Sweet Viburnum (Viburnum odoratissimum) is thought of as being an ironclad landscape shrub, generally a rapid, healthy grower free of insects and disease. However, this spring, many Sweet Viburnum specimens across the Panhandle have experienced varying degrees of dieback, from individual shoots to entire sections of shrubs, caused by the fungal pathogen Botryosphaeria – commonly known as Bot Rot.
Typical symptom of Bot Rot on Sweet Viburnum. Photo courtesy of Daniel Leonard.
Bot Rot almost always appears after some kind of major stress event that impacts susceptible plants – drought, pruning wounds, nutritional deficiencies, or another environmental stress. We haven’t been afflicted lately with any serious drought conditions and the disease occurrences are too widespread to have been a result of isolated pruning or poor plant nutrition. However, the Panhandle did experience a major environmental event around Christmas 2022 that was plenty stressful for landscape plants, a weeklong Arctic blast of extreme cold. This abrupt hard freeze event in an otherwise mild winter is my best guess for what brought about the increased incidence of Botryosphaeria we have experienced this spring.
The Botryosphaeria fungus enters plants via wounds – in this case one probably caused by cold – and begins destroying the plant’s vascular system in the area. As the pathogen progresses, it eventually causes sunken cankers to appear, girdles the affected branch, and cuts off “circulation” in that stem. The first symptom of Bot Rot that gardeners notice is shoots rapidly wilting and exhibiting a blighted appearance, with brown, dead leaves holding onto affected limbs. Unfortunately, dieback isn’t always limited to individual shoots and can spread back into plants to eventually encompass whole branches. Entire plants dying from Bot Rot infection is not uncommon.
While there aren’t any fungicides that are effective in controlling or preventing Bot Rot, gardeners can arrest its spread by pruning out infected branches. To completely rid the plant of the fungus, make sure to prune 4” or so below the last infected plant tissue (symptomatic tissue will appear dark and discolored; healthy tissue will appear light and greenish). After pruning each affected plant, it is important to sanitize pruning equipment with either a 10% bleach solution or 70%+ isopropyl alcohol to prevent spreading pathogens to other healthy plants! Plants that have been irreparably disfigured by Bot Rot or outright killed may be pulled and discarded offsite.
While this year’s Bot Rot infestation has been extremely frustrating and similar future freeze events can’t be ruled out, gardeners should not give up on Sweet Viburnum, an excellent specimen or screening shrub. Keeping plants healthy with proper pruning, good fertility, and adequate irrigation is the best defense to ward off future infection when we experience harsh environmental conditions! For more information on Bot Rot, Sweet Viburnum, or any other horticultural topic, contact your local UF/IFAS County Extension Office! Happy Gardening.
Twigs dangle from a branch, held in place by horsehair fungus rhizospheres. Photo credit: Carrie Stevenson, UF IFAS Extension
Imagine walking through the woods, enjoying the fresh air and birdsong, when you notice a twig just dangling midair. On first glance, you might presume it is stuck in a vine or a large spider web. However, this material is thinner than a vine and there are no spiders or webs to be found. On closer inspection, you see the twig is entangled in a dark knot of string, slightly thicker than a human hair. Following the source of the black string leads to a lacy network of the same material, flat against and around the branch from which the twig hangs. Fifteen feet down the trail, you come across the same thing—more twigs hanging from “string” in the trees, dangling from a knot of thin black material. What is going on here?
After some research, my conclusion is that we have come across horsehair fungus (Mirasmius spp). Mushrooms and other fungi have long, thin, rootlike structures called rhizomorphs. Rhizomorphs are composed of large numbers of microscopic hyphae, which are tubular and exchange nutrients and water among other parts of the fungus. The hyphae are typically located underground, but in this instance, they are visible in the open.
The ropy network of rhizospheres is visible on the branch, then turns into thin fibers that can trap plant debris. Sample provided by Suzy Marshall.
Aerial rhizomorphs are an excellent example of an evolutionary strategy known as resource partitioning. By forming a netted trap to intercept falling leaf material on a tree, the fungus gets a jumpstart on the decomposition process, taking nutrients from dying plant material before it falls to the ground. The tree-dwelling fungus is thus not competing directly with fungi on the ground, and both types of fungi can thrive. In human terms, this is loosely analogous to co-workers labeling their own food in a shared refrigerator. Everyone eats, but no one has to compete with another for nutrition.
A bird’s nest in Argentina is composed primarily of strong horsehair fungus fibers. Photo credit: Danny Newman
The common name, “horsehair fungi” applies to many species around the world. They are most common in nutrient-poor subtropical and tropical forests, where any available nutrients in the soil are used up quickly by the lush tree growth. They can be found here along the Gulf Coast, up to the Appalachian Mountains, and as far away as equatorial rainforests. In a Malaysian study, researchers found that the aerial fungi trapped up to 225 pounds of fallen plant material per acre! This network of material also supported a significant population of arthropods, which were crucial parts of the overall ecosystem by providing pollination, herbivory, and serving as detritivores. One study of spruce-fir forests in the northeastern United States showed that birds are known to use the material as a nest lining, as it is lightweight but very sturdy. In this particular research, 85% of the nests (particularly of warblers and thrushes) examined utilized the material.
Houseplants can soften up the interior of your home and help clean the air. They can also supplement your holiday decorations and help create stunning focal points. To help determine what plants do best under certain conditions and to give pointers on plant care, this month’s Gardening in the Panhandle LIVE! was all about houseplants.
A spider plant on a coffee table. Photo Credit: University of Florida/IFAS
Environmental Conditions for Houseplants
Unless you live in a glass house, you’ll probably want to choose houseplants that do well in low light conditions. A guide for what light level different houseplants prefer can be found on the Gardening Solutions Light for Houseplants page. This page also provides useful tips on supplemental lighting.
Some houseplants are better at cleaning the air than others. A list of houseplants that do a good job improving indoor air quality can be found on the Gardening Solutions Houseplants That Clean the Air page.
The best way to determine if your houseplants need water is your own green thumb or whatever finger you choose to stick in the potting mix, but for some interesting information on outdoor soil moisture meters check out this informative publication on soil moisture sensors.
Houseplants need a good quality, well-drained potting mix to thrive. Tips on selecting a potting mix can be found on the Gardening Solutions Container Media page.
Houseplant Pests
One of the best ways to rid houseplants of insect pests is to set the plants outside for a few days and let the pests move on. For some information on pest control products in and around the home check out the publication Natural Products for Managing Landscape and Garden Pests in Florida.
Fungus gnats are mainly a nuisance, but some species can feed on living plant tissue. Darkwinged fungus gnats are known to feed on ferns, orchids, and geraniums.
Houseplant Propagation
One way to increase your houseplant population and save a few dollars is to propagate your own plants. The University of Florida/IFAS created the Plant Propagation Glossary to help with any propagation questions you may have.
Air layering is a propagation technique that not only allows the prospective plant to thrive from the nutrients of the mother plant, but it also saves space.
A moth orchid (Phalaenopsis spp.) outdoors. Photo Credit: University of Florida/IFAS
Specific Species Info
Orchids in the genus Phalaenopsis are easier to care for than other genera of orchids. The American Orchid Society provides some great tips on caring for orchids indoors. Some people choose to water their orchids with ice cubes. The Ohio State University has a publication that provides some more insight on watering Phalaenopsis orchids with ice cubes.
A lot of cacti do well indoors. A popular cactus during the holiday season is Christmas cactus. Christmas cactus have interesting foliage, but their blooms are what people want to see. Some tips on getting your Christmas cactus to bloom on time and general care information can be found in this Christmas Cactus Preparation fact sheet.
Have you ever wanted to grow fruit trees indoors or do you want some tips on bringing containerized fruit trees indoors for the winter? The Growing Fruit Crops in Containers publication provides some good tips on growing fruit trees indoors.
Unless you have a house with a lot of windows or a sunroom, plumeria don’t make the best houseplants. They need at least six hours of sunlight per day and need to be at least three years old to bloom. If you are interested in propagating plumeria, then check out this publication on propagating plumeria from cuttings.
Did you know that the first virus discovered was in plants, not in humans? As early as 1857, tobacco farmers in the Netherlands recognized a new disease of tobacco. It wasn’t called a virus at the time as the causal agent was unknown. In 1886, Adolph Mayer, a German agricultural chemist, determined that the “tobacco mosaic” disease could be transmitted to healthy plants by rubbing them with infected leaf sap.
Tomato leaf with tobacco mosaic virus. Photo credit: UF Plant Pathology Department
When taking a plant pathology course in college, it amazed me that viral organisms were ever discovered. They are extremely small. So small that it was not until the development of the electron microscope in the late 1930s that scientists were able to see the structure of the tobacco mosaic virus. Viruses are 20 – 250 nanometers in diameter, about 100 times smaller than bacteria.
The discovery of fungi and bacteria came before the discovery of viruses as the cause for plant diseases. In most cases, we have many chemical options for control of fungal and bacterial diseases in plants. But there are few to no chemical options to control viral organisms in plants. By the nature of how a virus operates in a plant cell, chemical control results in death of the host cell, surrounding tissue and possibly the whole plant.
Control of viruses in plants involves eliminating the source of the virus such as nearby weeds, control of insect vectors that transmit the disease such as aphids and thrips and use of resistant plants.
Historically, plant diseases have caused major impacts on humans. In 1845, the Potato Famine in Ireland was caused by the fungus Phytophthora infestans. This disease was responsible for the death of more than 1 million people as it devastated the production of the potato as a major food source at the time and is credited for the beginning of plant pathology as a science. Cryphonectria parasitica, an introduced fungus, essentially wiped out the American chestnut as the dominant tree in the eastern U.S. forests during the early 1900s. The excellent wood was used to build homes and the nutritious nuts were eaten by humans and fed to livestock. Currently, UF/IFAS plant pathologists are working to solve Citrus Greening, a bacterial disease that has severely damaged the citrus industry and has the potential to completely eliminate citrus production in Florida.
If it were not for the land-grant university system, of which Extension Agents are a part, there would be few to no plant pathologists in the United States. It is these land-grant universities, like the University of Florida and others, that provide plant pathology courses, training, research, development and ultimately that graduate with degrees those who go on to careers in the field of plant pathology, discovering new diseases and developing controls for these diseases.
Hydrangea leaf spot disease Photo credit: Larry Williams
It is not uncommon to see leaf spots on your hydrangeas during late summer and fall.
These spots are caused by a number of fungal diseases. Plant fungi and wet weather go hand-in-hand. Florida’s high humidity, heavy dews and frequent rains during spring and summer provide perfect conditions to allow fungal diseases to flourish. Bacterial leaf spots can be part of this foliage disease mix, too.
Common foliage diseases seen on hydrangeas this time of year include Phyllosticta leaf spot, Target leaf spot, Bacterial leaf spot, Botrytis and Cercospora leaf spot.
These foliage diseases are the norm rather than the exception as we move into the wet summer months and on into fall. As a matter of fact, you would be hard-pressed to find any hydrangea in our area without some evidence of infection now.
This late in the year it is more of a “grin and bear it” problem. In other words, it’s too late to do much about the fact that your hydrangea plant has leaves covered in ugly spots. By now many of the infected leaves are turning brown, withering and dropping prematurely from the plant.
Cercospora leaf spot is one of the most common foliage diseases of hydrangeas. Along with most of the leaf spot diseases, it begins as small dark-colored specks on the leaves. The small specks generally go unnoticed. But as the spots continue to slowly enlarge, mostly maintaining a circular shape, they become more obvious. With heavy infection, individual spots can coalesce forming larger irregular shaped brown areas on individual leaves. The individual spots may have a purplish halo with gray center.
There are some fungicides that can help prevent these leaf spots. But you’d have to begin treatment early in spring before any leaf spots exist and spray the plant every 10 to 14 days during favorable disease development (humid, rainy weather), which is pretty much our spring and summer months. These types of diseases are prevented, not cured. That’s the “grin and bear it” part of waiting until now.
The fungus survives on infected leaves. So, the best thing to do now is to remove and dispose of infected leaves. Also, be careful to not wet the leaves when irrigating the plants during the growing season.
New leaves of spring should be spot/disease free as they emerge. But the cycle of life for these leaf spot diseases will again result in spotted/diseased leaves on your hydrangeas next summer and fall without persistent treatment.
The good news is that these leaf spot diseases normally do not cause permanent/long-term damage for hydrangeas. They just make the plant look ugly.
