What to Do When Roses Get Black Spot Disease? A Detailed Guide to Prevention and Control Methods
Rose black spot disease, also known as brown spot disease, is a worldwide rose disease.
Symptoms
Rose leaves, young shoots and pedicels can all be affected. The spots on the leaves first appear as small purplish-brown to brown dots, then expand into round spots 1.5 to 13 mm in diameter, black or dark brown, with fringed edges, though on some varieties the edges may also be neatly smooth. The spots are often surrounded by a yellow halo. Under a magnifying glass, small black blister-like bodies can be seen on the diseased areas. The spots often join together, and the leaf tissue around the diseased areas turns yellow over a large area, making the spots appear as “small islands” with green edges. Diseased leaves fall off easily, though some rose varieties do not shed them. On young shoots and pedicels, purple to black streak-like spots appear, slightly sunken. When the disease is severe, all the leaves on the lower and middle parts of the whole plant drop off, leaving only a few new leaves at the top.
Pathogen
It is a fungus of the genus Actinonema [Actinonema rosae (Lib.) Fr.]. The acervuli are borne beneath the cuticle of the host and later protrude through the epidermis. They measure 108 to 198 microns; the conidiophores are short and indistinct. The conidia are long-oval or swim-bladder-shaped, measuring 18 to 25 x 5 to 6 microns, two-celled, smaller above and larger below, with a slight constriction at the septum. The apex is beak-like and often bends to one side.
Transmission route
It overwinters as mycelium on diseased branches, diseased leaves or fallen diseased leaves. In early spring of the following year, acervuli form and produce conidia that spread and cause damage. Conidia are also one of the sources of primary infection. Conidia spread and cause damage via wind and rain and splashing water droplets, so the disease is prone to occur when there is much rain, fog and dew. According to experiments, when there is lingering water on the leaves, the spores can germinate and invade within 6 hours.
Prevention and Control Methods for Rose Black Spot Disease
Rose black spot disease, also known as brown spot disease, is a worldwide rose disease.Symptoms
Rose leaves, young shoots and pedicels can all be affected. The spots on the leaves first appear as small purplish-brown to brown dots, then expand into round spots 1.5 to 13 mm in diameter, black or dark brown, with fringed edges, though on some varieties the edges may also be neatly smooth. The spots are often surrounded by a yellow halo. Under a magnifying glass, small black blister-like bodies can be seen on the diseased areas. The spots often join together, and the leaf tissue around the diseased areas turns yellow over a large area, making the spots appear as "small islands" with green edges. Diseased leaves fall off easily, though some rose varieties do not shed them. On young shoots and pedicels, purple to black streak-like spots appear, slightly sunken. When the disease is severe, all the leaves on the lower and middle parts of the whole plant drop off, leaving only a few new leaves at the top.
Pathogen
It is a fungus of the genus Actinonema [Actinonema rosae (Lib.) Fr.]. The acervuli are borne beneath the cuticle of the host and later protrude through the epidermis. They measure 108 to 198 microns; the conidiophores are short and indistinct. The conidia are long-oval or swim-bladder-shaped, measuring 18 to 25 X 5 to 6 microns, two-celled, smaller above and larger below, with a slight constriction at the septum. The apex is beak-like and often bends to one side.
Transmission route
It overwinters as mycelium on diseased branches, diseased leaves or fallen diseased leaves. In early spring of the following year, acervuli form and produce conidia that spread and cause damage. Conidia are also one of the sources of primary infection. Conidia spread and cause damage via wind and rain and splashing water droplets, so the disease is prone to occur when there is much rain, fog and dew. According to experiments, when there is lingering water on the leaves, the spores can germinate and invade within 6 hours.
The optimum temperature for germination and invasion is 20 to 25°C, the pH is 7 to 8, and the incubation period is 10 to 11 days; the incubation period on old leaves is slightly longer, at 13 days. The pathogen can cause repeated infections many times, and the disease can occur throughout the entire growing season. Generally, the disease is severe in the plum rain season and typhoon season, while disease development is slow in the hot, dry summer season. Plants are prone to infection when they are weak. There are differences in disease resistance among varieties, but there are no immune varieties.
Prevention and Control Methods
1. Sweep up fallen leaves at any time and remove diseased leaves to reduce sources of infection. In winter, carry out heavy pruning on severely diseased plants to remove overwintering pathogens on diseased stems.
2. When growing in pots, do not place them too densely, and it is best not to place them directly on the ground, so that the potting soil does not become too wet when water accumulates on the ground; the best option is to place them on stepped plant stands. Improve the watering method and timing: water from the edge of the pot and avoid spray watering. Do not water in the evening, so that water on the leaves cannot dry quickly and favor pathogen invasion.
3. Chemical control. When the new leaves have just unfolded in summer, spraying should begin, generally once every 7 to 10 days. The agents used include 50% carbendazim wettable powder at 500 to 1000 times dilution, or 75% chlorothalonil wettable powder at 500 times dilution, or 80% zineb wettable powder at 500 times dilution, or 1:1:100 Bordeaux mixture, or 70% thiophanate-methyl at 1000 to 1200 times dilution.
After winter pruning, lime sulfur mixture at 3 to 5 degrees Baumé can also be sprayed to eradicate the pathogen.
Prevention and Control of Rose Black Spot Disease
Rose black spot disease is a widespread and seriously harmful disease. It not only harms roses but can also harm Rosa multiflora, roses and others, causing large numbers of yellow leaves, withered leaves and fallen leaves, affecting plant growth, flowering and ornamental value. It is especially severe when roses are placed and planted in large groups, and it occurs in most regions of China. Therefore, strengthening the prevention and control of this disease plays an important role.
1. Damage symptoms
This disease mainly harms leaves, and can also infect young shoots, petioles, veins and flower buds. In the early stage of leaf disease, there are small purplish-brown spots, which gradually expand into round, nearly round or irregular lesions, 4 to 12 mm in diameter. The lesions are purplish-brown or blackish-brown, with an indistinct radial pattern around the edge. In the later stage, the leaves around the lesions turn yellow, and small black dots appear on the lesions, namely the pathogen's acervuli. In severe cases, the lesions merge into patches, causing the leaves to turn yellow, the lower leaves to drop early, and the disease gradually spreads upward until finally the upper leaves all fall off.
The lesions on young shoots are long-oval and dark purplish-red. The lesions on petioles and veins are similar to those on young shoots. On flower buds, they are mostly purplish-brown oval spots.
2. Infection cycle and conditions for disease occurrence
The pathogen of black spot disease overwinters as conidia on fallen diseased residues on the ground or in the soil, or as mycelium on diseased branches. In the following spring, as the temperature rises, the pathogen lurking on diseased residues or in the soil or on diseased branches begins to become active. When humidity is high, the pathogen spreads via watering and rain splashes. When the temperature is suitable and there are water droplets on the leaf surface or branches, it invades through stomata or lenticels in 6 to 10 hours, and symptoms appear in 3 to 6 days. At 22 to 30°C, the incubation period is 3 to 4 days, and generally 10 to 11 days. In northern regions, leaf disease occurs in May to June when humidity is high, mostly infecting from the lower leaves. As the lesions expand, conidia are produced, which can spread with wind and rain, and after the lesions expand they cause reinfection. The disease is severe in August to September; when the temperature is around 24°C and the relative humidity is around 98%, the disease breaks out rampantly. Warm, continuous rainy, foggy and dewy conditions, and muggy weather after rain, are suitable for disease occurrence. Dense foliage between plants, pots placed too densely, poor drainage, ventilation and light transmission, as well as excessive application of nitrogen fertilizer, weak plants, and large, soft, thin leaves, lead to more severe disease. The degree of disease occurrence differs among varieties.