Citrus canker, caused by the bacterium Xanthomonas citri, is a disease affecting the leaves, fruit, and stems of most citrus trees. Citrus canker doesn’t typically kill healthy citrus plants, but it can damage fruit and cause your plant to look unappealing. Liquid copper fungicide sprays can be effective in managing citrus canker, and are considered an organic solution to disease problems in citrus trees.
Citrus Canker Symptoms
Symptoms of canker infections begin as raised lesions on both leaf surfaces, primarily on the undersides, and may later show on the fruit, stems and twigs. These lesions later become corky, with raised, water-soaked margins and sunken centers surrounded by a yellow halo. Older lesions may darken if they are colonized by saprophytic fungi. While almost all citrus varieties may be infected by citrus canker, grapefruit, early orange varieties and Mexican lime are particularly vulnerable. Mid-season oranges, tangerine hybrids, and Valencia oragnes are less susceptible, while tangerines are considered tolerant of canker infection.
Citrus canker infection usually occurs as new shoots are emerging, or when the fruit is in early stages of development. Warm, wet weather encourages the disease, so frequent spring or summer rainfall provides ideal infection conditions. While citrus canker is primarily a cosmetic disease that isn’t likely to kill your citrus tree, when conditions are highly favorable for infection, it can lead to leaf drop, shoot dieback and fruit drop, which can stress your tree and leave it vulnerable to other diseases and insect pests.
Liquid Copper Fungicides
Liquid copper fungicide is a protectant that you apply before the development of citrus canker to prevent infection. Copper fungicide is not systemic, so it can’t be carried through your citrus plant to kill the pathogen. Once applied, it remains only where it contacts your citrus, and will not spread across the fruit or leaf surface. This means that liquid copper fungicides may not be as effective in reducing existing infections as they are in preventing citrus canker from developing. The rapid expansion of young leaves tends to make the effectiveness of liquid copper fungicide short-lived, and you will need to reapply it for the best results.
To protect early orange varieties and grapefruit, spray five applications of copper fungicide at 21-day intervals. Three applications at the same interval are often enough to protect less susceptible Valencias and mid-season citrus varieties from infection. Keep in mind that application rates are estimates because growth rate and rainfall can cause copper residues to decay at different rates. To ensure you get the best coverage and optimum protection, read the product label and follow all recommendations for rates and timing. The application of liquid fungicide is most effective in preventing citrus canker when it is applied to coincide with leaf flushes, which typically require spraying about every three to four weeks from spring through fall. It is best to minimize copper fungicide usage whenever possible, as copper can accumulate in the soil and may damage your plants. More frequent applications of liquid copper fungicides at lower rates can decrease the risk buildup or damage.
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