Citrus canker is a highly contagious bacterial disease that threatens citrus species globally, including oranges, lemons, limes, and grapefruits. While the infection poses no risk to human health, the economic consequences for commercial growers are severe. Infected fruit is safe to consume, but blemishes make it unmarketable, leading to substantial financial losses and trade restrictions. The disease also affects tree vitality, often resulting in defoliation and premature fruit drop.
Identifying the Bacterial Threat
The specific cause of citrus canker is the rod-shaped bacterium Xanthomonas axonopodis pv. citri. This pathogen is an obligate parasite, requiring a living host to survive and multiply. The bacteria enter the plant tissue primarily through natural openings, such as stomata, or through wounds created by insects, weather, or mechanical damage.
Visual identification of the disease relies on recognizing the characteristic lesions on leaves, fruit, and stems. These spots begin as tiny, pinpoint blemishes that gradually expand to between 2 and 10 millimeters in diameter. On the leaves, the young lesions appear slightly raised and pustular, developing into a corky, rust-brown texture as they mature.
A distinctive feature of the canker is the bright yellow halo, or chlorotic ring, that typically surrounds the necrotic, raised tissue on leaves and fruit. The lesions often have an oily or water-soaked margin, which is a reliable diagnostic sign. On older leaves, the center of the corky spot may fall out, leaving behind a “shot-hole” appearance.
The lesions on stems and fruit are structurally similar to those on the leaves, appearing raised and corky with a crater-like center. Severe infection on stems can cause dieback, and fruit infection often leads to premature drop. While all common citrus varieties are susceptible, lesion size and severity vary depending on the host species and the tissue age at infection.
Mechanisms of Transmission
The bacteria reside and multiply within the canker lesions, and when these spots become wet, they ooze bacterial cells onto the plant surface. The primary mechanism for spreading this inoculum over short to medium distances is wind-driven rain. Strong winds paired with moisture create a dispersal mechanism, splashing the bacteria from infected surfaces to healthy plant tissue.
The disease thrives in environments characterized by high humidity, warm temperatures, and frequent rainfall, common in many citrus-growing regions. The bacteria can also be spread by human activity, including the movement of infected plant material, fruit, and clippings. Contaminated clothing, footwear, and farm equipment, such as tools and machinery, easily carry the bacteria between trees and groves.
Insects, particularly the citrus leaf miner, play a role in promoting infection by creating wounds. The feeding tracks of the leaf miner larvae provide a direct entry point for the bacteria, significantly increasing the probability of infection. Long-distance spread is most often attributed to the movement of infected nursery stock or propagation material, which can introduce the disease to entirely new regions.
Strategies for Disease Control
Control of citrus canker requires a multi-pronged approach combining chemical treatments, cultural practices, and regulatory compliance. Chemical management relies on the application of copper-containing bactericides, such as copper hydroxide or copper oxychloride. These compounds act as a protective coating on the plant surface, preventing new infections by killing the bacteria before they can enter the tissue.
Copper sprays are protective, not curative, meaning they cannot eliminate existing lesions. Effective use requires repeated, timely applications, particularly during periods of new leaf flush and fruit expansion, to ensure continuous coverage of susceptible tissues. Over-reliance on copper, however, carries the risk of bacterial resistance and can lead to environmental concerns regarding copper accumulation in the soil.
Cultural practices focus on reducing the conditions that favor infection and spread. Planting windbreaks around groves significantly reduces the force of wind-driven rain, limiting bacterial dispersal and reducing foliage wounds. Pruning infected branches reduces the inoculum present, but all tools must be thoroughly sterilized before moving to the next tree to prevent human-assisted spread.
In areas where citrus canker is not endemic, strict regulatory measures mandate the immediate eradication of infected trees. This mandatory removal is a core strategy to protect commercial agriculture and prevent the disease from becoming established. Strict quarantine zones prohibit the movement of citrus plant material, including nursery stock and fruit, from infected to uninfected regions. Selecting and planting tolerant citrus varieties is another proactive measure to minimize economic impact where the disease is endemic.