The rose has long been a symbol for love and romance, serving as an iconic and lucrative ornamental for commercial growers.
But as much as it has achieved universally beloved status, the rose can be a challenging crop to produce. With production occurring year-round, greenhouse and nursery crews must be vigilant about the pests and diseases biding their time to attack.
Comprehensive agronomic programs can be a critical tool to keep roses protected. An effective program includes insecticides and fungicides, as well as time-tested cultural practices – spacing, scouting, and more – to ensure a high-quality production cycle.
“Scouting and early diagnosis of infected plants are critical for preventing the spread of rose diseases and implementing effective disease control strategies,” says Fulya Baysal-Gurel, research assistant professor at Tennessee State University. “Humidity, temperature and light management, as well as spacing, sanitation practices, and preventive fungicide applications, are all important for controlling rose diseases.”
Scout for the signs
“Roses are cultivated in greenhouses and outdoor nurseries, with successful growing seasons characterized by careful planning,” notes Nancy Rechcigl, technical field manager for ornamentals at Syngenta. The key to preparation is knowing the signs of an infestation or infection, especially of frequent rose challenges such as mites, whiteflies, downy mildew and powdery mildew.
Mites, particularly the two-spotted spider mite (Tetranychus urticae), are voracious feeders that thrive in hot, dry environments. Their color can vary from a light yellow to yellow-green, with two dark spots on the abdomen. They can be found feeding on the underside of rose leaves.
Damage from two-spotted spider mites appear as tiny yellow-white spots or “stippling” on the upper foliage. Infestations that become severe often have noticeable webbing in the plant canopy, which serves as protection from predators. Heavily infested leaflets turn yellow and will drop from the plant.
If left untreated, foliage damage can lead to leaf loss and, eventually, plant death.
Whiteflies are another prolific nuisance for roses. The greenhouse whitefly (Trialeurodes vaporariorum) and the silverleaf whitefly (Bemisia tabaci) are the two most commonly seen in greenhouse and outdoor nursery spaces, with the silverleaf whitefly being the most dominant. There are two biotypes of the Silverleaf (B and Q) that can be found in operations. Biotype Q can be more difficult to control since it is more tolerant to some of the insecticides commonly used in operations.
“The adult and immature stages feed on the underside of leaves and cause leaves to turn yellow in a blotchy pattern,” says Rechcigl. “Whiteflies also excrete honeydew from their feeding. This drops to the upper surface of leaves, making them sticky and giving them a shiny appearance. You may notice ants crawling up and down the plant to feed on the honeydew.”
On the disease front, roses are a frequent target of downy mildew – caused by the oomycete Peronospora sparsa. This can be a very serious disease if roses become infected. Early symptoms include foliage that has a reddish or yellow blotchy or mottled appearance.
“If you turn the leaf over, you’ll likely see some gray-pink sporulation on its underside, which corresponds with the blotchy areas,” says Rechcigl. As the disease progresses, the plant will defoliate, leaving it weak and unsalable.
Another mildew that infects roses is powdery mildew, caused by Sphaerotheca pannosa var. rosae (also known as Podosphaera pannosa). As the name implies, this pathogen produces a powdery, white growth on the upper surface of leaves, stems and flower parts. This disease can spread rapidly, and if left untreated, the pathogen will stunt and disfigure leaves and rose buds, stopping them from opening and greatly reducing plant quality.
“Proper identification of a pest or disease problem is critical for effective control,” says Rechcigl. Without it, you run the risk of making a “control” application that will not actually help, instead letting the problem get worse.
For best results
A well-rounded agronomic program incorporates chemical products that can avert harmful infestations before they begin. A best-in-class management strategy features three or more products – including an effective miticide like Avid® 0.15 EC, or a fungicide such as Segovis® or Mural® – with different modes of action.
Not only does this optimize control, but it also helps decrease the chances that resistance will become an issue – a risk for highly produced ornamentals like roses exacerbated by overreliance on a single product.
“Pathogen resistance to fungicides is well known, and the performance of many fungicides has been affected to some degree by pathogens developing resistance,” says Baysal-Gurel. “Using different modes of action in a rotation program is important to minimize the risk for resistance development.”
To manage powdery mildew in roses, spray applications of a preventative fungicide like Mural are typically made on a one- to three-week interval, depending on disease severity and whether the crops are being grown in a greenhouse or nursery. “This way, growers can extend their treatment interval while maintaining good protection from the fungus,” says Baysal-Gurel.
Mural is a broad-spectrum fungicide for use in both greenhouses and outdoor nurseries. Due to its systemic and translaminar (or ability to distribute the chemical from upper to lower leaf surfaces) activity, Mural protects the entire plant, whether applied as a spray or drench.
Segovis fungicide, meanwhile, is a reliable treatment for downy mildew. Providing a month’s worth of control due to systemic properties, Segovis is best applied as a drench.
A study at Tennessee State University evaluated the efficacy of Segovis and additional fungicides on rose downy mildew, with drench treatments applied at the first sign of infection. While all fungicide treatments significantly reduced final disease severity ratings, Baysal-Gurel says, the severity of downy mildew as well as disease progression were significantly lower in plants treated with Segovis.
“Benefits of using a drench application of systemic products in a treatment program were clear: One application of Segovis provided excellent protection to rose plants for 30 days,” she says.
Battling mites by chemical means requires an industry-leading miticide like Avid, which also safeguards roses from the hungry depredations of whiteflies, aphids and thrips. The highly versatile, broad-spectrum pest control product targets the mite life cycle when swapped with insecticides carrying disparate modes of action.
“Avid’s use rate is about 8 to 16 fluid ounces per acre. For resistance management purposes, I recommend two applications, then rotate to a different product for two or three applications before going back to Avid,” Rechcigl says.
Prevention is key
Proper cultural practices can further mitigate the challenges growers are likely to face from insects and disease. For example, no matter the production scenario, removing plant and weed debris should be the first step taken by any grower.
Equally important is plant spacing, which is vital for allowing air flow between plants, effectively reducing humidity within the rose canopy and incidence of foliar disease.
Coupled with crop protection tools and a strong agronomic program, these routines can save growers time, money and stress.
“If you’re prepared, you’ll be ready to act when you see a problem or prevent a problem from occurring,” says Rechcigl. “Preventing a problem is always less expensive than making corrective applications.”
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