Blue disease in cotton

Also known as mosaic of ribs forms Ribeirão Bonito and an atypical virus, the most important disease caused by viruses in cotton plant is transmitted by aphids (Aphis gossypii). Your control in Brazil is based on the use of resistant cultivars and the use of insecticides against the vector.

At the moment known as “blue disease”, the mosaic of ribs forming Ribeirão Bonito was so called because it refers to the occurrence of a supposed more aggressive strain of the vein mosaic virus, observed by Costa and Carvalho (1962), affecting cotton plantations in the municipality of Ribeirão Bonito, in the state of São Paul. Later, a disease with the same symptom was found in the Paraguay, where local technicians associated it with the blue disease that occurred in Central African Republic. Thus, when the first outbreaks of this virus began to occur in Brazil, it was not known for sure which disease it was, of the blue disease found in Paraguay and associated with that which occurred in Africa or the mosaic of ribs forms Ribeirão Bonito. The doubt persisted for a long time period and, even so, improvement programs were carried out to resistance to the disease and virus control strategies have been established. In 2005, sequencing of the coat protein gene and part of the virus polymerase gene and, according to comparative analyzes of sequences, associated with a Polerovirus from the family Luteoviridae having been proposed nomenclature Cotton leafroll dwarf virus, CLRDV, ending the existing controversy. Therefore the most An important cotton virus in Brazil is Blue Disease, caused by the virus CLRDV.

The disease is characterized by causing shortening of the internodes, mosaic or pallor throughout of the veins, best visualized by observing the leaves against the light and the The rest of the leaf appears bluish green or even purplish in color. Roughness and intense curvature of the limbus from the edges is also checked. It is common in the most In severe cases, there is redness on the limbus reaching the veins and petiole. O Shortening of the internodes causes stunting of the plant, whose growth ceases. A production is completely compromised.

The virus of Blue disease is transmitted by aphids (Aphis gossypii) and plants exposed to feeding by these infected insects may develop symptoms around 18 days after contact with the vector.

In harvest 2005/2006, outbreaks of blue disease were observed in areas where it was being resistant varieties cultivated. Plants typically did not exhibit the signs characteristic of blue disease, and yes some atypical symptoms such as reddish colored leaves and plants with little or no reduction in growth. Studies carried out later allowed us to conclude that atypical symptoms were also caused after bitten by the same vector, which could characterize it as a virus atypical. The observations made about the reaction of different cotton varieties in relation to atypical viruses and blue disease led to the belief that it could be different pathogens for different diseases. Studies aimed at characterizing viruses associated with the two forms of the disease indicated a difference following a protein that may have given the CLRDV virus the ability to overcome resistance of some cotton cultivars. Based on these results the cotton genetic improvement programs, currently conducted in Brazil, take into account the existence of two pathogens, which probably have distinct characteristics. At the same time, care must be taken in a more incisive way with regard to aphid control on cultivars resistant to blue disease and susceptible to atypical viruses.

How to control

The control of blue disease in cotton is based on two principles: the use of cultivars resistant and chemical control of the vector, in this case the aphid Aphis gossypii. In cultivars resistant to both blue disease and atypical viruses, aphid levels can reach up to 60% of plants with colonies so that the insecticide applications for control. In cultivars with resistance intermediate aphid infestation levels should not exceed 40% of plants with colonies. The period of application of insecticides on cultivars resistance must extend from 20 days to 70 days after emergence and from 110 days to 130 days. Sampling should be carried out to detect aphids on the face bottom of the leaves and inspect the highest open leaf on the plant (pointer). The control level is reached when the infestation is 20 aphids per leaf or 50% to 60% of plants with aphids.

At the In the case of planting susceptible cultivars, the control period should be longer broad and extend from five days to 100 days after emergence and from 110 days to 135 days. Sampling must be carried out dividing the plant into three parts and examining the underside of a leaf of the pointer, the intermediate part and the lower part. The level of control is established from one to 10 aphids per leaf or from 5% to 10% of plants with three to five aphids. Table 1 shows the damages caused by blue disease in a susceptible cotton cultivar, not subjected to the chemical control. The values ​​allow us to observe the extent of the losses caused to production in case of carelessness on the part of the producer to adopt protective measures control within the appropriate period. The grade given to the disease level varies from 1 to 5 with 1 being the plant without symptoms and 5 being the plant with the most severe symptoms.

Is important highlight that the application of insecticides after 100 days is mainly aimed at the control of aphids as a pest and not exactly as a vector of viruses, a since the insect's sugary feces can cause the phenomenon of caramelization of the fiber or “cotton candy” which suffers from severe industry restrictions textile.

It is recommended that producers seek reliable information from breeding companies, on the resistance levels of cultivars to be planted in their areas of production, with regard to blue disease and atypical viruses to establish control strategies that minimize risks to the yield of their crops.

Article published in issue 227 of Cultivar Grandes Culturas, April 2018.