Pecan pest and disease management

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Authors: H.C. Ellis, Randy Hudson, and Paul Bertrand, University of Georgia

Contents

Scouting

The term “scouting” is currently being used to indicate some form of systematic monitoring of crops. The first organized scouting or monitoring program was started on cotton insects about 1946. After gradual refinement and expansion, scouting has become an essential part of pest management programs being conducted on numerous crops. Regardless of semantics, systematic scouting or monitoring offers several advantages applicable to pecan.

The major goal of scouting is to accurately detect and estimate pest populations and their potential for damage in order to determine the need for control. Systematic monitoring allows control treatments to be timed for maximum effectiveness within the framework of overall operations. The primary purpose of scouting is not to reduce pesticide applications. However, improving timing and, consequently, the efficacy of controls is becoming increasingly important with the current situation of short residual chemicals, pest resistance and rising production costs. In addition, scouting is useful in:

  1. evaluating the effectiveness of control efforts;
  2. detecting unusual or secondary pest problems and allowing alteration of control efforts to meet special needs;
  3. comparing pest problems from orchard to orchard and maintaining an accurate orchard history;
  4. comparing populations from year to year and detecting trends helpful in planning future control strategies.

Two essentials for effective scouting are consistent monitoring techniques and workable treatment guidelines. Monitoring techniques must permit accurate estimation of pest levels. Treatment guidelines should reflect, as nearly as possible, the population levels where control treatments will yield maximum beneficial results. In Georgia, monitoring procedures and treatment guidelines for pecan have been proposed and reviewed over a period of several years. Both the sampling techniques and the treatment guidelines have proved effective and workable but will be continuously revised to provide greater precision as new information becomes available. The authors feel that growers who wish to scout their orchards can use the described methods with confidence. For more information see the article on scouting procedures and thresholds for pecan.


Insect Pests

Diseases

Diseases, especially pecan scab, can be a major limiting factor in pecan production. Disease losses can only be prevented with a carefully planned spray program. All fungicides currently available for pecan disease control must be used as protectants to prevent infection. Fungicides will not have much effect on infections that occur before treatment. Fungicides are applied on a regular basis to maintain a protective barrier over the fruit and foliage of the pecan tree. The protective barriers may be internal, as in the case of Orbit, or external, as with TPTH and other pecan fungicides. The standard disease prevention in Georgia calls for fungicide applications on a 14 day interval from bud break until pollination (three applications). This is the period of most active leaf growth. A 14 to 21 day interval is suggested from pollination until shell hardening. This schedule will need adjustment depending on season, varieties, etc. For most older groves, bud break should be considered as the stage when green tissue is readily visible on the dominant variety. Most growers modify the standard program to best fill their individual needs. Even though fungicides are applied on a rather tight calendar schedule, regular orchard scouting can aid a grower’s disease control program in four ways:

The most obvious benefit of regular orchard scouting is to let the grower know if his disease control program is giving satisfactory results.

The fungicides commonly used to control scab and most other diseases do not control powdery mildew or zonate leafspot. These diseases occur occasionally and require special control measures. Regular scouting can aid a grower in knowing if and when these measures are needed.

Pecan varieties vary greatly in their susceptibility to scab. Historically, the general trend has been for varieties regarded as scab resistant to become susceptible with the passage of time. Many growers spray varieties believed to be scab resistant much less than varieties known to be scab susceptible. Orchard scouting can inform a grower if the disease control program used on a variety believed to be scab resistant is adequate. “Stuart” is the leading pecan variety in Georgia. It has been considered more or less scab resistant for many years. There has, however, been a continuous increase in scab on Stuart. Some growers have suffered losses that could have been avoided with closer observation.

The importance of pathogen populations becoming resistant to fungicides has been clearly demonstrated with pecan scab. Regular scouting should provide early clues to problems of this nature.


Weeds

Control of weeds in pecan orchards is essential for maximum production of pecans. Weeds interfere with pecan production by (1) competition for nutrients and water, and (2) decreasing mechanical harvesting efficiency.

Weeds require soil moisture and nutrients just as do the pecans. Any period of drought stress on pecans is enhanced further by competition by weeds for that limited moisture. Fertilizers applied to the pecans are used by weeds, thus adding to the expense of fertilizer use and potential for fertilizer deficiencies.

The presence of weeds at harvest may also affect the harvesting efficiency of mechanical sweepers. Weeds will prevent many pecans from being swept into the windrow, thus leaving pecans on the ground to either be wasted or require hand harvesting.

The standard weed management practices used for pecans in Georgia are a weed-free herbicide treated strip, 14 feet wide, along each tree row (7 feet on each side), and close mechanical or chemical mowing of the reminder of the orchard floor. Use of a herbicide treated strip reduces much of the competition of the weeds. Close mowing of the remainder of the orchard floor allows for an added reduction in weed competition while retaining a sod for traction of spraying and harvesting equipment and reducing the potential for soil erosion.

A preemergence herbicide application should be made in the fall of the year (November 1 - December 15) to provide control of winter annual weeds and permit delayed use of a spring herbicide treatment. The delayed use of a spring treatment insures that weed control is maintained for a longer period of the year. Postemergence treatments may be required for control of perennial weed such as bermudagrass {Cynodon dactylon (L.) Pers.}, johnsongrass {Sorghumhalepense (L.) Pers.}, wild garlic (Allium vineale L.), and annual weeds that emerge after the breakdown of the preemergence herbicide.

Orchard monitoring improves weed management by letting growers know when orchards need mowing and by identifying those difficult to control weeds that require special herbicide treatment. Textural analysis of soil samples also aids growers in selection of the proper herbicide rate for their specific situation.


Beneficial Insects

All plants and animals have natural enemies which attack their various life stages. Insects, due to their size, are particularly vulnerable to a vast array of natural enemies. These biological deterrents are probably the single most important factor keeping plant feeding insects from overwhelming the world. Control of plant feeding insect pests by biological agents is usually the first line of defense in an insect control strategy. There is literally an army of beneficial insects, spiders and disease organisms present in pecan orchards to help control pecan pests. The utilization and augmentation of these biological agents in traditional insect control programs can greatly enhance pest control and eventually profits. Many of these agents are commercially available.

Beneficial insects can be broadly categorized into those that are parasitoids and those that are predators. Parasitoids are typically smaller than their prey and usually require only one host to complete their development. Adults lay eggs and the developing larvae feed either internally or externally, depending on the species. A parasitoid generally kills its host, but not as suddenly as a predator. A good example of a parasitoid is a braconid wasp. Predators are generally larger than their prey and each predator normally requires several hosts to complete its development. They are free-living and found in great abundance in nature. Lady beetles, lacewings, spiders, and mantids are good examples of predators.

Predators

Damsel bugs are slender, cigar-shaped insects, tan to brown in color and about one-half inch (25 mm) long. The wings are light smokey-colored. The front legs are thick and made for grasping and holding prey. Damsel bug nymphs resemble the adults except they have no wings and appear very fragile. Both adults and nymphs feed on small, soft-bodied insects, worms and eggs.

Bigeyed bug adults are black with clear, silvery appearing wings. They have large conspicuous eyes on the sides of their head. Adults are less than one-fourth inch (6mm) in length. Nymphs also have enlarged eyes, are silvery or gray in color with small, black specks on their body. Both adults and nymphs feed on small, soft-bodied insects, worms, and eggs.

Minute pirate bug adults are about 1/16 inch (1.5 - 2 mm) long, mostly black in color with white markings on their wings. Adults have an “X” pattern on their backs. Nymphs are small, orange-colored insects with the same general body shape as the adult, but are wingless. Both adults and nymphs are important predators of small, soft-bodied insects, eggs and worms.

Lady beetle adults are oval-shaped insects that vary in color but usually have black or orange-red spots on their wing covers. Lady beetle larvae are elongate, spindle-shaped, with spiny backs. They are black, blue and orange in color with thick stubby legs. Both adults and larvae are very active and feed on small, soft-bodied insects and insect eggs. They are very effective aphid predators. Lady beetle eggs are yellow to orange in color and are laid in clusters of five to twenty eggs. Lady beetles are the most important insect predators on pecan

Lacewing adults have many veins in their wings, giving them a net-like appearance. The wings are held roof-like over the back. Adults are green or brown in color and some have characteristic golden eyes. Adults are not predaceous. Larvae are about one-half inch (25 mm) long, spindle-shaped, and have long, sharply-pointed, mandibles protruding from the front of their head. Larvae are tan and white in color with a warty or spiny appearance. Lacewing larvae eat small, soft-bodied insects, eggs and worms. The eggs of lacewings are small, green to whitish in color and are laid on a slender, thread-like stalk. Lacewings are very important predators on pecan in late season.

  • Syrphid fly

Syrphid fly adults are flies which are generally banded or spotted with bright yellow markings on a black body. They are often confused with wasp or bees and have a characteristic hovering flight. Adults are not predaceous. Syrphid fly larvae are elongate, legless, slug-like maggots which are tan to greenish in color and about three-eighths of an inch (10 mm) long. They are important predators of aphids and other small, soft-bodied insects.

  • Assassin bug

Assassin bug adults are flattened, oval bugs with narrow heads and a slender neck. They are brown or black in color with some species having red-colored markings. They range in length from one-half to one and one-half inches (12-36 mm). Nymphs are similar in shape, form, color and habits but have no wings. Both adults and nymphs feed on small soft-bodied insects, worms and eggs.

  • Spiders and Mites

Spiders and mites are nor insects, but these arthropods are very important predators in the orchard. Many types of spiders are found in pecans. They vary greatly in size and color. Spiders prey on almost any insect that comes within their range. One group of mites, Phytoseilus spp., are particularly helpful in controlling plant feeding mites in pecans.

Parasites

Several species of small parasitic wasps and flies may be observed in pecan orchards. Parasitic wasps vary in size. Colors differ but most are brightly colored. Wasps are often seen flying in and out of the plant canopy searching for hosts on which to lay eggs. Larvae of parasitic wasps develop within the body of the pest insect, ultimately killing the host. Parasitic flies resemble houseflies but are somewhat hairier. Adults may be seen resting on foliage or on flowers in which they feed. The flies lay their eggs on the skin of caterpillars. The eggs hatch and larvae eat into the caterpillar where they develop. Aphid parasites are very important in the pecan ecosystem.

Pathogens

Another group of organisms that aid in suppressing insect pests are disease causing organisms or pathogens. Pathogens occur naturally and can be a very significant factor in reducing insect populations. Insect diseases may be caused by bacteria, fungi, nematodes, protoza, rickettsiae, or viruses. All of these live abundantly on and in the bodies of insects. However, in nature, insect disease epidemics depend upon rather precise conditions of temperature, humidity, and host density. It is quite common to see insects in an orchard that have been killed by disease. Fungal pathogens often reduce pecan aphid populations very rapidly.

Direct utilization of the array of biological controls is not currently possible. However, a pest management approach allows us to gain maximum benefit from naturally occurring beneficial organisms.


Bird and Animal Damage Control

Birds and animals cause Georgia pecan growers to lose 12 to 16 million pounds of pecans each year. This represents considerable monetary loss. The greatest damage is caused by crows, bluejays, and squirrels. Some damage is done by deer, raccoon and rats.

A crow eats approximately one ounce of food per day (about 6 percent of its body weight). If pecans yield 50 percent meat, then a crow must eat two ounces of pecans to get one ounce of food. The loss is greater because some nuts are contaminated by pecking or are carried off and lost. Assuming three times as many pecans are contaminated, wasted or lost as are eaten, then a crow can damage one-half pound of pecans per day -- 15 pounds per month.

Blue jay consumption, wastage and loss are about half that of the crow . Thus blue jay damage amounts to 7 1/2 pounds per week and waste or bury about another two pounds. Squirrel damage will amount to 14 pounds of pecans per month per squirrel.

These three pests will take nuts from the tree and from the ground. Squirrels begin eating as soon as the kernel forms (about September 1). Crows and blue jays wait about a month longer -- until the shucks open. Nuts will be eaten until harvest is complete -- usually the first of the year.

Animal Damage per month (lbs./mo.) Vulnerable Time (months) Expected Damage (lbs./animal) Remarks
Squirrel 14 4 56 To estimate total damage, multiply the expected damage for each species by the number present and add for all species. Damage due to other species is usually not great. However, a complete damage control program should include them since there would be no additional costs.
Crow 15 3 45
Bluejay 7.5 3 22.5

Damage Control Program

The best control program is one that incorporates several different methods. It might include:

  1. alteration of the habitat so as to reduce available cover,
  2. fencing to preclude entry,
  3. scare devices and repellents to chase out intruders,
  4. direct reduction when necessary and
  5. a rapid harvest of the crop to reduce the time available for depredation.

The method chosen will depend upon several things; the amount of damage, the species causing damage, the money available for control and the particular situation in terms of the orchard and owner.

Pecan Fruiting Habit

Catkins:

Catkins (the male part of the flower that produces pollen) are produced on wood that grew the previous year. (They will occur at the tip of the one-year-old wood where the current twigs originate.) Catkins buds are produced in late summer before they bloom the next year.

Female Flowers:

Female flowers (the part that produces nuts) are produced on current season's growth at the end of the twigs. Therefore, trees have to make all the year's growth before the crop of nuts is set. This is why it is so important to apply fertilizers approximately six weeks before trees bud.

Trees in production should be fertilized to maintain a minimum annual terminal twig growth of six or more inches. Optimum growth is approximately six to eight inches. Because nuts do not appear until trees have most of their annual growth, the pecan crop is rarely killed from late spring frosts. Female flowers are pollinated by wind born pollen from the male catkins during the first two weeks of May.

Reasons for Premature Dropping of Nuts

Natural Nut Drop:

Pecan trees will naturally drop nuts several times during a growing season:

  • Mid-May after pollination
  • Mid to late June
  • Late July to early August

If you observe a nut drop, it should be determined if it is from a natural shed or one of the following factors.

Disease:

Early scab infection of young nuts will cause nuts to drop. Leaf disease such as scab, scorch, mildew, blotch, brown spot, and downy or vein spot also may cause nut shedding.

Insects:

The pecan nut casebearer probably causes more nut shedding than all other insects collectively. However, insects such as black aphids, May beetles, shuckworms, stink bugs, and pecan weevils may cause dropping.

Moisture and Plant Foods: Nuts may drop early due to lack of moisture and plant nutrients. Variation in moisture available to trees can cause early nut drop, such as a period of high rain fall following a dry spell. Moisture can be conserved by mulching, mowing grasses and weeds, and irrigation.

Weather:

Mechanical injury to leaves and nuts from hail and wind may cause premature dropping.

Tree Condition:

Poor physical condition of trees may cause premature dropping of nuts. This condition may be caused by inadequate fertilization for several years, continued drought for several years, or insect and disease damage. Trees may also be in poor condition if they are located in soil not adapted to high water table, or that is under-lined with solid rock.

Premature defoliation of any type (insect, disease, drought) will cause nut drop and low quality. Every effort should be made to maintain the foliage of the tree.

Pollination:

Too much rain during pollination season will cause poor pollination of female flowers, and those not pollinated will drop. Also, male pollen sometimes becomes mature and right for pollination before or after the female flower is receptive.


Soil and Leaf Sampling Procedures

Soil and leaf samples are required for making fertilizer recommendations. A soil or leaf analysis is no better than the procedures used to collect the sample. For samples to be representative of the area tested, follow these steps for sampling:

Soil Sampling Procedures

  1. Soil samples may be conveniently taken when leaf samples are pulled. Soil sample bags are available from your county agent. They should be used for submitting samples to the laboratory. Supply all the information asked for on the soil sample bags.
  2. Use a spade, trowel, soil sampling tube, auger or other tool which can take a thin vertical slice of soil to a depth of 8 - 12 inches.
  3. Take at least 12 or 15 cores or thin slices at random over the area to be sampled. In general, one composite sample consisting of 12 - 15 cores should be taken for each block of trees. If possible, sample under the predominant variety. (For example: Stuart.) Place samples in a clean plastic bucket or other non-metal container and mix well. Fill the soil sample bag at least 3/4 full. Do not use a galvanized bucket if the soil is to be analyzed for zinc or other micronutrients.
  4. Cores should be pulled within the drip line, not between rows. The area included in one sample should have been uniformly fertilized and limed in the past. When collecting the sample, avoid high or low spots, eroded areas, and areas along roads and fences. Sample problem areas within an orchard separately.

Leaf Sampling Procedures

  1. Obtain plant analysis mailing kit from the county agent’s office. One mailing kit per sample is required.
  2. Sample trees between July 7th and August 7th. (Sampling can be extended into mid-August without affecting the results.)
  3. Collect 100 middle-pair of leaflets from the middle leaf of this year’s growth (see illustration). Use terminal shoots exposed to the sun. Avoid twigs from the interior of the tree. Collect leaflets from all sides of the tree. Avoid leaflets damaged by insects and diseases.
  4. Abnormal trees or trees not representative of the area should be sampled and sent separately. A complete and accurate description of abnormalities should accompany such samples.
  5. Sample trees of the predominant variety in a given block. If Schley is the main variety, sample Schley; if Stuart is the main variety, then sample Stuart, etc.
  6. Immediately upon collection, wipe leaves (entire surface, both top and bottom) with a damp cellulose sponge or cheese cloth to remove dust and spray residue. Do not allow the leaves to come into contact with rubber or galvanized containers. Partially air dry and place in the large envelope of the mailing kit.
  7. Complete the questionnaire obtained in each mailing kit. Place the completed in the smaller envelope together with a check made payable to The University of Georgia to cover any charges and mail it to the Plant Analysis Laboratory.
  8. If recent soil test data is not available, it would be advisable to collect a soil sample and have it sent to the Soil Testing Laboratory.

Poisoning

For information on these hazards and their solutions, see Prevention, Symptoms and Treatment of Poisoning.


Record Keeping and Forms

The scouting techniques described should provide accurate orchard surveys that will allow assessment of the damage potential of insect and disease populations in an orchard. Accurate reporting of survey results is just as important as an accurate orchard survey. Every item on a scouting form (see example) is there to assist the grower in making a management decision concerning pest control. The information on scouting reports will be used to make management decisions that can either save or cost hundreds or thousands of dollars. Incomplete or illegible information on the form increases the chances of an incorrect management decision. A scout must make accurate orchard surveys and provide the grower or manager with an accurate and complete copy of each survey report. The information on the survey form is also used to inform decision makers of developing and impending insect and disease problems and to assist them in the development and improvement of management programs. The scout is the heart of the pest management program.

The data forms that you use should enable efficient collection of pertinent survey information, be easy to understand, and easy to fill out and use. Forms should also enable reliable computerization (if desired) of a representative sample of the survey data so that efficient and timely monitoring of pest populations across the state could occur. Every form should be filled out completely and legibly.

All information entered on the Pecan Survey Report should always be expressed in the appropriate units and/or coded as indicated on the form. If 50 yellow aphids are found on a total of 5 compound leaves on each of the trees surveyed and you put the number 50 in the aphid column rather than the average number of aphids, which was 10, you erroneously told the grower he needs to treat. Be precise and use the correct units to report your findings. It is important to report a zero or somehow designate on the form that you actually checked for the insect, disease or damage rather than leaving the column blank. If a column is blank, it might be assumed that you did not check for that insect, disease or damage! If the column is blank, then no one can be sure of your survey results.

A Pecan Tally Card is also available for your use while surveying an orchard. The front of the card should be used to record your findings as you check each tree. The back of the card contains information about the insect pests and when you are most likely to encounter them.


Other Resources


Originally compiled from

  • “Pecans in Georgia”, Extension Bulletin 609. 1975
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