Walnut

Description

Walnut, Juglans regia, is a deciduous tree in the Family Juglandaceae grown for its edible seeds. Walnut trees are large and vigorous with a wide-spreading canopy. The trunk of the tree can reach 2 m (6.6 ft) in diameter and mature trees possess smooth, silver-gray bark. Walnut leaves are composed of an odd number of smaller, oval shaped leaflets which are bright green in color. The tree produces male flowers on catkins and female flowers on terminal clusters where the fruit develops. The fruit of the walnut tree is a fleshy green drupe in which the nut is encased. The kernel of the nut is protected by a corrugated woody shell. Walnut trees can reach 25–35 m (82–115 ft) in height and can live for periods in excess of 200 years. Walnut may also be referred to as Persian walnut, English walnut, common walnut or European walnut and originates from central Asia.


Uses

Walnut kernels can be eaten fresh or processed for use in baked goods and confectionery. The timber of the tree is a highly valued as a wood for furniture making in Europe. Kernels may also be used as an animal feed or used to extract oil which was once a popular thinner for oil based paints.


Propagation


Basic requirements
Walnut can be grown successfully in a wide variety of climates but blossoms can be damaged by frost or high humidity. Must walnut varieties have a winter chilling requirement of 800–1000 hours below 7°C (45°F) to break dormancy and require a growing season of at least 200 frost-free days. Walnut trees should be planted in areas where there is good air circulation and drainage and will grow optimally when planted in a deep, fertile, well-draining loam soil with a pH between 5.5 and 6.5. Walnut roots systems can be extensive and It is not unusual for walnut roots to grow to depths of 4.5 m (15 ft). Water-logged soils will limit root growth and should be avoided. Walnuts will usually begin to produce in the fourth or fifth year following planting.

Propagation
Walnut trees are usually propagated vegetatively by budding or grafting, while saplings for rootstock are grown from seed. The planting site should be prepared by levelling the ground and eliminating weeds. Trees are planted by digging a hole that is twice as wide as the existing root ball. Aged manure or compost may be added to the soil at time of planting by incorporating it into the backfill soil. Trees should be spaced 10–15 m (33–50 ft) apart. In commercial plantations, trees may be spaced closer together. Grafted trees will begin to bear fruit in 5 years, while non-grafted plants may take 7-8 years. Newly planted young trees require staking as the new wood is soft and the tree can be damaged by winds.

General care and maintenance
Walnut trees should be trained to a central leader or modified leader system. Trees should be pruned in the Fall to prevent sap bleeding. Once the desired shape has been achieved, annual pruning should focus on thinning the tops of the trees to allow better air and light penetration to the canopy. In commercial orchards, lower limbs may be removed to facilitate the movement of machinery through the orchard. Newly planted walnut trees should be provided with supplemental irrigation for the first two years following planting. Trees should be watered deeply and the soil allowed to dry out between waterings. Irrigation is also required in the period after bloom in order to maximize nut production. In addition to an initial application of manure at planting, trees should be provided with 100–400 g of nitrogen per tree until the fifth year. In the sixth year, trees may be provided with up to 100 kg of nitrogen per hectare.

Harvesting
Walnuts in the home garden can simply be collected when they drop from the tree. In commercial orchards, walnuts are commonly harvested with the used of a mechanical shaker. Once the nuts are collected, they must be immediately hulled (the process of removing the nut from the green flesh) and dried.


References

Beddes, T., Murray, M., Kuhns, M. & Goodspeed, J. (2011). Walnuts in the home orchard. Utah State University Cooperative Extension. Available at: https://extension.usu.edu/files/publi.... [Accessed 21 April 15]. Free to access

CABI Crop Protection Compendium. (2013). Juglans regia (walnut) datasheet. Available at: http://www.cabi.org/cpc/datasheet/29063. [Accessed 21 April 15]. Paid subscription required.

Jaynes, R. A., Martin, G. C., Shreve, M. L. & Sibbett, G. S. Growing English walnut, pecans and chestnuts. United States Department of Agriculture National Agricultural Library. Available at: http://naldc.nal.usda.gov/naldc/downl.... [Accessed 21 April 15]. Free to access


Common Pests and Diseases

Walnut blight
Xanthomonas campestris

Symptoms
Small, water-soaked spots on immature fruit which darken and rapidly enlarge; bacterial exudate may be present during wet weather; if infection occurs prior to shell hardening then the kernels shrivel; infections which occur later may cause kernel discoloration but the fungus does not usually invade the kernel; catkins which become infected are are dark and shrivelled; new shoots may also be attacked and lesions may girdle the stem, killing the shoot above; lesions may form on tree bark and may also extend into the pith causing cankers to form; lesions on leaves are brown with a green-yellow perimeter; leaf lesions may coalesce to form large necrotic areas
Cause
Bacterium
Comments
Bacteria overwinter in dormant buds; following bud break the following year, bacteria infect surrounding leaves and young fruit; emergence of walnut blight is favored by periods of wet weather
Management
The primary method of controlling walnut blight is the application of copper containing bactericides such as Bordeaux mixture; bactericides should be applied weekly to protect new growth during periods of wet weather

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
Gnomonia leptostyla

Symptoms
Brown to black lesions on leaves, petioles, shoots and/or husks which fade toward the center and may be surrounded by a chlorotic halo; spots may coalesce to form large necrotic patches, usually located close to leaf margins; lesions on shoots, petioles and leaf midribs become elongated and sunken
Cause
Fungus
Comments
Disease affects English walnut and is widespread in Europe; disease is also known to occur in the Pacific Northwest of the United States
Management
Control of anthracnose in pistachio relies on the application of appropriate fungicide sprays; cultural practices can help to reduce the severity of the disease and include: removing leaf debris from around plants, avoiding wetting foliage when irrigating, spacing trees adequately to increase air circulation and applications of nitrogen fertilizer in Spring to delay leaf maturity and reduce the development of lesions

Armillaria root rot (Oak root fungus)
Armillaria mellea

Symptoms
Small, discolored leaves which drop early; death of branches; death of plant; clusters of honey-colored mushrooms may sprout at base of plant
Cause
Fungus
Comments
Fungus survives on dead roots in soil
Management
Armillaria root rot cannot be effectively controlled once it has become established in an orchard; diseased or dead plants should be uprooted and removed; planting resistant rootstocks is the most effective method of preventing the disease

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
Disease emergence favored by moderate temperature, poor air circulation around plant and shady conditions
Management
Disease is not severe enough on walnut to warrant control

Phytophthora root and crown rot
Phytophthora spp.

Symptoms
Slow growing trees with reduced vigor; leaves of tree turning yellow and wilting; shoots and branches dying back; if tree is girdled at the trunk or root crown then death occurs, usually within one growing season; infected tree roots are necrotic and discolored black or brown; most roots eventually die; trees with crown rot may exhibit cankers of the root crown which extend above the soil line; cankers are visible as discolored bark and possess a zonate appearance when the bark is removed
Cause
Oomycete
Comments
Disease emergence favored by water saturated soils; disease is usually introduced to orchards through contaminated soil, water and plants
Management
Control should focus on minimizing soil wetness and saturation by planting trees in well-draining soils; drainage can be improved by levelling soil or installing drainage systems; avoid wetting tree trunks when irrigating; ensure graft union is several centimeters above the soil line when planting trees

Blackline disease
Cherry leaf roll virus (CLRV-W)

Symptoms
Gradual reduction in tree vigor; leaves are yellow and drooping; defoliation occurs prematurely and is followed by dieback of terminal shoots; small holes or cracks may be present at the graft union and underlying tissue may be discolored
Cause
Virus
Comments
Virus may be introduced from infected graft wood or infected pollen
Management
Introduction of the disease to uninfested areas can be prevented by using virus-free graft and budwood from English walnut; in areas where the disease is uncommon, immediate removal of trees identified as being infected can prevent spread