Black discoloration on watermelons signals a severe problem requiring immediate diagnosis. This blackening indicates either significant tissue decay caused by aggressive pathogens or a physiological breakdown within the fruit. Causes fall into two main groups: infectious diseases, primarily involving fungi and bacteria, and non-infectious issues related to environmental or nutritional imbalances. Identifying the exact nature and location of the dark spots is the first step toward effective treatment.
Fungal Diseases Causing Dark Spots and Rot
Fungal pathogens frequently cause dark lesions and rot in watermelon fields, flourishing under high humidity and warm temperatures. Two common culprits are Anthracnose and Gummy Stem Blight, both causing substantial fruit loss. Anthracnose, caused by Colletotrichum orbiculare, manifests on the fruit as circular, black, and sunken spots. During high moisture periods, these lesions may exude masses of salmon-pink spores visible on the surface.
The fungus also creates dark, often angular, spots on the leaves that can develop tan or gray centers. Anthracnose can overwinter in infected plant debris, making crop rotation and field sanitation important preventative measures. Protecting plants with preventative fungicides, such as those containing chlorothalonil or mancozeb, is often necessary, especially before symptoms appear.
Gummy Stem Blight, caused by Stagonosporopsis cucurbitacearum, also leads to dark spots on the foliage, which are typically larger, more rounded, and often start at the leaf margins. On the fruit, the disease causes a fruit rot phase, sometimes called “black rot,” where dark, water-soaked spots appear. The defining feature is the formation of cankers on the stems and leaf petioles that may ooze a characteristic amber-colored, gummy substance.
Controlling Gummy Stem Blight involves reducing surface water on the plants, as the pathogen spreads easily with splashing water. Growers should implement a minimum two-year rotation with non-cucurbit crops to break the disease cycle. Proper air circulation around the plants, achieved through strategic pruning and spacing, helps foliage dry faster after rain or irrigation.
Bacterial Infections and Slimy Black Lesions
Bacterial infections often present differently than fungal ones, frequently resulting in a slimy texture to the black lesions. The most severe bacterial issue causing blackening is Bacterial Fruit Blotch (BFB), caused by Acidovorax citrulli. Initial symptoms on the fruit are small, water-soaked spots that appear dark green and “oily” on the upper surface.
These lesions expand rapidly, coalescing into a large, dark green to black blotch. As the infection progresses, the black area may crack, creating an entry point for secondary decay organisms that lead to slimy, internal rot. The bacterium is often introduced via contaminated seeds or infected transplants, making BFB a serious seed-borne disease.
Chemical control for bacterial diseases is more challenging than for fungi, but copper-based bactericides can help suppress BFB spread. The most effective strategy centers on prevention, including using certified, disease-free seed and avoiding overhead watering systems. Eliminating splashing water prevents the bacteria from spreading from infected leaves to developing fruit.
Non-Infectious Causes: Water Stress and Nutrient Issues
Not all black spots are caused by living pathogens; some result from a physiological disorder called Blossom End Rot (BER). This condition is not contagious and is easily distinguished because the black or dark brown lesion only appears on the bottom end of the fruit, where the flower was attached. The affected area is dry, sunken, and leathery, contrasting with the soft, slimy rot caused by infectious agents.
Blossom End Rot is directly linked to a localized calcium deficiency within the developing fruit tissue. This deficiency occurs even in calcium-rich soils when the plant cannot move the nutrient effectively due to inconsistent water supply. Erratic irrigation, such as periods of drought followed by heavy watering, interferes with the continuous transport of calcium to the fruit’s rapidly growing cells.
To manage BER, maintaining a consistent soil moisture level is necessary, often accomplished through mulches and drip irrigation systems. The soil pH should be kept between 6.5 and 6.8 to ensure calcium is readily available for uptake. Avoiding high-nitrogen fertilizers is also recommended, as excessive nitrogen can promote rapid vine growth that outpaces the plant’s ability to supply calcium to the fruit.