According to the University of Missouri, the spots that sometimes form on the leaves of tropical plants do not often cause major problems or kill the plant. Tropical plants are often grown as houseplants in areas without a tropical climate. In Hawaii, Florida and other areas of USDA climate zones 10 and 11, tropical plants survive outdoors year-round, but can be more susceptible to diseases than houseplants.
The cause of many types of leaf spot is a fungal disease, although other factors can influence their development. Insects, bacterial infections and even air pollution can cause tropical plants to form spots of various types on their leaves. Spots can begin as tiny dark spots and can grow to cover an entire leaf. Dead areas will occur on affected leaves. These areas can become tan, red, brown or black in color, sometimes having a purple or red border around the outside of the spot. Depending on the cause of the leaf spotting, defoliation of some or all of the plant sometimes occurs.
Diseases That Can Cause Leaf Spots
In tropical environments, anthracnose is a common fungal disease that causes leaf spots on many types of plants, such as mango trees. Another tropical plant disease is called shot hole fungus, which first manifests itself as spots on plant leaves and later causes actual holes to form. Tobacco mosaic virus and other mosaic viruses can attack many tropical plants, including tomatoes; it causes spots to form that resemble a mosaic pattern. The cassava bacterial blight disease also causes spots on the leaves of tropical plants, as does the malanga bacterial leaf spot disease, according to the University of Florida. The government of Queensland, Australia reports on its website that a leaf spot disease known as yellow Sigatoka is a serious disease that affects commercial banana plantations.
Types of Plants That Can Be Affected
Many tropical plants can develop diseases that cause leaf spotting. The University of Illinois reports that a bacterial disease known as Xanthomonas leaf spot or blight can affect anthuriums, dieffenbachia, philodendrons and syngoniums, which are all commonly grown as houseplants. Common garden vegetables that sometimes develop diseases that cause leaf spotting include tomatoes and peppers. These two vegetables can develop similar leaf spots due to several diseases: peppers sometimes develop water-soaked spots due to bacterial leaf spot and tomatoes become subject to tobacco mosaic virus.
Preventing Leaf Spot Diseases
Good air circulation is important when you grow tropical plants, either outdoors or inside your home. Do not place them or plant them too close to other plants because air will be prevented from flowing around the plants' leaves. Controlling insect pests such as aphids is another way to help prevent many types of plant diseases because insects such as these introduce fungal spores or bacteria responsible for causing leaf spot diseases. If you remove any leaves that begin to develop leaf spots, you can help to halt the infection: make sure to sterilize your cutting tool with a solution of bleach and water between cuts, however, to avoid spreading the disease. Reducing the humidity where your plants grow is another preventative measure you can take.
Controlling Leaf Spot Diseases
Fungicides are available commercially that effectively eliminate many fungal diseases. You can also spray your affected plants with a solution of organic sulfur and water. If the shot hole fungus affects kava kava plants, cut them almost to ground level and then spray with organic sulfur. Always follow product label instructions when you use any plant products, whether they are chemical or organic in nature.