Yellow-skin shoot rot, also known as yellow dead skin disease, is a serious citrus disease commonly found in regions such as Guangzhou, Nanhai, Dongguan, Sanshui, Huizhou, and Shantou. Affected plants may experience reduced yields or even complete destruction of the entire orchard. In severe cases, the infection rate can reach 70 to 90 percent, making it a major threat to citrus crops.
**Symptoms**
The disease primarily affects young shoots, causing brown necrosis and rot at the tips. The top of the shoot turns black-brown or black, and the affected area becomes shriveled, resembling a cigarette. When wet, white mold and orange-red viscous spores appear on the surface. Leaf blade infections usually begin at the leaf edges, spreading inward and forming dark brown margins between healthy and diseased tissue. Fruit infections show round, water-soaked, brown lesions, with white mold appearing when damp. Infected fruit eventually rots. On branches, the damage appears as spindle-shaped, raised, and sunken lesions, ranging from 3 to 12 mm in length, with a corky surface.
**Pathogen**
The pathogen responsible for yellow-skin shoot rot is *Fusarium lateritium* var. *longum*, which belongs to the subphylum Ascomycota. The aerial mycelium is sparse, colorless, and turns brick red over time. The center of the colony contains orange-orange spores. The conidia have five septa. Young shoots and fruits are particularly vulnerable after artificial inoculation.
**Infection Cycle and Disease Conditions**
The pathogen survives the winter in infected plant parts, including mycelia and conidia. In the following spring, large numbers of conidia are produced and spread by rain to new, tender shoots. Contaminated spores re-infect the plants, leading to fruit and branch rot. Infected seedlings can introduce the disease to new areas. The disease can occur throughout the year, but its peak incidence occurs between April and August. Spring shoots are more susceptible than autumn shoots. Tender, newly emerged shoots are highly vulnerable, while mature shoots are more resistant.
**Control Methods**
Prevention of yellow-skin shoot rot involves two main steps:
1. During winter, maintain a clean orchard by pruning and removing all diseased shoots, leaves, and branches. Apply 1 to 2 sprays of 50% carbendazim WP diluted at 500 times.
2. Regularly inspect new shoots and sprouts for early signs of the disease. If symptoms are detected, apply fungicides promptly. Options include 50% carbendazim WP at 500–600 times dilution, 70% thiophanate-methyl wettable powder at 800–1000 times, or 40% copper oxychloride suspension at 500 times. Consistent monitoring and timely treatment are essential for effective disease management.
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