Begonia sinensis is commonly affected by a disease known as pear rust, which is caused by a fungal pathogen. This disease requires two different host plants to complete its life cycle. The primary hosts are coniferous trees such as cypress, juniper, and other related species. The secondary hosts include plants like hawthorn, escarped sea buckthorn, and similar species.
Once the fungus infects the cypress or similar hosts, it first manifests as small, pale yellow spots on the leaves or twigs during the first year. These spots gradually enlarge and develop into swollen areas. By the second and third months of the following year, the infected areas produce brown, rice-like structures that break through the leaf surface. These are called teliospores, and they form in clusters or singly, causing the shoots to swell.
When the secondary host, such as hawthorn, becomes infected, symptoms appear in early April to May. Initially, yellow-green spots appear on the leaf surfaces, which then expand into round, yellow lesions. Small yellow dots may appear within these lesions, eventually turning black. As the infection progresses, the back of the leaves thickens, develops grayish-white hairs, and by late summer to early autumn, turns into a yellow-brown powdery substance. In severe cases, the leaves become deformed, their surfaces uneven, and they eventually fall off, potentially leading to plant death.
To manage this disease, it's important to avoid planting the primary host (such as cypress) near the secondary host. During the early stages of infection, apply 20% Fenbuconazole EC at a 400-fold dilution, or use 50% Triadimefon WP diluted 800 times, and spray once a month. Reduce or avoid spraying during dry weather. Additionally, in mid-March, when the teliospores are about to mature, spray with a mixture of 0.3% sodium pentachlorophenate and lime sulfur, creating a 1° Baum solution. Apply this treatment 2–3 times over a two-week period for effective control.
Understanding the life cycle of this disease is crucial for timely intervention. Regular monitoring of both host plants can help detect early signs of infection and prevent widespread damage. Proper cultural practices, such as pruning and maintaining good air circulation, also play an important role in reducing the risk of disease outbreaks.
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