Goodbye to leprosis

Citrus leprosis, caused by the citrus leprosis virus (Citrus leprosis virus - Rhabdoviridae) and transmitted by the mite Brevipalpus phoenicis, is currently considered the main disease caused by viruses in Brazilian citrus production due to the frequent damage it causes in orchards, substantially increasing costs of citrus fruit production. Depending on the severity of the attack, it can cause a reduction of 30 to 100% in the year's production (reduce 30 to 100% of the year's production), due to the depreciation of fruits for the market and premature fall of fruits and leaves, and in the following years, due to drought of branches and decreased plant life. As a result of this damage, there is an increase in production costs with disease and vector control, recovery of weakened trees and premature renewal of the orchard. Annual expenditures on acaricides alone to control mites in citrus farming are estimated to be around 60 million dollars, which represents around 35% of input costs and around 15% of the total cost of an orchard in production. However, these costs may be higher if the costs of acaricide application and pruning operations are considered. (Doesn't the 15% include the costs of applying acaricides?)

The disease is severe and is widespread in or through (?) almost all of South America, including countries such as Argentina, Brazil, Bolivia, Paraguay, Uruguay and Venezuela, and, more recently, it was introduced in Central America (Panama and Costa Rica), worrying the North American citrus industry, which estimates serious losses if the virus reaches the orchards in the states of California and Florida in which the mite vector is already present (it is not better to mention that it is a different species!). In Brazil, it was initially found in 1933 in orchards in the State of São Paulo and has also been reported in the States of Rio Grande do Sul, Santa Catarina, Paraná, Rio de Janeiro, Minas Gerais, Bahia, Tocantins, Piauí, Pará, Rondônia and Federal District. In the orchards of the State of São Paulo, the world's largest producer of orange and concentrated juice, the disease is widely disseminated throughout the citrus growing area of ​​the São Paulo plateau, being more severe in the north and northwest of the State, where the warmer climate and the period The autumn-winter period with prolonged dry periods favors the increase in the leprosy mite population and makes its control difficult.

The importance of this disease is even greater, because more than 90% of the tops planted in the State of São Paulo are sweet orange trees (Citrus sinensis) of the 'Pera', 'Natal' and 'Valência' varieties, which are highly susceptible to leprosy. In addition to sweet oranges, the disease also manifests itself with less severity in varieties of Sour orange (C. aurantium), true lemons (C. limon), sour limes (C. aurantifolia), sweet limes (C. limettioides), tangerines ( C. reticulata), citrons (C. medica) and pomelos (C. paradisi). The 'Murcote' mandarin and tangelos (C. reticulata x C. paradisi) have proven to be quite resistant to the disease.

Symptoms of the disease appear 17 to 60 days after transmission of the virus by the leprosy mite on leaves, fruits and branches, always in a localized manner, that is, around the feeding sites of the vector mite. The leaves, generally the oldest, initially present chlorotic and smooth lesions on both sides, which grow in size and become reddish-brown, still smooth, with or without a necrotic center and, sometimes, with gum exudation forming concentric zones. In green fruits with a diameter above 3 cm, the lesions appear as shallow yellowish spots, which become depressed, dark, with a concentric pattern and surrounded by a yellowish halo. In fruits at a more advanced stage of ripening, there are dark, depressed spots surrounded or not by a greenish halo. Depending on the number of lesions and the time when symptoms appear, there is intense defoliation of the plant and premature fruit drop. On the branches, smooth chlorotic lesions initially appear that evolve into raised reddish-brown lesions. As the lesions increase, they become corky, leading to peeling of the bark and subsequent drying out of the affected branches, making them unproductive.

Specific characteristics of this pathosystem (leprosis virus-Brevipalpus mite-citrus) make the leprosis mite population and the presence of virus source plants key to managing the disease. Due to the transmission of the virus only through the vector; which makes its dissemination totally dependent on the action of the vector, on the virus-vector relationship being persistent and propagative, that is, after acquiring the virus by the leprosis mite, the vector can transmit the virus throughout its life, and that all the developmental stages of the leprosy mite (larva, protonymph, deutonymph and adult) are capable of acquiring the virus and transmitting it with an efficiency that varies from 10 to 50%, control of citrus leprosis can be achieved by avoiding the introduction and dissemination of the leprosy mite in the orchard and hindering the population increase of the vector mite. Preventive measures such as planting seedlings free from the leprosy mite or leprosis virus, disinfesting harvest boxes, equipment and vehicles and implementing a windbreak or hedge with species that are not host to the mite (pine, spring or crown -de-cristo) prevent the introduction and spread of the leprosy mite in the orchard. Early harvesting of all fruits, the use of measures that favor the population of natural enemies of the leprosy mite (predatory mites such as Iphiseiodes zuluagai, and Euseius concordis and Agistemus sp.), the elimination of shelter and oviposition sites for the mite of leprosis (dry branches, warty fruits), the control of weed plants that host the leprosis mite and the use of species less favorable to the leprosis mite as green cover (mentrast and cambará) are measures, complementary to the application of acaricides, which hinder the population growth of the leprosy mite. Even today, the main practice adopted by citrus growers to control the leprosy mite and, consequently, the disease has been the spraying of plants with acaricides when (2 to 10%) 10% of fruits or branches sampled in (1 to 2% ) 2% of the plants in the plot present at least one leprosy mite in inspections carried out every 10 days. Among the main acaricides used are cyhexatin, fenbutatin oxide, azocyclotin, hexythiazox, dicofol, bromopropylate, propargite, fenpropathrin, bifenthrin, acrinathrin, fenpyroximate, quinomethionate, dinocap and sulfur. (Italicized products are no longer used). In general, 1 to 2 applications per year of specific acaricides are made after the end of the rainy season and during the dry period when the mite population begins to grow. Spraying must be carried out with turbo atomizing equipment or with a spray gun using a high volume of spray so that the branches and fruits deeper in the canopy are well wetted. Rotating products from different chemical groups and modes of action has been recommended to avoid the selection of resistant populations of leprosis mites to intensively used acaricides.

Other characteristics of this disease, such as the lack of transmission of the virus between generations of the mite, the acquisition of the virus by the vector mite only after feeding on infected tissue and the local character of the virus in the plant (particles restricted to tissue cells close to infection sites), allow citrus leprosis to be controlled, also, by eliminating the sources of virus inoculum. This sanitization of the orchard is achieved by planting virus-free seedlings, removing symptomatic, fallen and early fruits and cleaning pruning, eliminating branches affected by leprosis.

The joint adoption of measures to control the leprosis mite population and eliminate virus sources makes it possible to keep the damage caused by leprosis at acceptable low levels.

Renato Beozzo Bassanezi, Marcel Bellato Spósito and Pedro Takao Yamamoto

fundecitrus

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