Citrus canker is a highly contagious bacterial disease that impacts citrus plants worldwide. It is caused by the bacterium Xanthomonas citri subsp. citri, a pathogen that can devastate citrus groves and reduce production. The disease spreads rapidly, particularly in warm, humid regions, making it a concern for commercial growers and home gardeners.
Recognizing Citrus Canker
Citrus canker is identified through distinctive lesions on the leaves, stems, and fruit of affected plants. These lesions typically begin as small, pimple-like spots, about 1 millimeter in diameter, with a yellowish color. As the disease progresses, spots enlarge, becoming raised, brown, and corky in texture, often with sunken centers and raised edges.
A yellow halo or ring often surrounds the canker spot, making them more easily identifiable. Older lesions can grow up to 1 centimeter in diameter. Symptoms appear within 14 days of exposure to the bacteria.
How Citrus Canker Spreads
Citrus canker spreads through several mechanisms. Wind and rain contribute to its dispersal, as splashing water carries the Xanthomonas citri bacterium from infected to healthy plants. Strong winds propel bacteria-laden water droplets or infected plant debris to new locations.
Human activities also play a role in long-distance spread. Contaminated tools, clothing, and equipment used in citrus groves can transfer bacteria between trees or properties. The movement of infected plant material, such as nursery stock, cuttings, or fruit, is another pathway for dispersal. Insect pests, like the citrus leaf miner (Phyllocnistis citrella), can create wounds on leaves, providing entry points for the bacteria.
Controlling and Preventing Citrus Canker
Controlling citrus canker involves a multifaceted approach, beginning with quarantine measures. In areas affected by the disease, regulations often restrict the movement of citrus plant material, including fruit, branches, and green waste. These measures are designed to exclude the pathogen from uninfected regions and contain outbreaks where they occur.
Cultural practices also contribute to managing the disease. Proper tree spacing can improve air circulation and reduce humidity, reducing bacterial growth. Pruning infected branches, while sterilizing tools between cuts, helps remove sources of inoculum. Sanitation practices, such as cleaning equipment and vehicles that have been in contact with infected plants, prevent mechanical transmission.
In cases of severe infection or initial outbreaks, an eradication approach may be implemented, involving the removal and destruction of severely infected trees or entire groves. This measure aims to eliminate the bacterial source. Chemical control involves copper-based bactericides, which act as a protective barrier on plant surfaces and reduce disease development. However, these treatments are preventative and do not cure already infected plants. Ongoing research focuses on developing citrus varieties with natural resistance, offering a long-term solution.
Effects of Citrus Canker
Citrus canker primarily affects the appearance and marketability of citrus fruit. Infected fruit develops unsightly blemishes, characterized by the raised, corky lesions, which significantly reduce its commercial value, even though the fruit remains safe for human or animal consumption. The disease can also lead to premature fruit drop, further impacting yields for growers.
Beyond the fruit, citrus canker impacts the overall health of the tree. Severe infections can result in defoliation, where the tree loses a significant number of its leaves. Twig dieback, the death of small branches, can also occur, weakening the tree and reducing its productive lifespan. While the disease does not make the fruit harmful to eat, these combined effects can severely diminish the vitality and productivity of citrus trees over time.