Choose right onion varieties to avoid viral disease

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Are you an onion farmer and you have failed to get good yields from your onion crop due to a strange disease? Research now shows that the seed varieties you have been buying for planting could be susceptible to Iris Yellow Spot virus (IYSV) disease, which is transmitted by thrips.

Farmers should choose the right seed varieties for planting to avoid attracting thrips which carry the disease. IYSV can damage onions and cause great loss of income for onion farmers. Research also shows that onion farmers in Kenya lose up to 75 per cent of their crop but many of the farmers do not know that the onion varieties they choose are responsible for their losses.

Thrips attracted by onion colour and shape

Due to their colour and shape, some onion varieties tend to attract thrips, the major vectors (or carriers) of IYSV that devastates most onion varieties. According to a study conducted by scientists at ICIPE and the Department of Plant Science and Crop Protection at the University of Nairobi titled Resistance to Iris Yellow Spot Virus and onion thrips among onion varieties grown in Kenya (S. Subramanian, R.K. Birithia and others, 2012), camouflage and shape of the onion is the first line of defence against the insect-transmitted viral disease.

The Iris Yellow Spot Virus (IYSV) is transmitted by Thrips tabaci, which burrows into the  layers of the leaves of the onion plant. The pest saps the from the plant and passes on the virus that causes IYSV.

Common local onion varieties tested

Due to their colour and shape, some onion varieties tend to attract thrips, which are the major vectors for IYSV disease. Kenyan farmers have lost up to 75 percent of their onion crop due to the disease.

In the study conducted by ICIPE scientists over two seasons, five varieties of onions commonly planted by Kenya farmers including Red Pinoy, Red Creole, Green Bunching, Texas Grano and Bombay Red varieties were planted in nursery beds for 7 weeks and later transferred into the main experimental field. All the necessary conditions for proper growth were maintained to keep them free of other diseases.

In the experiment, all the five varieties had different leaf colour, leaf cross-section, shape and leaf erectness. The varieties were divided into three groups based on leaf colour, glossiness and erectness. Red Pinoy, Green bunching and Red Creole varieties had blue-green, non-glossy and erect leaves. Texas Grano variety had green, glossy and prostrate leaves while Bombay Red variety had a light green glossy and erect leaves. The five varieties showed different resistance levels to both thrips tabaci and IYSV.

Texas grano and Bombay red resistant

The Red Pinoy variety had the highest number of thrips per plant while the Texas Grano and Bombay Red had the least in both seasons. Among the five onion varieties, Texas Grano was least affected by the disease in both seasons. In season 2, Bombay Red variety showed no much difference in disease incidence to Texas Grano. The varieties with a semi-circular shape in their cross section were more infested with thrips and had high levels of IYSV disease than those with a circular cross-section.

In both seasons, Texas Grano variety produced the highest marketable onion yield than all the other varieties screened. The Red Pinoy and the Red Creole were found to be more susceptible to the damage caused by thrips and IYSV. Among the five varieties tested for resistance to Thrips tabaci and IYSV, only Texas Grano and Bombay red were found to have moderate resistance.

The reason why Texas Grano showed resistance to onion thrips could be its shape and structure; the two innermost leaves exposed the onion thrips to unfriendly environmental conditions and also their natural predators. Onion thrips look for narrow spaces on plants such as the leaf sheaths where they live and reproduce.

Green colour repelled thrips

The green colour of Texas Grano variety leaves also plays a major role in its resistance. This can be explained by the high number of thrips found in Red Pinoy, Red Creole and Green Bunching varieties which have blue-green non-glossy leaves. The Texas Grano and Bombay Red which have green glossy leaves had less number of onion thrips and lower rate of disease incidence.

The pest Thrips tabaci is more attracted to blue colour than the green colour. This is one of the reasons why there was a large number of onion thrips and a high IYSV damage in Red Pinoy, Red Creole and Green Bunching onion varieties. The study established that leaf colour is a key factor that determines the level of onion thrips and damage by Iris Yellow Spot Virus. With this information, we hope farmers can select the right onion varieties to grow to improve their production and income.

Symptoms of IYSV disease

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. 


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