Everything you need to know about controlling diseases of watermelon.
1. Anthracnose (Colletotrichum Orbiculare):
Anthracnose is a destructive fungal disease of watermelon. This fungus affects all above ground parts of the plant, and may infect at any stage of plant growth. The symptoms are first noticed as round to angular reddish brown spots on the oldest leaves. Spots may later dry, turn almost black and tear out, giving the leaf an irregular appearance. Often the leaves at the center of the plant die first, leaving the stem and a portion of the runners bare.
Light brown to black elongated streaks develop on the stems and petioles. After a few days of warm humid weather, every leaf in an entire field may be killed, giving the field a burnt-over appearance. Round, sunken spots may appear on the fruits also. Spots first appear as water-soaked, and later, turn dark green to brown colour. Pink-colour ooze is often noticed in the center of sunken spots.
This disease at Bangalore is limiting factor for the cultivation of watermelon. In watermelon, Sohi (1975) estimated a yield loss up to 90% and Amin and Ullasa (1981) 63% due to this disease. Being seed born disease the seeds extracted from infected fruits carry the primary inoculums for further infection.
i. Follow long crop rotation with non-host crops.
ii. Use disease free healthy seed so collect only from disease-free plants.
iii. Keep the field clean by collecting and burning crop debris.
iv. Maintain proper drainage in the field.
v. Treat the seeds with 0.25% carbendazim @ 2.5 g/kg of seed before sowing.
vi. Spray chlorothalonil 0.2% for good control of this disease.
vii. Also, spray systemic fungicide such as carbendazim 0.1%.
2. Downy Mildew (Pseudopernospora Cubensis):
Downy mildew is not a very serious disease of watermelon but due to continuous cropping and the prevailing climate suitable for mildew sometimes causes serious damage. Downy mildew has not been a problem in watermelon in the past few years in eastern Uttar Pradesh.
The fungus attacks only the leaves of watermelon, and first symptoms appear on the oldest crown leaves as yellow lesions or mottled spots with indefinite borders blending gradually into healthy portions of leaf. Older lesions are dark brown with a slight yellow border. As the disease progresses, brown areas coalesce, causing the leaves to curl inward toward midribs.
Under favorable conditions, disease develops rapidly, resulting in a scorched appearance over entire field. The pathogen is airborne and spreads very fast. The first symptom on leaves resembles as symptom of mosaic mottling. The pale green areas are separated by darker green boundary.
Soon the well- defined spots become irregular, yellow colour and are restricted by the veins of upper surface. A purplish downy growth appears on lower side of these spots in the moist weather, however, the growth is not so pronounced. The entire leaves die quickly. Young leaves are less susceptible than older ones. Infection occurs more readily on lower surface than on upper surface.
i. Use well-drained field.
ii. Use disease free healthy seeds by collecting from disease free healthy fruits.
iii. Sow the seeds at wider spacing.
iv. Follow field sanitation by collecting and burning crop debris to reduce the inoculums.
v. Air movement and sunlight exposure helps in checking the disease initiation and development.
vi. Spray mancozeb 0.25% at seven days interval.
vii. Spray metalaxyl + mancozeb 0.2% in severe conditions.
viii. Spray copper oxychloride at 10 days interval.
3. Fusarium Wilt (Fusarium Oxysporum F. Sp. Niveum):
The symptom characterized by yellowing of lower leaves, which gradually progresses on upper leaves can occur at any stage of crop growth. Infected plants develop wilt symptoms on one or more plants, usually beginning at their tips. Immediately after infection, the plant starts drooping followed by wilting, and later, wilting becomes permanent.
Characteristic symptom was observed after splitting of roots and lower portion of stem as vascular browning. Severity of disease is more if soil is infected with root knot nematode.
The pathogen causing wilt was determined by Sohi and Sharma (1998). The pathogen spreads to new areas through either seed or soil relocated by field implements, drainage water, and man. This disease is more common in sandy soil. Application of higher doses of nitrogen increases the disease severity, however, higher doses of potash reduces it.
i. Use disease free healthy seed.
ii. Avoid contamination of new fields with soil from Fusarium infested fields.
iii. Drench the soil with systematic fungicides.
iv. Minimize disease by applying Trichoderma 5-8 kg/ha, depending on soil structure, into the soil.
v. Treat the seed with carbendazim 0.25 % (2.5 g/kg seed) before sowing.
4. Gummy Stem Blight (Didymella Bryoniae):
A disease caused by fungus is now becoming severe in watermelon, especially when the sowing is done in infected field. This fungus can also cause damping-off, crown rot, leaf spot, stem canker and fruit rot of watermelon.
Lesions on cotyledons and leaves are round or irregular and brown in colour, and on crown and stem brown that usually turn white. Early infection normally comes from diseased seeds. On older leaves, brown to black spots develop between the leaf veins. The spots first appear on lobes of leaves. The disease spreads from the center of plant to outward.
As the season advances, it attacks vines, causing elongated water soaked areas that become light brown to gray. Vine cankers are most common near the plant crown. Gum oozes from stein cracks, and plants usually die one at a time. It is unusual to find gummy stem causing fruit rot in watermelon.
i. Follow long crop rotation with non-host crops.
ii. Plough the field in hot summer and follow Trichoderma application.
iii. Grow resistant or tolerant variety to reduce the risk.
iv. Use disease free healthy seeds.
v. Maintain proper drainage and aeration in the field.
vi. Treat the seed with 0.25% carbendazim @ 2.5 g/kg of seed.
vii. Drench the soil with carbendazim 0.25% near collar region.
viii. Handle the fruits carefully to avoid any injury and reduce risk of post-harvest rotting.
5. Watermelon Mosaic Virus:
Watermelon mosaic virus I and II are now known as papaya ring spot virus-watermelon type (PRSV-W) and watermelon mosaic virus (WMV), respectively. These are two common viruses in watermelon all over the world. Several other viruses also affect the watermelon and all have similar symptoms. In India WMV-2 is very common and destructive for watermelon cultivation.
Proliferation of shoots around the crown protrudes stiffly above normal level of the vines, and leaves become smaller and show deformation and blistering. Flowers are of unequal size and greenish colour. The first symptom is usually a bumpy and mottled appearance of fruit surface, and disease symptoms are expressed strongly in period of extended high temperature. This virus (WMV-2) is sap transmitted by aphids and leaf minor, which can spread during the entire growing season.
i. Avoid the source of infections (other cucurbitaceous crops).
ii. Use seed free from virus diseases.
iii. Control transmitting vectors by spraying systemic insecticides.
iv. Spray dimethoate or metasystox @ 1.0 ml/l at 10 days interval to check the vectors activity.
6. Watermelon Bud Necrosis:
Now bud necrosis virus is becoming the main limiting factors of watermelon cultivation in India. The virus is, generally, transmitted by sap of already infected plant and thrips.
The first symptom appears on the apical buds, which turn brown to black and die. This is followed by longitudinal streaks on the vine starting from tip to downward. Streaks first appear brown then turn to black colour. In certain cases, broken or complete concentric rings are also develops on fruit.
i. Use seed free from virus diseases.
ii. Spray systemic insecticides (dimethoate or metasystox @ 1 ml/liter) periodically up to flowering to check the vector activity.