Keeping it under control

Special Section - 2022 Horticultural Industry Leadership Awards

Utilize these tips to limit the spread of soil-borne and bacterial disease pathogens.

July 13, 2022

Xanthomonas on Zinnia, 2016 - Rechcigl
All photos are either the property of Syngenta or are used with permission

Ornamental plants are challenged by a number of fungal pathogens. These pathogens cause leaf spots, leaf blights, powdery mildew, leaf gall, root and stem rot, and vascular wilt. Accurate diagnosis of these diseases is crucial in their treatment, as misidentification can lead to an unmarketable crop. In the day-to-day running of a nursery or greenhouse, growers must tackle soil-borne pathogens such as Fusarium spp. as well as bacterial diseases caused by Pseudomonas spp. and Xanthomonas spp. pathogens.

To understand what’s attacking your plants, let’s begin with the basics. Fungi are multi-celled microbes that feed on living plants, producing spores that cause infection when carried by wind, water, insects and even production tools.

Bacteria are single-celled organisms that reproduce by simple cell division. Like their fungal counterparts, bacteria build up quickly under warm and humid weather conditions, spreading from plant to plant through water droplets.

Both fungal and bacterial pathogens can cause leaf spots, which often vary in size, shape and color. Some leaf spots have distinct margins surrounded by yellow halos, while others may be angular and blotchy.

If left untreated, these leaf spots may enlarge and spread over the entire leaf and to adjacent foliage. As leaf spots become more abundant, foliage will typically yellow and drop off. Growers should watch for tiny dot-like structures or moldy spore growth starting on lower leaves and progressing up the plant. A small hand lens or microscope with 10X-20X magnification may be needed to catch these symptoms before they progress.

Diseases caused by Fusarium spp. present in several ways: as a foliar blight, root and crown rot and as a vascular wilt. Indications of root rot generally appear on above-ground plant parts as wilting, leaf discoloration or loss of vigor. A lack of results from fertilizing and watering is another sign of this fungal infection. To diagnose root rot, soil must be carefully washed from roots, which will likely be mottled with black or brown decay and spongy to the touch. In addition to Fusarium spp., other pathogens such as Rhizoctonia and Thielaviopsis spp. and the water molds Pythium and Phytophthora spp. are the most common root rotting organisms.

Vascular wilt diseases, meanwhile, can be caused by several different pathogens including Fusarium, Verticillium and Ophiostoma. These pathogens restrict water flow to stems and leaves, causing individual limbs or branches to wilt and die. Fusarium and Verticillium spp. infections usually begin in the roots and spread internally throughout the plant.

Growers too often discover infections after the damage is done. Although the pathogen invades crops in early production stages, infections often go undetected until root and crown damage is evident.

“Fusarium spp. is also a very good saprophyte, meaning it does not even need a plant to live on,” says Dr. Ann Chase, owner of Arizona-based Chase Agricultural Consulting. “Once Fusarium spp. has infested, you can never get rid of it from the potting medium or from soil.”

First steps to safeguarding ornamental crops

First, growers should also be familiar with how diseases develop. Unrooted cuttings and young seedlings require warm temperatures, frequent misting or high humidity, which enable rooting into plant media. This environment also provides pathogenic spores with the optimal conditions to germinate and invade ornamentals via wounds and tender tissue.

Second, consider the condition of each plant. For example, Fusarium wilt, caused by Fusarium oxysporum, occurs more frequently in stressed plants, so it is vital for growers to pay close attention to environmental conditions and implement proper growing practices to prevent added stress as the plant develops. “The list of plants most susceptible to Fusarium spp. and bacterial diseases is long; the most common susceptible plants include cyclamen, mums and dracaena,” says Chase.

Third, have a clear understanding of the symptoms of common diseases. Consider bacterial leaf disease for example. Many ornamental and edible plants display dark, necrotic-looking spots on their leaves, which may include black edged lesions, brown spots with yellow halos, or light and dark areas on foliage. Leaf spots can be found on leaf edges as well, where tissue appears brownish-yellow and becomes papery and delicate.

Bacterial leaf spot pathogens — Pseudomonas spp., Xanthomonas spp. and others — normally do not pose a serious threat to plant health. However, they can negatively impact a crop’s overall appearance when the water-soaked spots turn dark with a greasy appearance.

2019 - Krasnow, Vero Beach Research Center

“Bacterial leaf spots are more common on tropical plants simply due to the amount of water needed to produce them,” Chase says. “We see Xanthomonas spp. infections on hibiscus and geranium, and cruciferous vegetables such as cabbage and broccoli, along with a multitude of others. Pseudomonas spp. has been common in the past on mums, hibiscus, canna lily and a lot of foliage plants.”

Control damaging diseases with proven innovations

Ornamental growers are far from defenseless when it comes to safeguarding their crops. Postiva fungicide, the latest from Syngenta, provides broad-spectrum and long-lasting protection against challenging fungal and bacterial diseases.

Postiva features two modes of action in FRAC Group 7 and FRAC Group 3. Postiva migrates from the leaf surface into the wax layer upon application, becoming rainfast to create a protective coating. Within hours, Postiva begins to penetrate plant tissue, providing systemic disease control. Applications can be made by spray, drench, chemigation or cold fogging.

For the most effective control, growers should apply Postiva either prior to or at the first sign of disease. “Preventive applications will decrease populations of pathogens more effectively,” says Steve Dorer, brand manager for fungicides at Syngenta. “Early in their lifecycles, ornamentals create new foliage, so it’s easier for them to grow through those early symptoms. But when it’s late in the growth cycle, you don’t want symptoms that would make the plant unsellable. Prevention is always the key.”

With two modes of action, Postiva provides a strong defense against multiple pathogens while helping delay resistance development. Growers can rotate Postiva with Mural®, Palladium® and Daconil® brand fungicides from Syngenta, for robust protection against most diseases that threaten production.

“When you rotate or mix different modes of action, you’re improving your ability to control a pathogen in more than one way,” says Dorer. “Pathogens may develop a resistance to a particular pathway, so it’s important to limit the opportunity to develop any resistance to your chemistry.”

“Postiva can be used on ornamental crops, vegetable plants and non-bearing fruit and nut plants grown for resale within nurseries and greenhouses. The chemical delivers a high level of control for diseases, impacting plants from root to canopy,” according to Chase.

“We have not seen any instances of phytotoxicity or plant damage yet, but you should always test new products on crops under your conditions to be sure,” Chase says. “I also never recommend curative applications, regardless of the kind of disease or the fungicide. They all work better preventively.”

Even the heartiest pathogen is not safe from Postiva, largely due to its robust ingredient combination and ability to halt pathogens before they can completely take over a crop.

“There are other products able to help control many different fungal and bacterial pathogens, but they have not proven to give the same high a level of control, or they cannot be used as foliar and drench applications like Postiva,” Chase adds.


All photos are either the property of Syngenta or are used with permission.

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