Black arm, boll rot and wilting in cotton plants

There is much hope for South Africa’s cotton industry as new players enter the market. However, as with all crops, there are several diseases that can cause serious economic losses. Janine Ryan examines a few.

- Janine Ryan

FAST FACTS

Bacterial blight can lead to various diseases, such as black arm. 

Over-watering and heavy soil with higher levels of silt can lead to an increase in disease incidence of Verticillium wilt.

Boll rot is caused by several fungal pathogens.

BLACK ARM

Black arm is caused by the pathogen Xanthomonas axonopodis pathovar malvacearum. It can affect the cotton plant during all growth stages, and infects stems, leaves, bracts and bolls. The same pathogen can also cause seedling blight, leaf spot, and black vein, among others. Infection can lead to seedling death. The lesions can cause chlorosis (yellowing), necrosis and distortion, and may eventually lead to defoliation.

Signs and symptoms

Symptoms may first appear on leaves as small, green, water-soaked spots, which turn brown as they mature. These spots may appear translucent when held up to the light. They eventually reach a size of around 5mm in diameter. The spots first appear on lower leaves, and will eventually spread to upper leaves.

The cotton plant may present with dark brown to black lesions on the leaves of the plant, which may eventually coalesce. This causes necrosis, leading to the blockage of the plant’s vascular system at infected sites, which hinders the movement of water and nutrients. As a result, leaves may begin to droop, while the stem may crack. The eventual breaking of the stem appears black, and it is from this that black arm was given its name. The pathogen also reportedly causes blackening of the veins and veinlets, giving a typical ‘blighting’ appearance.