Red Spots on My Gardenias

Fungal Leaf Spot

• Often induced by overly humid growing conditions, fungal leaf spots can vary in color. Advanced fungal leaf spot is usually a sunken red-brown area with a yellow halo. Anthracnose can be distinguished from other fungal spots by a pink or orange ooze coming from damaged tissues. Treat all fungal leaf spots with a general-purpose fungicide, such as chlorothalolnil, mancozeb or cupric hydroxide.

Bacterial Leaf Spot

• Bacterial leaf spot can be difficult to distinguish from fungal leaf spot. It, too, causes red-brown lesions, but the spots tend to run together to form irregularly shaped areas of damage. Avoid overhead watering to slow the spread of bacterial leaf spot. Little can be done once the infection has started, since it is often spread by infected cuttings or tools. Always take cuttings from clean plants and avoid using soil or pots that have not been sterilized.

Algal Leaf Spot

• Algal leaf spot is not an uncommon pest of gardenias. They, too, create red spots on leaves, but these spots are often raised and appear to have a velvet-like texture and feathering around the edges. The spots will change colors as the algae enters its reproductive phase -- reddish-brown coloration indicates that they are creating reproductive structures. Apply copper fungicide to gardenias with algal leaf spot at the first sign of a problem and again every 10 to 14 days until the algal leaf spot has been destroyed.

Scale Insects

• Gardenias are host to a specific scale insect that is reddish brown, called pyriform scale. These insects are about 1/10 inch long and are often surrounded by a cottony substance. Scales are usually seen on the undersides of leaves, where they feed but do not move. Scale can be treated using horticultural oil or insecticidal soaps. Systemic insecticides are often effective, but take more time to show results.