Watermelon

Citrullus lanatus

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Description

Watermelon, Citrullus lanatus, is a vining annual plant in the family Cucurbitaceae grown for its fleshy fruit. Watermelon vines are thin, grooved and covered in tiny hairs. Vines are branching and possess deeply lobed pinnate leaves. The plant produces solitary yellow flowers and and a large spherical to oblong fruit. The fruit is a 'pepo' - a fleshy fruit protected by a thick leathery rind. The fruit is smooth, light to dark green in color and can be striped, marbled or solid green. The flesh of the fruit is usually red in color but some cultivars produce green, orange or white flesh and contains numerous seeds which are usually black or dark brown in color. Watermelon vines can reach a length of 3 m (10 ft) and as an annual, survives only one growing season. Watermelon originates from Africa.

Uses

Watermelon is usually consumed as a fresh fruit. In Africa it is sometimes cooked before eating and may also be used as an animal feed.

Propagation


Requirements
Watermelon is a warm-season crop, requiring lots of sun and good drainage to develop optimally and growing best at temperatures between 18 and 28°C (65–82°F). Watermelon will yield best if grown in a light, well-draining soil, rich in organic matter and with a pH between 6.0 and 7.0. Watermelon should be planted in full sun and heavy feeders. They need to be provided with even soil moisture and fertilized regularly. Vining varieties can grow to very large sizes and require a good deal of space.

Sowing seeds
Watermelon can be direct seeded in areas with a long, warm growing season but in more Northern climates it should be sown indoors and transplanted. If direct seeding,seeds should be sown after the last frosts and when the soil has warmed to at least 18.4°C (65°F). Allow 90–120 cm (~3–4 ft) between seeds in a row and 150–180 cm (~5–6 ft) between rows. If transplanting, seeds should be sown approximately 3–4 weeks before the last frost date in your area and transplanted after the plants develop their first set of true leaves. Sow seeds in 3–4 in pots using a sterile seed starting mix and planting to a depth of 1–2 cm (~0.5 in). Thin seedlings once they are established. Seeds sown both indoors and out require lightly moist soil for germination, care should be taken to avoid overwatering as seeds are prone to rotting. Seeds should germinate in 3–10 days depending on the soil temperature.

Transplanting
Watermelon seedlings should be transplanted when all danger of frost has passed and the soil has warmed to at least 18.4°C (65°F). Covering soil with dark plastic or mulch a week prior to planting outdoors can help bring the soil temperature up more quickly in colder regions, allowing earlier planting. Beginning approximately 7–10 days before transplanting, plants should be set outside to harden off (see https://www.plantvillage.com/posts/264). The planting site should be prepared by incorporating plenty of organic matter to encourage vegetative growth. When transplanting seedlings, allow 90–120 cm (~3–4 ft) between plants and 150–180 cm (~5–6 ft) between rows. Drip or soaker irrigation is preferred to overhead irrigation and plants should be watered evenly to keep them moist.

General care
Watermelon vines are sprawling and require plenty space to grow. Vines can be trained to grow on a trellis or fence to save space. Watermelon plants Watermelons have a very deep root system which allows them to access soil moisture efficiently. However, if conditions are very dry for prolonged periods then additional water should be provided.. Plastic mulches are highly recommended in areas where conditions are not ideal for growth as they conserve soil moisture and black polyethylene has the advantage of warming the soil. Floating row covers can be beneficial while the plants establish to protect against insect pests.

Harvesting
Watermelons are ready to be harvested when the the tendrils closest to the fruit are beginning to dry out and turn brown. The underside of the melon should be beginning to turn yellow

Diseases

Alternaria leaf blight Fungus Alternaria cucumerina

Symptoms

Small, yellow-brown spots with a yellow or green halo which first appear on the oldest leaves; as the disease progresses, lesions expand and becone large necrotic patches, often with concentric patternation; lesions coalesce, leaves begin to curl and eventually die

Comments

Disease is prevalent in growing areas where temperatures are high and rainfall is frequent

Management

Cucurbits should be rotated with another crop every 2 years to reduce levels of inoculum; crop debris should be removed from the field as quickly as possible after harvest or plowed deeply into the soil; applications of appropriate protective fungicides can help to slow the development of the disease; water plants from the base rather than from above to reduce periods of leaf wetness which are conducive to the development and spread of disease

Alternaria leaf spot (Brown spot) Fungus Alternaria spp.

Symptoms

Irregularly shaped or circular dark brown lesions on leaves; lesions may occur in concentric circles

Comments

Spores survive on plant debris; transmitted via wind and rain

Management

Rotate crop with non-cucurbit for at least 2 years; plow crop debris deeply into soil after harvest; avoid overhead irrigation, water plants at base; apply appropriate protective fungicides

Angular leaf spot Bacterium Pseudomonas syringae

Symptoms

Small water-soaked lesions on leaves which expand between leaf veins and become angular in shape; in humid conditions, lesions exude a milky substance which dries to form a white crust on or beside lesions; as the disease progresses, lesions turn tan and may have yellow/green edges; the centers of the lesions dry and may drop out leaving a hole in the leaf

Comments

Spread through infected seed, splashing rain, insects and movement of people between plants; bacterium overwinters in crop debris and can survive for 2.5 years

Management

Use disease-free seed; do not grow plants in field where cucurbits have been grown in the previous 2 years; protective copper spray may help reduce incidence of disease in warm, humid climates; plant resistant varieties

Anthracnose Fungus Colletotrichum orbiculare

Symptoms

Angular dark brown or black lesions on leaves with yellow border; elongated lesions with sunken centers on stems and fruit

Comments

Spread by wind and rain

Management

Rotate crops with non-cucurbits every 1-2 years to prevent disease build-up; plant only disease free, treated seed

Aphids (Peach aphid, Melon aphid) Insects Myzus persicae
Aphid gossypii

Symptoms

Small soft bodied insects on underside of leaves and/or stems of plant; usually green or yellow in color, but may be pink, brown, red or black depending on species and host plant; if aphid infestation is heavy it may cause leaves to yellow and/or distorted, necrotic spots on leaves and/or stunted shoots; aphids secrete a sticky, sugary substance called honeydew which encourages the growth of sooty mold on the plants

Comments

Distinguishing features include the presence of cornicles (tubular structures) which project backwards from the body of the aphid; will generally not move very quickly when disturbed

Management

If aphid population is limited to just a few leaves or shoots then the infestation can be pruned out to provide control; check transplants for aphids before planting; use tolerant varieties if available; reflective mulches such as silver colored plastic can deter aphids from feeding on plants; sturdy plants can be sprayed with a strong jet of water to knock aphids from leaves; insecticides are generally only required to treat aphids if the infestation is very high - plants generally tolerate low and medium level infestation; insecticidal soaps or oils such as neem or canola oil are usually the best method of control; always check the labels of the products for specific usage guidelines prior to use

Aster yellows Phytoplasma Aster yellows phytoplasma

Symptoms

Foliage turning yellow; secondary shoots begin growing prolifically; stems take on a rigid, upright growth habit; leaves are often small in size and distorted, may appear thickened; flowers are often disfigured and possess conspicuous leafy bracts; fruits are small and pale in color

Comments

Disease is transmitted by leafhoppers and can cause huge losses in cucurbit crops

Management

Remove any infected plants from the field to reduce spread; control weeds in and around the field that may act as a reservoir for the phytoplasma; protect plants from leaf hopper vectors with row covers

Bacterial fruit blotch Bacterium Acidovorax avenae

Symptoms

Small water-soaked lesions on top or sides of fruit which enlarge over surface; lesions on fruit may turn reddish or brown and crack

Comments

Spread through infected seed or water splash; disease emergence favors wet conditions

Management

Use pathogen-free seed and transplants; rotate crops; avoid the use of overhead irrigation

Blossom-end rot Nutritional disorder

Symptoms

Symptoms first appear on immature fruits as small light brown spots close to the blossom end of the fruit; as fruit grow, the spots enlarge, resulting in dark leathery lesions sunken into the fruit

Comments

Watermelon varieties that produce long fruit are more susceptible to blossom-end rot

Management

Blossom-end rot is caused by a lack of calcium in the developing fruit and it occurs when the uptake of nutrients to the plant is disrupted; factors which disrupt nutrient uptake include drought, root damage or high soil salinity; application of excess nitrogen fertilizer may also contribute to the development of blossom-end rot as it promotes vigorous growth of vegetative tissues and depletion of calcium in the soil; watering plants evenly and regularly reduces the incidence of blossom-end rot

Cabbage looper Insect Trichoplusia ni

Symptoms

Large or small holes in leaves; damage often extensive; caterpillars are pale green with a white lines running down either side of their body; caterpillars are easily distinguished by the way they arch their body when moving; eggs are laid singly, usually on the lower leaf surface close to the leaf margin, and are white or pale green in color

Comments

Insects overwinter as pupae in crop debris in soil; adult insect id a dark colored moth; caterpillars have a wide host range

Management

Looper populations are usually held in check by natural enemies; if they do become problematic larvae can be hand-picked from the plants; an organically acceptable control method is the application of Bacillus thuringiensis which effectively kills younger larvae; chemical sprays may damage populations of natural enemies and should and should be selected carefully

Cercospora leaf spot Fungus Cercospora citrullina

Symptoms

Initial symptoms of disease occur on older leaves as small spots with light to tan brown centers; as the disease progresses, the lesions enlarge to cover large areas of the leaf surface; lesions may have a dark border and be surrounded by a chlorotic area; the centers of the lesions may become brittle and crack

Comments

Fungus survives on plant debris; spread by wind and water splash; occurs mainly in tropical and subtropical growing regions

Management

Any diseased plants should be removed and destroyed to prevent further spread; crop debris should be removed after harvest or plowed deeply into the soil to reduce inoculum

Cucumber green mottle mosaic Virus Cucumber green mottle mosaic virus (CGMMV)

Symptoms

Early symptoms on young plants include vein-clearing and the development of crumpled leaves; older plants develop bleached and/or chlorotic leaves. As the infection progresses, leaves develop mottling and become blistered and distorted. Leaf symptoms are very difficult to distinguish from other mosaic viruses of Cucurbits/ Severity of symptoms varies depending on the strain of the virus.

Comments

All Cucurbit species are susceptible to the virus, some cucumber varieties have been developed which have some resistance to the disease and are available in Canada and Europe.

Management

As the virus is spread primarily by infected seed, only disease-free seed from a reputable supplier should be planted. Seedlings and plants infected with the virus should be removed and destroyed to prevent spread. All seedlings/plants within a 3-5 ft radius of the infected plant should also be destroyed. The virus can be spread mechanically via tools and on hands, good sanitation should be practiced at all times to prevent virus transmission - disinfect all tools and equipment between uses by dipping in a solution of bleach or using a commercially available disinfectant such as Virkon.

Cucumber mosaic Viruses Cucumber mosaic virus (CMV)

Symptoms

Plants may e severely stunted; foliage is covered in distinctive yellow mosaic; leaves of plant curl downwards and leaf size is smaller than normal; flowers on infected plants may be deformed with green petals; fruits become distorted and are small in size; fruit is often discolored

Comments

Transmitted by aphids; virus has an extensive host range; can be mechanically transmitted via tools etc.

Management

Control of the virus is largely dependant on the control of the aphid vectors; reflective mulches can deter aphid feeding; aphid outbreaks can be treated with mineral oils or insecticidal soap applications; some resistant varieties are available

Cutworms Insects Agrotis spp.
Peridroma saucia
Nephelodes minians
and others

Symptoms

Stems of young transplants or seedlings may be severed at soil line; if infection occurs later, irregular holes are eaten into the surface of fruits; larvae causing the damage are usually active at night and hide during the day in the soil at the base of the plants or in plant debris of toppled plant; larvae are 2.5–5.0 cm (1–2 in) in length; larvae may exhibit a variety of patterns and coloration but will usually curl up into a C-shape when disturbed

Comments

Cutworms have a wide host range and attack vegetables including asparagus, bean, cabbage and other crucifers, carrot, celery, corn, lettuce, pea, pepper, potato and tomato

Management

Remove all plant residue from soil after harvest or at least two weeks before planting, this is especially important if the previous crop was another host such as alfalfa, beans or a leguminous cover crop; plastic or foil collars fitted around plant stems to cover the bottom 3 inches above the soil line and extending a couple of inches into the soil can prevent larvae severing plants; hand-pick larvae after dark; spread diatomaceous earth around the base of the plants (this creates a sharp barrier that will cut the insects if they try and crawl over it); apply appropriate insecticides to infested areas of garden or field if not growing organically

Downy mildew Fungus Pseudoperonospora cubensis

Symptoms

Yellow mottling on leaves; dark brown lesions on leaves; leaves curling inwards;

Comments

Spread by airborne spores and water splash

Management

Do not overcrowd plants; avoid overhead irrigation, water plants from base; apply appropriate fungicide

Flea beetles Insects Epitrix spp.

Symptoms

Small holes or pits in leaves that give the foliage a characteristic “shothole” appearance; young plants and seedlings are particularly susceptible; plant growth may be reduced; if damage is severe the plant may be killed; the pest responsible for the damage is a small (1.5–3.0 mm) dark colored beetle which jumps when disturbed; the beetles are often shiny in appearance

Comments

Younger plants are more susceptible to flea beetle damage than older ones; older plants can tolerate infestation; flea beetles may overwinter on nearby weed species, in plant debris or in the soil; insects may go through a second or third generation in one year

Management

In areas where flea beetles are a problem, floating row covers may have to be used prior to the emergence of the beetles to provide a physical barrier to protect young plants; plant seeds early to allow establishment before the beetles become a problem - mature plants are less susceptible to damage; trap crops may provide a measure of control - cruciferous plants are best; application of a thick layer of mulch may help prevent beetles reaching surface; application on diamotecoeus earth or oils such as neem oil are effective control methods for organic growers; application of insecticides containing carbaryl, spinosad, bifenthrin and permethrin can provide adequate control of beetles for up to a week but will need reapplied

Fusarium wilt Fungus Fusarium oxysporum

Symptoms

Wilting plants; wilting confined to one or more vines; foliage has a dull gray-green appearance and turns yellow as the disease progresses; vascular tissue has a red discoloration

Comments

Disease can be spread through infected seed or via contaminated water and/or equipment

Management

Plant in well draining soils and avoid waterlogging; plant fungicide treated seed; rotate crops on 4 year rotation

Gummy stem blight Fungus Didymella bryoniae

Symptoms

Round or irregular brown lesions with faint concentric rings on cotyledons; brown or white lesions on crown and stems; soft, circular brown lesions on fruit; lesions on stems and fruit may be oozing an amber colored sticky substance

Comments

Fungus can be spread b infected seed, air currents or water splash; survives on plant debris in soil; disease emergence is favored by warm, wet conditions

Management

Use disease free seed; treat seeds prior to planting; rotate crops every 2-3 years to a non-cucurbit to reduce disease build up in soil; reduce crop residue in soil by plowing plant debris into soil after harvest; application of preventative fungicides are usually required to control the disease successfully

Powdery mildew Fungi Podosphaera xanthii
Podosphaera fuliginea

Symptoms

Reddish or bronze appearance of older leaves; obvious patches of white powdery growth on leaves

Comments

Disease emergence favored by dry weather and high relative humidity

Management

Plant in sites with good air circulation and sun exposure; do not overcrowd plants; sanitize equipment regularly

Thrips (Western flower thrips, Onion thrips, etc.) Insect Frankliniella occidentalis
Thrips tabaci

Symptoms

If population is high leaves may be distorted; leaves are covered in coarse stippling and may appear silvery; leaves speckled with black feces; insect is small (1.5 mm) and slender and best viewed using a hand lens; adult thrips are pale yellow to light brown and the nymphs are smaller and lighter in color

Comments

Transmit viruses such as Tomato spotted wilt virus; once acquired, the insect retains the ability to transmit the virus for the remainder of its life

Management

Avoid planting next to onions, garlic or cereals where very large numbers of thrips can build up; use reflective mulches early in growing season to deter thrips; apply appropriate insecticide if thrips become problematic

Verticillium wilt Fungus Verticillium dahliae

Symptoms

Symptoms generally appear after fruit set; chlorotic leaves which develop necrotic areas; leaves collapsing; symptoms only on one side of vine; discoloration of vascular tissue in roots

Comments

Fungus can survive in soil for many years; disease emergence favored by cool or mild weather in Spring

Management

Do not plant in areas where other susceptible crops have been grown previously; delay planting until temperatures are warmer

Watermelon mosaic Virus Watermelon mosaic virus (WMV)

Symptoms

Symptoms vary widely depending on species, cultivar, virus strain and environmental conditions; symptoms on leaves may include green mosaic patternation, green vein-banding, chlorotic rings and disfigured leaves

Comments

Virus is found in almost all Cucurbit growing regions in the world; virus is spread by over 20 aphid species

Management

Treatments that control populations of aphid vectors can also reduce the incidence of the virus; spraying plants with mineral oils or insecticidal soaps can help to reduce aphid numbers

Zucchini yellow mosaic Virus Zucchini yellow mosaic virus (ZYMV)

Symptoms

Infected plants are severely stunted and leaves can exhibit a variety of symptoms including yellow mosaic patternation, severe deformation, blistering, reduced size and necrosis; fruits are deformed

Comments

DIsease can cause devastating epidemics when present

Management

Use of resistant varieties, where available, is usually the most effective method of controlling the virus; control of aphid populations on the plants can be achieved through the use of mineral oils and insecticidal soaps but is rarely effective at controlling the virus

References

CABI Crop Protection Compendium. (2010). Citrullus lanatus (watermelon). Available at: http://www.cabi.org/cpc/datasheet/13678. [Accessed 21 April 15]. Paid subscription required.

Roberts, W., Motes, J., Damicone, J., Duthie, J. & Edelson, J. Watermelon production. Oklahoma Cooperative Extension Service. Available at: http://pods.dasnr.okstate.edu/docusha.... [Accessed 21 April 15]. Free to access.

Saha, S. (2014). Watermelon. University of Kentucky Cooperative Extension Service & University of Kentucky College of Agriculture, Food and Environment. Available at: http://www.uky.edu/Ag/NewCrops/intros.... [Accessed 21 April 15]. Free to access.

Zitter, T. A., Hopkins, D. L. & Thomas, C. E. (1996). Compendium of Cucurbit diseases. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopap.... Available for purchase from APS Press.