What to Do When Roses Get Black Spot? Detailed Guide to Prevention and Control Techniques

Rose black spot occurs in many places. The disease harms roses, causing leaves to turn yellow and drop early, triggering a second flush of leaves in the same year, weakening the plant's growth vigor and reducing cut flower yields.

I. Symptoms

Rose black spot mainly infects leaves, but also infects petioles, veins, young shoots and other parts. In the early stage of the disease, small brown spots appear on the upper surface of the leaves, gradually expanding into circular, nearly circular or irregular lesions. The lesions are 2-12 mm in diameter, blackish-purple, with radiating edges. In the later stage, the central tissue of the lesions turns grayish-white, scattered with many small black dots. In some varieties, the lesion tissue turns yellow, and the lesions connect with one another, causing the leaves to turn yellow and drop.

II. Disease Development Patterns

The way the pathogen overwinters varies with the planting method. For open-field roses, the pathogen overwinters as mycelium on bud scales, leaf scars and fallen leaves; for protected cultivation roses, the pathogen overwinters as conidia and mycelium in diseased parts. The route of infection is spread by splashing rain and irrigation water. After conidia germinate, they can directly penetrate the plant tissue through the epidermis; under suitable conditions the latent period is 3-4 days, and fruiting bodies are produced 15 days after inoculation.

1. Relationship between disease occurrence and climate

For open-field roses, generally speaking, in years when the rainy season comes early, the disease occurs early, and when rainfall is heavy, the disease is severe. For greenhouse-grown roses, below 25°C, the higher the relative air humidity, the more favorable it is for disease occurrence.

2. Relationship between disease occurrence and cultivation management

For roses grown as cut flowers, a spacing of 35 cm is preferable; too dense makes them prone to disease, while too sparse reduces greenhouse utilization and economic returns. For open-field rose cultivation, determine the plant and row spacing according to use and seedling age, preferably so that plants are not crowded and ventilation and light penetration are good. In fertilization, pay attention to a reasonable balance of nitrogen, phosphorus and potassium fertilizers, and never apply nitrogen fertilizer alone.

3. Relationship between disease occurrence and variety

Common rose varieties can all be infected by black spot, but disease resistance differs markedly among varieties. Cultivation experience summarizes: Peace, Elizabeth, Black Centifolia, David, Thompson and other varieties are highly resistant varieties.

4. Relationship between disease occurrence and leaf age

Tests have shown that old leaves are relatively resistant, while new leaves are susceptible, and leaves are most susceptible when they have been expanded for 6-14 days.

III. Prevention and Control Measures

1. Reduce sources of inoculum

In autumn, thoroughly remove dead branches and fallen leaves, and in conjunction with winter pruning, cut off diseased and pest-infested branches, overly dense branches and crossing branches, and burn them in a central location. During the dormant period from after leaf fall in winter to before new buds break in the following spring, from November to February, spray 1% copper sulfate solution to kill the overwintering inoculum on diseased plant residues.

2. Scientific management

Apply fertilizer and water scientifically and reasonably: increase phosphorus and potassium fertilizers, apply nitrogen fertilizer in moderation, and apply more organic fertilizer to enhance the plant's disease resistance. For watering, drip irrigation or watering along the edge of the pot is best; avoid sprinkler irrigation. Water in the morning on sunny days to keep the leaves dry. For large-scale plantings or potted flowers, do not place them too densely, so as to facilitate ventilation and light penetration and reduce disease occurrence.

3. Chemical control

In the early stage of the disease, spray 80% zineb wettable powder at 500 times dilution, or 70% thiophanate-methyl wettable powder at 1000 times dilution, or 50% carbendazim wettable powder at 500 times dilution. Spray once every 7-10 days, for 2-3 consecutive sprays, and the control effect is relatively good. To prevent the pathogen from developing resistance, it is best to alternate the chemicals.