Myrothecium Leaf Spot Of Watermelon: What Is Watermelon Myrothecium Leaf Spot
By: Bonnie L. Grant, Certified Urban Agriculturist
There’s a fungus among us! Myrothecium leaf spot of watermelon is a mouthful to say but, luckily, it does minimal damage to those sweet, juicy fruits. It is the leaves that take the brunt of the fungi’s attack. Watermelon Myrothecium leaf spot is a fairly new disease, only recognized in 2003, and is also rather rare. Just like most fungi, this character needs moisture to grow and cause trouble.
Symptoms in Watermelon with Myrothecium
Korean plant growers first spotted Myrothecium on watermelon plants growing in a greenhouse. The disease has rarely been observed in field grown melons, probably due to the humid conditions in the enclosed plants. The disease is a leaf and stem rot fungus that attacks foliage first and can progress to the stem over time. It resembles many other fungal diseases, such as damping off in seedlings or Alternaria blight.
The diagnosis may be difficult due to the disease’s similarity to many other fungal problems. Symptoms begin on stems and appear as dark brown lesions. These will coalesce into larger spots. A very close look may reveal the black spores on the surface of the spots. The leaves will also become infected with necrotic black to tan irregular spots.
Once the diseased tissue has produced fruiting bodies, it will break away from the rest of the plant, leaving shot holes in leaves. In watermelon with Myrothecium, the fruit is unaffected. The development of seedlings and young plants is halted and no fruit will be produced, but on mature plants, growth may slow in fruit but no lesions will occur.
Watermelon Myrothecium Leaf Spot Causes
Humid, rainy weather contributes to most fungal organism’s growth. Myrothecium on watermelon has similar requirements. Warm, wet weather conditions favor the development of the fungus Myrothecium roridum. Overhead spraying or excessive rains that keep leaves consistently wet are ideal conditions for the development of the spores.
The fungus is harbored on host plants and in soil, especially in areas that were previously cropped by melons. In addition to melons, the fungus seems to inhabit soybeans. Poor sanitation practices and favorable weather conditions are the biggest contributing factors to the disease. It does not appear to attack seeds of the fruit.
Control of Myrothecium
The easiest way to avoid this disease is by crop rotation since the fungus is harbored in decaying pieces of melon plants. Clean up the sight at the end of the season and compost any leftover plant material.
Avoid overhead watering during periods of the evening when leaves will not dry completely, especially when conditions are humid and warm.
Apply copper fungicide by spraying leaves early in the season when seedlings have at least two sets of true leaves and again just as flowering commences. Install plants far enough apart that adequate circulation is possible.
Good care of plants and removal of affected leaves can also minimize the spread of Myrothecium leaf spot of melons.
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Anthracnose: Anthracnose is caused by the fungus Colletotrichum species and causes a browning and blotching of the leaves. Initially, the infected leaves have pale yellow spots with distinct black margins. As the disease progresses, the center of the spots becomes dry and tan and may have a concentric ring pattern. Flower petals of infected plants may be spotted and develop abnormally. Seriously infected plants may die.
Prevention & Treatment: Combat anthracnose by digging up and destroying severely infected plants. Pick off infected leaves as soon as they appear. Reduce disease development by avoiding overhead watering. Apply a thin layer of mulch around plants to help prevent fungal spores from splashing onto leaves from the soil. For serious infections of anthracnose, fungicide sprays containing either chlorothalonil or mancozeb are available for homeowner use. Apply at 7- to 14-day intervals until conditions no longer favor disease development. See Table 1 for examples of products containing these fungicides. As with all pesticides, read and follow all label instructions and precautions.
Other Leaf Spots: There are many fungi (Alternaria species, Cercospora species, Myrothecium species and Ramularia species) that can cause unsightly spots on pansy foliage. Leaves may have transparent tan, brown or black spots. Often these spots may grow together to form larger patches of dead tissue. Reduce leaf spot development by picking off and destroying infected leaves as soon as they appear. Avoid overhead irrigation. Moist leaf surfaces are ideal locations for these fungi to thrive. A layer of mulch will help to prevent the fungi from splashing from the soil onto plants.
Cercospora leaf spot commonly occurs in fall landscape beds. It appears as a dry, brown blotch or an irregular purple lesion, especially during cool weather. For serious infections of Cercospora leaf spot, fungicide sprays containing thiophanate-methyl can be used. Apply at 7- to 14-day intervals until conditions no longer favor disease development. See Table 1 for examples of products containing these fungicides. As with all pesticides, read and follow all label instructions and precautions.
Black Root Rot: This disease is caused by the fungal organism, Thielaviopsis basicola, which can affect a wide range of ornamental plants. Older plants affected by the disease turn yellow and have small crinkled leaves. Close observation reveals a black discoloration moving up from the tips of the roots. Diagnosis may be difficult without professional help.
Prevention & Treatment: Remove and discard any infected plants. Provide good drainage to plants and avoid overwatering. The disease can be suppressed, but not cured, with regular applications of fungicides containing thiophanate-methyl. See Table 1 for examples of products containing these fungicides. As with all pesticides, read and follow all label instructions and precautions.
Root & Crown Rot: Many fungi (Phytophthora species, Pythium species, Rhizoctonia species and Fusarium species) live in the soil, which can infect the roots or the base of the plant (crown) at the soil line. Plants may wilt and suddenly die or the leaves may simply turn yellow. A dark sunken area may be seen on the stem at or near the soil line. Roots may appear rotted. Some plants may survive but remain weak and stunted.
Prevention & Treatment: The fungus thrives in areas with poor drainage and warm soils. Always choose locations that have good drainage for planting. The drainage of existing areas can be improved by using raised beds. Avoid applying too much water since many of these fungi thrive in moist conditions. Always allow the soil to dry between each watering. Promote drying of the soil by not setting plants too close or applying too much mulch around plants. Prevent future infection by always removing and destroying diseased plants.
Fungicides can be effective on a preventative basis only, and repeat applications are required. Fungicides containing potassium salts of phosphorous acid (such as Monterey Agri-Fos, Fosphite, Agri-Fos 400) or mefenoxam (Subdue Maxx) can be applied as a drench in the home landscape, and will suppress, but not cure infected plants. Due to product cost and for accurate application, homeowners may want to hire a licensed landscaper to apply products containing these fungicides. Apply all chemicals according to directions on the label.
Gray Mold (Botrytis Blight): This disease is caused by a fungus, Botrytis species, which produces a fuzzy, gray coating on the flowers and stems of many plants. When infected flowers are picked, a puff of gray spores can usually be seen. Infected areas of the plant will eventually be soft, slimy and decayed.
Prevention & Treatment:Reduce disease development by keeping plant surfaces dry, removing aging flower blossoms and providing good air circulation. Do not overcrowd plants. Fungicide sprays containing chlorothalonil, mancozeb or copper fungicides are available for serious infections. Repeat every 7- to 14-days when conditions favor disease development. See Table 1 for examples of products containing these fungicides. As with all pesticides, read and follow all label instructions and precautions.
Watermelons are grown throughout Florida. The greatest production region is the north to north-central region, including Suwannee, Gilchrist, Alachua, Lake, Levy, Marion, and Sumter Counties. In 2002, this region comprised 36.6% of the harvested acreage and 26.7% of the state’s watermelon producing farms. The west-central region (including Manatee, Hardee, DeSoto, and Highlands Counties) comprised 20.5% of the acreage and 5.7% of the watermelon producing farms in that year, while the southwest region (including Lee, Hendry, and Collier Counties) held 19.3% of the watermelon acreage and 5.5% of the farms. An additional 18% of acreage was found in the northwest region (including Jackson, Jefferson, Santa Rosa, Washington, and Holmes Counties), containing 5.6% of the state’s watermelonproducing farms. Remaining watermelon production is distributed among 40 of the state’s other counties (USDA/NASS 2009).
Watermelon production regions in Florida.
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Watermelon (Citrullus lanatus) is known to be affected by a variety of both seed-borne and soil-borne fungi. In routine screening of watermelon seed samples, sporodochia of Myrothecium verrucaria were observed. The fungus was isolated and the spore suspension was inoculated onto healthy seedlings of watermelon. The resulting symptoms confirmed Koch’s postulates.
Watermelon [Citrullus lanatus (Thunb.) Matsum and Nakai], is a commercial crop grown for its fleshy, refreshing fruits. It is widely cultivated in tropical and subtropical countries throughout the world. The crop is susceptible to a variety of fungal, bacterial and viral diseases at various stages of its growth (Sharma and Khan 1991 Roberts and Kucharck 2003). In this study, seeds of two popular varieties, Arka Manik and Sugar Baby, were incubated on wet blotters following the standard procedures of ISTA. Routine examination of the seeds showed a high incidence of a pathogenic fungus Myrothecium verrucaria (Albertini & Schewein) Detmar ex Fr. Infected seeds showed many cushioned, light green sporodochia with a white hairy margin. M. verrucaria showed hyaline to very light green coloured, aseptate spores with acute ends (Fig. 1A, B). Myrothecium species generally cause round dark brown leaf spots in cucurbits, which on later stages coalesce to form blighted areas on the leaves (Belisario et al. 1999).
To evaluate the pathogenicity of the fungus, 10-day-old seedlings raised in wet sterilised sand beds in plastic trays were collected without damaging the roots. The seedlings were inoculated by dipping the roots in the freshly harvested spore suspension (1 × 10 6 spores/mL). The spore suspension was obtained from 8-day-old sporulated colonies of the M. verrucaria watermelon isolate, which was grown on potato dextrose agar medium at 22 ± 2°C. The inoculated seedlings were then transplanted to pots containing sterilised moist soil and sand in a 1 : 1 ratio. Seedlings not inoculated with the fungal spore suspension, but transplanted and maintained in similar conditions were used as the control. For both treatments, the pots were maintained in the greenhouse at 25 ± 2°C under diffused light conditions.
In another experiment, the seeds were inoculated with the same isolate of the fungus by rolling on 10-day-old sporulating colonies and were sown in the sterilised wet soil : sand bed as described above. On the 10th day after inoculation, the seedlings were carefully observed for the occurrence of disease symptoms. All the seedlings showed damping-off. Necrosis of cotyledonary leaves was also observed 7 days after inoculation, and was found to be initiated at the tip and marginal portions of the leaves (Fig. 1C). These necrotic cotyledons were harvested from the seedlings and incubated on wet blotters in plastic plates at 22 ± 2°C. On the fourth day of incubation, the necrotic portions showed luxuriant, well-developed light green sporodochia (Fig. 1D). The shoot portions also showed similar sporodochia (Fig. 1E), and the entire seedling showed damping-off symptoms (Fig. 1F). These sporodochia were inoculated on to PDA and incubated for a period of 8 days. The 8-day-old sporulating cultures were used to inoculate the watermelon seeds as described above. The inoculated seeds were then sown in the sterilised wet soil : sand beds and incubated. The growing seedlings were observed for the occurrence of similar symptoms. The repetitive symptoms on the cotyledonary leaves and the occurrence of sporodochia on different parts of the seedlings proved Koch’s postulates.
Yang and Jong (1995) have described the host range of M. verrucaria, isolated from leafy spurge. Belisario et al. (1999) have also reported the occurrence of the fungus on the muskmelon seeds under in vitro conditions. However, this study reports the only known description of the occurrence of M. verrucaria on the seeds of watermelon.
This study confirms that the watermelon symptoms were due to infection by M. verrucaria. This observation provides important information for the growth of watermelon crops: better crop growth may be achieved by the selection of seeds free from this pathogen. Seed screening may provide a complementary treatment to the use of seed treatments to control this fungal disease.
Belisario A, Forti E, Corazza L, Van Kesteren HA (1999 ) First report of Myrothecium verrucaria from muskmelon seeds. Plant Pathology 83, 589.
Roberts R , Kucharck T (2003) ‘Florida plant disease management guide: cucumber.’ (Edis, Florida Cooperative Extension Service, Institute of Food and Agricultural Science, University of Florida: Gainesville, FL)
Sharma GK, Khan W (1991 ) Observation on occurrence and identity of powdery mildew of cucurbits in Tamil Nadu. Indian Phytopathology 48, 314–324.
Yang SM, Jong SC (1995 ) Host range determination of Myrothecium verrucaria isolated from leafy spurge. Plant Disease 79, 994–997.
Black spot is a fungus that often attacks hibiscus plants when they are left in damp conditions. Symptoms include black and brown spots on leaves. The leaves may eventually begin to yellow and fall off. Good air circulation, proper watering techniques and careful disposal of affected plants or plant parts will prevent and control black spot. Copper sulfate is also used to treat black spot.
- Phytophthora breaks out when the soil is left dry in the warm months and plants are allowed to sit in sodden soils throughout the winter.
- Good air circulation, proper watering techniques and careful disposal of affected plants or plant parts will prevent and control black spot.