In Florida, the bacterial disease Asian citrus canker (Xanthomonas axonopodis) has devastated citrus trees (Citrus spp.), a major agricultural crop in the state. Citrus grow in much of Florida, in U.S Department of Agriculture plant hardiness zones 9 through 11. The bacteria is highly contagious and can be spread by windborne rain, landscaping equipment, people carrying the bacteria on their clothing or hands, flooding, overhead watering, or by moving infected fruits or tree parts. The disease enters into the tree through wounds and through the plant’s leaf pores, called stomata. There is no cure. Approximately 16.5 million trees in Florida have been destroyed as a result of citrus canker.
Citrus canker symptoms include brown, raised lesions on leaves and fruits that are surrounded by a water-soaked margin and yellow ring. Old lesions on leaves may dry up and fall out, leaving holes. Lesions can appear on twigs and branches as well. Severe infections can leave trees without leaves, and the fruit often falls prematurely. Symptoms appear seven to 14 days after infection, depending on the temperature. Internal fruit quality is not affected, and fruits can still be used for juice or eating.
Florida has had a number of outbreaks. First detected in 1912 on seedlings imported from Japan, it was eradicated by 1933. Discovered again in 1986, it was declared eradicated in 1994. It was discovered again in 1995, with circumstances that made eradication nearly impossible. Despite removing 1.56 million commercial trees and 600,000 residential trees, the disease raged on due to severe storms in 2004 and 2005, and the invasion of the Asian citrus leafminer. The storms spread the bacteria to wider area than could reasonably be contained, while the leafminer created opportunity for the disease to enter the tree through the holes bored into the leaves by the insect. By 2006, the USDA withdrew all funds for eradication and issued a statewide quarantine, prohibiting the movement of any citrus without a limited permit.
Florida has implemented several strategies to avoid further destruction of trees. All citrus must be bought through nurseries registered with the state of Florida, and homeowners cannot propagate trees. Homeowners are encouraged to avoid planting susceptible citrus, such as grapefruits (Citrus paradisi), key limes (Citrus aurantiifolia) and certain sweet oranges (Citrus sinensis). No plants or plant parts can be moved out of the state. Similar to the practices commercial orchards use, you should place windbreaks around trees and follow sanitation practices that include sanitizing clothing, hands and tools. These practices, along with pruning and destroying infected plant parts and applying copper-based sprays, reduce the spread of the canker.
Citrus canker has had great socioeconomic impact in Florida. Because the disease disfigures the fruit, entire crops were rendered less valuable or unsaleable. Because the disease posed an economic risk to the industry, in 2000, a regulation was implemented saying trees that were within 1,900 feet of an infection were to be removed. More than 600,000 residential trees were removed before an injunction stopped the activity. This removal often met with strong resistance from homeowners, and beginning in 2000, many filed lawsuits. Some final judgements found in favor of the homeowners as late as 2012, but some were still pending as of 2013. Eventually, more than 850,000 residential trees were destroyed, along with more than 4 million nursery trees and more than 11 million orchard trees.
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