Warning against greening in citrus in Bahia

Citrus farming is an activity of great relevance for Brazilian agribusiness, responsible for generating 6,5 billion dollars per year (Neves & Trombin, 2017). Brazil is the leader in orange production, with 16.713.534 tons (harvested area of ​​589.139 hectares), followed by China and India, with 9.103.908 tons (harvested area of ​​504.683 hectares) and 8.367.000 tons (harvested area of ​​613.000 hectares). hectares), respectively (FAO, 2018).

The Southeast region is the first producer, with 13.912.388 tons, accounting for 83% of national production, followed by the South region, with 1.233.191 tons, the Northeast, with 1.147.793 tons, and the North, with 251.601 tons. , and the Central-West, with 168.561 tons, corresponding to 7,4%, 6,9%, 1,5% and 1%, respectively, in 2018 (IBGE, 2018). In terms of state production, the main producing states are: São Paulo (12.889.878 tons), Minas Gerais (948.129 tons), Paraná (834.513 tons), Bahia (604.023 tons) and Rio Grande do Sul (367.725 tons) (IBGE , 2018).

However, phytosanitary threats compromise the sustainability of this important production chain. Currently, the greening, Also known as huanglongbing of citrus (HLB), is identified as the most serious problem in citrus farming in Brazil and in the world, due to the high economic losses it causes. Infected plants may become economically unviable as the disease progresses.

The first record of the disease in Brazil was in the state of São Paulo, in Araraquara orchards, in 2004. Since then, there has been a very rapid spread of HLB, which worried citrus growers about this unexpected situation. Currently, HLB is present in the producing regions of the state of São Paulo, the Triângulo Mineiro, the North of Paraná and the Southeast of Mato Grosso do Sul.

Thus, the state of Bahia, which is the leader in orange production in the Northeast region, is considered an area free of the disease. However, unlike the business-based citrus industry practiced in the Southeast region, mainly in São Paulo, in the state of Bahia, most of the producing areas have a typical family-based citrus culture structure, with small properties (10ha on average). This characteristic can be seen as a factor of extreme vulnerability of the state in the imminent introduction of HLB. Considering a 20-year scenario in the absence of adoption of disease management measures, losses of approximately R$1,8 billion are estimated with the introduction of HLB in the state, which could cause severe losses to producing families.

In this context, the important role of inspectors who work in agricultural defense agencies at state or federal level and of professionals dedicated to technical assistance and rural extension activities is highlighted. Integrated actions are necessary to provide information in the field in order to alert producers, mainly family citrus growers, whose production areas are located in areas still free from the disease and to establish measures to prevent the introduction or contain the spread of the disease.

In this sense, the State Agency for Agricultural Defense of Bahia (Adab) has intensified phytosanitary surveillance actions, such as biannual inspections of orchards, in the various production centers: Litoral Norte, Recôncavo, Chapada Diamantina, Extreme South and West of Bahia . It is worth noting that inspections follow the methodology established in federal legislation published by the Ministry of Agriculture, Livestock and Supply (Mapa).

The initial symptoms of HLB in infected plants are characterized by the appearance of branches with young yellowish leaves. When mature, the leaves take on a mottled tone, that is, the presence of irregular yellow spots on the leaf blade. The leaf vein may become thicker and lighter in color. The fruits of infected plants are also affected, presenting limitations to their development (they become smaller, deformed and asymmetrical), do not ripen normally and fall prematurely. The occurrence of aborted (necrotized) seeds and the albedo (white part of the shell) thicker than normal are also observed. Furthermore, the flavor of the juice from the fruits of infected plants becomes more acidic and bitter, which makes its industrial use unfeasible.

In Brazil, the disease is associated with bacteria Candidatus Liberibacter asiaticus and Ca. L. americanus. These bacteria are transmitted through infected plant material and can therefore be transmitted through grafting. This mode of transmission ends up putting citrus growers at risk when they use seedlings of unknown origin or of questionable quality. Therefore, the first recommendation to be made to citrus growers is that citrus seedlings are only purchased from reputable nurseries. The transport and sale of clandestine citrus seedlings are activities that expose citrus growers, especially family members, to a situation of extreme risk in terms of the introduction not only of HLB, but also of other pests.

With the aim of combating the transit and trade of propagative material of citrus plants (“horses”, buds and seedlings), as well as the ornamental species of sweet myrtle or false myrtle (Murraya paniculata), Adab operates in the inspection of the state's entry points, on highways and gardens, which usually results in the seizure and destruction of this material without phytosanitary certification of origin (CFO) or in the absence of the compliance statement and invoice.

Considering the importance of the citrus seedling in the process of spreading the greening, the state of Bahia published Ordinance No. 243, of 2011, standardizing the production system in a protected environment (screened nursery). Some screened nurseries were implemented by business groups, both on the North Coast and in the West of Bahia. However, most citrus growers, although they recognize the quality of the seedlings produced with the safety of screens, still purchase propagative material produced in the “open sky”, sold in open-air markets and in the rural area itself.

Insect-vector

One of the main ways the disease is transmitted is through an insect vector, which passes the bacteria to healthy plants after having fed on an infected plant. In Brazil, this vector role is played by an insect known as the citrus psyllid - diaphorina citri Kuwayama, 1908 (Hemiptera: Liviidae).

This insect occurs in several Brazilian states and was first recorded in Rio de Janeiro, in 1942, long before the discovery of HLB. Before the introduction of HLB in Brazil, the importance of the insect was negligible, it was practically not even considered a pest. However, the situation has changed, and it is currently considered one of the main pests in citrus farming.

In the state of Bahia, this insect is easily found, especially in urban areas, as it is associated with the ornamental plant sweet myrtle (M. paniculata). This plant is considered a very favorable host for the insect's development. As mentioned for citrus seedlings, the transport and sale of myrtle seedlings, infected with the bacteria Ca. Liberibacter, are a potential risk to the introduction of HLB in Bahia.

The citrus psyllid is a sap-sucking insect from plants, generally located on the underside of leaves and buds. It goes through the egg, nymph and adult stages. Its cycle (egg to adult) is completed in a period of 20 to 40 days, depending on the host and also the temperature. The eggs are yellow and placed in the shoots of plants, therefore, this factor (presence of shoots) limits the population increase of the pest. The nymphs are yellowish and remain on the branches and leaves of plants and go through five instars. The adults, shortly after emergence, are whitish. Later, they take on the characteristic color of adults. When colonies of nymphs or adults are located on the lower surface of the leaves, it is common to observe the presence of sooty mold on the upper surface of the leaves located just below the colonies.

Both adults and nymphs of D. citri can transmit the bacteria, however the transmission efficiency is higher when the adult insect acquires the bacteria while still in the nymph stage. The adult has a longevity of 90 days to 120 days, a period in which it can transmit the bacteria, which multiplies inside the insect. To identify the presence of adult insects on properties, sticky traps are used. Due to the strong attraction exerted by the yellow and yellowish green color on D. citri, traps (panels or cards) of these colors are recommended for monitoring this pest. The distribution of these traps must be done, preferably, on the periphery or on the plants on the edges of the plot or property. They must be clearly visible, being positioned outside the plant canopy. Visual inspections of the branches of citrus plants are also carried out by trained pest pests to identify eggs, nymphs and adults.

Recently, Adab installed an experimental area called Regional Management Area - “Arma”, where, every fortnight, the yellow sticky traps will be replaced and the number of insects captured will be verified. This activity is the result of a partnership established with Embrapa Mandioca e Fruticultura, which has produced solutions with the help of biomathematics. The “Weapon” will produce information on the population dynamics of the insect vector in the citrus growing area of ​​Recôncavo and will serve to feed a Phytosanitary Alert System – an important tool for managing HLB, should the disease become established in the region.

In areas where HLB already occurs, control of the insect is recommended to lower the pest population. Typically, chemical control is used. In areas subject to chemical control, sticky traps are the most effective method for quantifying and detecting low psyllid populations. However, in areas of family farmers, which constitute a large part of the citrus growers in the state of Bahia, there is no prospect of using the recommended chemical control, due to low local purchasing power and productivity, with biological control being more pertinent.

One of the problems with managing HLB is that symptoms can take anywhere from six months to a year and a half to appear. However, these infected, but asymptomatic, plants are already a focus of infection and may serve as a source of bacteria associated with HLB for the citrus psyllid. Thus, there is a trend of progressive increase in the incidence of cases.

Given this condition of silent infection, in which the plant can be a source of inoculum without necessarily showing symptoms, for three years Adab has been monitoring a possible invasion of the bacteria from the greening on Sentinel Routes – routes that may represent an increase in the risk of the pest establishing itself, thanks to the presence of hosts (citrus and sweet myrtle plants), human traffic or even commerce in vegetable gardens. Specimens of the insect vector are collected quarterly and are subjected to molecular testing (PCR) to detect the presence/absence of the bacteria. This is a strategic action for Agricultural Defense, as, depending on the results, Greening contingency measures are adopted, even before symptoms appear in hosts.

The phenomenon of the introduction of bacteria into production areas from insects coming from outside the orchard is called primary infection and results from the insect's long-distance movements, up to 3,5km. The spread of bacteria from plant to plant within the same orchard or plot is called secondary infection, characterized by short movements of the insect (up to 25m-50m). Typically, there is a concentration of affected plants in peripheral areas of properties and plots, known as the “edge effect”. For the reasons explained above, it is essential, in regions where there are already affected orchards, that periodic inspections be carried out every three months to allow the identification of symptomatic plants, which must be eradicated. Special attention should be given to residential orchards, citrus plants located in urban areas. Abandoned orchards constitute a serious threat to the health of the citrus industry.

There is no known cure for the disease. Therefore, the measures that must be adopted to reduce the damage caused by the disease are considered as the basis for managing HLB, that is, the use of healthy seedlings produced in protected nurseries, monitoring and control of the insect vector and elimination and /or replacement of symptomatic plants in the orchard.

Marilene Fancelli, Embrapa Mandioca e Fruticultura; Suely Xavier de Brito Silva, Bahia State Agricultural Defense Agency; Gabriela Souza de Oliveira, Juliana Nascimento dos Santos, Davi Ferreira Amorim; Federal University of Recôncavo da Bahia

Growing Vegetables and Fruits December/January 2021

With each new edition, Cultivar Hortaliças e Frutas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in vegetable and fruit cultivation, the Magazine helps farmers in the search for management solutions that increase their profitability. 

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