The Meyer lemon tree is a dwarf citrus cultivar that grows well in home landscapes as well as a container plant indoors. The small, compact growth habit makes Meyer lemon a popular ornamental plant. Healthy trees can produce lemons throughout the year but are most productive in spring and fall. Though a resilient specimen plant in general, Meyer lemon trees are prone to greasy spot infestations that cause dark spots to appear on the plant's foliage.
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Meyer lemon trees are not true lemons, but rather a compact hybrid between lemon and mandarin orange. Originally cultivated in China, the Meyer lemon was imported to the United States by the agricultural explorer Frank Meyer. Meyer lemons only grow between 6 and 10 feet tall and develop yellow lemons that are rounder than traditional lemon varieties and have a less acidic taste. The trees have deep green, shiny foliage and develop from purple buds on the tree.
Causes of Dark Spots
Dark spots on Meyer lemon trees can indicate fungal infection on the leaf surface, a disease commonly called greasy spot. Greasy spot develops as spores from the fungus Mycosphaerella citri germinate and colonize on the underside of Meyer lemon tree leaves. As the fungus develops, it penetrates the leaf tissue and causes the plant cells to swell, which appear as yellow spots on the surface of the infected leaf. Eventually, the cells collapse and die, causing small brown spots to appear on the leaf surface. Greasy spot can cause leaf drop in infected lemon trees if left untreated.
Several fungicide treatments are readily available to combat greasy spot on Meyer lemon trees. Horticultural oils and copper-based fungicides are the most common treatments, according to the University of Florida. Regular monitoring and identification of spots on the leaves is important for treating greasy spot on Meyer lemon trees, before the fungus spreads to other leaves on the tree and causes leaf drop. Always follow the instructions on fungicides, as labeled, for best results.
Meyer lemon trees' leaves are prone to greasy spot infections throughout their lifespan. The trees are most vulnerable when temperatures are above 77 degrees and relative humidity is above 90 percent. Keep potted Meyer lemon trees in an area in your home in a sunny area of your home where the relative humidity is low to prevent the fungus from germinating on the leaves. Keep the area under the Meyer lemon trees (if planted outdoors) free from leaf litter or other organic debris to reduce the suitable habitat for the first stage of the fungi to develop around your plant. A preventative fungicidal spray can be applied to Meyer lemon trees in the spring and summer months.