Yellow-skin shoot rot, also known as yellow dead skin disease, is a serious citrus disease commonly found in areas such as Guangzhou, Nanhai, Dongguan, Sanshui, Huizhou, and Shantou. This disease can lead to significant losses, with mild cases causing reduced yields and severe cases potentially destroying entire orchards. The disease incidence can reach as high as 70-90% in affected areas.
**Symptoms**
The disease primarily affects young shoots, which show brown necrosis and rot. The tip of the shoot turns black-brown or black, and the affected area shrinks, resembling a cigarette. When wet, a thick layer of white mold and orange-red sticky spores appears on the surface. Leaf blade infections typically begin at the tips and edges of leaves, spreading inward, often with dark brown margins between healthy and diseased tissue. Fruit infections start as round, water-soaked, brown spots, and when moist, they develop a white mold covering, leading to fruit rot. Infected branches may show spindle-shaped, raised, and sunken lesions, ranging from 3 to 12 mm in length, with a corky surface.
**Pathogen**
The causal agent of yellow-skin shoot rot is *Fusarium lateritium* Nees ex LK. Var. *Longum* Wollenw., which belongs to the subphylum Ascomycota. The aerial mycelium is sparse and colorless, turning brick red over time. The center of the colony contains orange-orange spores. Conidia have five septa. Artificial inoculation has shown that young shoots and fruits of citrus are highly susceptible to infection.
**Infection Cycle and Disease Conditions**
The pathogen survives the winter in infected plant parts as mycelia and conidia. In spring of the following year, large numbers of conidia are produced and spread by rain to young shoots. Infected spores then reinfect new tissues, leading to fruit and branch rot. The disease can occur throughout the year, but it peaks between April and August, with more frequent outbreaks on spring shoots than autumn ones. Young, tender shoots are more vulnerable, while mature shoots are more resistant.
**Control Methods**
Prevention of yellow-skin shoot rot focuses mainly on two key steps:
(1) During winter, maintain a clean orchard by pruning and removing diseased shoots, leaves, and branches. Apply 1–2 sprays of 50% carbendazim WP diluted at 500 times.
(2) Regularly inspect new shoots and sprouts for early signs of the disease. If symptoms appear, treat promptly using 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. These measures help reduce disease spread and protect the orchard effectively.
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