Times of Oman
BY SARAH MACDONALD | DECEMBER 15, 2014 , 9 : 57 PM GST
There are some new lime trees planted nearby, but Al Zadjali predicts they will soon be infected, too, and die within a few years. –Photos, videos OK Mohammed Ali
Muscat: Since the 1970s Oman has lost more than one million lime trees to a disease with no known cure, but now a scientist from the Ministry of Agriculture and Fisheries has found a way to grow disease-free trees to rehabilitate the Sultanate’s lime production.
Dr Abdullah Dawood Al Zadjali, a plant pathology researcher, is a leading expert on Witches’ Broom Disease of Lime (WBDL), a disease which has devastated Omani citrus trees since it first emerged in the 1970s. But the tides may be turning for citrus farmers in Oman, as he has found a way to overcome the disease by growing tissue cultures from disease-free blossoms and grafting them onto new disease resistant root stocks.
Walking through a farm in Al Rumais, a few kilometres from his research lab, he looks at some lime trees that are infected by the disease.
“The tree will die completely in four to five years. This part was green before, but if it dies, the limes will die, too,” he says, examining of a lime tree that has some normal branches and others that are dry and leafless, much like the witches’ broomstick the disease was named after.
Al Zadjali, who has a PhD from the Tokyo University of Agriculture and Technology, points to a part of the tree with small, bushy leaves that look similar to flowers, and explains that this is another symptom of WBDL, which is caused by a microorganism bacteria called ‘phytoplasmas.’
The ‘phytoplasmas’ are carried from plant to plant by insects, usually leaf-hoppers. They cause leaf-like structures to grow in place of flowers, yellowing leaves, and dead branches that look like witches’ brooms. The fruits that grow are small. Eventually, the disease causes the tree to die.
The ‘phytoplasmas’ are an obligate parasite, which means they can only survive if they are inside the host plant or the vector, which is the insect that carries the disease from tree to tree. They can’t live in the soil or in the air.
There are some new lime trees planted nearby, but Al Zadjali predicts they will soon be infected, too, and die within a few years. As the trees die off from the disease, new ones are planted. But these, too, soon die as they are in the same area as infected ones and the disease is easily transferred. Many of the new trees live just four to six years before dying.
“Annually, we’re losing a lot of trees. The Ministry is distributing new trees to the farmers but those trees, in two to six years, die, too. Once the farmer sees the tree showing some symptoms he should eradicate the tree, even the roots, so as not to transfer the disease to other trees,” he advises.
One way to control the plant is to control the vector, in this the leaf-hoppers, but Al Zadjali says this is much too difficult. “We found the only way to overcome the disease is to grow clean material, disease-free, and distribute those to the farmers so we can reduce the disease in the environment. Or we can clean the areas of citrus plants and replant the area with 100% certified, clean material, free from diseases,” explains Al Zadjali.
Back in his lab he has thousands of disease-free trees which are growing at different stages. Some are still new tissue cultures growing in test tubes; others are new seedlings measuring a 10 or 20 centimetres, while some are even producing fruit.
The success rate for the technique he and his team uses and is incredibly low, no more than 5%, but slowly they are making progress and now have about 1,500 mother plants from which more trees can be grown.
“We are so proud and so happy we succeeded to get the mother plants and also to find good results in the field where we’re using the resistant root stocks,” he mentions.
Now they are moving on to the mass production of seedlings. By the end of 2015 the aim is 15,000 seedlings, and another 20,000 by the end of 2016.
“The project is aiming to protect disease-free mother plants. Phase two is the mass production of seedlings. We need to produce at least 10,000 seedlings annually. Now we have here more almost 15,000 root stock seedlings,” he says, wandering through a green house which has row after row of young trees.
Not only have Al Zadjali and his assistants managed to grow of Omani limes, they have been successful in growing other varieties of disease-free citrus including oranges, grapefruits, lemons and others fruits from Oman in the past four years, as well as some international varieties.
“We’re very proud because our target was Omani lime, but we’ve succeeded to produce over 15 different varieties,” he comments.
Al Zadjali says it’s important to use Omani specimens for growing clean trees, because they save Oman’s agricultural history and identity that way.
“If we import disease-free plants from other countries it costs us too much money and we will lose our indigenous species,” he says.
Now, Al Zadjali hopes the Ministry of Agriculture will be able to provide farmers with disease-free limes and teach private nurseries how to grow and sell clean trees, too.
His project, which started in 2010 and is funded by the Agriculture and Fisheries Development Fund, aims to produce enough disease-free trees to replenish the lime crops in Oman, bringing the industry back to its former glory days before WBDL appeared in the Sultanate.
“This is the kind of solution that helps us get our lime industry back, and be part of the income for the farmers. Economically it will be important,” he says.
For generations many Omani farmers depended on limes as a major part of their livelihoods. According to Al Zadjali’s research, in 1973, Port Sultan Qaboos alone exported OMR36 million worth of limes. The export of limes was the third largest part of the national income after oil and dates, he says.
“If we have a big number of lime trees we can make not just a fruit industry, but we can produce factories for producing concentrating juices. We can have incense, perfumes, soaps. We can use limes for many purposes, so that will create a lot of job opportunities for Omani people,” he says.
For Al Zadjali, who has been studying WBDL since the early 1990s and make it the subject of both his Master’s degree and PhD, finding a way to tackle the disease and help Omani farmers and national economy has become more than a job. It has become a personal passion and mission.
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