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Oranges on tree

Citrus Fertilizer for Greening Affected Trees

Declining Citrus with Greening

People often ask me what they can do with their declining citrus to make them productive again. There are numerous diseases that cause citrus to decline, with the latest being citrus greening. Greening is a bacterial disease of citrus now found in almost every citrus tree in the area. It does not affect humans, so don’t worry about that! It is spread by a small insect, the Asian Citrus psyllid, that pierces the leaves to suck sap and spreads the bacteria from one plant to another much like a mosquito spreads malaria in humans. The bacteria multiply inside the plant and plug the vascular system of the tree so the roots starve for carbohydrates from the leaves and the leaves starve for nutrients from the roots. This gradually kills the tree and makes the fruit small and bitter, and there is currently no cure for the disease. Research continues on finding a cure and how to keep the trees productive even with the bacteria. Some growers have been successful in keeping trees in production although the fruit quantity and quality is not what it used to be because the trees are stressed with the disease. Current guidelines to keep trees in production are expensive and many growers cannot afford to continue to grow citrus with low returns and high expenses. Hence you see many declining groves and groves being pushed and burned.

Is There a Cure?

It may take years before researchers produce disease-resistant citrus cultivars that can fully replace the cultivars currently grown. In the meantime, citrus growers may push out their dying citrus and maintain the land in agriculture waiting for the new resistant citrus to plant, or they may try expensive treatments to keep the trees producing. There are no requirements to push out infected trees like the unsuccessful attempts to control bacterial canker several years ago. However, if a grower wishes to push the trees and keep the land in agricultural production, there is a program called the Abandoned Grove Initiative in which the Florida Department of Agriculture certifies the grove should be pushed, then certifies it has been pushed. With this certification the grower can go to the county property appraiser to keep the land in agricultural exemption for five years. More details on the Abandoned Grove Initiative.

Controlled Release Fertilizers

What are the treatments to keep infected trees alive? Several have been tried, but the latest with research-based success is the use of controlled release fertilizers. Controlled release fertilizers (CRF) are produced in such a way that the nutrients held in the granule of fertilizer slowly release through pores in the polymer coating or through the microbial breakdown of a sulfur coating. CRFs are produced to continue to release nutrients for various lengths of time, depending on the number of pores or the thickness of the sulfur coating. Regular fertilizer granules may dissolve and release all their nutrients in the first heavy rain or irrigation event, causing the nutrients to be lost to the deep soil where there are no roots, or worse, to the ground water. CRFs are recommended in all landscape treatments because there is less chance of water pollution (less algae blooms on water bodies) and the plants respond better to continuous slow release of nutrients. However, they are more expensive than the regular fertilizers.

What Do Trees Want?

What do citrus trees want to keep growing? Dr. Tripti Vashisth’s research, with the University of Florida Institute of Food and Agricultural Sciences, a multi-year study that is only a few years in, indicates that using a balanced Controlled Release Fertilizer (CRF), like a 12-4-16 with Ca and Mg, gives the best results for citrus affected with Citrus Greening (HLB). For best results, this fertilizer should be amended with a Mn-Zn-Fe-B micronutrient fertilizer. Using a four month CRF, divide about 2 1/2 pounds of this type of fertilizer (12% N) plus micronutrients per mature tree into three equal amounts to apply in in February, June, and September. The fertilizer should be applied in the wetted zone of the tree’s root system, not just put in a single spot or narrow ring around the tree. Micronutrients are the nutrients needed at much lower amounts in plant growth, while macronutrients, including the nitrogen, phosphorus and potassium in most fertilizers, are the major nutrients required for plant growth. Care should be taken not to over-apply micronutrients because too much may cause a toxic response. Younger, smaller trees require less fertilizer, so use the table below as a guide to amounts. If you use a CRF with a different release rate, just divide the amount up so that the total amount in a year is the same. If your fertilizer has a different analysis than 12-4-16 (%N-P-K) then you will need to calculate the total amount to apply based on the percent of nitrogen using the table below for pounds of N/tree/year.

If you cannot find a CRF, or prefer not to pay the increased cost of this product, a regular granular fertilizer may be used, but the same total amount of nitrogen will need to be divided up and applied monthly during the period February through September. Others may prefer using a soluble fertilizer and applying even smaller amounts frequently. Information on using liquid fertilizer. Organic fertilizers may be naturally slow released if the nutrients are tied up in composted organic matter that must be broken down by microorganisms or they come in very low analysis (%N-P-K) that must be applied frequently. You can still calculate the amount to apply based on the total pounds of N/tree/year. You should not apply more than 1 pound of N (remember the fertilizer is not pure N) per year per tree.

For best citrus growth, the soil pH should be between 6 and 7, with irrigation water pH between 5 and 6. Irrigation water pH is not something most homeowners can control, so be aware that if your water pH is high, it will cause your soil pH to increase over time and will contribute to poor plant response to any fertilizers. Using a sulfur coated CRF will help to reduce the soil pH.

How Much to Apply
Tree Age Lbs of N/tree/ year Lbs. of 12-4-16/tree/yr
1 0.3 2.5
2 0.6 5
3 0.9 7.5
4 1 8.3
5+ 1.6 13.28

More information on nutrition management for citrus trees.

More information on growing citrus trees at home.

Sources:
  1. Enhanced CRF root nutrition to maximize HLB tolerance. 2019. Jude Grosser.
  2. Nutrition Management for Citrus Trees. 2018. K.T. Morgan, D.M. Kadyampakeni, M. Zekri, A.W. Schumann, T. Vashisth and T.A. Obreza. UF/IFAS Florida Citrus Production Guide.

 

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