Plum and prune

Description

Plum, Prunus domestica, is a deciduous tree in the family Rosaceae grown for its edible fruits. The plum tree has an erect growing habit with a spreading canopy. It possesses large, thick, oval-shaped leaves which are darker in color on the upper surface than on the lower and which often have a serrated edge.The tree produces buds on terminal spurs on the branches with each bud generally producing 3–5 flowers. The fruit is a fleshy oval fruit with a single seed contained within a stone. The color of the fruit varies with variety and fruits can be purple, blue, green, red or yellow. Plum trees can attain a height of between 6 and 10 m (20–33 ft) and can live for periods in excess of 50 years if properly maintained. Plum may also be referred to as European plum and originates from Southwest Asia


Uses

Plum fruits are commonly consumed fresh or used to make jams or jellies. Plums may be dried to produce prunes.


Propagation


Basic requirements
Plums grow best in areas with warm summers and require a summer temperature between 20 and 30°C (68-86°F) for the fruit to mature. The trees also have a chilling requirement to break dormancy. Plum trees grow best in well-draining sandy loams in areas that receive full sun but can will grow in a variety of soils as long as water does not sit on the surface after heavy rainfall. It can be beneficial to plant the trees on elevated land to allow cold air to drain away. Trees will grow optimally in soils with a pH between 5.5 and 6.5

Propagation
Plum trees are propagated vegetatively to maintain the desirable genetic characteristic of the parent. Plum rootstocks are commonly used but plum may also be grafted onto peach, Japanese apricot and almond rootstocks. Plum trees should be planted in full sun. Plant bare root trees in a pre-dug hole which is slightly wider than the root ball. Backfill the hole so that the tree is planted to its original planting depth ensuring that the bud union is above the soil line. It is usually possible to identify this from changes in the color of the bark. If planting multiple trees they should be spaced 2.0–2.5 m (6.6–8.2 ft) apart. Most plum varieties are self fruitful but cross pollination may be required for fruit set. Plums are pollinated by honeybees and it can be beneficial to maintain bee hives in orchards.

General care and maintenance
Plums should be pruned annually, including the year of planting and are best trained to an open center. When the tree is bearing fruit, it is important to thin the fruits to prevent the tree from over-bearing. Aim to have 1 fruit every 15–20 cm (6–7 in). This allows fruits to become larger and prevents the tree from reducing production the following year. Trees should be watered regularly during the growing season to aid with fruit development. During dry periods, water trees every 10 to 14 days. Apply water deeply and widely, to at least the width of the canopy. Trees will also benefit from the application of a nitrogen fertilizer in Spring.

Harvesting
Plum fruits should be allowed to mature on the tree. Fruits can be picked by hand when the skin has turned the color typical of the variety being grown.


References

CABI Crop Protection Compendium. (2014). Prunus domestica (plum) datasheet. Available at: http://www.cabi.org/cpc/datasheet/44278. [Accessed 30 March 15]. Paid subscription required.

Ogawa, J. M., Zehr, E. I., Bird, G. W., Ritchie, D. F., Uriu, K. & Uyemoto, J. K. (eds) (1995). Compendium of Stone Fruit Diseases. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopap.... Available for purchase from APS Press.

Parker, D. (1999). Plum. Clemson Cooperative Extension. Available at: http://www.clemson.edu/extension/hgic.... [Accessed 30 March 15]. Free to access.

Strang, J. (2012). Plums. University of Kentucky Cooperative Extension Service. Available at: http://www.uky.edu/Ag/NewCrops/intros.... [Accessed 30 March 15]. Free to access.






Common Pests and Diseases

Bacterial canker
Pseudomonas syringae

Symptoms
Cankers on twigs at bases of flower and leaf buds, in pruning wounds or at the base of spurs which exude amber colored gum; cankers spread upwards and form sunken areas in winter; if pathogen enters dormant buds they may be killed or open normally in Spring before collapsing in early Summer; infected buds may be symptomless
Cause
Bacterium
Comments
Bacteria are spread by water splash; disease emergence favored by high humidity and low temperatures in spring
Management
Ensure that a suitable plum variety and rootstock is chosen based on geographic location and environmental conditions to prevent stress to tree which predisposes tree to canker disease; apply protective copper spray to trees before flowering; prune trees in early summer to decrease likelihood of infection

Crown gall
Agrobacterium tumefaciens

Symptoms
Galls on root and/or crown of tree which can range in size from so small they are not visible to the naked eye up to 10 cm (4 in) in diameter; galls first become visible as white, fleshy swellings that grow rapidly and become tan to brown in color; galls typically develop at the site of a wound and new galls form adjacent to old ones the next year
Cause
Bacterium
Comments
Infection with crown gall begins at the site of plant wounds; disease emergence is favored by poorly-drained, alkaline soils and previous feeding damage by nematodes
Management
Chemical control of the disease is generally ineffective; an effective bacterial biological control is available for commercial production; cultural control methods include: planting only certified, disease-free material, planting plum in well-draining soil, rotating infected fields with a non-host before plum is planted and also using good sanitation practices

Bacterial spot
Xanthomonas campestris

Symptoms
Water soaked, angular gray lesions on the underside of the leaves which turn purple and necrotic in the center and cause a shot hole appearance if lesion center drops out; if lesions are present in high numbers on leaves they may become chlorotic and drop from tree; cankers develop on twigs either as raised blisters or as a dark area surrounding a bud that fails to open; in years of severe infection the entire fruit crop may be lost; lesions on fruit begin as small brown, water-soaked lesions which may exude gum
Cause
Bacterium
Comments
Periods of frequent rainfall during late bloom and early petal drop increase likelihood of fruit and leaf infection; infection is rare during hot, dry weather
Management
Avoid planting susceptible varieties in areas where disease is known; once disease is visible it can be difficult to control, protective copper applications in the Fall prior to leaf drop and/or application in early growing season may help prevent the disease; care should be taken as peach trees are very sensitive to copper

Brown rot
Monolinia spp.

Symptoms
Death of young blossoms and associated twigs and leaves; small tan cankers with dark margins on twigs; gummy exudate at base of flowers; brown spore masses on flowers in humid conditions; infected fruit usually exhibit a rapidly spreading brown rot but may also take the form of small necrotic spots; infected fruits usually remain attached to the tree
Cause
Fungi
Comments
Fungus survives in mummified fruit on the tree, blighted blossoms, cankers and infected twigs; blossom and twig blights are promoted by periods of wet weather
Management
The currently most effective method of controlling brown rot is through the application of appropriate protective fungicides timed so that they are applied when the susceptible flower parts are exposed or after a wet period; avoiding sprinkler irrigation protects the leaves and flowers from wetness that promotes the disease. Cultural control methods include: removing mummified fruit from tree, pruning infected twigs and reducing plant stress by providing adequate levels of water and fertilizer

Powdery mildew
Sphaerotheca pannosa
Podosphaera tridactyla

Symptoms
White powdery growth occuring in circular patches on fruit; scabby, dry patches on fruit; white powdery growth on leaves in Fall
Cause
Fungi
Comments
Disease emergence favored by cool, moist nights and warm days
Management
Management of powdery mildew is reliant on the application of appropriate fungicides and cultural practices which promote good air circulation around tree canopies to lower humidity

Armillaria root rot
Armillaria mellea

Symptoms
If tree is infected after it has reached 5 years of age then typical symptoms include poor terminal growth and small leaves; around midsummer the whole tree suddenly collapses; in orchards trees usually die in a circular pattern; infected trees often have a fan-shaped white fungal mat growing between the bark and wood of the crown
Cause
Fungus
Comments
Fungus survives in dead roots; symptoms similar to Phytophthora root rot; aqvoid planting plum in areas where oak has been planted recently
Management
Once a tree is infected there is no treatment and it should be removed, fumigants do not control fungi in soil adequately; do not plant apricot in newly cleared forest or on the site of old orchards with a history of Armillaria

Black knot
Apiosporina morbosa

Symptoms
Elongated swellings (knots) on woody parts of tree which can reach up to 30 cm (12 in) long; knots are initially olive green in color with a corky texture but turn black in color and become hard and brittle; knots grow in length each year
Cause
Fungus
Comments
Infections occur on new shoots after rainfall and knots develop rapidly in second year
Management
Prune knots on twigs and branches 8-10 cm (3-4 in) below the swelling and remove the pruned branches from the orchard; remove knots on older branches by removing the knot plus 2 cm (0.8 in) of surrounding tissue; removal of knots is most effective when done in midsummer; disease can be controlled by application of appropriate fungicides, if available, during shoot elongation

Rust
Tranzschelia discolor

Symptoms
Pale yellow-green spots on both upper and lower leaf surfaces which are angular in shape and turn bright yellow in color; spots on lower leaf surface develop orange-red spores
Cause
Fungus
Comments
Fungus overwinters in twigs or in leaves which remain attached to the tree
Management
Rust can be prevented by spraying trees with protective fungicides; application is usually carried out one, two and three months before harvest in areas prone to early season outbreaks of the disease and after harvest in areas where disease is less problematic or emerges later in the season

Phytophthora root and crown rot
Phytophthora spp.

Symptoms
Poor new growth; leaves chlorotic, small in size and sparse; fruit may be small, brightly colored and susceptible to sunburn; shoots may suffer from dieback and tree will often die within weeks or months of first signs of infection or decline gradually over several seasons; root crown may show signs of decay which develops into a canker; bark of infected crown tissue turns dark brown; cankers may occur on aerial parts of plant
Cause
Oomycete
Comments
Severity of disease is linked to soil moisture content; water-saturated soils promote development of fungus
Management
Management of phytophthora is reliant on good management of water: plum trees should be planted in well-draining soil to minimize the frequency and duration of water saturated soil; trees should be propagated from resistant rootstock and application of appropriate systemic fungicides may provide some protection from the disease

Plum pox virus
Plum pox virus (PPV)

Symptoms
Pale green chlorotic spots, rings and lines on leaves which appear in early summer; the spots may become necrotic; infected fruit have rings or spots on the surface and may have red rings or spots on the stone
Cause
Virus
Comments
Virus is transmitted by aphids but most common method of spread is diseased plant material
Management
Plant certified healthy material; remove infected trees from orchard; chemical sprays to control aphids may prolong spread of virus