Improving pest control with nutrition
By Dana Venrick, Volusia County Extension and
Richard Tyson, Seminole County Extension
University of Florida, IFASDid you know that many pest problems might be promoted by fertilization? Pest populations may increase when crops are fertilized indiscriminately without careful consideration of the existing nutrient status of the plants. Not only do pest populations increase, but also do more damage when plants have been weakened by poor, unbalanced nutrition. Some of the pests increased by various fertilizer materials are mites, aphids, psyllids, scale insects, whiteflies, some caterpillars and grubs as well as diseases. Only after soil testing and/or leaf tissue analysis (for long term crops) indicates the need for specific nutrients, should they be applied to a plant and then only as much as needed. Fertilization should be made in conjunction with any necessary pest treatment or made after an existing pest population has been reduced. Fertilization itself does not cause insect and disease problems to increase, but are the result of less than ideal nutritional inputs. The correct amount of balanced fertilizer actually reduces pest/disease damage.
How do fertilizers promote increases in pests? Large amounts of various quick release fertilizer materials, especially those with a high salt index of over 50, cause weak, succulent growth and may stress the plant with soluble salt injuries. Certain materials such as ammonium sulfate are very acidifying and could drive the pH below a desirable level. Low pH may create mineral toxicities or cause some minerals to become unavailable. Supplying a mineral to a plant that already has a surplus of that mineral or not supplying a deficient mineral will cause a mineral imbalance in the plant to become worse. A balanced supply of all the essential nutrients the plant needs is the best way to increase resistance to various insects and diseases. Essential minerals, other than carbon, hydrogen and oxygen that are in air and water, include nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, iron, zinc, manganese, copper, boron, molybdenum, chlorine, nickel and for certain plants - silicon.
As a guide to assist growers with the use of materials that have a high potential for increasing pests, here is a list of some of the most frequently used fertilizers giving their salt index, pH reaction and rate of release (how soluble). Remember to use materials with a salt index over 50 in smaller doses or amounts and with care.
* (Commonly used in liquid fertilizer)
Calcium (Ca) is very important for disease protection. Ca provides strength to cell walls and membranes. The incidence of fungal and bacterial infections is inversely proportional to the amount of Ca in the leaf tissue. In other words, as the level of Ca increases, the incidence of diseases caused by fungi and bacteria decreases. Enzymes that are released by pathogens that cause disease are strongly inhibited by Ca.
Silicon (Si) helps provide pest resistance and is usually not even considered in a fertilizer program. Cell walls containing Si act as a mechanical barrier to the stylets (sucking mouth parts) and mandibles of sucking and chewing insects. Foliar sprays of silicon reduce aphid populations on leaves. Si reduces water loss, increases resistance to freezing temperatures and decreases root and foliar diseases. Plants growing in media without soil are particularly vulnerable to Si deficiency, but this may be a problem in sandy soils as well. Generally, this is not a problem in clay soils. If Si is available to a plant, it will accumulate to leaf tissue levels approaching that of nitrogen. The need for silicon supplementation is still being researched. Existing research shows a positive response to Si amendments by some plants but not others.
Nitrogen (N) is important in that increasing N supply increases feeding by insects with piercing/sucking mouthparts, such as aphids. The form of N affects silicon content and disease incidence. For example the use of ammonium sulfate permits more uptake of silicon than the use of calcium nitrate. Increasing the supply of N causes increased incidence of fungal diseases but reduces the incidence of bacterial diseases. There are a few exceptions, such as red thread, pink patch Anthracnose and dollar spot in turf, which are increased by N deficiency. What balance of N sources should you use? Most terrestrial plants grow best with a 75:25 ratio of nitrate to ammonia. For water and acid loving plants the ratio is reversed. With warm temperatures and a pH above 6, ammonium nitrogen is converted to nitrate nitrogen (the form absorbed by plants) relatively quickly. However, conversion is inhibited at lower temperatures and at pH levels below 6.
Potassium (K) is very important for pest resistance and is often below optimum levels in the soil and in the plant tissue. Many plants need a higher level of K in the plant tissue than nitrogen. Increasing K level within the plant increases plant resistance to fungal and bacterial infections. Always consider mineral balances within plants. As with nitrogen and other highly soluble elements, K must be supplied frequently with small doses as a water- soluble fertilizer or as a combination of immediate release and longer-term release materials (such as coated K and sulfate of potash magnesia) in dry, surface applied fertilizers, especially in sandy soils. The plant must not ”run out” of potassium or other essential nutrients, if minerals are to be kept at their optimum levels within the plant tissue and optimum pest protection by nutrient balancing is to be achieved. The down side of a high K supply is that it suppresses the uptake of silicon, calcium, magnesium, and production of sugars and amino acids. Elevating supplies of nutrients suppressed by potassium (and other minerals) is very important.
Boron, manganese, copper, zinc, selenium, cobalt and sometimes sodium also play important roles in pest protection for plants even though they may be essential or beneficial in extremely small amounts. Some research indicates that traces of various other elements are essential as well. Make sure that trace elements are available and are being taken up by the plant. This is particularly true for boron, since it is the only trace element that has high solubility and will readily leach to ground water. Manganese is deficient in almost all Florida soils, and because of its importance in disease protection, careful attention must be given to supplying adequate levels of this nutrient.
When using overhead irrigation systems, providing a uniform distribution of water is essential for providing the correct balance of nutrients, whether it is watering in granular fertilizer, or fertigating and chemigating for existing pests. An irrigation system with a distribution uniformity (DU) of less than 60% will not produce a quality crop because uniform absorption of inputs by plants in such a situation is not possible. The DU should be 80% or better. How do you determine the efficiency of your system? DU is determined by spacing “cans” (straight-sided, flat- bottomed containers) in a grid spaced the same distance apart in the irrigation zone. The irrigation system is run for a set amount of time. Then the average depth of water in all the cans is determined. Then the lowest one-fourth of the measurements are selected and an average depth is determined. Then:
DU = (average low quarter depth / overall average depth) x 100%
This exercise will also help determine the correct amount of water to apply to prevent leaching of fertilizer salts below the root zone.
Because fertigation may cause an increase of existing pests, it should be coordinated with chemigation or other means of pest control, if pests are present. The correct sequence to follow when fertigating/chemigating is to run the water, inject compatible fertilizers (short duration- don’t leach nutrients to the ground water), inject compatible pesticides and turn the water off as soon as the pesticide clears the lines. Injection time can be determined by running and timing a dyed fertilizer (or dye) first and then timing the pesticide injection time accordingly. Fertilizer should be “rinsed” off the plants while pesticide should not be washed off; therefore the irrigation system should be designed so that the main line goes to the center of the production area with irrigation lines then fanning out to the corners. With the correct design, materials clear all the end nozzles at the same time and no pesticides are retained in the lines. To avoid incompatibilities, fertilizers and pesticides should be injected from two different tanks at two different points. Some fertilizer combinations must be avoided. Never mix calcium or magnesium with phosphorus or sulfur. Water could also be too high in calcium or magnesium to work with phosphorus or sulfur compounds. Don’t inject ammonium polyphosphate into water high in calcium and magnesium. Ammonium thiosulfate and magnesium nitrate can be mixed in warm conditions, but never mix when the temperature may be less than 50 degrees Fahrenheit.
Account for the nutrients that pesticides may contain. For example, Dithane or Mancozeb contains zinc and manganese. Phosphorus acid (phosphite), which controls various fungal diseases, is usually blended with potassium as well as phosphorus. Do leaf tissue analysis to determine the extent to which these nutrients are being taken up by the plant. Account for the minerals dissolved in water as well. Again, testing is the only way to know the extent to which these minerals are being absorbed. In soils and media, pH may be increased with the incorporation of dolomitic limestone (calcium magnesium carbonate) and decreased temporarily with wettable sulfur.
High pH water can cause line clogging and tie-up of nutrients. Fertilizers and pesticides generally work better with an acidic pH. Hard water with a high pH should be tank acidified with ammonium sulfate or injection acidified with sulfuric acid or some other acid, depending on conditions. Of course, it is of utmost importance to adjust pH in soils or growing media to the desirable range for the crop being grown; otherwise nutritional and pest control programs will likely be ineffective.
Remember, plant nutrition has been demonstrated to affect pest and disease problems. Fertilizer inputs should be based on actual nutrient needs as determined from soil and, when appropriate, plant tissue testing. A balanced nutrient supply ensures optimum plant growth and resistance to pests. Fertilization may promote existing pest populations and should be coordinated with appropriate pest control measures to include scouting. In some instances, the best option may be to reduce an existing pest population before fertilization.
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