Iron chlorosis on trees: what extension services describe

This page describes what the named extension services and the Missouri Botanical Garden say. It does not diagnose a tree, say whether any tree is safe or unsafe, or tell anyone to treat, replace or remove one. Chlorosis is a plain word for yellowing, and the sources stress that many different things cause it. Where they differ, the page says so.

What the sources call it

Colorado State University Extension (Iron Chlorosis of Woody Plants) defines chlorosis as general yellowing of leaves and iron chlorosis as the yellowing that comes from too little iron in the leaf tissue. Utah State University Extension calls it one of the most common abiotic disorders affecting plants in Utah. The Missouri Botanical Garden says the iron does not have to be missing from the soil: it may be present but in a form the roots cannot take up.

All three sources point at soil chemistry. Utah State (Michael Kuhns and Rich Koenig, forestry and soils specialists) says Utah soils hold plenty of iron, but in soil with a pH above 7.0 that contains lime, chemical reactions turn the iron into solids that roots cannot absorb. It says that is why rusty nails or iron shavings do not fix the problem. Colorado State describes the same reaction in its alkaline, lime-rich soils and puts the pH of its calcareous soils above 7.5.

What it looks like

Colorado State and Missouri both describe leaves that turn yellow between the veins while the veins stay green. In severe cases Colorado State says the leaf turns pale yellow or whitish with only a greenish cast left along the veins, angular brown spots can form between veins, and leaf edges can scorch. Because iron does not move easily inside a plant, Colorado State says the yellowing shows first and worst on the newer growth at branch tips. Leaves can be smaller than normal, curl, dry up and fall, and in severe cases individual limbs or the whole plant can die. Missouri adds that twigs can stunt and die back and that plants may fail to flower or fruit.

Colorado State and Utah State both note that chlorosis often shows on a single branch or on one half of a tree while the rest looks normal. On pines, junipers and other evergreens Colorado State says it usually appears as overall yellowing of the needles.

Which plants the sources name

Missouri says iron chlorosis is most common on pin oak, sweetgum and river birch, and on rhododendrons and azaleas. Colorado State lists many woody plants with high susceptibility, among them aspen, birch, beech, honeylocust, linden, London planetree, pin and red oak, red maple, Amur maple, spruce, pine and juniper. Utah State names silver maple and quaking aspen as popular trees with serious chlorosis in high-pH soils, and says even trees that usually tolerate moderately high pH, such as Norway maple and Scots pine, can show it on especially bad sites. Utah State says aspen is native to cooler, wetter mountain sites with lower soil pH and is not well adapted to the low-elevation sites where it is often planted.

Conditions the sources say make it worse

Colorado State says the factors are complex and not fully understood. It lists overwatering and soil that is too wet or too dry, soil compaction and surface crusting, and trunk-girdling roots, which it links mainly to planting too deep. It also lists competition from turf under the canopy, winter injury and cankers, high soil salt levels, extreme soil temperature and light, and too much phosphate, manganese, copper or zinc. Utah State adds cool soil, plastic sheet mulch and water-saturated soil, and says chlorosis is often worse where topsoil has been removed and lime-rich subsoil is exposed, for example on eroded ground or land levelled for new housing.

Look-alikes the sources name

Colorado State says a suspected iron problem in high-pH soil may really be iron and manganese deficiency together. It says zinc and manganese deficiency look similar but usually show first on older, interior leaves, that nitrogen deficiency yellows the whole leaf including the veins and starts on older leaves, and that soil-sterilant weed killers can leave tissue along the veins green. It lists root and trunk damage, some viruses, phytoplasmas and vascular wilt diseases as other possible causes of similar leaf symptoms. Utah State lists insect and disease problems, herbicide misuse and a history of overwatering as causes of yellowing to rule out first, and notes that some cultivars are bred to have yellow foliage. Missouri lists nutrient deficiency, root damage, temperature extremes, herbicide misapplication, too much light, too little or too much water, insect feeding and disease. For injury from weed killers see the page on herbicide injury.

What the sources say about treatment

None of the sources calls this easy. Colorado State says there is no easy, inexpensive or long-term correction, that treatments can be expensive with disappointing results, and that reducing springtime overwatering and soil compaction can ease it in some situations. It names four general approaches: lowering soil pH, putting iron in the soil, spraying the leaves and injecting the tree, each with variable results. Utah State lists the same four and says results vary. Missouri says a foliar iron spray corrects chlorosis only for the season and does not change the underlying soil cause. The treatments involve chemical products. Who may apply pesticides, and where, is set out in the guide to pesticide licences.

The other pages in this group are listed on the weather, soil and chemical damage page. For how roots take up water and minerals, see how a tree works.