Watermelon Diseases
Back to
Watermelon Page

Black rot symptoms on watermelon, honeydew melon, and butternut squash
Fig. 1 
Black rot symptoms on watermelon, honeydew melon, and butternut squash

Sporulation of Pseudoperonospora cubensis on underside of pumpking (cv. Howden) leaf
Fig. 7
Sporulation of Pseudoperonospora cubensis on underside of pumpking (cv. Howden) leaf

Watermelon leaf showing an even distribution of powdery mildew (Podosphaera xanthii) over the entire leaf surface
Fig. 10
Watermelon leaf showing an even distribution of powdery mildew (Podosphaera xanthii) over the entire leaf surface

Various stages of fruit rot of watermelon caused by Phytophthora capsici. Bottom, early symptoms; top, advanced symptoms
Fig. 12
Various stages of fruit rot of watermelon caused by Phytophthora capsici. Bottom, early symptoms; top, advanced symptoms

Mature fruit showing lesion, cracking, foaming bacterial ooze and copper spray residue on fruit surface. Bacterial ooze has been colonized by the fungus Geotrichum candidum
Fig. 16
Mature fruit showing lesion, cracking, foaming bacterial ooze and copper spray residue on fruit surface. Bacterial ooze has been colonized by the fungus Geotrichum candidum

Alternaria leaf blight, many foliar lesions of varying ages
Fig. 20
Alternaria leaf blight, many foliar lesions of varying ages

Watermelon transplant showing a water-soaked lesion on the lower stem. Symptoms are consistent with infection by Rhizoctonia solani
Fig. 23
Watermelon transplant showing a water-soaked lesion on the lower stem.  Symptoms are consistent with infection by Rhizoctonia solani.

Fusarium wilt symptoms on watermelon stem
Fig. 24 
Fusarium wilt symptoms on watermelon stem

Note the cracking of necrotic tissue, irregular lesion margins and shot hole effect
Fig. 27
Note the cracking of necrotic tissue, irregular lesion margins and shot hole effect

Back to
Watermelon Page


The most important fungal diseases are gummy stem blight (caused by Didymella bryoniae/Phoma cucurbitacearum) and downy mildew (caused by Pseudoperonospora cubensis). Some consider gummy stem blight to be the most troublesome disease on watermelons in Florida. Other occasional or minor diseases include phytophthora blight (caused by Phytophthora capsici), bacterial fruit blotch (caused by Acidovorax avenae subsp. citrulli), alternaria leafspot (caused by Alternaria cucumerina), seedling blight (caused by Pythium spp., Rhizoctonia solani, and Fusarium spp.), fusarium wilt (caused by Fusarium oxysporum f.sp. niveum), angular leafspot (caused by Pseudomonas syringae), anthracnose (caused by Colletotrichum orbiculare), rind necrosis (usually caused by Erwinia spp.), and powdery mildew (caused by Erysiphe cichoracearum). Blossom end rot, a physiological disorder related to calcium deficiency and water stress, is also an occasional problem. Finally, Cercospora leafspot (caused by Cercospora citrullina) and southern blight (also called southern stem rot or white mold and caused by Sclerotium rolfsii) are only rarely seen on watermelons in Florida (Hochmuth et al. 1997; Maynard 2003; Roberts and Kucharek 2005). 1



Gummy stem blight
(Fig. 1)
caused by Didymella bryoniae/Phoma cucurbitacearum

Gummy stem blight (caused by Didymella bryoniae/Phoma cucurbitacearum) is one of the most important diseases of watermelons in Florida. It can affect most aboveground parts of the watermelon plant. Symptoms may be difficult to distinguish from other foliar diseases and include brown to black leaf spots, stem cankers, or fruit spots. Lesions eventually develop black spots, which are the fruiting bodies of the fungus (pycnidia). A brown, gummy material may be produced on the surface of lesions. On watermelon, the disease is referred to as black rot. It produces water-soaked spots on the fruit, which leak gummy material (Roberts and Kucharek 2005; Maynard and Hopkins 1999). 1

Necrosis on cantaloupe transplants due to gummy stem blight Water-soaked regions on the stem of cantaloupe transplants due to gummy stem blight Canker on watermelon stem due to gummy stem blight
Fig. 2 Fig. 3 Fig. 4
Gummy red exudation from a D. bryoniae-infected cantaloupe stem. Brown, irregularly shaped lesions containing a concentric ring pattern can be seen on cantaloupe and watermelon leaf margins and interveinal areas with high moisture retention
Fig. 5 Fig. 6

Fig. 2. Necrosis on cantaloupe transplants due to gummy stem blight
Fig. 3. Water-soaked regions on the stem of cantaloupe transplants due to gummy stem blight
Fig. 4. Canker on watermelon stem due to gummy stem blight
Fig. 5. Gummy red exudation from a D. bryoniae infected cantaloupe stem
Fig. 6. Brown, irregularly shaped lesions containing a concentric ring pattern can be seen on cantaloupe and watermelon leaf margins and interveinal areas with high moisture retention

Further Reading
Management of Gummy Stem Blight (Black Rot) on Cucurbits in Florida from the University of Florida pdf 8 pages
Gummy Stem Blight and Black Rot from the University of Minessota Extension pdf



Downy Mildew (Fig. 7)
caused by Pseudoperonospora cubensis

Downy mildew (caused by Pseudoperonospora cubensis) is another key fungal disease of watermelons in Florida. It occurs principally in South Florida, and its likelihood of occurrence in the state decreases with distance to the north. It occurs every year in South Florida, while in North Florida its presence is sporadic. Ideal conditions for the development of downy mildew include nighttime temperatures of 55 °F–75 °F (12.8 °C–23.9 °C) and relative humidity above 90%. In South Florida, watermelons planted in the fall and spring may be infected with downy mildew as early as the appearance of the first true leaves (Roberts and Kucharek 2005). 1

Sporulation of Pseudoperonospora cubensis on underside of pumpking (cv. Howden) leaf Downy Mildew on Melon Leaf
Fig. 8 Fig. 9

Fig. 8. Sporulation of Pseudoperonospora cubensis on underside of pumpkin (cv. Howden) leaf
Fig. 9. Downy Mildew on Melon Leaf

Further Reading
Management of Cucurbit Downy Mildew in Florida from the University of Florida pdf 9 pages
Downy Mildew of Pumpkin from Purdue University Extension pdf



Powdery Mildew (Fig. 10)

Powdery mildew (caused by Podosphaera xnathii or Erysiphe cichoracearum) is a common and serious disease of watermelons in Florida. Since no resistant cultivars are available for this disease, fungicides are the primary means for management. Typical symptoms are the production of white, powder-like colonies on the leaf surface which are found on the leaves and petioles. Yellow spots can also be noticed the leaf, which can be confused with other diseases, such as downy mildew or bacterial leaf spots. In severe cases, the fungus will colonize the whole leaf, leading to death. 1
Dry conditions, usually followed by periods of free moisture at night, tend to promote the reproduction and spread of this pathogen. Spores are produced in chains on the leaf surface and are removed from the leaf by wind. These spores then infect watermelon leaves and the cycle repeats multiple times throughout the season. The pathogen can develop and spread quickly under favorable conditions with symptoms appearing three days after infection. 1

Squash plant infected with powdery mildew. Notice how old leaves are completely covered with talc-white powdery mildew (arrows), whereas new leaves appear to be free of this disease (circle). Underside of cantaloupe leaf showing many powdery mildew lesions Very heavy powdery mildew sporulation on watermelon leaf
Fig. 11 Fig. 11a Fig. 11b

Fig. 11. Squash plant infected with powdery mildew. Notice how old leaves are completely covered with talc-white powdery mildew (arrows), whereas new leaves appear to be free of this disease (circle). 



Phytophthora blight (Fig. 12)
caused by Phytophthora capsici

Phytophthora blight (caused by Phytophthora capsici) occurs sporadically in Florida, but the disease can spread rapidly if the weather is favorable to the fungus, causing serious losses. In 1998, the disease was widespread and severe on several vegetable crops in Florida. In the southwest region of the state (Lee, Collier, and Hendry Counties), 25% of watermelon plants in some surveyed fields were found to have the disease, and disease incidence on watermelons in Manatee County was 36%. Previously, phytophthora blight had affected only the fruit of watermelons, but during the 1998 outbreak, many watermelon plants were affected and died in Manatee County, regardless of plant age. 1
Watermelon foliage is less susceptible to P. capsici than that of summer squash. Foliar symptoms of Phytophthora blight in watermelon are generally limited to water-soaked leaf blotches, which dry and turn brown, and dieback. However, all stages of watermelon fruit are highly susceptible. Early symptoms of fruit rot include rapidly expanding, irregular, brown lesions which become round to oval. Concentric rings may occur within a lesion. The centers of rotted areas are covered with grayish fungal-like growth, while the outer margins of lesions appear brown and water-soaked (Fig. 12). The entire fruit eventually decays. 2

Cross-section through watermelon fruit showing the depth and breadth of a Phytophthora blight lesion Fruit lesion Fruit with several large lesions
Fig. 13 Fig. 14 Fig. 15

Fig. 13. Cross-section through watermelon fruit showing the depth and breadth of a Phytophthora blight lesion
Fig. 14. Fruit lesion
Fig. 15. Fruit with several large legions

Further Reading
Vegetable Diseases Caused by Phytophthora capsici in Florida from the University of Florida pdf 5 pages



Bacterial Fruit Blotch (Fig. 16)
caused by Acidovorax avenae subsp. citrulli

Bacterial fruit blotch (caused by Acidovorax avenae subsp. citrulli) occurs sporadically, affecting only a limited number of fields, but it can cause severe losses where it occurs. Also called greasy fruit spot or watermelon fruit blotch, this bacterial disease is a relatively new problem in Florida watermelon production. The pathogen produces both a leafspot and a fruit spot, and symptoms can occur on seedlings, leaves, and fruit. When it attacks seedlings, water-soaked lesions form, and the seedling can collapse and die. Found principally along the major veins, lesions on the leaves are light to reddish brown and can occur throughout the season. Although they are generally inconspicuous and do not usually contribute to defoliation, leaf lesions serve as reservoirs of the bacteria that later infect the fruit. On the watermelon fruit, lesions start as very small, water-soaked areas that usually do not appear until close to harvest. Later, they enlarge, and within two weeks, they may cover the entire upper surface of the fruit. Cracks may later appear on the rind, which may show internal discoloration, and the whole fruit may rot within, as secondary pathogens enter the open lesions (Roberts and Kucharek 2005; Somodi et al. 1991; Hopkins, Cucuzza, and Watterson 1996; Hopkins 1991; Frankle, Hopkins, and Stall 1993). 1

Mature fruit showing lesion, deep cracking, bacterial ooze and copper spray residue on fruit surface Cross section through mature fruit shows external and internal damage Cross section through mature fruit shows external and internal damage
Fig. 17 Fig. 18 Fig. 19

Fig. 17. Mature fruit showing lesion, deep cracking, bacterial ooze and copper spray residue on fruit surface
Fig. 18,19. Cross section through mature fruit shows external and internal damage

Further Reading
Bacterial Fruit Blotch from the Queensland Government pdf
Watermelon Bacterial Fruit Blotch from the University of Missouri Extension pdf 5 pages



Alternaria Leafspot (Fig. 20)
caused by Alternaria cucumerina

Alternaria leaf spot (caused by Alternaria cucumerina) is a minor disease on Florida watermelons. Symptoms begin on the upper surface of older leaves as very small yellow or tan spots that may be surrounded by light green or yellow halos or by a water-soaked area. The spots later grow up to ¾ of an inch (2 cm) in diameter and turn brown in color. Similar in appearance to gummy stem blight, the lesions are the source of spores spread primarily by the wind. Under severe infestations, the disease produces leaf curling, defoliation (which leaves the fruit susceptible to sunscald), and premature ripening. Lower yields, lower fruit sugar, and fruit deformity may occur (Roberts and Kucharek 2005). 1

Alterneria leaf blight of cantaloupe Alternaria leaf blight on cucumber
Fig. 21 Fig. 22

Fig. 21. Alterneria leaf blight of cantaloupe
Fig. 22. Alternaria leaf blight on cucumber

Further Reading
Alternaria Leaf Blight of Cucurbits from the University of Maine Extension pdf
Alternaria Leaf Spot or Blight of Cucurbits from the University of Illinois Extension pdf



Seedling Blight (Fig. 23)
caused by Pythium spp., Rhizoctonia solani, and Fusarium spp.

Seedling blight (caused by Pythium spp., Rhizoctonia solani, and Fusarium spp.) can kill seedlings before or after they emerge. Rot symptoms, either wet or dry, are observed in the presence of this disease. The lesions resulting from seedling blight caused by Rhizoctonia solani (Fig. 23) are reddish-brown to orange and appear sunken. Pythium spp. cause shoots or roots to appear gray and water soaked. The incidence of seedling blight is higher when watermelons are planted in cool soils because seedlings that emerge slowly are more susceptible to pathogenic fungi (Roberts and Kucharek 2005). 1



Fusarium Wilt (Fig. 24)
caused by Fusarium oxysporum f. sp. niveum  

Fusarium wilt symptoms on watermelon Symptoms of fusarium wilt in a watermelon field
Fig. 25 Fig. 26

Fig. 25. Fusarium wilt symptoms in a watermelon field
Fig. 26. Symptoms of fusarium wilt in a watermelon field

Further Reading
Fusarium Wilt of Watermelon from Purdue University Extension pdf
Fusarium Wilt of Watermelon from the University of North Carolina Extension pdf 5 pages
Fusarium Wilt in Seedless Watermelons from the University of Florida, university of Maryland and the US Department of Agricultural Research Service pdf 6 pages



Anthracnose (Fig. 27)
caused by Colletotrichum orbiculare

In the past, anthracnose (caused by Colletotrichum orbiculare) was a serious disease in Florida watermelon production, but the use of resistant varieties has limited its impact. When it does occur, anthracnose can destroy the entire field if not controlled, particularly after several days of warm, rainy weather (Bertelsen et al. 1994; Roberts and Kucharek 2005).
All above ground plant parts may be affected by this fungal disease, and infected plants may die under severe conditions. Early symptoms of the disease include angular, brown to black leaf spots on older leaves, similar in appearance to those of gummy stem blight or downy mildew. Tan, oval-shaped lesions may appear on the stems. Spores from leaf and stem lesions later infect the fruit, producing sunken, water-soaked spots. 1

Anthracnose on watermelon stems. Note the black stromata in the lesion at the center of the frame Cracking of lesion is characteristic of anthracnose Cracking of lesion is characteristic of anthracnose
Fig. 28 Fig. 29 Fig. 30
Anthracnose (Colletotrichum orbiculare) (Berk. & Mont.) Arx. Fruit lesions
Fig. 30a

Fig. 28. Anthracnose on watermelon stems. Note the black stromata in the lesion at the center of the frame
Fig. 29, 30. Cracking of lesion is characteristic of anthracnose
Fig. 30a. Anthracnose (Colletotrichum orbiculare) (Berk. & Mont.) Arx. Fruit lesions

Further Reading
Anthracnose of Cucumber, Muskmelon and Watermelon from Purdue University Extension pdf



Viruses transmitted by Aphids

Aphids damage watermelon plants directly by feeding, as well as indirectly by transmitting viruses. The three principal viruses affecting watermelons in Florida (i.e., papaya ringspot virus type W, watermelon mosaic virus 2, and zucchini yellow mosaic virus) can be transmitted by aphids that colonize watermelons and by a number of aphid species that do not reproduce on watermelons. These include A. middletonii, A. spiraecola (spirea aphid, also known as green citrus aphid), and U. pseudambrosiae (Webb 2003). 1

Watermelon Mosaic Virus 2

Watermelon mosaic virus 2 (WMV-2) is another potyvirus that causes regular problems for watermelon production, particularly in Central and North Florida during spring production. Incidence of watermelon mosaic virus 2 in Central Florida rarely exceeded 5% during the 1960s and 1970s. However, in the late 1980s, the virus caused severe losses to the spring watermelon crop in Central and North Florida, with incidence in fields of up to 100%. Infection early in the season results in yield loss and reduced fruit quality because of blemishes, particularly rings and spots on the watermelon rind. However, if the virus does not enter a field until the time of fruit set, little to no yield difference is likely (Webb, Kok-Yokomi, and Voegtlin 1994; Webb and Linda 1993). 1

Papaya Ringspot Virus Type W

Formerly referred to as watermelon mosaic virus 1, papaya ringspot virus type W (PRSV-W) is a greater problem in South Florida, where it can severely damage the crop. Epidemics of papaya ringspot virus have been linked to weeds as the primary source of inoculum (Roberts and Kucharek 2005). Papaya ringspot virus has been shown to overwinter in wild cucurbits such as wild balsam apple and creeping cucumber, and it is spread from these to spring-planted watermelons in South Florida. During most years, the virus reaches Central and North Florida in early summer (Webb and Linda 1993). 1

Yellow Zucchini Mosaic Virus

Zucchini yellow mosaic virus (ZYMV), the third potyvirus affecting watermelon production in the state, was first observed in Florida in 1981 and identified in 1984. At that time, researchers demonstrated that it caused mosaic symptoms in watermelons and other cucurbits and could be transmitted by the aphids Myzus persicae and Aphis spiraecola (Purcifull et al. 1984). The incidence of zucchini yellow mosaic virus is generally lower than the other two potyviruses that affect watermelons. In Central Florida, zucchini yellow mosaic virus is most common in late spring and fall. The virus causes more severe symptoms than the watermelon mosaic virus 2, with resulting discoloration and distortion of fruits (Webb and Linda 1993). 1

Further Reading
Zucchini Yellow Mosaic from CTAHR University of Hawai'i at Manoa pdf 5 pages
Squash Vein Yellowing Virus, Causal Agent of Watermelon Vine Decline in Florida from the Florida Departmemt of Agriculture & Consumer services pdf 4 pages



Further Reading
Florida Crop/Pest Management Profile: Watermelon from the University of Florida pdf 26 pages
Foliar Diseases of Cucurbits from the University of Kentucky Extension pdf 4 pages
Cucumber, Squash, Melon & Other Cucurbit Diseases from Clemson University Extension pdf 7 pages


Bibliography

1 Elwakil, Wael M. and Mossler, Mark A. "Florida Crop/Pest Management Profile: Watermelon." edis.ifas.ufl.edu. This document is CIR1236, one of a series of the Horticultural Sciences Department, UF/IFAS Extension. Publication date Oct. 2000. Revised July 2017. Web. 28 July 2017.
2 Gevens, Amanda J., Roberts, Pamela D., McGovern, R.J. and Kucharek, T.A. "Vegetable Diseases Caused by Phytophthora capsici." edis.ifas.ufl.edu. This document is SP159, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date May 1994. Reviewed Aug.2011. Web. 27 Oct. 2014.

Photographs

Fig. 1 Kucharek, Tom. Black rot symptoms on watermelon. N.d. edis.ifas.ufl.edu. Web. 25 Oct. 2014.
Fig. 2,3,4,5 Dankers, Hank. Management of Gummy Stem Blight (Black Rot) on Cucurbits in Florida. N.d. edis.ifas.ufl.edu. Web. 25 Oct. 2014.
Fig. 7,8 Holmes, Gerald. Sporulation of Pseudoperonospora cubensis on pumpkin (cv. Howden) leaf. 2010. California Polytechnic State University at San Luis Obispo. bugwood. org.  Web. 26 Oct. 2014.
Fig. 9 Downy Mildew on Melon Leaf. 2010. Clemson University - USDA Cooperative Extension Slide Series. bugwood. org. Web. 26 Oct. 2014.
Fig. 10 Holmes, Gerald. Watermelon leaf showing an even distribution of powdery mildew (Podosphaera xanthii) over the entire leaf surface. 1995. Under (CC BY-NC 3.0 US). California Polytechnic State University at San Luis Obispo. bugwood. org. Web. 26 Oct. 2014.
Fig. 11 Powdery Mildew of Cucurbits in Florida. N.d. edis.ifas.ufl.edu. Web. 26 Oct. 2014
Fig. 11a Holmes, Gerald. Underside of cantaloupe leaf showing many powdery mildew lesions. 1996. California Polytechnic State University at San Luis Obispo. bugwood. org. Under (CC BY-NC 3.0 US).  Web. 26 Oct. 2014.
Fig. 11b Holmes, Gerald. Very heavy powdery mildew sporulation on watermelon leaf. 2000. California Polytechnic State University at San Luis Obispo. bugwood. org. Under (CC BY-NC 3.0 US). Web. 26 Oct. 2014.
Fig. 12 Vegetable Diseases Caused by Phytophthora capsici. N.d. edis.ifas.ufl.edu. Web. 27 Oct. 2014.
Fig. 13,14,15 Holmes, Gerald . Phytophthora bligh. 2000, 2001. California Polytechnic State University at San Luis Obispo. bugwood. org. Web. 27 Oct. 2014.
Fig. 16,1718,19 Holmes, Gerald . Bacterial fruit blotch. 1999. bugwood. org. California Polytechnic State University at San Luis Obispo. Web. 28 Oct. 2014.
Fig. 20 Holmes, Gerald . Alternaria leaf blight. 1999. California Polytechnic State University at San Luis Obispo. bugwood. org. Web. 28 Oct. 2014.
Fig. 21,22 Brock, Jason. Alternaria leaf blight. 2012. University of Georgia. bugwood. org.  Web. 28 Oct. 2014.
Fig. 23 Fusarium wilt symptoms on watermelon. 2003. Clemson University - USDA Cooperative Extension Slide Series. bugwood. org. Web. 28 Oct. 2014.
Fig. 24 Holmes, Gerald. Watermelon transplant showing a water-soaked lesion on the lower stem. Symptoms are consistent with infection by Rhizoctonia solani. 1996. California Polytechnic State University at San Luis Obispo. bugwood. org. Web. 28 Oct. 2014.
Fig. 25,26 Brock, Jason. Fusarium wilt symptoms. 2012. University of Georgia. bugwood. org. Web. 28 Oct. 2014.
Fig. 27 Holmes, Gerald. Anthracnose (Colletotrichum orbiculare) (Berk. & Mont.) Arx. Note the cracking of necrotic tissue, irregular lesion margins and shot hole effect. 2000. California Polytechnic State University at San Luis Obispo. bugwood. org. Under (CC BY-NC 3.0 US). Web. 28 June 2016.
Fig. 28 Holmes, Gerald. Anthracnose on watermelon stems. Note the black stromata in the lesion at the center of the frame. 2000. bugwood. org. California Polytechnic State University at San Luis Obispo. Web. 31 Oct. 2014.
Fig. 29,30 Holmes, Gerald. Anthracnose (Colletotrichum orbiculare) (Berk. & Mont.) Arx. Cracking of lesion is characteristic of anthracnose. 2000. California Polytechnic State University at San Luis Obispo. bugwood. org. Under (CC BY-NC 3.0 US). Web. 28 June 2016.
Fig. 30a Holmes, Gerald. Anthracnose (Colletotrichum orbiculare) (Berk. & Mont.) Arx. Fruit lesions. 2000. California Polytechnic State University at San Luis Obispo. bugwood. org. Under (CC BY-NC 3.0 US). Web. 28 June 2016.

Published 21 Oct. 2014 LR. Last updated 28 July 2017 LR
© 2013 - growables.org
about credits disclaimer sitemap updates