Pests
Symptoms Small yellow beetles with stripes or spots chew on the leaves and flowers of young plants and can scar the fruit. They can also spread disease, though watermelon handles the wilt disease better than cucumbers or cantaloupe.Small striped or spotted yellow beetles chew young leaves, stems, flowers, and the rind of developing fruit, and can enter the soil to feed on seedlings below ground. The bigger threat is indirect: cucumber beetles can transmit bacterial wilt and spread mosaic viruses, though watermelon is far less prone to bacterial wilt collapse than cucumber and muskmelon. Cause Cucumber beetles spend the winter as adults and move onto young plants early. They feed on the leaves and can carry diseases from plant to plant, and seedlings are most at risk.Striped and spotted cucumber beetles feed on cucurbits and overwinter as adults, colonizing plants early in the season. They carry the bacterial wilt organism in their gut and spread mosaic viruses as they feed; seedlings are most vulnerable. Organic treatment Cover young plants with a light row cover (take it off when flowers open so bees can reach them), pick beetles off by hand, and clear away weeds. Yellow sticky traps help, and strong young plants shrug off some feeding.Protect seedlings with floating row cover removed at flowering for pollination, hand-pick beetles, and clear weeds that shelter them. Yellow sticky traps help monitor and reduce numbers; keep young plants vigorous so they grow past the most vulnerable stage. Prevention Cover seedlings until they flower, keep them growing strong, clear away weeds and old plants, and do not plant where melons or squash grew last year.Exclude beetles from seedlings with row cover, keep plants growing fast, clear weeds and debris, and rotate away from where cucurbits grew. Aim control at the early-season colonizers.
Symptoms Tiny soft insects gather under the leaves and on new growth, making leaves curl and yellow, often with a sticky film. They can also spread viruses that damage the plant.Colonies of small soft-bodied insects, often melon aphids, cluster on leaf undersides and growing tips, curling and yellowing the leaves and coating them in sticky honeydew that grows sooty mold. Beyond direct feeding, melon aphids transmit mosaic viruses that mottle and distort the foliage and can ruin a crop. Cause Aphids are small insects that suck the plant's juices and multiply quickly, especially on soft, over-fed growth. The sticky honeydew they leave grows black mold, and they can carry plant viruses.Melon and other aphids suck sap and multiply rapidly in warm weather, building up fast on stressed or over-fertilized plants, and typically colonize once the vines begin to run. The honeydew they excrete fosters sooty mold, and several species spread cucurbit mosaic viruses. Organic treatment Blast them off with a strong spray of water and treat the undersides of the leaves with insecticidal soap, repeating every few days. Ladybugs and other helpful insects eat aphids, so welcome them.Knock colonies off with a strong jet of water and spray insecticidal soap or horticultural oil with thorough coverage of leaf undersides, repeating every few days. Conserve ladybugs, lacewings, and parasitic wasps, which usually bring aphids under control; reflective mulch deters them from young plants. Prevention Do not over-fertilize with nitrogen, check the undersides of leaves regularly, and let helpful insects do their work. A reflective mulch around young plants helps keep aphids away.Avoid excess nitrogen, which drives the soft growth aphids favor, scout leaf undersides and growing tips, and support natural enemies rather than spraying broadly. Reflective or aluminum-foil mulch deters aphids from young plants.
Symptoms The leaves get a fine yellow or bronze speckling, and in bad cases you see tiny webs underneath. Spider mites love hot, dry weather and can build up fast on watermelon in summer.Pale yellow and reddish-brown stippling appears on the upper sides of leaves, ranging from small specks to large whitish, stippled areas, and fine webbing shows on leaf undersides in bad infestations. Two-spotted spider mites explode in hot, dry weather, exactly watermelon's season, and heavy feeding bronzes and dries the foliage, cutting fruit size and sugar. Cause Tiny mites that thrive in hot, dry summers and multiply fast, especially after broad insecticide sprays kill the helpful bugs that normally keep them down. They feed on the undersides of the leaves.Two-spotted spider mites are tiny sap-feeders that proliferate during hot, dry weather and whose populations explode rapidly when their natural enemies are killed off by broad-spectrum sprays. They feed mostly on leaf undersides. Organic treatment Spray the plants, especially the undersides of the leaves, with a strong stream of water, and treat with insecticidal soap or horticultural oil if needed. Avoid spraying broad insecticides, which kill the good bugs that eat mites.Spray plants with a strong jet of water to dislodge mites and raise humidity, and apply insecticidal soap or horticultural oil with thorough leaf-underside coverage before populations peak, repeating as needed. Avoid broad-spectrum insecticides, which kill the predatory mites and insects that control them. Prevention Keep plants watered so they are not stressed in the heat, and avoid spraying strong insecticides that kill helpful bugs. Check under the leaves during hot, dry weather and treat early if you see speckling.Keep plants well watered and not heat-stressed, since drought-stressed plants are hit hardest, and avoid broad-spectrum insecticides that wipe out natural enemies. Scout leaf undersides through hot, dry spells and act early.
Symptoms Leaves get yellow spots, then brown and wilt, and you find clusters of shield-shaped gray-brown bugs and their bronze eggs under the leaves. Watermelon is not their favorite, but they can still bother young plants.Leaves develop yellow specks that brown and wilt, and heavily fed runners can wilt and turn brittle. Flat bronze egg clusters appear on leaf undersides along the veins, and gray-brown adults and pale nymphs cluster near the crown and under debris. Watermelon is less affected than winter squash, but young plants can still be hurt. Cause Squash bugs pierce the leaves and stems and suck out the juices, which makes the vines wilt. They hide in plant debris over winter and prefer squash to watermelon, but can still attack young plants.The squash bug feeds with piercing-sucking mouthparts on cucurbit foliage and stems, injecting saliva that wilts and browns the tissue. There is one generation a year in much of the South, and adults overwinter in garden debris. It prefers squash and pumpkin to watermelon. Organic treatment Pick the bugs off by hand and scrape off the egg clusters under the leaves. A board laid on the ground overnight draws them underneath, then lift it in the morning and destroy them. Treat the young bugs with insecticidal soap.Hand-pick adults, crush the bronze egg masses on leaf undersides, and trap bugs by laying a board near the plants overnight, then lifting it in the morning and destroying the cluster underneath. Target young nymphs with insecticidal soap, since adults are tough to kill. Prevention Clean up old vines after harvest so the bugs have nowhere to overwinter, move your melons around the garden each year, and check under the leaves for eggs early in the season.Clean up vines and debris at season's end to cut overwintering sites, rotate cucurbits, and scout leaf undersides for the bronze egg clusters from early in the season, acting before nymphs mature.
Symptoms On many squash, a healthy plant suddenly wilts in summer with sawdust-like specks at a hole near the base of the stem, and a fat white grub inside. Watermelon rarely gets this, since the borer mostly leaves it alone, but it is still worth a look.On susceptible squash a vine wilts suddenly in midsummer with sawdust-like frass oozing from a hole low on the stem and a fat white caterpillar inside. Watermelon is the fortunate exception: the borer rarely attacks watermelon or cucumber, so this collapse is uncommon, though heavy local pressure can occasionally produce damage. Cause A moth lays eggs at the base of squash stems in early summer and the grubs tunnel inside, but the moth strongly prefers thin-stemmed squash like zucchini and mostly leaves watermelon alone.The larva of the squash vine borer moth bores into thin-stemmed Cucurbita pepo squash; it largely avoids watermelon and cucumber, so watermelon planted near a borer-ravaged zucchini usually sails through untouched. Where pressure is severe, an occasional larva may still enter. Organic treatment Watermelon seldom needs treatment. If you do find a borer, slit the stem with a knife, dig out the grub, and cover the spot with damp soil so it can re-root.Rarely needed on watermelon. If a borer does enter, slit the stem lengthwise, remove the larva, and bury that node under moist soil to re-root; remove badly damaged vines. Prevention Growing watermelon is half the battle, since the borer mostly skips it. If borers are bad where you live, cover young plants with a light row cover until they flower, then take it off so bees can pollinate.Choosing watermelon is itself most of the prevention, since the borer mostly avoids it. Where pressure is severe, row cover from planting until flowering excludes the egg-laying moths, and tilling in late winter and rotating disrupts overwintering pupae.
Diseases
Symptoms Dark brown-black spots show up on the older leaves and grow into ragged patches, and the fruit can get sunken dark spots on the rind. In wet weather the spots may look pink-orange. Bad cases strip the leaves off the vine.Small brown-black spots appear first on the older crown leaves, expanding into ragged, sometimes water-soaked lesions; in damp weather the spots carry orange-pink spore masses that turn gray as it dries. On fruit it causes sunken, dark spots on the rind that can keep developing in transit and storage, and severe infection defoliates the vines. Cause A fungus that thrives in warm, wet weather and spreads from infected seed and old plant debris. It is worse on late plantings and in rainy spells.Anthracnose is caused by the seed-borne fungus Colletotrichum, favored by warm, damp weather and prolonged leaf wetness, and it hits late plantings especially hard. It overwinters on infected debris and seed. Organic treatment Treat early, at the first spots, with a labeled fungicide and remove badly affected leaves and fruit. Water the soil, not the leaves, and stay out of the patch when the vines are wet.Treat at the first spots with a labeled fungicide such as chlorothalonil or mancozeb, covering both leaf surfaces, and remove the worst-affected leaves and fruit. Avoid overhead watering and working among wet vines. Once it is widespread, control is mostly damage limitation. Prevention Choose resistant varieties and clean seed, rotate where you grow melons so they are not in the same spot every year, water at the base, and remove sick leaves and fruit early.Plant resistant varieties and disease-free seed, do not soak seed before planting, rotate watermelon and other cucurbits out of the bed for several years, water at the base, and start a protective spray schedule before the disease appears in wet seasons.
Symptoms You see round black spots on the leaves and dark sunken patches on the stems, sometimes with a brown gummy ooze. The leaf spots grow fast into big tan blotches, and in warm, wet weather the vines can collapse.Round, black, wrinkled spots form on young leaves and dark, sunken, sometimes girdling lesions develop on the stems, often with a brown gummy ooze on the surface. Brownish circular leaf spots expand rapidly into large tan lesions, and a brown streak can appear at the blossom end of the fruit. In warm, wet weather the foliage and vines can collapse quickly. Cause A fungus that spreads in warm, wet weather, especially in the South, and survives on seed and old plant debris. Long rainy spells make it move fast.Gummy stem blight is caused by the seed-borne fungus Didymella bryoniae (Stagonosporopsis), a warm-weather, southern cucurbit disease driven by prolonged rainy periods and leaf wetness. It survives on seed and infected debris. Organic treatment Use a labeled fungicide at the first sign and remove the worst vines. Give plants room so air moves between them, and water at the base to keep the leaves dry.Start a protective fungicide schedule at the first sign, covering leaf undersides, and remove and destroy badly affected vines. Improve airflow, avoid overhead irrigation, and keep the foliage as dry as possible. Prevention Pick resistant varieties and clean seed, rotate where you grow melons, give plants room, water at the base, and keep an eye out in warm, humid weather.Plant resistant varieties and clean seed, rotate cucurbits out of the bed for several years, space plants for airflow, water at the soil line, and avoid working among wet vines. Scout closely in warm, humid weather.
Symptoms The leaves turn yellow and wilt, often on just one runner at first, and the plant dies even though the soil is wet. You may see a pinkish mold near the base of a dying stem. It is the most serious soil disease of watermelon.Foliage turns yellow and wilts, often starting on one side or one runner, and infected plants decline and die soon after symptoms appear even though the soil is moist. A pinkish-white cottony growth can show near the soil line on dying stems, and a cut stem reveals browned, discolored vascular tissue. It is the most damaging soilborne disease of watermelon. Cause A soil fungus that only attacks watermelon, blocking the plant's water pipes so it wilts. It builds up in warm, sandy soil and survives for years, which is why long rotations matter.Fusarium wilt is caused by the soilborne fungus Fusarium oxysporum f. sp. niveum, which is specific to watermelon and clogs the water-conducting tissue. It thrives in warm, sandy soils typical of watermelon ground and persists in soil for many years; several races exist, and resistance covers only some of them. Organic treatment There is no cure once a plant has it, so pull and destroy infected plants. You cannot spray it away, so prevention is everything.There is no cure once a plant is infected; remove and destroy wilting plants. Management is preventive and cultural, since fungicides do not control a soilborne vascular wilt. Prevention Choose resistant varieties, do not grow watermelon in the same ground for at least five years, and where the disease is bad, look into grafted plants on resistant roots. Keep plants healthy and do not overwater.Plant resistant varieties, follow a long rotation of at least five years out of watermelon and related cucurbits, and consider grafting onto resistant rootstock where the disease is established. Avoid moving infested soil, and keep plants vigorous but not over-watered.
Symptoms A white, powdery coating spreads over the leaves until they yellow, brown, and die. The plant fades early, and the melons may not size up or sweeten well.White, powdery patches appear on the upper and lower leaf surfaces and stems, spreading until leaves yellow, brown, and die back ahead of schedule. Early defoliation cuts the season short, reduces fruit size and sugar, and exposes melons to sunscald. Cause A fungus that spreads on warm, humid days, especially where plants are crowded and air does not move well. It does not need wet leaves to take hold.Powdery mildew fungi thrive in warm conditions with high humidity but do not need leaf wetness, spreading by wind-blown spores. Crowded plantings, poor airflow, and shade worsen it, and it is a common late-season cucurbit disease. Organic treatment Treat early, at the first white spots, with a labeled fungicide like sulfur or horticultural oil, and remove badly affected leaves. Water the soil, not the leaves.Treat at the first spots with a labeled fungicide such as sulfur or a horticultural oil or biofungicide, covering both leaf surfaces, and remove the worst leaves. Avoid overhead watering. Once it is widespread, control is mostly damage limitation. Prevention Pick resistant varieties, give plants plenty of room and full sun, water at the base, and start checking in midsummer, treating the first white spots early.Choose resistant varieties, space plants for airflow, plant in full sun, and avoid excess nitrogen and overhead watering. Scout from midsummer and treat at the first sign.
Symptoms Yellow, angular patches show on top of the leaves with fuzzy gray-purple growth underneath, mostly in damp, humid weather. The spots brown and spread, and badly hit leaves can die within a couple of weeks.Small yellowish, angular areas bounded by the leaf veins appear on the upper leaf surface, water-soaked underneath with a grayish or purplish fuzzy spore growth in humid weather. Lesions turn from yellow to brown and several infected leaves can die within 10 to 14 days, rapidly defoliating the vines. Cause A mold-like disease that blows in on the wind during mild, rainy weather and needs wet leaves to infect. It can move through a planting quickly.Downy mildew is caused by the air-borne, fungus-like water mold Pseudoperonospora cubensis, which needs leaf wetness and humidity to infect and arrives on wind currents during mild, rainy stretches, often spreading north through the season from overwintering sites in the far South. Organic treatment Open up the planting so leaves dry faster, remove the worst leaves, and use a labeled fungicide when wet weather sets in. Do not water over the leaves.Improve airflow, let foliage dry, remove badly affected leaves, and apply a labeled fungicide covering leaf undersides when conditions favor the disease. Avoid overhead irrigation, which keeps leaves wet. Prevention Give plants room so air moves between them, water at the base, choose resistant varieties if downy mildew is common where you live, and watch closely in humid weather.Space for airflow, water at the soil line rather than overhead, choose resistant varieties where available, and start protective sprays when the disease is reported moving into your area. Scout closely in humid weather.
