By: Joseph P. Krausz and Thomas A. “Chip” Lee Jr.
For pecan trees to be healthy and vigorous and for nut quality and yield to be satisfactory, producers must establish sound disease-management programs. Producers can prevent losses from diseases and insufficient zinc by implementing effective grove management practices.
Throughout the year, producers need to pay close attention to their groves and be able to recognize the factors—including weather, plant stages and disease development—that can contribute to disease outbreaks during the different seasons.
Spring and early summer: The fungi that cause pecan diseases require moisture and mild temperatures when the spores are germinating and infecting the immature leaves and nutlets. To reduce losses to these fungi, producers should follow cultural practices that shorten the time that leaves and young nutlets are wet after rain, dew or irrigation.
Also during this period, the leaves and nutlets are immature and most susceptible to the pecan scab fungus. In susceptible cultivars, the foliage is vulnerable also to downy spot fungus. Even when sound cultural practices are followed, producers may need to apply a protective fungicide in some locations and on scab-susceptible cultivars. Continue these applications on 14-day intervals as long as weather conditions favor infection.
Mid summer: In summer, rain and dew are less likely to occur for extended periods, which reduces the chance of infection. If you carefully monitor weather conditions and disease development, you can reduce your fungicide costs by reducing rates and increasing intervals between applications when the weather is dry.
Late summer and early fall: Producers must continue to monitor weather conditions during the late summer and early fall. Although the foliage is mature and no longer susceptible to the scab fungus, the shucks surrounding the nuts are immature and vulnerable to late-season infections.
Factors influencing disease development
As you develop a spray program, consider these factors:
- The susceptibility of certain pecan varieties to diseases, especially pecan scab
- Current weather conditions
- Predicted weather conditions for next 1 to 2 weeks
- Status of disease pressure in and near the orchard
- Tree spacing
- Age of the trees
- History of disease in the orchard
- Date of last fungicide applications
- Last fungicide applied
Periods when pecan diseases are common
In most locations, producing high-quality pecans that are both appealing and disease free requires multiple sprays during the growing season. See Table 1 for periods when pecan diseases are most often observed and when zinc can be used effectively.
For information on suggested fungicides approved for use on pecans, see Table 2.
Fungicides differ in how they act on fungi. Two types of fungicides are contact fungicides and systemic fungicides. Contact fungicides are those that act on the fungus when it is on the surface of the leaf or nutlet. Systemic fungicides are taken in by the plant and become part of the sprayed tissue.
Systemic fungicides must retain their activity even after they are taken in by the tissue. On pecans, the movement of the fungicides is restricted—it does not move freely throughout the tree.The fungus is exposed to the fungicide both on the surface of the tissue and below the epidermal cells (Table 3).
Some systemic fungicides can act on a disease- causing organism, or pathogen, even after it has penetrated the leaf and started to develop outward. This is known as “kickback.” Although the length of time after infection that the fungicide will still control the pathogen varies with fungicides, it is normally 24 to 72 hours.
To help prevent fungicide resistance:
- Rotate a contact fungicide with any other group.
- Rotate a first-generation fungicide with any other group.
- Rotate a second-generation fungicide with any fungicide that does not contain a second- generation fungicide.
- Rotate a third-generation fungicide with any fungicide that does not contain a third-generation fungicide
Although all the products listed in this publication are approved for use on pecans, label restrictions and environmental concerns may restrict their use. You must consider these when selecting products. Always read the label and follow it closely.
Pecans require zinc for normal stem and leaf growth. Trees not receiving zinc do not produce enough indoleacetic acid (IAA), which is a naturally occurring growth hormone responsible for shoot elongation, leaf development and other critical plant functions. In trees lacking in zinc, the inter- nodes between the leaves and stems are shortened.
This compaction of the annual growth is known as zinc rosette. Leaves are smaller, thickened and somewhat distorted. When the deficiency is severe, the affected leaves develop necrotic areas between the veins. Fungi are often associated with these necrotic lesions. Zinc sprays are essential for early-season pecan growth. The most effective applications are early and frequent. Several formulations of zinc are approved for use on pecans (Table 4).
Elemental zinc is the most toxic to plants other than pecans and grapes. To protect nearby plants from poisoning, avoid drift. If drift is a possibility, use NZNTM or a similar formulation.
Do not use any zinc product at a rate higher than that specified on the product label. If you are applying more than one zinc spray within 2 weeks, reduce the rate by half. Never spray young trees that are not actively growing.
Aerial application of fungicides either by fixed wing or rotary aircraft has not proven as effective as are ground applications. However, it may be necessary to make an aerial application when the disease potential is severe and weather conditions prevent the use of ground equipment.
To achieve satisfactory disease control, take into consideration the tree height, the density of leaf canopy and the requirements for maximum coverage.
Chemical use precautions
For the most effective, safe, economical control, use suggested materials. All suggested materials are poisonous, but proper handling reduces the hazards associated with use. Read and comply with the manufacturer’s label directions for storage and handling of toxic chemicals.
The EPA has established pesticidal residue toler- ances on pecans. These regulations establish the amount of a specific chemical that can be present in or on pecans at harvest. Always consult the product label for specific restrictions, and be sure the pesti- cide used is registered for use on pecans and is used only in accordance with specific application instruc- tions.
All pesticides are poisonous; some are poisonous to humans, animals, nontarget crops, etc. Use them carefully, and store them out of reach of children, irresponsible persons, livestock and household pets. Dispose of leftover spray solution and empty con- tainers properly.
Avoid spray drift into urban areas, rivers, lakes or crops that will be used for food for human consumption or livestock feed, unless the product is approved for use on that crop and the rate and timing follow the label recommendations.
To reduce the chances for pesticide drift, follow these guidelines:
- When using an air blast sprayer, direct the force of the air blast away from the two or three rows next to a sensitive area.
- Avoid spraying when the wind is blowing in the direction of a sensitive area.
- Take care when spraying toward county or state highways. Drift onto vehicles traveling on the road can cause accidental exposure and concern.
- Do not spray when it’s foggy. Pesticides can be captured in the fog droplets and carried far from the application site.
- When spraying around sensitive areas, use the least toxic materials.
- Use buffer zones to prevent drift onto sensitive areas. A buffer zone can be:
- An open space where native or planted grass can be used to catch drift.
- A dense grass buffer between the sprayed trees and surface water that can filter out much of the pesticide runoff from pecan trees.
- Hedge rows or closely planted trees that do not require spraying. Research has shown that in some instances, the drift will be carried by air currents over the plant material.
- Cut off the spray in open spaces between trees.
- Make sure that the upper two-thirds of the nozzles are directed to the upper 70 percent of the top canopy, but do not go above the tree.
- Remember that smaller droplets are more likely drift.
- If there is a drift problem, stop and consider precautions to avoid movement of pesticides into sensitive areas.
Symptoms of pesticide poisoning include headaches, nausea, cramps, blurred vision, weakness, muscular twitching and diarrhea. If any of these symptoms occurs during or after handling a pesticide or spraying, consult a physician immediately. Always maintain a copy of the label and the Material Data Sheet.
Spray application considerations
It is essential that the foliage and nuts be covered completely with fungicide. For conventional, high- volume hydraulic sprayers, a general rule for the volume of finished spray required is ½ to 1 gallon of spray mixture per foot of tree height.
Most high-volume sprayers require a pressure of 300 to 400 pounds per square inch. Low-volume sprayers (such as mist blowers, air blast sprayers and speed sprayers) use forced air as the carrier to deliver a concentrated spray mix and require proportionately less water. Concentrated spraying saves water and time but not pesticides because the same amount of pesticide is needed to achieve control on each tree.
The pecan foliage and nuts must be thoroughly covered with spray solution. The fungi that affect pecans infect the tissue where they land on the leaf or shuck. Good coverage is important for both contact and systemic fungicides.
Before making any pesticide application, calibrate the sprayers. This is necessary to determine the amount of chemical to add to the spray tank.
One of the easiest ways to determine the amount of water being applied per acre is to spray half an acre with clear water. Record the amount of water required to refill the tank. Multiply this amount by 2 to determine the amount applied per acre. Table 5 lists the number of trees per acre and per half-acre at different spacing.
If the trees in an orchard are not planted in rows but spaced randomly, determine the number of acres of land involved and the number of trees being sprayed. To calculate the average number of trees per acre, divide the number of trees into the acreage. Although not exact, this will give you an approximate number of trees.
If the trees vary in size, spray representatives of each tree size when determining the amount of water being applied. This is the amount of water used to spray 1/2 acre of trees. Multiply this number by 2 to determine the amount of water used per acre.
Using this rate and the rate of chemical per acre, you can determine the amount of pesticide applied per 100 gallons of water. Below are two examples (Table 6).
Effective disease management programs require that producers use a wide variety of practices.
- Select varieties that are productive and resistant to pecan scab fungus.
- Select orchard sites that have good air circulation.
- Space the trees to allow for air circulation and sunlight. At midday, 25 percent of the orchard floor should be in sunlight.
- Manage the orchard floor to remove diseased shucks and leaves.
- Apply zinc as a foliar spray.
- Take soil samples and follow fertilizer recommendations.
- Remove trees that are susceptible to pecan scab and cannot be sprayed properly.
- Apply insecticides when pest populations reach levels that require treatment. (Refer to current Extension publications that have pest thresholds.)
- Apply fungicides as needed.
Download a printer-friendly version of this publication: Commercial Pecans: Controlling Rosette, Diseases and Zinc Deficiency
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