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Watermelons are hardy, but how can you effectively root out diseases?

By George Michaels Mbakahya | November 7th 2015
Farmers harvest watermelons at Kong'ato Clan land in Elgeyo marakwet County. The produce does well in hot conditions. 13-10-2015. PHOTOS BY: KEVIN TUNOI

Watermelons are generally pretty tough crops, but once in a while they develop problems that seem insurmountable. Treating watermelon problems is often a simple matter, but the first thing you’ve got to do is figure out what’s causing your problems.

If your watermelons are having problems, they may be suffering from one of these common watermelon diseases, but don’t worry, we’ve got plenty of tips for watermelon disease control.

Anthracnose – This seed-borne fungus is hard to detect initially, as it may only appear as small spots on your plants and fruits.

As it grows, these spots expand and turn black or gray and new sunken areas may appear on your fruit. Anthracnose is noticeable along the leaves and the veins as small lesions. These dark, sunken lesions may also be found on stems, flowers and fruits.

In order to distinguish between anthracnose and other leaf spot diseases, you should carefully examine the undersides of leaves for a number of small tan to brown dots, about the size of a pin head. If you are unsure about diagnosing anthracnose, consult your local extension office for assistance and additional anthracnose disease information.

Anthracnose control begins with practicing good sanitation. Picking up and disposing of all diseased plant parts, including twigs and leaves, from the ground or from around the plant is important. Crop rotation combined with an aggressive treatment of neem oil will help preserve this and future harvests from anthracnose.

Keeping plants healthy by providing proper light, water and fertiliser will strengthen the plant’s ability to ward off a fungus attack. Stressed plants have a difficult time recovering from anthracnose fungus. mancozeb, captan, and ziram provide a significant level of disease control and are generally rated as moderately effective for anthracnose control.

Bacterial fruit blotch – The bacterium is often responsible for seedlings and young plants and fruits with water-soaked spots that spread and become necrotic. The leaves may be brown, but the most dramatic sign is on fruit. The rind may crack and ooze a sticky, yellow liquid. Copper fungicide can control symptoms if it’s applied as soon as symptoms of bacterial fruit blotch are detected.

Downy mildew – Downy mildew is notable for the angular leaf spots it creates as it works its way through watermelon leaves. They may start as yellow areas, but soon turn brown with purple spores on the undersides of infected leaves.

Fortunately, downy mildew won’t attack fruit, but it can reduce yields by weakening your plants. The best control of downy mildew is to make sure that your plants do not get it in the first place. Because downy mildew needs water to survive, the very best thing you can do to prevent downy mildew is to water your plants from below.

Water that sits on the leaves of the plant gives the downy mildew a way to infect and spread on the plant. The spore of downy mildews spreads by literally swimming through water until they come across live plant material to infect. If there is no water on your plant leaves, the downy mildew cannot travel to or infect your plants. Good garden hygiene is also crucial to stopping downy mildew from developing in your garden. Neem oil can control this nasty mildew.

Mancozeb, Captan, and Copper fungicides are recommended. Ridomil Gold MZ and Ridomil Gold/Copper are by far the most efficacious fungicides available for control of downy mildew. Ridomil is locally systemic and has good post-infection or curative activity.

Gummy Stem Blight – Gummy stem blight is a fungal disease of melons, cucumbers and other cucurbits. It is a contagious disease which can spread across a field of fruits.

The fungus damages the tissues of the stem at all stages of development. Stem blight treatment must start before you even plant the seeds to be entirely effective. Find out what is gummy stem blight so you can prevent this problem in your vegetable garden. Gummy stem blight fungus is most active during periods of warm, wet weather.

The spores of the fungus can spread in soil or by air. Infection signs and symptoms can be seen on all parts of the plant except for the roots. Yellowing of the leaf margins (chlorosis) is an early symptom on the plant, and light- to dark-brown spots (necrosis) can appear on the seed leaves.

Lesions can also form on the stem that enlarge and girdle the main stem. Cracking is often visible on the stem, accompanied by gummy ooze. Cankers develop on the stem that can be red, brown, or black, and a red to amber gummy substance can exude from this region.

The first stages of a disease-free cucurbit crop are preparation and rotation. Never plant cucumbers, melons or other susceptible plants in the same area as the previous season’s crop. The plant debris, and even seeds, left over in the soil will harbour the spores of black rot fungus.

In most cases, removal of old plant debris, rotation and resistant species will prevent the appearance of gummy stem blight. In climates with warm, moist bloom conditions, the fungal spores are carried on the wind and you may have to combat the disease even if you took preventative steps. The most common method is the use of fungicides as a stem blight treatment. Dusts or sprays of fungicides useful for preventing and combating powdery or downy have been shown to be effective against gummy stem blight disease. Control is difficult, but copper fungicides can be effective if used as soon as gummy stem blight appears.

Powdery Mildew – One of the most common diseases of plants in general, powdery mildew doesn’t spare watermelons. Leaves will appear to have a white powdery substance on them when the infection is active, though fruits aren’t generally affected. As the powdery mildew moves through the plant, leaves brown and die, leaving fruits to sunburn and weakening plants. Neem oil is an excellent treatment, but increasing air circulation around your watermelon plant by pruning can be equally effective. Copper fungicides (fixed coppers or Bordeaux mixture) have been rated moderately effective against powdery mildew; however, care must be taken when using copper due to the danger of foliage injury (phytotoxicity). Under heavy disease pressure, copper fungicides may not provide adequate control.

{The writer is an expert on sustainable agriculture/agricultural innovations}

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