Pecan

Description

Pecan, Carya illinoinensis, is a large deciduous tree in the family Juglandaceae grown for its edible seeds (nuts). The pecan tree has a thick gray-brown trunk which can reach 2 m (6.6 ft) in diameter and a rounded canopy that spreads . The bark is ridged and has a scaly appearance. The twigs of the tree are red-brown in color and the foliage is dark green. The leaves are pinnately divided and composed of 9–17 oblong-lanceolate leaflets. Each leaflet has as serrated margin and a downward curving tip. The tree produces separate male and female catkins with the male producing pollen and the female developing into the fruit. The male catkin is slender and pendulous, measuring 22–50 cm (8.7–19.7 in) in length and the female catkins are much smaller and possess 3–6 flowers in a cluster. The fruit is an oval–oblong drupe containing a single seed surrounded by a thin shell. The outer husk (shuck) is green, turning brown-black when mature at which point it also splits open to release the nut. The edible kernel of the nut is light brown in color and ridged longitudinally. Pecan trees can reach a height of 40–50 m (131–164 ft) and can live for several hundred years in a wild state. Commercial trees usually have an economic lifespan of approximately 25 years. Pecan originates from the North America, specifically, the South.


Uses

Pecan nuts are consumed fresh or after roasting as a snack food or may be used as an ingredient in baked goods or confection


Propagation


Basic requirements
Pecan trees have a high water requirement and within their native range they are found growing in deep soils where their extensive roots can reach the water table or along river banks or streams. Pecan trees thrive in areas with long hot summers and cool winters. Trees can be grown in a range of soils but will grow optimally in a deep, well draining clay loam or sandy loam with a pH 6.0–7.5. Growing multiple pecan trees requires a great deal of space as the trees can reach a very large size.

Propagation
Pecan trees can be propagated from nuts or by budding or grafting. Nuts for propagation should be collected in the Fall from trees which gave good fill. The seeds require stratification prior to planting. Stratification is the name given to a cold treatment which breaks the dormancy of the seed, speeding the germination process. Nuts are stratified at 2–5°C (35.6–41°F) for a period of 3- to 90 days and then incubated at room temperature. Nuts should be soaked in water for 24 hours prior to the treatment and kept moist throughout by mixing the nuts on moist vermiculite. The nuts can be planted outdoors in late winter if they are planted in place. if growing the seedlings in containers it is best to wait to plant until all threat of frost has passed. Seedling or grafted trees should be planted in full sun to part shade although trees are sensitive to shading and care should be exercised to ensure that the trees are not too shaded. Trees should be spaced 9–10 m (30–35 ft) apart

General care and maintenance
Newly planted trees must be supplied with adequate moisture to meet their growing requirements in the first few years following planting. Trees must be supplied with 10 to 15 gallons of water weekly. This water can come from rainfall, irrigation, or a combination of both. Fertilization rates should ideally be based on the results of a soil test prior to planting but generally, 4 lbs of a complete fertilizer should be applied in a circular area around the base of the tree measuring 25 square feet. Fertilizer should never be placed directly in the planting hole as it can cause damage to the tree roots. A subsequent similar application in the summer should be enough to meet the growth requirements of the tree. The following year, apply 4 lbs of complete fertilizer per inch of tree diameter. Fertilizer should not be placed within 30.5 cm (12 in) of the trees trunk.

Harvesting
Pecans should be harvested as soon as the nuts have reached maturity, when the shuck has lost its green color and has started to split. in commercial plantations, hydraulic shakers may be used to dislodge the nuts from the tree.




References

Anderson, P. C. & Crocker, T. E. The Pecan tree. University of Florida IFAS Extension. Available at: https://edis.ifas.ufl.edu/pdffiles/HS.... [Accessed 26 March 15]. Free to access.

CABI Crop Protection Compendium. (2008). Carya illinoinensis (pecan) datasheet. Available at: http://www.cabi.org/cpc/datasheet/17234. [Accessed 26 March 15]. Paid subscription required.

Carroll, B. Starting pecan trees. Oklahoma Cooperative Extension Service. Available at: http://pods.dasnr.okstate.edu/docusha.... [Accessed 26 March 15]. Free to access.

Polomsky, B. & Shaughnessy, D. (1999). Pecan planting and fertilization. Clemson Cooperative Extension. Available at: http://www.clemson.edu/extension/hgic.... [Accessed 26 March 15]. Free to access.

Teviotdale, B. L., Michailides, T. J. & Pscheidt, J. W. (eds) (2002). Compendium of Nut Crop Diseases in Temperate Zones. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopap.... Available for purchase from APS Press.


Common Pests and Diseases

Bacterial leaf scorch
Xylella fastidiosa

Symptoms
Tan to brown necrotic lesions with distinct dark brown line separating the diseased tissue from healthy; lesions occur on leaf margin or apex; entire leaves becoming necrotic; leaflets may drop from tree; symptoms may be confined to one limb or scattered throughout canopy
Cause
Bacterium
Comments
Vector of pathogen unknown, likely spread by xylem feeding insects
Management
Control of the disease should focus on limiting stress to the trees by providing adequate irrigation, thinning trees and controlling damaging insects

Crown gall
Agrobacterium tumefaciens

Symptoms
Galls of various sizes on roots and root crown below the soil line; galls may occasionally grow on the trunk; galls are initially light colored bulges which grow larger and darken; galls may be soft and spongy or hard; if galling is severe and girdles the trunk then young trees are weakened due to constricted vascular tissue; trees may be stunted and rarely die
Cause
Bacterium
Comments
The bacterium enters host plants through wounds and causes plant cells to proliferate and cells to be undifferentiated, leading to the formation of a gall
Management
Only plant disease-free nursery stock; plant trees in well-draining soils; avoid wounding the plants as much as possible; fresh wounds can be treated with a biocontrol agent (Agrobacterium tumefaciens K84), if available, to prevent the bacterium colonizing

Anthracnose
Colletotrichum gloeosporoides

Symptoms
Shiny, dark brown sunken lesions on green fruit which may coalesce to cover the whole fruit; may be pink colored fungal masses on lesions during wet weather; yield may be reduced
Cause
Fungus
Comments
Fungus overwinters on tree; disease emergence favors warm temperatures and high rainfall
Management
Disease can be controlled by regularly applying appropriate fungicides; mummified fruit and infected branches should be removed to reduce inoculum; avoid cover crops which are an alternative host for the fungus; plant anthracnose resistant varieties

Downy spot
Pseudocercosporella caryigena

Symptoms
Circular, pale yellow spots on lower surface of leaves; spots may be covered with fuzzy white growth during wet; lesions develop through leaves and become visible on upper leaf surface; lesions on upper leaf surface yellow-brown; leaflets may drop from tree; nut yield may be reduced die to defoliation
Cause
Fungus
Comments
Fungus survives in infected leaves from previous season; disease occurs sporadically in Southeastern U.S. and is common in the U.S. state of Texas
Management
Downy spot is controlled by the fungicides that are applied to protect against scab

Powdery mildew
Phyllactinia guttata

Symptoms
Small, powdery white spots on leaves and fruit; spots spread to cover entire leaf; small black fungal fruiting bodies may be visible in the white growth; young leaflets may crinkle as they mature
Cause
Fungus
Comments
Some pecan varieties are susceptible to powdery mildew and disease is not controlled by scab fungicides and plants should be scouted for the disease over the summer months
Management
Trees showing symptoms should be treated with appropriate sulfur-containing fungicides

Scab
Cladosporium caryigenum

Symptoms
Small, rough or velvety olive green to black spots on leaves, green twigs or fruit; lesions may coalesce to form large dark areas with an irregular shape; leaflets may drop from plant if they are infected at the base of the petiole; lesions on young leaves may dry out and crack, resulting in a "shot hole" appearance as the leaves expand; lesions on shucks are virtually identical to those on the leaves ; severe infection of fruit can cause fruit to stop developing
Cause
Fungus
Comments
Fungus overwinters in tissues infected previous year
Management
Scab is usually controlled by a series of fungicide sprays between budbreak and shell hardening

Vein spot
Gnomonia nerviseda

Symptoms
Dark brown to black lesions associated with vascular tissues e.g. along leaf veins or leaf midribs; leaves may fall from trees
Cause
Fungus
Comments
Fungus survives in plant debris on the ground
Management
Fungicide applications made to control scab should also be sufficient to control vein spot; there are some pecan varieties that are less susceptible to vein spot such as Melrose and Cherokee

Zonate leaf spot
Grovesinia pyramidalis

Symptoms
Lesions with concentric rings on leaves which are tan to light brown on the leaf underside and gray-brown on the upper leaf surface; a crystalline substance may be present on the surface of the lesion; infected leaves dry out by late summer and drop from the tree prematurely; defoliation may be severe
Cause
Fungus
Comments
Disease can develop rapidly after wet periods in the summer months and symptoms are worse in low-lying orchards
Management
The disease can be controlled by application of appropriate fungicides

Pecan nut casebearer
Acrobasis nuxvorella

Symptoms
Holes in nutlets at base which usually is associated with black frass that protrudes from the hole; frass and damaged nutlets are held together by silken thread; adult insect is a small, light gray moth; larvae are initially white with a brown head and mature to olive or jade green
Cause
Insect
Comments
Larvae pupate to adult moths within the nut; insect overwinters as larva and may undergo several generations per year with the total number determined by location
Management
The primary method of controlling the pecan nut casebearer is through the application of an appropriate insecticide; insect growth regulators are preferable to organophosphates, carbamates or pyrethroids which disrupt populations of beneficial insects; Bacillus thurengiensis can also be an effective method of controlling the moth larvae but application must be timed well in order to be effective

Black pecan aphid
Melanocallis caryaefoliae

Symptoms
Bright yellow angular spots on leaves between veins; the spots turn brown and, if there are a few present on the leaflet; the leaflet will drop from the tree; premature dropping of leaves causes reduced nut yields; insect is small and soft bodied and ranges in color from various shades of green to black; nymphs are usually lighter in color than the adults
Cause
Insect
Comments
The characteristic stickiness of plants which is usually present during aphid infestation cannot be used as an indicator of black pecan aphid attack as this aphid does not produce honeydew
Management
Trees should be inspected for aphids regularly due to their potential to cause severe damage; an appropriate insecticide which does not harm beneficial insects should be applied to reduce the aphid population

Pecan weevil
Curculio caryae

Symptoms
Adult feeding on nuts prior to nut hardening causes nuts to drop from the tree; adult feeding causes a dark brown stain where the insect's mouthparts puncture the nut shell; larvae feed inside the nuts and can completely destroy the kernels; there are often little outward symptoms of larval feeding; shucks may remain attached to the nut shell; adult insect is a light brown-gray beetle with long snout; larvae are creamy white grubs wit brown heads
Cause
Insect
Comments
Trees should be monitored closely for signs of weevil infestation, particularly in dry years or if trees have a history of weevil infestation
Management
Pecan weevils can be controlled through the use of traps (there are several commercially available traps for pecan weevils) or by wrapping sticky bands around the tree trunk to prevent adult weevils climbing the trees,

Shuck and kernel rot
Phytophthora cactorum

Symptoms
Rotting fruit with rot starting at stem end of fruit; dark brown rot with light brown margin spreading to cover entire fruit; kernels have a dark seed coat and bitter taste
Cause
Oomycete
Comments
Disease outbreaks sporadic but can be very damaging
Management
Appropriate fungicides should be applied at first sign of infection to reduce the severity of the infection; cultural control methods include thinning the canopy to promote good air circulation around the foliage and planting pecan in well-draining soils