WHAT IS CHLOROSIS?
Chlorosis can be identified by a yellowing of plant leaves typically between veins (interveinal) usually a sign of a micro-nutrient deficiency. The most common deficiencies are iron and manganese. If chlorosis is left unchecked it can lead to entire leaves turning yellow, including the stems, browning of the edges of the leaves as these leaf cells die. Over a prolonged period even entire branches or individual stems may die. The lack of green color in the leaves is due to a lack of chlorophyll, a compound manufactured by plants necessary for photosynthesis to occur. Manganese is the core of every chlorophyll molecule. No manganese leads to no chlorophyll. Iron is required during the process of chlorophyll manufacturing. Just remember that it is a critical component of the energy transfer process during this chlorophyll production. It should be getting easier to understand now just how critical micro-nutrient deficiencies are to the well being of plants.
WHAT CAUSES CHLOROSIS?
The soil itself could simply be deficient in nutrients but that is usually not the most common cause of chlorosis symptoms. Soil tests many times reveal plenty of iron and manganese in the soil but something is interfering with the plants ability to absorb these nutrients from the soil. Alkaline soils (over a pH of 7.0) tend to interfere with the availability of iron and manganese. This problem is not totally understood but it seems in higher alkaline soils that iron remains more of a solid and manganese is bound tightly by soil particles. This situation will remain like this indefinitely if something is not done to alter the soil chemistry. Also our urban soils are usually stripped of all the beneficial fungi (mycorrhizae) during development. Additionally our urban soils never get enriched each year from decaying leaves as is the case with our forest soils.
WHAT PLANT SPECIES ARE THE MOST SUSCEPTIBLE?
The most problematic species in our area are pin oaks and river birch. Pin oaks thrive in well drained gravely soils with a pH closer to 6.0. When we plant them in our poorly drained, saturated clay soils with a pH of typically 8.0, it should come as no surprise that we need to deal with chlorosis. Add azaleas and rhododendron to this problematic list also. In fact nearly all acid loving plants are not going to like the average upper Midwest soil types.
ARE THERE DIFFERENT METHODS OF TREATMENT FOR IRON CHLOROSIS?
Treatments for Iron chlorosis fall into three general classes – trunk injection, soil treatment, and foliar spraying. They all have varying levels of costs and length of time they are effective. Foliar spraying works quickly but does not last very long. Trunk injections last longer but can cause some damage to the trees if applied through any drilling methods. Soil treatments can be more cost effective but depending on the products chosen might not be as effective nor last very long. The problem with chlorosis treatment comes when you fail to address the underlying soil chemistry issues.
WHY IS IT IMPORTANT TO FOCUS ON SOIL CHEMISTRY WHEN LOOKING FOR A CHLOROSIS SOLUTION?
I like to explain this problem in the following manner. If we inject our trees with the nutrients they need we will encourage them to become “junkies”. They will become even lazier when it comes to looking for what nutrients they need on their own. You will now need to inject them repeatedly, sometimes yearly, to maintain the non-chlorotic look you are after. While injections might be needed at first if you have waited too long to address the chlorosis issue, I still prefer to address the soil problem first. If you can solve the problem through a proper soil treatment it will last longer, at a lower overall cost, and cause less damage to our trees. As mentioned earlier nutrient deficiency might not be the problem with our soils. We need to deal with bigger issues of Soil pH, compaction, poor drainage, and a different measure of soil health called Cation Exchange Capacity (CEC). The iron and manganese ions that our plants need are being “held” by the soil sometimes in forms that are unavailable to the plants roots to absorb. I will detail the better solution in the next few paragraphs.
HOW DOES UNDERSTANDING THE SYMBIOTIC RELATIONSHIP BETWEEN PLANTS AND MYCORRHIZAE HELP WITH THE TREATMENT OF CHLOROSIS?
We do not need to manage forest soils. This is because there are billions of miles of fungal roots in just a cubic foot of healthy forest soil. But many of our urban soils are lacking these beneficial fungi. They are extremely important to the plants ability to absorb and even find nutrients stored in the soil around them. Mycorrhizae build colonies around a trees roots and attach to them. Then they develop their own root system hundreds of times more dense than the tree’s own root system. These fungi then gather nutrients, water, and minerals and then “trade” the tree for carbon in the form of sugars and dead root cells that are shed as the tree grows. 15% of a trees’ energy derived from photosynthesis can go into this trade in the soil with these mycorrhizal colonies of fungi. This trade occurs in nature’s own “flea market”. The soil around these roots and mycorrhizae actually help facilitate this trade. How good the soil is at handling this trade is measured as the “Cation Exchange Capacity” of soil. Generally the more organic material in the soil, the healthier it is and the higher this measurement is. If we can increase the health of the soil around a chlorotic pin oak by establishing new mycorrhizal colonies, they will help gather the very nutrients that the tree is deficient in. As the tree “exudes” sugars to trade with the fungi, this exudate (especially from oaks) is acidic. As a result the healthy oak tree itself will actually help to lower the pH of the soil around it.
WHAT ARE OUR RECOMMENDED TREATMENTS?
Our Board Certified Master Arborist, Wayne White, will personally recommend the treatment best suited to your chlorotic tree or shrub. If the trees are severely chlorotic they might need both a trunk injection and a soil treatment the first year. The goal though is to wean them off of any need for a trunk injection in the future by treating the soil around the tree with a combination of fertilizers with the necessary micro-nutrients, a bio-root stimulant, and billions of beneficial fungal spores of mycorrhizae. They need to be applied with a deep root feeder to be effective.