Plant Doctor Archive

Yellow leaves on Rhododendrons

My rhodos have developed a yellowing leaf with patches of ginger staining on the underside. How do I treat them?

From your description, it sounds like your rhododendron has rust. Rust is a fungal disease that is common in periods of wet weather. Once rust has set in, it can be hard to control. Prune out infected shoots. Clean up any fallen leaves around the plant and destroy by burning or placing in the rubbish bin (not the compost heap). Spray with an appropriate fungicide such as Fungus Fighter — a broad spectrum systemic fungicide. Systemic sprays are absorbed by the plant and carried in the sap giving better control. Spray at 7-10 day intervals for complete control. A maximum of 4 applications per growing season is recommended as disease resistance can build up. Alternate sprays with Bravo, an effective protectant spray.

Greenguard is an alternative to Fungus Fighter. It has a protectant/systemic activity and is a mixture of two different sprays so disease resistance is less likely to build up.

The chance of plants becoming infected with fungal diseases can be reduced by providing optimum growing conditions — good air circulation around the plant, a cool, moist root run, mulching plants in late winter and mid summer. Feed plants with a specially blended plant food for acid-loving plants in late winter and again in mid summer.

Another possibility is a root-rot disorder. If the leaves start to wilt but remain on the plant, turn yellow and eventually die back, it is possible the plant has a root-rot disorder. This is caused by poor drainage and the roots sitting in water-logged soil. There is no remedy. The plant should be removed and destroyed. Avoid replanting into the same hole.

To avoid root-rot, provide good drainage — plant into raised beds if necessary and incorporate plenty of peat, compost or organic material into the planting hole.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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