Anthracnose, zinc deficiency and rust.

The pecan nuts of a Pawnee tree were not ripe yet; the frost came before the pecan nuts were filled.

Anthracnose:

A tree with Anthracnose is a condition of black or brown spots but zinc deficiency and rust is also a possibility. The cultivars most likely for these deficiencies are: Kiowa, Gloria Grande, Pawnee and Mohawk.

Pecan anthracnose is caused by the ascomycete Glomerella cingulata (Stoneman) Spauld. & H., it is a widespread disease throughout the industry. Pecan Anthracnose has been reported as far back in time as 1914 (Rand 1914), and as far away as Argentina (Mantz, Minhot et al. 2010). Glomerella cingulata has two anamorphs which cause disease on pecan trees, Colletotrichum gloeosporioides and Colletotrichum acutatum (Latham 1995). The occurrence of Colletotrichum on pecans has contributed to a significant decline in pecan production in various years.

An increase in Pecan Anthracnose incidence is highly correlated with heavy rainfall, especially in early spring. Severity increases as the season progresses, the disease will often cause leaf drop in the late fall; fall defoliation is linked to lower yield and nut quality (Brenneman 2010).

The financial loss due to Pecan Anthracnose in 2009 in Georgia was estimated at 3.4 million USD (Brock 2010). However, the actual loss due to the disease is difficult to quantify due to the nature of carbohydrate storage in perennials. The damage to pecan production is also likely to present itself in a lower yield in the year directly following a disease outbreak (Worley 1979).

Pecan Anthracnose is reported to have an unusually long latent period; it can take weeks to months from the time of initial inoculation to symptom development. Both ascospores and conidia can be found in the field and in culture, and both of these spore types can cause infection (Rand 1914). Pink conidial oozes can be observed emerging from acervuli with setae on leaves and shucks.

The Life Cycle of Anthracnose in layman’s terms

Anthracnose (Black Rust) spores are very common and can also be present in the soil where new trees are planted.

There is a high presence of Fungus in existing orchards.

Unicellular condia or ascospores are spread during the summer, during or after a rain storm when conditions are favourable for formation of fungi.
Fungi are A-sexual in the growing season and spread by forming stringy roots that breaks down during a rain storm. They called it condia spores and they are A- sexual or a clone.

Formation of fruiting bodies on leaves occurs during the growing season and fungi cause damage when the soft tissue on the growing points died backwards. This results that the following year’s crop will potentially be as nuts grow on first year wood that grow out of second year wood. Infection is spread when an comes in contact with an adjacent tree.

Some cultivars like Pawnee, Mohawk and Kiowa are more susceptible to the fungus than others. Trees that are more tolerant to the fungus like the Ukulinga, Wichita and Western are also infected when branches comes into contact with infected tree. During the winter infected leaves drop to the ground. The fungi that were A-sexual during the growing season, changed and become sexual.

When two mycelia meet, they swap chromosomes and formed an ascus with a Y shape and looks like a small toadstool. Theseascior fruiting bodies, protect the spores which were formed during the winter regardless the severity of the winter.
Once the growing season started, the fruiting bodies burst and the spores shoot high into the air and spread by hot upward air movement and wind. Some fungi can also survive in the wood of the tree without forming fruiting bodies. There they grow roots (condia spores), which break down when it rains.

The assumption that zinc deficiency are the only reason for the dying growth points, is not entirely correct. There are reported cases were observed under Wichita’s as well. During the next growing season, trees were badly infected and the growing tips died backwards.
To date, many farmers were under the impression that the symptoms of the dying growth points are due to a severe zinc deficiency.

Corrective action:
Spraying of Copper as a fungicide is important.
Spraying low burette urea on trees suppress formation of fungus.
Spraying ordinary Urea on soil, suppresses the formation of asci

Zinc deficiency in pecan nut trees

The main cause is definitely zinc deficiency.
Smaller deficiencies appear as soon as a tree undergoes the stress of zinc deficiency

Nickel deficiency

Some of the trees between three to seven years of age show symptoms of Nickel deficiency on new growth during December months.

Green stinkbug

Introduction:

The green stinkbug is called a green vegetable bug, (Nezara viridula).
The southern green stink bug, Nezara viridula (Linnaeus), is in the order Hemiptera or “true bugs.” Stink bugs are in the family Pentatomidae and adults are recognized by their shield-shape, five-segmented antennae, and their malodorous scent. The southern green stinkbug is a highly polyphagous feeder, attacking many important food crops.
The southern green stink bug is believed to have originated in Ethiopia. Its distribution now includes the tropical and subtropical regions of Europe, Asia, Africa and America.

Eggs have been found as early as the second week of April and as late as December 12th. The eggs are deposited in masses that range from 30 to 130 eggs per mass. The female ovi posits on the undersurface of leaves in the upper portions of canopied crops and weeds. Weeds that are favored by the southern green stink bug include rattlebox, Mexican clover, wild blackberry and nut grass. The eggs are firmly glued together and to the substrate. The eggs are white to light yellow in color and barrel shaped with tops that are flat with a disc shaped lid. There are 28 to 32 finger-like projections around the lid called chorial processes. The egg is 1/20 of an inch in length and 1/29 inch wide. The incubation time for the eggs is five days in the summer and two to three weeks in early spring and late fall. As incubation continues the eggs turn pinkish in color.

The nymph hatches from the egg by opening the disc shaped cap. The nymph slowly wiggles out of the shell. Each hatchling takes five to six minutes to escape from the egg, and the entire pod hatches in 1.5 hours. The first instars aggregate by the empty eggs and do not feed. The possible benefits of aggregation are to deter predation from the pooling of their chemical defenses. The nymphs are light yellowish in color with red eyes and transparent legs and antennae. The time until the next molt is three days. Feeding begins with the second in star. The second in star has black legs, head, thorax, and antennae. The abdomen is red and so are the spaces between the second, third, and fourth antennal segments. The thorax has a yellow spot on each outer side. The length of time spent in the second in star is five days. The third and fourth instars differ from the second in size and an overall greenish color becoming apparent. The length of each of these instars is seven days. Wing pads mark the arrival at the fifth in star. The abdomen is yellowish green with red spots on the median line. The southern green stink bug usually spends eight days as a fifth in star before the final molt to an adult.

The southern green stink bug has piercing-sucking mouthparts. The mouth consists of a long beak-like structure called the rostrum. Salivary fluid is pumped down the salivary duct and liquefied food is pumped up the food canal. All plant parts are likely to be fed upon, but growing shoots and developing fruit are preferred. Attached shoots usually wither, or in extreme cases may die. The damage on fruit from the punctures is hard brownish or black spots. These punctures affect the fruit’s edible qualities and decidedly lower its market value. Young fruit growth is retarded and it often withers and drops from the plant. In addition to the visual damage caused by southern green stink bug feeding, the mechanical transmission of tomato bacterial spot may also result.
Research confirmed that damage to pecan nuts before the shell hardens, causes the nuts to fall from the tree.
This damage causes black spots on the pecan nuts, if the shell is already hard by late November.

To get rid of the bugs you have to do it while it is still in the nymph stadium.
There are registered solutions on other cultivars in South Africa.
The use of trap crops is not a widely accepted idea for control of the southern green stink bug, but it has excellent potential as a type of control. The choice for trap crops in the summer would be leguminous plants such as cowpeas and beans. In the late fall and early spring cruciferous plants are recommended. The trap crop should be sprayed or plowed under before the developing southern green stink bugs become adults to prevent them from migrating to the main crop. Insecticides are commonly applied at blossom and fruit formation.

Diseases and pests.

Diseases and insects are often the limiting factors in the production of pecans. The crop requires about 7 months developing and during that time is subject to attack by a variety of pests. A good disease and insect control program is important, not only in protecting the maturing crop, but it is essential to the production of high yields year after year. Trees, which are prematurely defoliated by insects, mites, and diseases, frequently produce nuts of low quality and set a light crop the following year. Pecan varieties differ in resistance to scab and other diseases. Scab susceptible varieties will require more fungicide sprays than resistant varieties. Check with your county extension director to determine the most suitable varieties for local conditions. Although today’s pecan grower is fortunate to have effective fungicides, insecticides, and improved spray equipment available to him, spraying for pest control is not an easy job. It requires attention to many details that on the surface may seem unimportant. Even the best spray program can be improved if the following cultural and sanitary practices are followed:
1. Provide better air circulation in orchard; mow or disc weeds, and prune low hanging limbs. Keeping the tree row weed-free by use of herbicides is recommended.
2. Maintain tree vigor by following recommended fertility practices.
Perhaps the three most important factors in a spray program are timing, coverage and rate (discussed in subsequent sections).

Timing of Spraying

Some insects occur in orchards at particular and somewhat predictable times. Timing of sprays for control of these insects is very important and each grower should learn to recognize the vulnerable stages of these insects and time sprays accordingly. Remember that about 75% of nuts abort naturally from the tree. If pests such as phylloxera occurred last year, control may be necessary this year. Apply control for leaf phylloxera when the new growth is about 1/2 inch in length. Nut case bearer sprays should be timed to control first generation larvae before they enter the nuts. Use the new pheromone traps to monitor for adult nut case bearers emergence and population density. Nut case bearer populations differ from orchard to orchard and year to year. Scout for eggs on the small nuts after pollination. Controls should be applied 14 – 21 days after first male is captured in the traps.
The pecan weevil can be a serious pest of pecans. The adults emerge from the soil beginning in July and can continue emerge into November. Adults feed on the nuts until the shell begins to harden and then they begin laying eggs. Feeding prior to shell hardening prevents nut development and causes the nut to drop.
Growers should monitor for adult weevils starting about July 15 using the circle traps. Contact your county agent for more information. Home owners can also use these traps to suppress damage from pecan weevils.
Spray when adult weevils are found and continue at 7 to 10 day intervals during weevil emergence. Spraying need not start until the shell begins to harden, if the early population does not appear large enough to cause serious nut drop. Drought periods will delay weevil emergence.
Pecan aphids have two population peaks: one in May – June and a larger one in August – October. Winter cover crops of hairy vetch with crimson clover planted in row middles are recommended to augment beneficial. In most years beneficial will build up in the cover crops in February – April and then suppress the May – June aphid populations in pecan. A newly introduced lady beetle, Harmonia axyridis, is an important biological control of pecan aphids. It is suggested that pyrethroids not be used alone or in combination in early or mid season and further do not use more than 2 applications per season.
Excessive use of insecticides should be avoided as it may create insect problems that otherwise would not exist, as well as adding to environmental pollution.

Coverage

In applying spray materials, all leaves, twigs, and nuts should be covered. Hydraulic machines are designed to use large volumes of water to carry the chemicals to the trees. Sprays should be applied until water runs off leaves in the upper portion of the tree. Do not attempt to concentrate or use low volume sprays with hydraulic equipment.

Air blast and mist blowers are designed for low volume applications. Air blast equipment should be adjusted before the spray season begins to deliver the desired volume of spray in the proper pattern as is dictated by tree size. Consult instruction manuals or spray machine representative for advice on correct placement of spray nozzles.

Remember, when spraying with air blast sprayers, the pesticide is carried to the tree in a small volume of water which is diluted by a larger volume of air. Too fast a rate of travel will result in insufficient coverage where the trees are not filled with spray-laden air; thus poor coverage results.

Fig tree borer

Order: Coleoptera

Biology

Family: Cerambycidae Genus: Phryneta Species: spinator (Fabricius,1792)

  • Indigenous beetle
  • 35mm in length has become a serious pest of fig or pecan trees
  • The larvae bore into the wood and in severe cases kill the tree, particularly if it is old and lacking in fig or pecan trees.
  • The large size, long antennae, mottled appearance and the prominent spines on each side of the thorax.
  • The life cycle takes three years!

Larvae

  • Each egg is laid in a T-shaped slit that
    the female makes near the base
    of a branch.
  • Ovi position from mid-November through
    to mid- March.
  • Eggs take 10-18 days to develop.
  • The hatched larva initially feeds on the bark, later enters the wood and chews its way through the wood, forming tunnels.
  • This diet of wood is not particularly nutritious.
  • Larvae are cannibalistic.
  • The larva packs excrement (‘frass’) and castings behind it a sit burrows and pushes some of them through small holes in the bark so that one can often find small heaps of wood fragments at the base of the tree.
  • Larval development takes from 2.7-2.8 years.

Pupa

  • When fully grown, the larvae tunnel to just beneath the bark where they molt into a pupa.
  • By pupating under the bark, they can make an easy exit when they emerge as adults.
  • The pupa is enclosed in a chamber consisting of calcium carbonate from the Malpighiantubules together with silk and gum.
  • The tunnel leading from pupation chamber is blocked with frass and wood chips. The papal development period lasts 89-99 days.
  • The adult form develops inside the pupa and on maturity emerges.
  • The female then needs to mate with a male and find suitable sites on host plants for laying her eggs.

Damage

  • So adults have been found to damage the following plants:
  • Cupressaceae: Cupressus lusitanica and C. horizontalis (feed on bark).
  • Meliaceae: Syringa Melia azedarach (feed on bark).
  • Moraceae: fig trees Ficus spp. (feed on bark and eat rind of unripe figs).
  • Rosaceae: Pyrus spp. (e.g. apples and pears), Prunus spp. (e.g. apricot, nectarine, peach, plum) (feed on bark).
  • Salicaceae: Willow Salix mucronata (= S. capensis). Other Salix species (e.g. Weeping willow Salix babylonica)? (Feed on bark.)
  • Vitaceae: Grapevine Vitis vinifera (feed on bark).
  • Feeding by larvae. Larvae bore into wood of the following species of trees.
  • Rosaceae: Occasionally feeds in wood of Pear Pyrus communis and Peach Prunus sp.
  • Salicaceae: The native host plant is the Willow Salix mucronata (= S. capensis) but it also attacks the introduced Weeping willow Salix babylonica.
  • Moraceae: Domestic fig Ficus carica. It is unclear whether it attacks native Ficus species or

Control

  • Locate the tunnels.
  • After the larva has been located it can be destroyed either by inserting a strong flexible piece of wire into the burrow.
  • If the larva has burrowed deeply into parts below the surface of the soil, it is more difficult to destroy it.
  • For this purpose, the wire should be thrust deeply into the burrow until it comes into contact with the larva. The slightest touch of the wire is sufficient for its destruction.”
  • An alternative to his method of killing the larvae would be to locate the tunnels (as he advises) and then spray an insecticide with fumigant properties down the hole..
  • After the larvae have been destroyed, any damage you made to the tree should be painted over with a pruning sealant.
beta – cyfluthrin
EC
50 g/l
2 Bulldock
4162
Bayer
pecan nuts
pecan stem borer, bark borer
beta – cyfluthrin
SC
125 g/l
Bulldock
2125SC
7612
Bayer
(Pty) Ltd
pecan
nuts
pecan stem borer, bark borer
carbaryl
SC
480 g/l
Sevin
SLR
2Plus
5783
Bayer
(Pty) Ltd
pecan
nuts
stem & bark borer
carbaryl
SC
480 g/l
2 Karbasol
6072
Efekto
pecan
nuts
stem borer, bark borer
carbaryl
WP
850 g.kg
Sevin
2850 WP
1450
Bayer
(Pty) Ltd
pecan
nuts
stem borer, bark borer

Scab

Scab is the most important disease in pecan nuts in South Africa, and is caused by a fungus.

Early symptoms would be the appearance of many small, brown to black spots, occurring especially on the underside of the leaves. The spots progressively become larger and merge with each other until the whole leaf turns black. Immature leaves drop off.

Similar spots are also visible on the shuck of the nut. These nuts suffer from delayed development and become misshapen. Immature nuts may drop off and will have no commercial value.

The fungus winters on branches and old shucks that have dropped. Fungal spores develop rapidly in the spring and spread by wind and rain. New spring growth on the trees become infected when the leaf surfaces are wet, especially after rain.

Susceptibility for the disease will vary in different cultivars. Ukulinga, Shoshoni, Moore and Barton are regarded as highly tolerant, while Mohawk, Wichita and Chocktaw are susceptible.

Pests

Bark borer

        • Larvae of bark borer feed on the living bark of pecan-nut trees, especially in young trees. They later bore into the hard wood.
        • Penetration is usually where branches are formed and can occur in branches of any thickness.
        • The holes in the branches serve as shelter for the larvae, and when the larvae reach maximum size, are about 70 mm long and 5 mm in diameter.
        • Feeding marks on the bark are covered with a “House” of larval excreta spun together with threads in such a way that the larvae are able to move freely underneath the threads. As the larva feeds, this “House” becomes bigger and can be found around a branch. Although infested trees do not die, the branch is ring-barked and it could die back.

Control:

        • It is possible to achieve good control can be with a registered chemical, even if only the lesions on the branches are treated.
        • Its not necessary to remove the excreta from the branches before spraying.
        • It is not recommended to spray the whole tree.

Pecan weevil (Curculio caryae )

The weevil damages the nut both in the orchards and storage. The adults feed on the nuts in late July and August, puncturing the nuts and laying eggs at around the time when kernels start filling up. The grubs then feed on kernels. The nature of damage caused by weevil may be premature nut drop, black spots on the kernel, shrivelled kernels and /or the destruction of the kernel by the larvae.

Control:

Apply five foliar applications of Carbaryl at 2.7 kg a.i./ha at a of 7-10 day interval. Apply the first spray as soon as the nuts begin to harden in the early cultivar.

Hickory shuckworm (Laspeyresia caryana)

This moth is a serious pecan pest. It lays its eggs on foliage and green nuts. The caterpillars then attack the immature nuts which fall to the ground prematurely. The caterpillars the bore into the shell when it becomes hard, thus hampering the kernel development.

Control:

All the affected nuts must be collected and destroyed.

Pecan nut case bearer (Acrobasis caryae)

This pest causes great damage. The larvae start feeding on the buds which begin growth in spring. They later bore into new shoots and pupate there. The moth emerges in April-May and lays greenish white eggs on the blossom end of developing nuts.

Control:

An effective method in controlling the pest is spraying Parathion and Malathion. This should be done when about 10% moths have emerged after pupation or when the tips of small nuts have become brown.

Pecan leaf case bearer (Acrobasis juglandis)

The caterpillars, found closed in leaf case, feed on buds, leaves and flowers and later enter in the shoot. The moth lays eggs on the underside of a leaf along mid-vein. The main sign of their presence would be observing broken shoots.

Control:

The pest can be controlled by pre-pollination spray of either Parathion or Malathion.

Aphids

Many aphids cause injury to pecans. They feed on both sides of leaves and cause large yellow blotches which turn brown and cause premature leaf fall.

Control:

One application of Aldicarb @ 28 kg a.i./ha during spring season results in rapid control of aphids.

Armored scales

These scales commonly attack the nuts. They suck up the plant sap and devitalize it. The main sign of their attack is the presence of dead bark and twigs.

Control:

One spray of systemic insecticides such as Malathion, Metasystox, etc. in early spring is sufficient to kill the pests.

Fall web worm (Hyphantria cunea)

A loose silken web which covers the leaves, twigs and small branches is made during summer. The caterpillars then attack limbs and can cause defoliation. The insect damages a number of fruits. The insecticidal sprays used for shuck worm are known to eradicate this pest as well.

Mites (Tetranychus hicoriae)

Mites are able to cause serious injury to foliage. They mainly feed on the lower side of leaflets, along the midrib forming a brown discolouration. This covers the entire leaflets. The most effective chemicals used to control mites would be sulphar and Cyhexatin. Insecticides such as Aldicarb can also be used to control the mites for the entire season.

Serpentine leaf miner (Stigmeta juglandifoliella)

The leaf miner feeds between the lower and upper surfaces of leaflets and creates characteristic designs which causes defoliation. It can be controlled by the application of Dimethoate, Diflubenzuron or Fenvalerate.

Birds

Crows and blue jays are known to cause huge losses to pecans. Crows damage the nuts right after kernel filling either on the tree, or by carrying them away. The damage is more serious to small, well filled and thin shelled nuts. To scare away the crows, a dead crow should be hanged high on the top of a pecan tree.

Diseases

Pecan scab (Cladosporium coryigenwn)

This is one of the most serious fungal diseases to the pecan. It primarily attacks the growing leaves, shoots and fruits. The damage is accentuated with rains during spring and early summer. The symptoms are irregular, light green to black spots on leaves or young shoots and small dark brown to black circular lesions on the nut.

Control

        • Burn all of the diseased leaves, twigs and nuts.
        • Spray fungicides such as Bordeaux mixture, Zineb, Cyprex and Benlate in middle of April and again at a 3 week interval.

Leaf blotch (Mycospharella dendroides)

The disease is only serious on neglected, declining trees and nursery plants. The symptoms would be the appearance of live green, velvety tufts on the under surface in early summer and yellow spots on the upper surface of leaflets in late season. Severe infection may fall on on the leaves . The black pimple like structures give the leaflets a black shiny blotched look in the mid summer. The scab control measures are also applicable to this disease.

Vein spot (Gnomonia nerviseda)

This is a foliage disease and has been found to be more serious on several cultivars in some regions. The fungus attacks the vascular tissue at the junction of petiole to the rachis and base of rachis, causing premature leaf fall.

Control:

Benomyl is the most effective fungicide against vein spots.

Brown leaf spot

This disease is found mostly in humid regions. Circular, reddish brown spots occur on the underside of mature leaves in June and July. These spots later attain an irregular shape, and upon sever infection leaves may fall.

Control:

Repeated sprays of Bordeaux mixture, Zineb and Dodine are rather effective in preventing the spots.

Crown gall (Agrobacterium tumefaciens)

This is a bacterial disease that occurs worldwide in distribution. Tumors or wort like growths develop on the collar and adjoining roots of the tree, and often protrude out of the soil. This disease kills nursery plants and mature trees.

Control:

        • All diseased small plants must be destroyed.
        • The injury to plants by cultivation must be avoided.
        • The affected tissues can be painted with an Elgetol-Wood Alcohol mixture after removing the gall.

Bunch disease

This disease is caused by a virus that is transmitted by insects that suck. Once characteristic symptom of bunch disease would be the bushy growth of slender shoots. The branches of a plant that is severely affected gets clustered with thick bloomy sucker growth.

Control:

  • The affected shoots must be pruned and burnt.
  • You can grow resistant cultivars such as Stuart.
  • Diseased wild plants growing in the area around and within the orchard must be removed.
  • Do not grow highly susceptible cultivars in bunch disease prone regions.

Parasitic plants in pecan-nut trees

Parasitic plants, Tapinanthus spp (bird-lime), occur in most of the pecan-nut producing areas within South Africa. These plants have don’t have a root system and parasites the host plant. In doing this, they debilitate the tree and reduce the bearing area.

The plants are easily seen in the tops of pecan trees by their red and yellow flowers, especially during winter and the month of September.

Control:

Unfortunately the only way to get rid of these parasitic plants is to prune them, as there is no chemical control method. The branch on which the bird-lime grows must be cut off and removed from the orchard.

Cold damage

Cold damage mainly occurs in trees that grew well. The strong sap flow resulted that the tree did not accumulated sufficient reserves in time. In blocks where such cases occurred, only large trees have died but smaller trees, that grew less vigorous start budding again after winter and show no damage.

Pecan Disorders (1):

Abstract
Pecan (Carya i1/;no;nens;s), as do all crops, exhibits certain maladies when under cultivation. Among the most prominent of these are a) alternate bearing, b) nutrient deficiencies, c) fruit-drop, d) pre-harvest sprouting, and e) canopy parasites. These are briefly discussed below, and will be discussed in greater detail during the oral presentation.

Alternate Bearing:
The alternate bearing problem is exhibited to some degree by all cultivars and is accentuated by weather related stresses (e.g., drought, late spring or early autumn freezes, excessive cloud cover). It is typically the most important biological problem faced by pecan farmers. Natural selection operating over evolutionary time has produced pecan as an economically important species that exhibits pronounced biennial-like alternations in seed production as a strategy for ensuring long-term reproductive success. This year-to-year variation in flowering, and subsequent crop-load, is termed alternate bearing (i.e., AB). While AB linked variation in flowering likely increases individual fitness in natural habitats, it is also a major impediment to greater horticultural domestication, and is the primary biology-based impediment to horticultural enterprises. Excessive year-to-year variability in flowering limits tree and orchard profitability; thus, adversely affecting producers, processors and consumers via instabilities in nutmeat supply, quality and price. While the specific processes regulating AB in pecan remain ambiguous, the trait tightly links to floral initiation processes occurring within bud meristems within the tree’s canopy.

Horticultural manipulation of flowering and mitigation of AB in commercial pecan orchards currently targets minimization of tree stress, with orchard management strategies directly or indirectly targeting key exogenous biotic and a-biotic stressors. These include sunlight, nutrient elements, and water as essential resources; and pathogens, arthropods, and weeds as potentially harmful pests. Crop-load thinning prior to, or at the time of, inception of kernel (i.e., primarily cotyledon) filling of developing seeds also acts to moderate AB by increasing subsequent year flowering. This fruit/seed association implicates one or more seed-associated phloem mobile phyto-hormones in regulation of floral initiation (i.e., the production of meristems of clearly recognizable flower primordia, and includes all preceding reactions that are required if flowers are to be initiated).

Regulation of floral initiation in trees depends on processing of environmental and/or endogenous cues, with initiation in most large-seeded temperate woody perennial angiosperms being primarily controlled by endogenous cues consistent with processing via an autonomous flowering pathway involving phyto-hormones. Floral initiation in pecan is therefore likely to involve an autonomous flowering pathway as a key step in its floral initiation process. As with many other tree-fruit species, florally induced bud primordia on heavy crop-load trees (i.e., “on” year of alternate bearing cycle) are likely exposed to different phyto-hormonal environments than are primordia of induced buds on light crop-load trees (i.e., “off” year of alternate bearing cycle). This raises the possibility that timely application of phyto-hormones or bioregulators to tree canopies might alter the phyto-hormonal environment of primordia in such a way as to enable control of flowering by pecan farmers, but no such protocol has yet been developed.

The efficacy and horticultural potential of bioregulators to control the “on” and “off” flowering phases of pecan trees has not been developed despite considerable circumstantial evidence that endogenous phyto-hormones are involved in floral initiation processes. A variety of natural and synthetic bioregulators are efficacious for control of floral initiation processes in several polycarpic perennial crops, as well as for pecan, and involve timely usage of floral promoters [generally ethephon and prohexadione-Ca (P-Ca); and naphthaleneacetic acid (NAA) or gibberellin A4 (GA4) in certain situations in “on ” years to promote return flowering the following “off” year, and usage of floral inhibitors [gibberellic acids (GA3, 4, 7); and auxin analogues (e.g., NAA), in certain cases] in “off” years to decrease subsequent year flowering. Commercial pecan production enterprises need better horticultural tools for managing flowering and AB. Successful development and exploitation of such tools depends on acquiring better understanding of floral initiation processes operating in pecan. It appears that pistillate flower initiation in pecan involves three distinct phases of chromatin (i.e., DNA, RNA, and affiliated proteins) modification before new flowers appear in early spring.

It appears that in pecan there are three sequential phases of chromatin modification controlling flower initiation, beginning with a) a foliage produced phloem translocated florigen acting as a first level-signal to initiate phase-one chromatin modifying inductive processes in young bud primordia; b) then phase-two chromatin modification regulated by translocated phyto-hormones, from foliage and/or fruit, acting in the primordia environment during early post-induction as a “cytokinin-gibberellin ratio” based second-level-signal subject to modulation by auxin and ethylene, and c) an finally phase-three chromatin modification regulated by concentration of one or more non-structural carbohydrates (e.g. sucrose) acting in the primordia environment during vernalization as a third-level-signal enabling floral development in preparation for anthesis. Circumstantial evidence indicates that the above described process is very much influenced by assimilate and nutrient stress, with special emphasis on certain forms of nitrogen and also certain forms of carbohydrates held in storage pools.

From the practical standpoint of the pecan farmer the best that at present that can be done to control alternate bearing is to manage to minimize tree stress and excessive cropping. Trees need plenty of water, sunlight, and mineral nutrients. Canopies should be maintained in good health and longevity. Trees should be managed to favour fruiting, with excessive fruit being managed directly via mechanical fruit thinning (via trunk shaker in midsummer at time of the gel stage of kernel development) or indirectly via either selective limb or by mechanized hedge pruning (usually a 2,3 or 4 year cycle). Fertilization of trees with nitrogen forms that favour increased endogenous availability of ammonium (compared to nitrate) also increases the tendency to fruit. Large one-time application of fertilizer-N, regardless of N-form, typically results in rapid conversion to nitrate-N in the soil (after 2-3 weeks) and then luxury consumption of nitrate by the tree, which then favours vegetative over reproductive growth. Trees fruit best when exposed to small amounts of ammonium-N over the growing season.

Both ammonium sulphate and urea are sources of ammonium-No Urea converts to ammonium in the soil, if soil applied, and if foliar applied, it is easily absorbed by foliage and converts to ammonium-N within the plant if plant nickel nutrition is sufficient. So, from the above, growers can substantially manage alternate bearing in pecan trees if they manage all stressing factors simultaneously. Focusing on one stressor (e.g., water) while ignoring others (e.g., nutrient deficiency, shading, wrong N form, defoliating pests, mechanical crop-load thinning, etc.) will result in disappointment.

Preharvest Germination -Pecan exhibits a malady commonly termed either ‘vivipary’ or ‘preharvest sprouting’, in which nuts sprout while still on the tree. Vivipary is the premature sprouting emergence of a visible hypocotyls into or through the ovary, or shell, wall of the nut-of seed while still on the plant, ‘before’ dispersal.

Viviparous seed do not fully undergo the degree of internal desiccation, organellar dedifferentiation, membrane stabilization, and metabolic quiescence exhibited by non-viviparous seed; thus, viviparous seed have not fully completed the ‘maturation’ phase of seed development. Similarly, preharvest sprouting, is the germination of physiologically ‘mature’ seed on the parent plant when the environment is very humid. In pecan, germinating seed have not attained full physiological maturity prior to sprouting, nor is sprouting tightly linked solely to a humid environment; thus, the malady is most accurately described as ‘vivipary’.

Viviparous nuts quickly lose nut quality and marketability due to the germination process, which breaks down nutmeat tissues, especially around the embryo (i.e., the point where the two kernel sections, or cotyledons, are attached).

Pecan Disorders (2):

This breakdown blackens the embryonic region and is commonly termed ’embryo rot’ when germination processes have only minimally progressed. While many orchards rarely exhibit the malady, others irregularly exhibit vivipary related crop loss> 70%; thus, vivipary can be a major profit limiting problem. The economic impact of vivipary in North America is usually most severe in the lower San Joaquin Valley of California, lower elevations in Arizona, portions of the mid to lower Rio Grande Valley of Texas, and lower elevation arid regions of northern Mexico. It also occurs on a relatively intermittent basis in many orchards at other geographic locations. It occurs in most pecan cultivars if conditions are right, but there is genetic variability in expression of vivipary. It is especially common in the ‘Wichita’, ‘Western Schley’, ‘Burkett’ , ‘Mahan’, ‘Cheyenne’, ‘GraKing’, ‘Shawnee’, ‘Choctaw’, ‘GraTex’, ‘Oconee’, and ‘Pawnee’, but relatively rare in ‘Sioux’, ‘Caddo’ and ‘Squirrel’s Delight’.

The incidence of vivipary in pecan increases with crop-load, length of growing season, and duration of time nuts are in trees before harvesting. Observations indicate that vivipary is also influenced by night temperature, irrigation, soil characteristics, light conditions, and black margined pecan aphid (Monellia caryella). It can also be influenced by pollen source, with less vivipary occurring when southern adapted cultivars are fertilization by northern adapted genotypes. Anecdotal observations indicate that vivipary is a) closely associated with high temperature during the late stages of kernel filling (yet high temperatures alone do not necessarily lead to vivipary), and b) delayed shuck opening or splitting (yet vivipary but does not always occur under such conditions). Factors delaying shuck ripening or splitting often contribute to greater incidence and severity of vivipary if temperature is also relatively high. Nitrate within plants can modulate metabolism, growth and. Such signalling is involved in breaking of seed dormancy. Additionally, tissue moisture content typically remains high throughout ontogeny for viviparous seed. Because metabolic water is critical to embryo metabolism and development, its availability, and factors contributing to availability, favour vivipary. A combination of high N03-, high seed moisture, warm night temperatures, and low ABA concentration, appear to be an especially powerful trigger for vivipary.

Contemporary pecan orchard practices to minimize vivipary usually adopt one of more of the following:

  • ensuring that soil moisture levels are near field capacity during kernel filling (i.e. usually September and October for most cultivars and locations in the U.S.; although, it should be noted that waterlogged soils can actually induce vivipary);
  • using Temik (Aldicarb; 2-methyl-2-(methylthio)-proprionaldehyde-0-(methylcarbamoyl)-oxime) to advance early-ripening;
  • avoiding excessive crop-loads by either timely mechanical thinning or by indirect fruit thinning via hedge pruning; and
  • early harvesting using mechanical shakers

Field research on ‘Cheyenne’ pecan assessed the possibility of controlling vivipary via supplemental irrigation and nitrogen (N) management. The incidence of vivipary on irrigated trees was 25-fold greater than that on non irrigated trees, and incidence on trees receiving excessive N fertilization was 21-fold greater than that of trees not receiving N. There was a strong ‘Irrigation x N’ interaction, with trees receiving ‘Irrigation + N’ exhibiting 41 -fold more vivipary than did ‘Non irrigated -N’ trees. The incidence of vivipary in orchards therefore appears amenable to management via an appropriate irrigation and N fertilization strategy. It is also likely that insufficient Mo and Cu contribute to vivipary in that these two metals are key to the production of abscisic acid, a growth regulator that seeds produce to inhibit germination.

Fruit-Split -Water stage fruit-split (WSFS) of pecan is often a major problem exhibited by thin-shelled cultivars (e.g., Schley, Oconee, Sumner, ‘Wichita’, ‘Frotcher’, and ‘Farley’) and, to a lesser degree, by certain relatively thick-shelled cultivars (e.g., ‘Cape Fear’ and ‘Elliott). The WSFS malady is highly erratic, with incidence and severity varying depending on cultivar, location, and year.

Crop loss can be severe in certain years and nearly absent in others. In pecan, WSFS occurs during the “late water stage”; a time when turgor pressure of the liquid endosperm is high and the shell is beginning to become rigid with lignin deposition. This typically occurs during mid-August for susceptible cultivars growing in the South Eastern U.S.

The malady is typically associated with rainfall occurring at the initiation of shell hardening. There are two episodes of WSFS for ‘Wichita’ pecan, the major episode being triggered by increased water availability due to rainfall (or potentially irrigation) and potentially a relatively minor event triggered by “high humidity/low light”. They also noted that a variety of cultural (e.g., irrigation scheduling and amounts), environmental factors (e.g., rainfall, relative humidity, sunlight/shading, soil characteristics), and tree characteristics (crop load and distribution, nutrient element status) potentially influence WSFS.

The endosperm of pecan fruit develops from the central cell of the megagametophyte after fertilization by the second nuclear sperm cell migrating from the pollen grain. This triploid nuclear endosperm then passes through coenocytic, cellularization, and differentiation stages, terminating in the maturity stage comprised of starchy endosperm cells, aleuron cells, transfer cells, and cells of the embryo surrounding region. This triploid nucleus is located in the proximal zone of a cytoplasm that encompasses a large central vacuole {i.e., endosperm coenocytes} that is filled with a pressurized aqueous solution comprised primarily of elemental ions and sugars. Turgor pressure generated by solutes forces the testa into the void generated as a result of ovary wall expansion as the fruit grows.

This solution filled central vacuole is present from soon after fertilization until about the time of shell hardening, when acceleration of centripetal growth of alveolus cell layers begins to completely fill the central vacuole to form the cotyledons, but is not particularly noticeable until within a couple weeks prior to the initiation of shell hardening. The initiation of endosperm cellularization within the endosperm coenocytes results with formation of radial microtubular systems attached to the surface of the endosperm nuclei, with further developmental processes leading to deposition of cell layer files {i.e., alveoli} via centripetal growth until the central vacuole is completely filled.

It is usually about the time of the formation of the radial microtubular system to deposition of a couple layers of alveroli that WSFS occurs {BWW, personal observation}. Control of turgor pressure exerted by the central vacuole of the nuclear endosperm against the various fruit tissues, is at least partially via movement of potassium (K +) in and out of the endosperm solution. The movement of K+ is potentially influencable by several factors, one apparently being the availability of B within cellular membranes. Thus, inadequate B within certain cells and tissues of the developing fruit potentially affects WSFS via its effect on rapid influx or efflux of K+ as the balance of availability of water between foliage and fruit varies according to rainfall, irrigations, and atmospheric conditions-i.e., potentially limiting the rate of K+ movement and rapid osmotic adjustment of the liquid endosperm solution that generates the turgor pressure that potentially ruptures fruit tissues.

A common aspect of B deficient plants is brittle cell walls, with supra-optimal B concentrations enabling greatly enhanced elasticity. Foliar applied B appears to be highly mobile in some tree species. Thus, timely application of foliar B may potentially influence cell wall elasticity of fruit tissues. Other potential nutritional factors affecting WSFS include the essential trace micronutrients that link either indirectly or directly to lignifications (Mn, Fe, In, Cu and Ni). Because xylem connections to developing fruit either are absent or poorly formed, their availability to the developing seed is primarily dependent on phloem transport.

Certain micronutrients are only variably mobilized in the phloem (Fe, In, Cu, Mo, Ni, and Co) whereas others are only conditionally mobile (Co, B, and Mn). This raises the possibility that developing fruit may be deficient in one or more lignification-associated micronutrients about the time of endosperm cellularization and associated lignification of the ovary wall; hence, interfering with the normal timely deposition of lignin.

Nematodes on Pecans

Substantial dieback of branches in the upper canopy. Examination revealed the presence of many small galls and egg masses on feeder roots—with female nematodes often protruding from root tissue. The worms were identified as root-knot nematodes, Meloidogyne partityla. Host range tests conducted in Texas, revealed that it prefers to feed on hickory, pecan, and walnut trees. Although M. partityla is not likely to kill the pecan trees, it will debilitate them and lower their productivity to a point where the orchard may no longer be profitable.

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