Choose right onion varieties to avoid viral disease

Are you an onion farmer and have failed to get good yields from your crop due to a strange disease? Research now shows the seed varieties you have been buying for planting could be the problem.

There are varieties susceptible to Iris Yellow Spot virus (IYSV) disease, which is transmitted by thrips. To avoid it, farmers are advised to choose the right seed varieties for planting. The IYSV disease can cause vast damage to onions and great loss of income.

Research shows that onion farmers in Kenya lose up to 75 per cent of their crop but many do not know that the onion varieties they choose is responsible.

Due to their colour and shape, some onion varieties tend to attract thrips, the major vectors (carriers) of the IYSV.

According to a study conducted by scientists at International Centre of Insect Physiology and Ecology (ICIPE) titled ‘Resistance to Iris Yellow Spot Virus and onion thrips among onion varieties grown in Kenya (S Subramanian, RK Birithia and others, 2012)’, camouflage and shape of the onion is the first line of defence against the insect-transmitted viral disease.

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The IYSV is transmitted by thrips tabaci, which burrows itself between layers of the leaves of the onion plant, sapping juice from the plant and passing on the virus.

In the study conducted over two seasons, five varieties of onions commonly planted by Kenyan farmers were studied. The varieties Red Pinoy, Red Creole, Green Bunching, Texas Grano and Bombay Red varieties were planted in nursery beds for seven weeks and later transferred into the main experimental field where all necessary management conditions for proper growth were maintained to keep them free of other diseases.

In the experiment, all the five 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.

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.

The average IYSV disease incidence in season 1 and 2 for all varieties ranged between 42 and 69 per cent.

Among the five onion varieties, Texas grano had the lowest disease incidence in both seasons while 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 had more thrips infestation and high IYSV disease severity 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 was its shape and structure.

Leaf colour

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 foliage and therefore a higher virus spread.

The Texas grano and Bombay red which have green glossy leaves had less number of onion thrips and lower rate of disease incidence. This shows that thrips tabaci is more attracted to blue colour as opposed to the green colour, a reason 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 the virus.

These findings can therefore help farmers to decide which 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.

 The writer is the editor Organic Farmer, published by International Centre of Insect Physiology and Ecology — ICIPE