disposal instructions. Most pesticide labels will contain a general statement to the effect of “do not contaminate, water, food, or feed by storage, disposal, or cleaning of equipment” and “store in original containers only”. This section of the label provides guidance on how to store the product safely, including temperature requirements, shelf-life, and precautions to prevent contamination. It also outlines guidelines for disposing of unused pesticides or empty containers to prevent environmental harm.
Pesticides can be a valuable tool if used correctly. The first step in using a pesticide safely is to read the product label to learn about how the product may be used. For more information on understanding pesticide labels or for assistance with interpretation, please contact your local Extension Office.
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.
