Iron chlorosis (lime-induced chlorosis)

Symptoms

The symptom affects acid-loving plants — hydrangea, rose, rhododendron, blueberry and others — grown on alkaline or calcareous soils. The leaf blade turns yellow while the veins remain distinctly green, producing the characteristic interveinal chlorosis. The symptoms appear first on the youngest leaves at the shoot tips, because iron is not very mobile within the plant and is not translocated into new growth. In extreme cases the leaves are almost white and their margins dry out. Crucially, the cause is usually not a lack of iron in the soil but too high a pH, which blocks its uptake by the roots. It is worth noting that not all susceptible plants are equally acid-loving — the rose prefers a slightly acidic to neutral reaction (about pH 6–7) and also develops iron chlorosis on strongly alkaline, calcareous soils, although it is not acid-loving in the way rhododendron or blueberry is. The pattern of yellowing is very regular: not only the main vein stays green but the whole fine network of veins, so that the leaf looks like green lace on a yellow-green background — this is the most reliable diagnostic feature, distinguishing interveinal chlorosis from the uniform, whole-surface yellowing typical of nitrogen deficiency. The symptoms intensify with each successive flush: older leaves at the base of the shoot may still be green or only slightly paled, and the younger the leaf, the paler it is, up to almost white, lemon-yellow tips. Under a long-lasting deficiency the margins and tips of the youngest leaves turn brown and necrotic, shoots grow more weakly and become smaller, and in blueberry the yield and fruit quality decline. Iron chlorosis is not contagious — it does not spread from plant to plant but appears simultaneously in all specimens growing on the same, overly alkaline site, which also helps to distinguish it from infectious diseases. Its severity can vary through the season: cold, wet and compacted soil, poor root aeration, excess calcium, a high bicarbonate content in the watering water, and over-fertilization with phosphorus additionally hinder iron uptake and aggravate the symptoms even when the element is not lacking in the soil.

Treatment

In the short term, spraying or watering the plants with chelated iron gives the fastest improvement, as it is readily available to the plant regardless of soil pH, although it acts only temporarily. A lasting solution is lowering the soil pH by acidifying it — incorporating garden sulphur, acidic peat or acidifying fertilizers, so that the iron already present in the soil becomes available. When choosing a chelate it is worth paying attention to its type, because this determines its effectiveness on a given soil: the Fe-EDTA chelate is stable only up to about pH 6.5 and quickly loses iron on calcareous soils, whereas the Fe-EDDHA chelate remains stable even at a high, alkaline reaction and it is the one that works on the most difficult sites. A soil application — watering the chelate into the root zone — lasts longer than a foliar spray, which gives quick greening but only on the sprayed leaves and needs to be repeated during the season. Because acidification with sulphur is a slow process (oxidation by soil bacteria takes weeks, and the full effect up to two seasons, and it requires warm, moist soil), the best results come from combining a stopgap chelate intervention with a lasting solution, that is gradual acidification. Before each adjustment it is worth measuring the soil pH and dosing the sulphur carefully so as not to lower the reaction too abruptly. The rooting conditions must also be improved — loosening and draining compacted, waterlogged substrate — because in cold, anaerobic soil iron uptake remains poor even at a correct pH. Plants in containers are most easily repotted into an acidic substrate for acid-loving (ericaceous) plants and watered with soft water. It is worth remembering that this is a physiological disorder, not an infection — fungicides are useless here, and the only treatment is correcting the nutrition and the soil reaction.

Prevention

The best prevention is planting acid-loving plants from the outset at a site with a suitably acidic reaction and avoiding calcareous substrates. It is worth watering with soft water, such as rainwater, instead of hard, calcareous water, and mulching the soil with bark, which slightly acidifies it as it decomposes and stabilizes the pH. Before planting it is worth measuring the soil pH and matching the species to the site rather than later fighting the soil chemistry by force — as a rough guide, blueberry needs a pH of about 4.5–5.5, and rhododendrons, azaleas and hydrangeas roughly 5.0–6.0; on a clearly alkaline soil it is more sensible to choose a tolerant plant than to grow a demanding acid-loving one. Acid-loving plants should be planted away from foundations, walls, concrete and building rubble, from which lime leaches out, and the soil in their vicinity should not be limed. On soils of borderline reaction it helps to mix the substrate beforehand with acidic peat or sulphur and to mulch regularly with pine bark, needles or acidic compost. It is worth using fertilizers intended for acid-loving (ericaceous) plants, avoiding over-fertilization with phosphorus, which blocks iron uptake, and not overwatering the plants while ensuring good drainage, because excess water and poor root aeration promote chlorosis. Regular observation of the youngest growth is also helpful — the first paling of the tip leaves is a signal to react before the chlorosis becomes established.

How to tell it apart from look-alikes

Control — organic and chemical methods

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