Osmanthus fragrans, a popular evergreen shrub or small tree belonging to the Oleaceae family, is widely cherished for its fragrant flowers and ornamental value. With straight trunks, rounded canopies, and persistent green foliage, it's a staple in traditional gardens. However, it is susceptible to several leaf spot diseases, such as brown spot, common spot, and anthracnose, which can lead to premature leaf drop, hinder growth, reduce aesthetic appeal, and cause significant economic losses.
1. Osmanthus Brown Spot
1.1 Symptoms
Initially, yellowish chlorotic spots appear on the leaves, gradually expanding into near-circular or irregular lesions due to vein constraints. These spots range from yellow-brown to gray-brown with a yellow halo, measuring 2–10 mm in diameter. In later stages, black moldy structures—conidia and conidiophores—develop on the lesions.
1.2 Pathogens and Disease Cycle
The pathogen responsible for brown spot belongs to the class of semi-known fungi, including species like *Mycetophora* and *Cystis*. The conidiophores are pale olive, unbranched, and measure 15–58 μm in length. The disease typically occurs from April to October, with older leaves being more vulnerable. Overwintering mycelia on infected leaves serve as the primary infection source. Under favorable temperature and humidity conditions, spores are released and spread via wind and rain, leading to new infections.
2. Leaf Spot Disease
2.1 Symptoms
The disease usually begins at the leaf tips or margins, starting as small light brown spots that grow larger over time, forming round or irregular necrotic areas. In severe cases, these lesions merge, causing the leaf to dry up by one-third to half. Lesions are gray-brown to reddish-brown with sharp red edges and sometimes crumbly textures. Black dots, representing conidia, may appear in later stages.
2.2 Pathogens and Disease Cycle
The causal agent belongs to the genus *Sporemiella*, with conidia that are nearly spherical, ranging from 100–150 μm in diameter. The optimal germination temperature is between 10–33°C, with 27°C being ideal. The disease is most prevalent from July to November. The bacteria survive winter on infected leaves and spread through wind and rain. High humidity, poor air circulation, and weakened plants increase the risk of infection.
3. Anthracnose
3.1 Symptoms
Early symptoms include small chlorotic patches that expand into round, semicircular, or elongated lesions. These lesions are light brown to gray-white with reddish-brown borders. As the disease progresses, small black dots (conidial discs) appear, along with pinkish, sticky spore masses.
3.2 Pathogens and Disease Cycle
The pathogen is classified under the genus *Colletotrichum*, with conidial discs that are brown or black, measuring 100–300 μm. Conidiophores are cylindrical, and conidia are produced beneath the leaf epidermis before breaking through. The disease is most active from April to June. The pathogen survives the winter in infected leaves and spreads via wind and water.
4. Control Methods
4.1 Reduce Infection Sources
Remove diseased leaves promptly, especially in autumn, and burn them to eliminate the initial infection sources. For nursery outbreaks, soak affected plants in a 1000-fold dilution of potassium permanganate for disinfection.
4.2 Cultural Practices
Improve cultivation techniques by avoiding waterlogged soil, increasing organic and potash fertilizer application, and ensuring proper spacing for better air circulation and sunlight penetration. This enhances plant resistance and reduces disease incidence.
4.3 Chemical Control
Apply fungicides early when symptoms first appear. For brown spot, use a 1:2:200 Bordeaux mixture, 50% benzene at 1000–1500 times dilution, or 65% mancozeb at 500 times. For anthracnose, use 50% thiophanate-methyl at 800–1000 times or 80% carbendazim at 600 times. Alternate treatments and apply them 2–3 times for best results.
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