What Is Wrong With My Althea Shrub?

Are there any angular or circular green to gray spots with brown borders on leaves?

Are the leaves pale on their upper side and are there any dusty blisters on the undersides?

Are the leaves covered with white powder?

Are the leaves and flowers covered with gray mold?

Does the trunk or the branches sunken areas of dead tissue?

Are the leaves curled and yellow?

Are the leaves skeletonized, metallic green-bronze beetles present?

These symptoms indicate a fungal disease - Cercospora leaf spot. This fungal disease only occurs in very warm climates. Spots develop a “frog-eye” appearance with light centers and dark edges. Spray plants with copper as soon as symptoms appear to prevent further symptom development.

Likely cause for these symptoms is a fungal disease - Rust. Provide good air circulation around plants. Avoid making leaves wet when watering. All infected leaves should be removed. Spray leaves with sulfur early in the season to prevent rust or to treat mild infections.

Powdery mildew is a fungal disease that usually appears in damp places. Encourage good air circulation around the affected plants. Water plants from below to keep foliage dry. Any affected leaves should be cut off and removed. The appropriate fungicidal spray will protect your plant from further infections.

Likely cause for these symptoms is a fungal disease - Botrytis blight. Other symptoms include brown to black spots that may appear on leaves and petals. You can only prevent Botrytis blight disease by planting sweet alyssums in areas with good air circulation. Water plants sparsely. Infected plants should be removed. Spray weekly with a copper fungicide until the disease is under control.

Sunken areas of dead tissue on Althea shrubs are caused my cankers. Cankers can be caused by several different kinds of fungi. Mildly affected plants will recover if you remove and destroy all affected branches. Heavily affected plants can't be cured and you should remove the whole plant. Avoid damaging your plant with lawnmowers, trimmers, or pruning tools because the wounds are the best place for cankers to start.

Distorted, curled leaves are caused by aphids. These tiny insects cluster under leaves and on growing tips where they feed on plant sap. Leaves, stems, and buds get distorted, and later leaves and flowers drop from the plant. You can control them by washing them off the plant with water spray. Insecticidal soap should be used with severe infestations.

Skeletonized leaves are caused by Japanese beetles. Handpick adult beetles into a can of soapy water but make sure to wear gloves as Japanese beetles are blister beetles. Apply parasitic nematodes to the soil to limit beetle grub populations. Use pyrethrin powder or neem oil as a last resort.

Yellow, stunted plants and roots that have swollen galls indicate root-knot nematodes. Other symptoms are plants wilting during bright, hot days. Prevent root-knot nematodes by applying chitin or introducing parasitic nematodes to the soil before planting. Solarizing the soil also helps in preventing root-knot nematodes.