We have raised hibiscus outside during the summer for years with hardly a problem. This year, our plants and several neighbors' plants continuously develop brown leaf spots which ultimately causes the leaves to drop. There has been no change in the large pots or watering from previous years. Malathion has been used to no avail. Thanks for any advice.
By Popa Jimbo
Brown spots can be cause by a number of things, I would first check the soil, is it too wet or dry? Did you add a lot of fertilizer this year? if you added too much, you could be burning the roots.
If they are a potted plant, they could be root-bound and need to be re-potted into a bigger pot. If this is the case, make sure you loosen the roots before you re-plant them, so the roots can have a chance to spread out and not keep strangling themselves.
If this is happening to neighbor's hibiscus too, it could be a bug or a type of fungus. Try contacting your local or nearest college and see if someone in the horticulture dept there can give you some advice. You may even be able to send in a sample of an infected leaf for them to get a better idea of what it is.
Taken from the Yardener:
Reddish-brown Spots on Leaves means Scale.
Several kinds of scale insects sometimes attack rose mallow plants. They bore into the surface tissue of the leaves and twigs, causing telltale red spots on the leaves. White scale becomes evident when the leaf tissue is ruptured. Spray infested foliage and twigs with carbryl (Sevin) according to directions on the package.
Leaf Spots Run Together; Exudates Seen indicates Bacterial Leaf Spot.
A bacterium sometimes infects rose mallow foliage, causing small, blister-like spots. They enlarge and run together. Ruptured spots release a bacterial ooze. In serious infections, plants may be defoliated and collapse. Remove and discard affected leaves as soon as spots appear. Dig up heavily infected plants along with the soil around their root ball and throw them in the trash. Keep the yard free of plant debris. Disinfect garden tools by spraying them with Lysol to prevent the spread of the bacteria. Spray affected plants weekly during rainy spells with a copper-based bactericide or Agrimycin. Control insects (such as aphids) that may transmit disease organisms. Increase air circulation by spacing plants more widely apart, and remove the lower 4 to 6 inches of foliage to avoid contact with wet soil.
Brownish-Orange Spots on Foliage, Stems means Rust.
Rust is caused by fungi that are parasites, requiring a host plant on which to live and carry out their life cycle. Sometimes they afflict rose mallow plants, causing brownish orange patches to appear on leaves and stems. Often rust can be ignored, but in cases where it is extensive, periodically spray a wettable sulfur solution on the affected tissues of the plants.
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