ISSUE 8 July 2, 2009
IRON CHLOROSIS IN TREES
One thing that trees don’t often face in North Dakota is nutrient deficiency. The exception is a problem called iron chlorosis. It affects trees in many parts of the state and it can be the first step in a decline spiral that eventually kills the tree. The main symptom of iron chlorosis is yellow leaves with green veins. Iron is used by the trees to make chlorophyll, the green pigment that traps sunlight for photosynthesis. There is no predicting if or when a tree will be affected by iron chlorosis; a tree can be healthy for many years, then suddenly become chlorotic.
Chlorotic leaves from a silver maple tree. Note the green
veins surrounded by yellow tissue. (Photo by J. Zeleznik)
Most of North Dakota’s soils have enough iron in them to support healthy plant growth. But sometimes the iron is in a form that’s not available to plants. The culprit is usually high soil pH. The soil is not acidic enough to keep iron in the available form. Low soil oxygen can also cause iron to be unavailable. Trees are more susceptible to iron chlorosis if they are growing in flooded or compacted soils. Even low temperatures can reduce iron availability.
Can chlorotic trees be "fixed" or "saved"? As with most things, the answer is "it depends." In the case of iron chlorosis, prevention is more effective than after-the-fact treatment. Determining the cause of the low iron availability is critical to developing a treatment approach. Obviously, low-temperature induced chlorosis can’t be prevented. However, flooding and compaction – and therefore low soil oxygen – can be mitigated by improving drainage or aerating the soil.
For chemical treatment products, there are a lot of manufacturers’ claims. What does the science say? Unfortunately, it’s not conclusive. Everything works some of the time, but nothing works all of the time.
If the cause of the chlorosis is high soil pH, then there are two options – lower the pH or add iron in an available form. Soil pH can be lowered by adding elemental sulfur or by adding acid-forming fertilizers. Some experts recommend combining these approaches by applying iron sulfate to the soil. Treatments must be repeated every two-to-three years. Iron can also be added as a chelate. Chelated iron comes in many formulations, so ask your local garden center what formulations are on hand.
Iron can also be added directly to the tree, either as a foliar spray or as a trunk injection into the vascular system. Iron chelate sprayed onto the leaves offers a quick fix to the problem, but the effects are often short-lived. Trunk injections have shown mixed results. Sometimes, injections are effective and a tree will regain its health and vigor. However, if tree health has severely declined before injections are employed, then the tree may not recover.
Iron chlorosis can be found in trees throughout the state. But with proper management, it can be prevented, and sometimes, cured.