Control onion diseases for more yields

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Onions grow well under many different conditions. But when grown in the rainy season, onions have a greater chance of contracting diseases. Onions get more diseases in warm humid weather and your onions are more likely to get infected in the parts of your field where there is water logging.

Different onion diseases can show different symptoms; however most diseases can be managed in the same way. The most common disease starts as a brown oval spot on the leaves. The spots then turn into yellow streaks that run along the leaf. Once the yellow streak turns brown, the leaves fold.

It is important for farmers to stay vigilant so as to be able to prevent and manage the diseases. After harvest, the germs that cause the diseases are found in the soil. This means that after growing onions, it is important to plant other crops in rotation. You should avoid planting onions in that field for three years until the soil is free of the diseases.

There are ‘improved’ varieties of onions that can cope better in wet conditions than other types. You can find out from your local dealer or extension agent what varieties to grow. Then you could plant small amounts to see how they perform on your farm.

Diseases can also be passed on in the seed, especially in untreated seed from the local market. To avoid this you should buy genuine seed from a trusted seed dealer. Diseases will often attack weak plants in poor soil. A way around this is to get healthy seedlings and try improving the quality of your soil. Adding organic compost or well-rotted manure to your seedbed is a step you can take to promote the growth of your crop. It is important to raise your beds especially in the rainy season for this will ensure proper drainage of excess water.

Plant the seeds in lines and leave enough space between the seeds, so each seedling can become strong. Picking seedlings is important, transplant only healthy seedlings and discard the sick or weak ones.

Onions need just enough water to grow well. If you water the crop too much, the onions may get infected. Under watering is not good for the plants either. This makes it vital for you to always check how moist the soil is before you water.

Water your plants early in the morning so that they have time to dry up during the day. If plants stay wet throughout the night they easily get infections. Weeds block air circulation. To avoid this regularly weed your field. When onions remain wet for longer this condition increases the likelihood of them contracting infections.

If you are vigilant and regularly observe your crop, you can prevent any disease from spreading. But if your field was attacked by a disease, remove or bury the remains of onion plants after harvest.

By keeping your onions healthy, you and your family will harvest more and enjoy a better income.


Control devastating IYSV viral disease

One of the most destructive diseases in onions is the Irish Yellow Spot Virus (IYSV) disease. The viral disease can wipe up to 75 per cent of onion crop. The symptoms of the disease include yellow to straw coloured lesions (wounds) on the onion leaves and stalks.

Dry long strips of lesions show on the leaves which may look like insects (thrip) damage. Late in the season the leaves may dry up and fall. Plant vigour (strength) is reduced and bulb size reduces affecting the overall onion yield in a farm.

Symptoms

Symptoms of Iris Yellow Spot Virus on onion include yellow to straw coloured lesions (wounds) on onion leaves and stalks. Dry, elongated lesions or flecks may resemble thrips injury. Lesions may be diamond shaped (this occurs rarely on leaves, more commonly on scapes). Late in the season, infected seed stalks and leaves may fall. Plant vigour and bulb size are also reduced including the onion yield.

Control measures

The viral disease is spread by thrips (Thrips tabaci). The number of thrips in an onion crop determines the severity of the disease. Volunteer onion from the previous crops transfer the disease to the new crop (this is why crop rotation with crops not prone to the disease is a good control measure). Seed transplants can also spread the disease. All crops that attract thrips act as hosts for the pest and the disease.

Studies have established that some varieties of onions are less prone to the disease. The disease- causing thrips are attracted by the colour of onions. For example Red Pinoy has been found to attract more thrips per plant while the Texas Grano and Bombay Red have least infection. The green colour of Texas Grano and Bombay Red leaves were found to be the cause of less infection. The Thrips tabaci is more attracted to blue colour than the green colour. Farmers are therefore advised to grow Texas Grano and Bombay Red onion varieties to reduce infection of their onion crop.

You can now watch a video of managing onions on http://www.accessagriculture.org/harvestingand-storing-onions


>>Share your experiences with TOF and fellow farmers. Send email to admin@theorganicfarmer.org,  leave a comment below this article.

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