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agriculture

The legs of the viruses

Aphids may spread the possible causal agent of citrus sudden death

Researchers from Alellyx Genomics have discovered two insects known as aphids that transmit a virus that they believe to be responsible for citrus sudden death, a disease that caused the elimination of some 400,000 diseased orange tress in the state of São Paulo – for the time being, this is the only way of holding back its advance. In experiments carried out in the hothouses of the Fund for Citrus Plant Protection (Fundecitrus) by the team from this company, the aphids Aphis spiraecola and Aphis gossypii infected microcuttings and orange trees with the Citrus Sudden Death Virus (CSDV), which Alellyx presented in October last year as the possible causal agent of the sudden death.

The researchers found that a third species, Toxoptera citricida, like the other two, can also carry the virus identified by the company, but the tests carried out so far have not indicated that this aphid is equally capable of transmitting the microorganism. “We have not ruled out the possibility that Toxoptera also transmits the virus that we believe to be associated with the sudden death”, says Ana Cláudia Rasera da Silva, a researcher with Alellyx.

There is still no evidence to make it possible to claim that it really is the causal agent of the sudden death, but in the last few months the team from the company has shown a direct relationship between the disease and this virus, found only in trees from regions affected by the sudden death, in the north of São Paulo and in the state of Minas Gerais.

For the time being, the possibility is being considered that the CSDV acts in conjunction with the virus responsible for another disease, citrus tristeza, which 60 years ago almost decimated São Paulo’s orange plantations and today, in more attenuated versions, has become practically endemic. One of the transmitters of the tristeza virus is precisely the aphid – a soft-bodied wingless insect, found by the dozen, sucking the sap from cotton plants, wheat, sugarcane, and apples, or from the foliage of greens like cabbage.

Detected for the first time in 1999, in the municipality of Comendador Gomes, in Minas Gerais, the sudden death consists of the gradual clogging up of the phloem, the vessels that carry nutrients and water from the roots to the crown, causing a sort of infarction. It has advanced very rapidly, according to a survey by Fundecitrus. In 2002, there were about 22,000 plants with symptoms of the sudden death in five municipalities of São Paulo. One year later, the area affected had expanded 60 kilometers, and 44,000 orange trees from 18 municipalities in the state were showing signs of this disease. Including Minas Gerais, there are about 2 million trees infected in 30 municipalities, with a loss of productivity close to US$ 40 million. The disease has very probably spread by means of insects like the aphids, the eradication is practically impossible, so abundant they are.

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