Question: I have several trees, shrubs and bushes that have leaves that are turning yellow but the veins remain green. I water them every other day and feel that they are receiving enough water. What can I do to correct this problem?
Answer: The leaf yellowing is called iron chlorosis and is caused by insufficient iron in the plant leaves. If you look closely you will see that the older leaves are not affected. Iron is necessary in the formation of chlorophyll, the green pigment of plant alkaline, meaning that they have pH's above 7.0. As soil pH increases above 7 iron forms other molecular complexes which are not taken up by plants. Water logged soils or anaerobic conditions can cause iron to be unavailable.
Control: Decrease the amount of water given to plants, checking the soil to see if watering is necessary. Lowering soil pH will make iron more available. Changing soil pH is a long term proposition but can be accomplished by adding acidifying materials such as soil, sulfur, and/or gypsum. Iron can applied to the soil or the leaves to help correct this problem. Iron applied to the soil is slower acting but is longer lasting than iron sprayed on the foliage. There are several dry products that can be soil and other forms of iron. These products are poured into holes that are about poked into the soil one to one and a half feet deep, placed around the drip line of the affected plant and serves as a reservoir of available iron. Foliar applied irons are usually liquids and can also come in chelated forms. Chelates are "chemical jaws" which protect the iron from becoming bound up with other chemicals before it is inside the plant.
Once near or inside the plant the chelating agent is dissolved and the chelated chemical is available. Chelates can be applied to the soil also, but only Sequestrene 138 (6% iron), will work well in our alkaline soils. It is red in color as a powder and when added to water has the color of blood and is as expensive as blood! The Sequestrene 330, (10% iron), is yellow in color and is applied to acid soil, if not it will become bound up and not available to the plant. Look at the product color to know which chelated iron is being used and how to properly apply it. Always read and understand the label before using chemicals.
Question: My mesquite tree has mistletoe in it. Is there any way to rid my trees of this growth?
Answer: There are several genera and species of mistletoe. In Arizona we have 5 species of Phoradendron (leafy mistletoes) and 3 species of Arceuhobium. The latter, called dwarf mistletoe, infests only conifers. Leafy mistletoes are considered as "hemiparasites", which means they produce some or all of their own energy through photosynthesis but depend on their host for water and minerals. Mistletoes elicit a disease response from most hosts and are considered pathogens. However, mistletoes seldom kill healthy hosts except dwarf mistletoe, which can cause severe damage in coniferous forests. Severely infested trees usually have been subjected to other stresses that increased their susceptibility such as drought, flooding, soil compaction, nutrient deficiencies, etc.
The "root' of a leafy mistletoe is directly connected to the hosts' xylem (that part of the plants' plumbing that conducts water and minerals from the root to the leaves). The "root" of a dwarf mistletoe is connected to the hosts' phloem as well as the xylem. The phloem conducts sugars and other products of photosynthesis from the leaves to other parts of the plant. So the dwarf mistletoe is highly parasitic, depending on the host, for photosynthate as well as water and minerals.
Leafy mistletoes can occur on several hundred host species. Mistletoe creates a drain on host resources that reduce growth, decreases vigor, and increases susceptibility to other diseases and insect pests. Local symptoms can include dieback, formation of witches' broom, and weakened branches. Dwarf mistletoe, in particular can cause spiketop, where the entire host crown dies and also causes witches' broom which increases the diversion of water, minerals, and nutrients to the site of infestation.
Control: Removal of the branch below the mistletoe remains an economical and fairly efficient method of mistletoe "control". However, control by pruning requires diligence.