leaf scald

leaf scald. A severe bacterial disease affecting sugarcane and related grasses, characterized by distinctive white pencil-line streaks on foliage and chronic systemic infection. The disease, caused by the pathogen Xanthomonas albilineans, poses a significant threat to global sugar production, leading to substantial yield losses and milling quality decline. First described in the early 20th century, it is now distributed across major sugarcane-growing regions including Australia, Brazil, Florida, and Southeast Asia.

Symptoms and signs

Initial symptoms often appear as one or more narrow, white, pencil-line streaks running parallel to the leaf veins, which can be observed on young leaves. As the disease progresses, these streaks may broaden into larger chlorotic bands, and severe infections cause leaf tip and margin necrosis, giving a scalded appearance. A key diagnostic sign is the emission of a bright blue fluorescence when the vascular tissue of a freshly cut stalk is viewed under ultraviolet light, indicating bacterial colonization. In acute phases, infected shoots may exhibit rapid wilting and death, a condition known as the "ratoon stunt" phase, while chronic infections lead to stunted growth and reduced stalk diameter.

Causal agents

The disease is exclusively caused by the gram-negative bacterium Xanthomonas albilineans, a member of the Xanthomonadaceae family. This pathogen is highly specialized, producing a potent toxin called albicidin that inhibits chloroplast development, leading to the characteristic leaf chlorosis. Genetic studies, including those conducted by CIRAD in Montpellier, have revealed significant strain diversity, with distinct pathotypes showing varying virulence across different geographical regions such as Queensland and Mauritius. The bacterium colonizes the xylem vessels, allowing it to spread systemically throughout the plant.

Disease cycle and epidemiology

The primary source of inoculum is infected planting material, or setts, which allows the pathogen to persist between growing seasons. Secondary spread within a field can occur via rain splash, irrigation water, and contaminated harvesting tools or machinery. The bacteria enter the plant through natural openings or wounds, subsequently invading the vascular system. Epidemics are favored by warm, humid conditions typical of tropical and subtropical climates, with notable outbreaks historically reported in Guadeloupe and Réunion. Long-distance dissemination is primarily through the movement of infected cane cuttings, as evidenced by inter-regional spread documented by the Food and Agriculture Organization.

Host plants and economic impact

The primary economic host is sugarcane (Saccharum officinarum and hybrid cultivars), but the pathogen can also infect other members of the Poaceae family, including some wild grasses like Imperata cylindrica. The disease has caused severe economic losses globally; for instance, in the 1990s, an epidemic in Bundaberg region of Australia led to the eradication of entire cultivars. Yield losses result from reduced tonnage and lower sucrose content, directly impacting mill efficiency and profitability for major producers like Brazil and India. The need for resistant variety development has driven major research programs at institutions like the South African Sugarcane Research Institute.

Management and control

An integrated management approach is essential. The most effective control is the use of resistant cultivars developed through breeding programs at organizations such as the United States Department of Agriculture and CSIRO. Cultural practices include using disease-free planting material from certified nursery sources, implementing strict quarantine protocols to prevent introduction into new areas, and roguing infected plants. Hot water treatment of setts, a technique pioneered in Taiwan, can reduce bacterial load. Chemical control is generally ineffective, though antibiotic sprays have seen limited use in some regions like the Philippines. Ongoing research focuses on biocontrol agents and understanding pathogenicity genes to inform durable resistance strategies. Category:Plant diseases and disorders Category:Agriculture