Watermelon Diseases - Solution Guide

Cucurbits, such as melons, squash, pumpkins, cucumbers, etc., are susceptible to many common diseases. Although all diseases are different, many symptoms can look very similar. Because of this, a qualified university or agricultural plant pathology laboratory specialist should make a full and proper diagnosis.

Although most pathogens are spread by wind, water splashing, soil contact, or insects, some diseases, such as Bacterial fruit blotch, can be seed-borne and unknowingly transmitted via infected seeds. Purchasing seeds from an established and reputable company can limit potential exposure to a disease like this.

For any crop disease to occur, three factors are needed:

1. a host plant
2. a pathogen
3. conducive weather conditions

Therefore, practical management efforts will successfully address these three factors. This guide provides symptom descriptions of specific diseases and practical management steps for avoidance and control measures for some or all of these three factors.

Common Watermelon Diseases

Alternaria leaf spot - Alternaria spp. (Fungal)

Symptoms: Older leaves first develop small, yellow to tan spots with a light green to yellow halo between necrotic and healthy green tissues. As the disease progresses, smaller spots congeal into larger dead patches or lesions, typically showing darker concentric ring patterns within the dying tissue. Leaves curl and eventually die off. Stems can show linear cankers with water-soaked splits or discolored sections. Overall, plants become stunted but do not immediately die.

Management: Spreads via wind or rain.

• No resistant varieties are currently available.
• Rotate with non-cucurbit crops for at least two years.
• Till residual crop debris under immediately after harvest.
• Avoid overhead irrigation and working with or around plants with wet leaves.
• Apply preventative fungicides such as Copper (Bonide® Copper) or a Bio-fungicide (Bonide® Revitalize®) as directed.

Fusarium wilt - Fusarium oxysporum (Fungal)

Symptoms: Wilting stems and leaves, typically unilaterally to one side of the plant or another. Foliage can appear dull, with initial gray-green coloration that turns more yellow as the disease progresses. Cut stem cross sections show reddish to brown vascular discoloration. Note that other diseases, such as Verticillium and Bacterial wilt, exhibit similar symptoms; however, bacterial wilt produces white vascular ooze when stems are cut.

Management: Soil-borne fungi spread on infected plant debris and infected soil. Fungi can live in soil for up to 10 years without a suitable host and inoculum can spread during localized flooding as well.

• Plant-resistant varieties such as 'Charleston Grey,' 'Crimson Sweet,' 'Jubilee,' 'Sangria,' or 'Sweet Eat'n' Hybrid.
• Purchase seeds from reputable, licensed, and certified companies.
• Plant non-cucurbit crops for 3-4-year rotation.
• Improve soil drainage and aeration.

Powdery Mildew - Podosphaera xanthii (Fungal)

Symptoms: Appears first on older, mature leaves as yellow blotching on surfaces with white sporangia on leaf undersides that is not easily detected by the naked eye. As the infection progresses, characteristic white powdery blotches become more noticeable, covering the upper and lower leaf surfaces. Eventually, the leaves brown, dry and drop off the vine. It is not ordinarily fatal to the plant, but infections should be prevented since defoliation results in production and leads to poor fruit quality.

Management: Spores are wind spread and can be great distances. PM is favored by cloudy, warm temperatures, often with higher humidity, but infections do not require wet conditions or initial infection. It can spread as quickly during dry weather. The fungi have a symbiotic relationship with their host plants and only thrive on living cucurbit plants. However, dormant spores can live on plant debris for long periods, causing eradication to be complicated.

• Prevention and early detection are vital to reducing major outbreaks.
• Scouting and continual monitoring as vine density increases is essential.
• Thorough sanitation after cucurbit crops is a critical step for prevention.
• Rotate out of cucurbits for 2-3 years.
• Fungicide applications can be helpful, especially early on, but avoid synthetic fungicides since PM can quickly develop resistance. Instead, natural fungicides like mineral-based or bio-fungicides are utilized for the best protection. Sulfur, for example, is effective and is allowed for organic growers. Read and follow all label instructions.
• Some PM-resistant varieties are available. 'Cal Sweet Bush' and 'Golden Crown' Hybrid are examples.

Anthracnose - Colletotrichum orbiculare (Fungal)

Symptoms: Angular dark brown to black spots or lesions with a yellow halo form on leaves, typically near the veins. Spots are small initially but can be numerous and coalesce to form larger spots on the leaf surface and undersides. The center of the spots can dry and drop out, causing a shot-hole appearance. Elongated sunken lesions form on stems and fruit surfaces. Secondary infections and mold form quickly from fruit lesions.

Management: Disease spreads via wind and rain. Some resistant varieties are available.

• Plant-resistant varieties such as 'Crimson Sweet,' 'Golden Crown' Hybrid, 'Jubilee,' 'Mini Love Hybrid,' 'Sangria' Hybrid, and 'Sweet Eat'n' Hybrid.
• Rotate with non-cucurbit crop every 1-2 years.
• Remove then deep till old crop debris after harvest.
• Avoid overhead irrigation or working around wet plants.
• Preventative fungicide applications should commence when vines start to run approx. once every 7-10 days, using copper or bio-fungicide following product label recommendations for specific spray intervals, especially during humid or wet weather conditions.

Cercospora leaf spot - Cercospora citrullina (Fungal)

Symptoms: Initial symptoms occur on older leaves as small dark spots with light tan to white centers. As the disease progresses, the lesions coalesce to cover larger leaf surfaces. Spots typically show darker margins with the center tissue becoming brittle or cracked, creating larger lesion areas with yellow halo or entire leaves turning yellow. Infections can occur on stems in conducive wet, warm, and humid weather, but the symptoms do not affect the fruits.

Management: The disease is spread via wind and rain. Spores can be carried long distances. The disease favors warm, humid weather conditions.

• No resistant varieties are currently available.
• Remove & destroy leftover plant material.
• Till under cucurbit debris after harvest.
• Rotate out of cucurbit crops for 2-3 years.
• Avoid overhead irrigation, and don't work with plants while the foliage is wet.
• Apply preventative fungicides like Copper, Chlorothalonil (Bonide® Fung-onil®), or bio-fungicide (Monterey® CDC)

Gummy stem blight - Didymella bryoniae (Fungal)

Symptoms: Round or irregular brown, water-soaked lesions starting from the leaf veins with faint concentric rings. On the stems, starting near the nodes, brown vine canker lesions will appear, eventually girdling the entire stem. Soft, circular brown lesions appear on the fruit. Lesions on stems or fruit may ooze sticky, amber fluid, or black rot similar in appearance to blossom end rot.

Management: Like others, this fungal disease is favored by high humidity and moisture. Spores spread via wind and water and can survive on cucurbit plant debris or debris from other garden host plants such as beans, okra, and tomatoes. Hypocotyls, cotyledons, and immature seedling stems of watermelon or cantaloupe are highly susceptible.

• No GSB-resistant varieties are available.
• Remove & destroy old cucurbit plant material after harvest.
• Deep till debris post-harvest to ensure proper decomposition of material.
• Rotate out of cucurbit or host plants for 2-4 years.
• Avoid overhead irrigation.
• Scout immature plants in the field for characteristic brown linear lesions on stems near the nodes. If discovered, remove and destroy potentially infected plants ASAP.
• Cultural controls are the best management practice. However, some preventative fungicide treatments, especially before conducive weather conditions, can help reduce exposure to GSB. Follow label instructions for listed fungicides such as Bonide® Captan® or Bonide® Fung-onil®, and rotate products from application to application.

Verticillium wilt - Verticillium dahlia (Fungal)

Symptoms: Chlorotic leaf symptoms appear shortly after fruit sets, with necrotic tissues developing and leaf dieback. Symptoms usually occur unilaterally on plants. Cut into cross-sections, the roots will show vascular discoloration.

Management: Soil-borne fungi can survive in soil for many years, entering plant roots through wounds or breaks. This disease favors cool, mild weather typical of early spring conditions. Over 300 known host plants exist across various vegetables, ornamental, woody, and herbaceous plants. No effective treatments exist after infection. Remove and destroy infected plants.

• No resistant varieties are available.
• Do not plant in areas where susceptible crops have grown previously.
• Avoid transferring soil from one area to another.
• Thoroughly clean soil off tools and equipment after use.
• Wait to transplant melons or cucurbits until soil and air temperatures are 70°F or higher.
• Dispose of potentially infected plant materials. DO NOT compost. Instead, burn or throw out debris.

Angular leaf spot - Pseudomonas syringae (Bacterial)

Symptoms: Small, water-soaked spots on leaves that expand between leaf veins, becoming angular in shape. In humid conditions, lesions exude a milky fluid that dries to form a white crust on or near the lesions. As the disease progresses, spots turn tan with a yellow halo. Centers dry to brown or tan, then typically drop out, creating a shot-hole appearance.

Management: This bacterium survives on infected plant debris or in the soil and is spread by rain, splashing water, or frequently by human contact with plants, tools, or machinery in the field, especially when the foliage is wet. It mostly occurs on cucumber plants but can be more of an issue on melons in warmer climates.

• No melon-resistant varieties exist.
• Avoid overhead irrigation; do not work around plants when the foliage is wet.
• Rotate out of cucurbits for at least two years.
• Remove and destroy potentially infected plants ASAP.
• Deep till plant debris after harvest.
• Preventative liquid copper applications may help reduce potential infection.

Bacterial fruit blotch (BFB) - Acidovorax avenae (Bacterial)

Symptoms: Small, water-soaked spots on leaf surfaces and undersides, often adjacent to leaf veins. As the disease progresses, dark spots congeal to become larger necrotic blotches. On fruits, small, water-soaked spots or lesions develop as spots grow. Coloration can look brown to reddish, with surface cracks and associated white bacterial ooze, especially during wet weather conditions.

Management: Heat, humidity, and wet conditions favor the spread of disease, which is primarily spread by water splash or direct contact.

• Currently, no resistant varieties are available; however, seedless (Triploid) varieties are less susceptible than seeded varieties. Some observational information concludes that dark-rind varieties are less susceptible than lighter-colored rind types.
• Rotate out of cucurbits for 3-4 years.
• Deep till cucurbit or potentially infected debris.
• Monitor immature plants for symptoms and remove any with symptoms for testing by a qualified plant pathologist for positive disease identification since many diseases can have similar or mimicking symptoms.
• Avoid overhead irrigation or working with plants when foliage is wet.
• Thoroughly clean and sterilize tools or equipment used in gardens or fields containing BFB-confirmed infection.
• If symptoms are observed on immature plants, Copper fungicide applications may help reduce spread. Read and follow all product label instructions for application rates and spray schedule, and maintain applications as directed until fruits have reached maturity. The best prevention with copper occurs when initial applications commence and the first male flowers appear.

Root-knot nematode - Meloidogyne incognita (Nematode)

Symptoms: Plants can show stunted or weak growth behaviors and/or develop yellow foliage. Often, lackluster growth will occur sporadically or randomly in a row or patch, which can indicate problems in the roots. Pulling up a symptomatic plant to investigate the roots is the most accurate method of determining if nematodes are the cause. Many other diseases, as well as abiotic factors, can cause similar-looking symptoms. If the roots are infested with nematodes, they will appear gnarled, with small, 1/4 - 1/2-inch galls noticeable on the roots.

Management: Nematodes exist in virtually all soils, with many microscopic worms occurring in different parts of the country and soil types. Some are beneficial, and others can cause problems in crop plants. Generally, nematodes are less problematic in cold weather areas with thorough ground freezing. Like other fungal diseases, the higher the population, the bigger the problem. Preventing damaging population build-ups or initial infestations is essential. Spreads via garden or yard soil transport on dirty tools, equipment, or transplants from one area to another.

• Currently, no nematode-resistant watermelon varieties are available.
• Clean soil off tools and equipment after use.
• Avoid moving soil from one area to another.
• Rotate out of nematode susceptible crops for 2-3 years. (More difficult with southern nematodes since so many host plants exist)
• Soil solarization in warm climates may be possible but should be done long enough to influence nematode populations. Have the soil tested to establish the initial population, then re-test after solarization to determine if the process was effective.
• No other chemical treatments are effective or recommended.

Mosaic viruses - Potyviruses

Symptoms: Many virus types exist that affect watermelon plants, including papaya ringspot virus (PRSV), watermelon mosaic virus (WMV), and zucchini yellow mosaic virus (ZYMV). All show similar symptoms, including stunted plant growth, dwarfing leaves, puckered or misshapen leaves, mottled yellow and darker green discolorations or blotches on foliage, reduced fruit set, or abnormal fruit development. Immature plants are most severely affected. Older plants that become infected are typically less affected. Viruses are uncommon unless you garden where more significant numbers of commercially grown melons or other cucurbits are grown.

Management: Viruses are spread by insects. Aphids are most responsible, with cucumber beetles being the second most probable vector, although viruses can also be spread plant to plant mechanically by tools or equipment. With insects, their piercing and sucking mouth parts that pick up infected sap transmit viruses from crop to crop. This is why topical insecticide sprays don't work to prevent viruses since the aphids can feed well before succumbing to any lethal contact chemicals. Infection rates typically increase as aphid populations increase during a planting season.

• Currently, no virus-resistant watermelon varieties are available.
• Controlling aphids or beetles as much as possible helps to limit plant exposure to viruses.
• Plant as early in the season as possible to avoid the highest aphid populations.
• Practice thorough weed management in adjacent areas since many weeds can be virus-host plants and attractive to aphids.
• Planting taller trap crops such as grasses or broom corns can effectively intercept migrating aphids.