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Disease Management

Exclusion: The first line of defense against citrus canker is exclusion. Citrus canker still does not exist in some countries or regions of countries where climatic conditions are favorable for pathogen establishment, which is probably because of rigid restrictions on the importation of propagating material and fruit from areas with canker. Unfortunately, with increased international travel and trade, the likelihood of X. axonopodis pv. citri introduction is on the rise as it is with many exotic pests and pathogens. Documentations of 6 separate introductions of citrus canker into Florida have occurred since 1985, demonstrating that even with eradication, reintroduction is a continual process and problem.

Sanitation: Numerous cases of new infections of citrus canker are linked to human and mechanical transmission. Humans can carry bacteria on their skin, clothing, gloves, hand tools, picking sacks, ladders, etc. Vehicles can become contaminated by brushing wet foliage or coming in contact with plant material. Machinery such as tractors, implements, sprayers, hedgers, etc. can similarly become contaminated and even inadvertently transport plant parts. In areas where citrus canker is resident, it is necessary to construct decontamination stations for personnel, vehicles and machinery which are sprayed with bactericidal compounds (Figure 30).

Figure 30

Eradication: Once introduced into an area, elimination of inoculum by removal and destruction of infected and exposed trees is the most accepted form of eradication. To accomplish this, trees may be uprooted and burned (Figure 31) or in urban areas, cut down and chipped (Figure 32) and the refuse disposed of in a landfill. In Florida, state law requires that all citrus trees within 579 m (1900 ft) of infected trees must be removed in both residential and commercial situations.

Figure 31

Figure 32

Disease Management: In countries where the disease is well established and severe, only the more resistant types of citrus, such as Valencia oranges and mandarins may be profitable. In regions where canker is endemic, certain cultural practices are used to reduce the severity of the disease. It is imperative to avoid working in infected orchards when the trees are wet from dew or rain. The reduction of wind is another primary concern. Wind speeds are reduced by deployment of windbreaks on the perimeter of the orchard or between the rows (Figure 33) Reduction of wind speed lowers the probability of direct penetration of stomates by bacteria as well as entry of wind-induced injuries on foliage and fruit.

Figure 33

Where canker is a major problem, control requires integration of appropriate cultural practices including sanitation, windbreaks and leafminer control with frequent applications of copper sprays. Copper sprays have been shown to reduce infection somewhat. Because the fruit is susceptible to canker during the first 90 days after petal fall, it is important to maintain a protective coating of a copper material on the fruit surface during this period. Two or three treatments may be needed for this purpose, depending on rainfall and cultivar susceptibility. Windbreaks can greatly reduce spread and severity of disease and increase the efficacy of copper sprays. Leafminer control is particularly important on young trees and certain cultivars that have a high proportion and greater frequency of vegetative growth flushes.

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