Keeping it under control – Greenhouse Management


Ornamental plants are threatened by a number of fungal pathogens. These pathogens cause leaf spot, leaf scorch, 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 culture. In the daily operation of a nursery or greenhouse, growers must control soil pathogens such as Fusarium spp. as well as bacterial diseases caused by Pseudomonas spp. and Xanthomonas spp. Pathogens.

To understand what is attacking your plants, let’s start with the basics. Fungi are multicellular 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, the bacteria accumulate rapidly in hot, humid weather conditions, spreading from plant to plant through water droplets.

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 mottled.

If left untreated, these leaf spots can enlarge and spread throughout the leaf and onto adjacent foliage. As the leaf spots become more abundant, the foliage usually turns yellow and drops off. Growers should watch for tiny dot-like structures or moldy spore growth starting on the lower leaves and working their way up the plant. A small magnifying glass or microscope with 10X to 20X magnification may be needed to detect these symptoms before they progress.

Diseases caused by Fusarium spp. presents in many ways: as leaf scorch, root and crown rot, and vascular wilt. Indications of root rot usually appear on the aerial parts of plants as wilting, discoloration of leaves or loss of vigor. A lack of fertilizing and watering results is another sign of this fungal infection. To diagnose root rot, the soil must be thoroughly washed of roots, which will likely be mottled with black or brown rot 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 organisms responsible for root rot.

Vascular wilts, on the other hand, can be caused by several different pathogens, including Fusarium, Verticillium, and Ophiostoma. These pathogens restrict the flow of water to stems and leaves, causing limbs or branches to wilt and die. Fusarium and Verticillium spp. infections usually start in the roots and spread inside the plant.

Too often growers discover infections after the damage has been done. Although the pathogen invades crops in the early stages of production, infections often go unnoticed until root and crown damage is evident.

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

First steps towards safeguarding ornamental crops

First, growers must also be familiar with how diseases develop. Unrooted cuttings and young plants require warm temperatures, frequent misting, or high humidity, which allows rooting in plant media. This environment also provides the optimal conditions for pathogen spores to germinate and invade ornamental plants 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. It is therefore vital that growers pay close attention to environmental conditions and implement proper cultivation practices to avoid additional stress as the plant develops. “The list of plants most susceptible to Fusarium spp. and bacterial diseases is long; the most common susceptible plants are cyclamen, chrysanthemums and dracaena,” says Chase.

Third, have a clear understanding of the symptoms of common diseases. Take for example bacterial leaf disease. Many ornamental and edible plants show dark, necrotic spots on their leaves, which may include black-edged lesions, brown spots with yellow halos, or light and dark areas on the foliage. Leaf spots can also be found on the edges of the leaves, where the tissues appear brownish-yellow and become papery and delicate.

Bacterial leaf spot pathogens – Pseudomonas spp., Xanthomonas spp. et al. — do not normally pose a serious threat to plant health. However, they can negatively impact the overall appearance of a crop when waterlogged spots turn dark with a greasy appearance.

2019 – Krasnow, Vero Beach Research Center

“Bacterial leaf spot is more common on tropical plants simply because of the amount of water needed to produce it,” Chase says. “We see Xanthomonas spp. infections on hibiscus and geranium, and cruciferous vegetables like cabbage and broccoli, plus a host of others. Pseudomonas sp. has been common in the past on chrysanthemums, hibiscus, canna lilies and many foliage plants.

Control harmful diseases with proven innovations

Horticulturists are far from defenseless when it comes to protecting their crops. Syngenta’s latest addition, Postiva Fungicide, provides long-lasting, broad-spectrum protection against tough fungal and bacterial diseases.

Postiva exhibits 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 rain resistant to create a protective coating. Within hours, Postiva begins to penetrate plant tissues, providing systemic disease control. Applications can be made by spraying, dipping, chemigation or cold fogging.

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

With two modes of action, Postiva provides strong defense against multiple pathogens while helping to delay the development of resistance. Growers can alternate Postiva with Syngenta’s Mural®, Palladium® and Daconil® brand fungicides, for robust protection against most production-threatening diseases.

“When you alternate or mix different modes of action, you improve your ability to control a pathogen in more than one way,” says Dorer. “Pathogens can develop resistance to a particular pathway, so it’s important to limit the possibility of developing resistance to your chemistry.”

“Postiva can be used on ornamental crops, vegetables, and non-bearing fruit and nut plants grown for resale in nurseries and greenhouses. The chemical provides a high level of disease control, affecting plants from root to canopy,” according to Chase.

“We haven’t seen any cases 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 disease type or fungicide. They all work best in prevention.

Even the most potent pathogen is not immune to Postiva, largely due to its robust combination of ingredients and its ability to stop pathogens before they can fully overrun a crop. .

“There are other products that can help control many different fungal and bacterial pathogens, but they haven’t been shown to give the same high level of control, or they can’t be used as foliar and drench applications like Postiva,” adds Chase.

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© 2022 Syngenta. Important: Always read and follow label directions. Some products may not be registered for sale or use in all states or counties and/or may have state-specific use requirements. Please check with your local extension service to ensure registration and proper use. GreenCast®, Daconil®, Mural®, Palladium®, Postiva™ and the Syngenta logo are registered trademarks of a Syngenta group company. All other trademarks are the property of their respective owners.

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