Roses are beautiful, diverse in form, fragrant, and bloom year-round. With numerous varieties and ease of propagation and cultivation, they are hailed as the "Queen of Flowers" and are one of China's ten most famous flowers. However, roses are often susceptible to pests and diseases, leading to poor growth and even death, severely impacting their ornamental value. Based on years of practical experience, the author has summarized the main rose pests and diseases and their control methods for reference.
1. Rose Black Spot Disease
: Leaves, young leaves, and flower stalks can all be affected. Lesions typically appear on the leaf surface, forming nearly circular or irregular spots about 1.5 mm in diameter. The edges are fibrous, black or dark brown, and often surrounded by a yellow halo. The mycelium branches radially beneath the cuticle and is dark purple. Small black raised dots, the conidiophores of the pathogen, are visible on the lesions. Several lesions often merge together. Large areas of leaves around the affected area turn yellow, and the lesions become "islands" with green edges. Lesions on petioles and young branches appear as elongated strips without obvious radial edges. In severe cases, diseased leaves become wrinkled and uneven, curling backwards; young branches bend downwards or die; and flower buds fail to open properly.
The pathogen is a fungus of the genus *Actinomyces*. Conidia are oblong or gourd-shaped, two-celled, and slightly constricted. The upper cells are small, with a beak-like projection, and tend to one side.
The pathogen overwinters as mycelium or conidiophores on diseased branches and dead leaves. Conidia are produced the following spring and spread by wind. Humid, foggy, hot, rainy, and poorly ventilated environments favor the occurrence and spread of the disease. Heavy dew, watering after dusk, or water accumulation in the foliage at night all provide favorable conditions for disease development and exacerbate the disease. Spores can germinate and invade within 6 hours when there is water on the leaves. Poor plant growth, especially in newly transplanted seedlings, leads to severe disease. High planting density in open fields, overcrowded flowerpots, and sprinkler irrigation all exacerbate the disease.
Control measures: (1) Eliminate the source of infection. Thoroughly remove dead branches and fallen leaves in autumn, and prune diseased branches in winter to eliminate overwintering pathogens on diseased branches.
(2) During the disease season, check frequently, remove and destroy diseased leaves in time to prevent the spread of the disease.
(3) Improve environmental conditions, strengthen cultivation management, improve the plant's disease resistance, and control the occurrence of the disease. Watering is best done by drip irrigation, furrow irrigation, or watering along the edge of the pot. Avoid sprinkler irrigation to prevent water droplets from splashing and spreading pathogens to nearby leaves. Watering is best done on sunny mornings to keep the leaves dry. Planting density and pot placement should be appropriate to facilitate ventilation; select locally resistant rose varieties; increase the application of organic fertilizer, phosphorus and potassium fertilizer, and nitrogen fertilizer in appropriate amounts to promote robust plant growth and improve disease resistance.
(4) When new leaves just unfold in summer, fungicide protection should begin until winter. During the rainy season, spray 1-2 times per week; otherwise, spray once every 7-15 days. The frequency of spraying depends on the development of the disease and the type of pesticide. Suitable pesticides include 75% chlorothalonil at 500x dilution, 50% carbendazim wettable powder at 500-1000x dilution, 80% zineb at 500x dilution, 1% Bordeaux mixture, or 70% thiophanate-methyl at 800-1000x dilution. Among these, 75% chlorothalonil at 500x dilution is the most effective. Spray once every 7-10 days. To prevent the development of pesticide resistance, pesticides must be used alternately.
Powdery mildew
affects the leaves, young shoots, buds, and flower stalks of roses. When young leaves are infected, they become wrinkled, curved, and deformed, sometimes turning purplish-red. When older leaves are infected, nearly circular, water-soaked, chlorotic yellow spots appear on the leaf surface, with no clear boundary between the spots and healthy tissue. A white substance appears on the underside of the leaves where the spots are located. In severe cases, the leaves wither and fall off. Affected parts of young shoots and flower stalks swell slightly, with their tips bending towards the ground. Infected flower buds fail to open or exhibit deformed flower shapes. The surface of affected areas is covered with a white powdery layer.
The pathogen is *Monoclonus roseus*, whose mycelium develops on the host surface and uses haustoria to penetrate the plant's epidermal cells and absorb nutrients. Conidiophores are short, erect, and bear conidia at the apex. The conidia are colorless, oval or barrel-shaped, and occur in chains of 5-10. The mycelium mainly overwinters as hyphae in the dormant buds of infected plants. The following spring, as soon as the buds unfold, they are covered with a white powdery coating. These conidia are dispersed by wind to young tissues, germinate under suitable conditions, and enter the epidermal cells through the cuticle and epidermal cell walls to cause damage.
Powdery mildew generally thrives in warm, dry, or humid environments. Rainfall is unfavorable for disease development. Excessive nitrogen fertilizer, soil deficiencies in calcium and potassium fertilizers, and overcrowded planting with poor ventilation and light penetration exacerbate the disease. Drastic temperature changes and excessively dry soil in pots lower the turgor pressure of host cells, weakening the plant's resistance and promoting disease development.
Control methods: (1) Strengthen cultivation management, pay attention to soil moisture, and water promptly; apply fertilizer rationally, avoiding excessive nitrogen fertilizer and appropriately increasing potassium and calcium fertilizers to enhance plant growth and improve disease resistance. Prune and shape the plant in a timely manner, removing diseased shoots and leaves to improve ventilation and light penetration between plants. Indoor potted roses should be placed in a well-ventilated and sunny location. (2) Chemical control: Spray with lime sulfur solution at 3-4 Baume degrees before bud break in early spring. During the growing season, spray with 70% methyl thiophanate at 1000-1500 times dilution or 15% wettable powder of triadimefon at 1000 times dilution, both of which have good control effects. The residual effect of triadimefon can last for 20-25 days. After spraying, the white powdery layer on the affected parts turns dark gray, dries, and disappears. The author has also used a 1:20 lime water solution for spraying, followed by rinsing with clean water a few minutes later, which also effectively controls powdery mildew.
3 Aphids are mainly rose aphids and peach aphids. Aphids gather on buds, tender leaves, and flower buds to suck sap. When there is a large infestation, there is a honey-oil-like black secretion. In spring and summer, they reproduce parthenogenetically, while in autumn they reproduce sexually and lay eggs to overwinter. In open-field cultivation, early spring and early summer are the peak periods for infestation.
Control methods: When the insect population is small, spray with clean water to rinse. If necessary, spray with 2.5% deltamethrin emulsion at a dilution of 4000-5000 times or 10% imidacloprid wettable powder at a dilution of 2000 times. Adding 1/1000 of neutral detergent to the spray can improve the control effect. Protect natural enemies such as ladybugs, grasshoppers, and hoverflies.
4. The main species of scarab beetles include the green scarab beetle, black velvet scarab beetle, white-spotted scarab beetle, and small blue-flowered scarab beetle. Adults often feed on new leaves, tender shoots, and flower buds, severely affecting plant growth and flowering.
Control methods: Utilize the adults' feigning death behavior by shaking them off and killing them at dusk; utilize their phototaxis by using black light traps; spray with a 1000-fold dilution of 50% malathion emulsifiable concentrate when adults are feeding.
5. Scale insects mainly include white-ringed scale, Japanese wax scale, red wax scale, brown soft wax scale, and cottony cushion scale, etc. Their damage is characterized by sucking the sap from the tender stems and young leaves of roses, leading to poor plant growth. This is mainly induced by high temperature and humidity, poor ventilation, and insufficient light.
Control methods: During the peak hatching period of nymphs, spray with a 2000-fold dilution of 25% imidacloprid wettable powder.
The rose stem wasp has one generation per year. The larvae overwinter inside the infested branches, and the adults emerge in early to mid-May of the following year. They prefer to lay eggs on the tender shoots of the current year's growth, especially on robust shoots emerging from the ground. Infested branches first wilt and then wither, bending downwards. In autumn, some burrow into the underground parts of the branches, while others burrow into thicker branches from the previous year to spin cocoons and overwinter.
Control methods: In spring, prune any wilting shoots to kill the larvae inside. Combine winter pruning with the removal of infested branches and burn
them. Also, protect natural enemies such as the parasitic wasp. Other pests that damage roses include tussock moths, rose leafhoppers, carmine spider mites, cotton bollworms, light moths, noctuid moths, bagworms, leafhoppers, and stink bugs. Appropriate control measures should be taken based on the characteristics of each pest species.