Prompt action keeps roses healthy

Special Section - 2022 Horticultural Industry Leadership Awards

Vigilance and education are the watchwords in preventing the spread of invasive diseases and pests.

July 13, 2022

Roses are a valuable crop in the greenhouse and nursery industry and a consumer favorite year after year. The plant’s inherent flexibility makes it suitable for growing as cut flowers, patio decorations or for transplant into the garden.

Unfortunately, a myriad of insects and diseases are often waiting to spoil their elegance and beauty. However, growers that take the proper preventive measures — from cultural practices to chemical mitigations — can nurture a strong crop while keeping their plants safe from insects and infection.

“The best way to maintain a clean crop is to constantly scout for issues, although that won’t always prevent the problem completely,” says Carson Cashwell, ornamental market manager at Syngenta. “Even the most detailed scouting won’t uncover every issue. Applying fungicides and insecticides preventively will ensure that the necessary steps are taken to provide a pest-free crop.”

What to watch for

Rose production occurs throughout the year in greenhouse and outdoor nursery environments. While year-round availability is great for growers and retailers, there are many different diseases and insects to watch for depending on the season. Spring through early fall is the domain of mites, aphids, black spot, downy mildew and powdery mildew, with overwintering mites taking residence beginning in October.

The diseases that can hinder rose production are caused by fungi, bacteria and plant viruses. Such infections can significantly harm rose health, and in worst-case scenarios, interfere with production and sales. In early spring, growers should begin checking plants for signs of downy mildew infections as this pathogen mostly prefers cool, moist conditions between 50°F and 75°F. This temperature range allows the infection to survive through spring and into the early summer months.

Caused by Peronospora sparsa, downy mildew attacks all types of roses in greenhouse, nursery and landscape settings. Symptoms are most commonly found on the foliage and stems. Early indicators are yellow-reddish blotches on the upper areas of leaves. Under humid conditions, downy mildew causes a fuzzy or downy growth on the underside of the leaves corresponding to the discolored areas. Infected leaves will defoliate causing the plant to become bare and weak.

Black spot on rose, Postiva™ fungicide 21 fl. oz. - Uber, 2020
All photos are either the property of Syngenta or are used with permission.
Black spot on rose, Untreated - Uber, 2020

In contrast, powdery mildew pathogens generally occur in early summer, characterized by white, powdery colonies that flourish on surfaces of leaves, shoots and buds. Early symptoms appear as chlorotic or reddish areas on the plant’s upper surface, which are quickly covered by the white clusters that give the disease its name. Powdery mildew is caused by the fungal pathogen Sphaerotheca pannosa var. rosae (also known as Podosphaera pannosa). Untreated, the disease will stunt and disfigure leaves and rose buds, stopping them from opening and greatly reducing plant quality.

Many different leaf spotting pathogens can challenge roses during the summer months. Black spot (Diplocarpon rosae) is one of the most serious leaf spot diseases on roses. Cultivators surveying their crop for signs of infection may find dark brown or black lesions with a feathery margin, often surrounded by a yellow halo. Infected leaves turn yellow and drop prematurely, weakening plants so they produce fewer flowers. Additionally, stem cankers can develop on canes, initially displaying as red-purple blotches that darken and crack with age.

Leaf spots caused by Cercospora spp. can also be problematic on roses and can often be confused with black spot lesions. Infections caused by Cercospora spp. cause tan circular spots surrounded by a distinct reddish-purple to dark brown margin. Leaf spots caused by Xanthomonas spp. produce dark brown or black angular spots with a greasy appearance and yellow halo. Leaf spot diseases are more problematic in warm summer months when rainfall or dew is abundant.

“Vigilance and education are the watchwords for growers aiming to differentiate the various plant ailments targeting their crops,” notes Fulya Baysal-Gurel, a research assistant professor at Tennessee State University.

“Early diagnosis is important, as is managing spacing, sanitation practice implementation and utilizing fungicides,” Baysal-Gurel says.

Fighting back against diseases and insect pests

“Preparing your nursery or greenhouse for the inevitable disease- or insect-related pressure starts with impeccable cultural practices,” says Cashwell. This includes scouting for signs of disease and insect pressure, as well as removing plant debris from growing sites, where insects and disease pathogens can linger. Additionally, newly purchased plants should be carefully inspected before they’re added to your inventory, as they can introduce many unwanted diseases and insects.

Another recommendation is to thin canes and remove foliage where possible to increase air circulation and reduce excess moisture. The same can be said for proper spacing of plants, with the added bonus of preventing disease spread.

After ensuring correct cultural practices are in place, plan ahead with a preventive agronomic program. “Meticulous production planning should include a preventive chemical plan,” explains Cashwell. “Syngenta offers growers numerous agronomic programs for the various production scenarios they will encounter throughout the year.”

Let’s take mites for example. Mites, particularly two-spotted spider mites (Tetranychus urticae), are hungry pests that flourish in dry, hot environments. Sharp growers will notice their specific coloring, which varies from a light yellow to yellow-green, with two dark spots near the abdomen. When feeding on the underside of rose leaves, spider mites leave yellow-white spots or “stippling” on upper foliage. Severe infestations leave webbing in the plant canopy, and heavily infested leaflets will turn yellow and fall from the plant.

When battling these pests, an agronomic plan that includes an industry-leading miticide like Avid® 0.15 EC, will help safeguard roses from not only mites but also whiteflies, aphids and thrips.

Downy mildew on rose - Rechcigl, Syngenta

A proactive approach to disease control will also give growers a leg up and begins with a program that includes the latest innovations. For example, the new Rose Agronomic Program from Syngenta includes Postiva™ fungicide. Recently introduced into the market, Postiva is powered by a unique combination of ingredients to control foliar and soil-borne diseases while suppressing various bacterial diseases. Postiva features two modes of action in FRAC Group 7 and FRAC Group 3. It 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.

Another proven Syngenta solution is Mural®, a broad-spectrum fungicide for use in greenhouses and outdoor nurseries. When applied at low use rates, Mural provides broad-spectrum disease control and observable plant-health benefits, such as increased root density. Baysal-Gurel explains further why a preventive plan is critical to producing a successful rose crop. “Roses can be latently infected with downy mildew pathogens. When conducive conditions such as cool weather and high humidity occur for disease development, symptoms may appear overnight,” says Baysal-Gurel. “Therefore, preventive fungicide applications in a rotation are critical to control downy mildew.”

Segovis® fungicide is a reliable treatment for downy mildew and is best applied as a drench because of its systemic properties that provide a month’s worth of control. An effective downy mildew rotation uses three or more modes of action to provide maximum benefits and thwart the onset of pathogen resistance.

“The biggest benefit of following a disease management program from Syngenta is that you’re covered from the beginning of production to the end,” Cashwell says. “Any program we provide will contain recommended products to attack the key insects and diseases a grower will face. Programs developed by Syngenta also rotate modes of action, which will delay the onset of resistance.”


Visit to access the Syngenta Rose Agronomic Program or use the QR code to the left to download the program.

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