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Common diseases of flowers - black spot
Symptoms: Black spot disease can affect various parts of the plant, including leaves, petioles, twigs, and pedicels, but it most commonly targets the leaves. There are two main types of symptoms. The first type begins with small reddish-brown to purple-brown spots on the leaf surface, which gradually enlarge into round or irregular dark black lesions. These spots often have a yellow halo surrounding them, with a radial or frayed edge. The diameter of these spots typically ranges from 3 to 15 mm. In later stages, small black structures known as conidial plates appear on the lesions, which are the reproductive spores of the pathogen. Severe infections can cause lower leaves to turn brown and fall prematurely, leading to branch dieback, as seen in rose black spot.
The second type of symptom appears as brown to dark brown, lenticular or irregularly shaped spots along the veins of the leaves, where black moldy growths—consisting of conidia and germs—develop. In severe cases, this leads to early leaf drop, which can hinder plant growth, as observed in the black spot disease of Prunus triloba.
Black spot disease is one of the most significant diseases affecting Chinese roses. It occurs widely and causes serious damage. The pathogen survives the winter in dead branches or in the soil, either as mycelium or conidial discs. The disease typically starts in mid to late May of the following year, with peak infection occurring between July and September. Spores are spread by wind, rain, or insects, leading to further infection and disease spread.
Rainfall is a key factor in disease outbreaks. In years with heavy rainfall, the disease tends to appear earlier and more severely. Conditions such as low-lying areas with stagnant water, poor air circulation, insufficient light, and improper watering or fertilization also favor the development of the disease.
Control Measures:
1. Choose and plant resistant varieties to reduce susceptibility.
2. After the growing season, remove and destroy dead branches and fallen leaves to minimize overwintering inoculum.
3. Implement proper cultural practices, including regular pruning, ensuring good air circulation and light penetration.
4. When new leaves begin to unfurl, apply fungicides such as 50% carbendazim WP at a dilution of 500–1000 times, or 75% chlorothalonil at 500 times, or 80% dexamethasone at 500 times. Repeat the application every 7–10 days for 3–4 times, depending on the severity of the infection.
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Common diseases of flowers - black spot