Cause A deficiency of some element, usually iron, in the plant. Iron may be lacking in the soil or in a form unavailable to plants. Lime-induced chlorosis, or leaf yellowing, is caused by strongly alkaline soil, which can make iron or manganese unavailable to plants. Rhododendrons grow normally in acidic soils. The evergreen azaleas R. macrosepalum var. hanaguruma, R. ripense and R. scabrum are tolerant of neutral to basic soil pH, showing very limited leaf damage.
Symptoms Leaves yellow, starting at the margin and progressing inward until, in severe cases, only the areas around larger veins remain green.
Cultural control Feeder roots are close to the surface, so work soil very lightly to avoid further injuring the plant.
- Adjust soil pH to 4.5 to 6. Adding sulfur or organic matter, especially conifer needle mulch, which is highly acidic, may help.
- Use ammonium sulfate as a nitrogen source especially if soil pH is on the high side. Repeated use can lower soil pH below the optimum range. Home gardeners should use products marketed as azalea, camellia and/or rhododendron food.
- Check plant's proximity to newly poured concrete. New concrete leaches lime into the surrounding soil, raising the pH.
- Use iron chelate, either as a soil treatment or by foliar feeding, to quickly help chlorotic plants.
Reference Scariot, V., Caser, M., and Kobayashi, N. 2013. Evergreen azaleas tolerant to neutral and basic soils: Breeding potential of wild genetic resources. Acta Hortic. 990:287-291.