Management strategies against greening

Nine years after the detection of the presence of Greening in Brazil, the disease continues to challenge citrus growers and research. Given the impossibility of eradicating it, there remain integrated management measures, adopted in a regionalized manner and with the sum of efforts and the greatest number of tools available to face the arduous battle against huanglongbing

March 2013 marked nine years since the first plants with symptoms of greening or huanglongbing (HLB) were found in Brazil, in the region of Araraquara, São Paulo. Since then, the incidence of the disease has increased in all producing regions of the state, despite the difference in terms of severity in different regions. In Brazil, in addition to São Paulo, the disease is also present in Paraná and Minas Gerais. In South America, in addition to Brazil, the disease was recently found in Argentina and Paraguay.

In Brazil, the symptoms of the disease are associated with bacteria Candidatus Liberibacter americanus and asiaticus. Until 2006, the first species predominated and from that year onwards the Asian species, the most aggressive and the one that is most quickly spread by the vector, the psyllid. diaphorina citri Kuwayama (Hemiptera: Liviidae). Until recently this species belonged to the Psyllidae family, but currently it belongs to the Liviidae family.

Incidence in São Paulo and Triângulo Mineiro

Regarding the current situation of the disease in São Paulo and the southern region of the Triângulo Mineiro, the last survey carried out by Fundecitrus, in 2012, indicated that approximately 64% of plots had at least one plant with symptoms. When compared to 2011, the increase in the number of plots with incidence of the disease was 20%. For the number of plants, the growth was even greater. In 2012, it was found that 6,9% of orange trees showed symptoms, an increase of 82,8% compared to 2011.

Comparing the citrus producing regions in São Paulo, including the Triângulo Mineiro in the North region, it was observed that the incidence is higher in the East region, with 14,8% of plants showing symptoms. In this case, the increase from 2011 to 2012 was approximately 107%, currently being the most affected region in the State. Another region whose incidence of greening remains high is Central, with 9,89% of orange trees showing symptoms of the disease. For the other regions, the incidence of symptomatic plants is less than 2%, in descending order North, West, South and Northwest, with 1,78%; 1,35%; 0,85% and 0,28% of symptomatic plants, respectively. The survey results indicate that the disease is present in all areas where citrus trees are planted, with differences only in incidence and severity. It always refers to symptomatic plants, because, using the tools currently available, it is not possible to identify asymptomatic diseased plants in the field.

The disease is cyclical throughout the year, with symptoms being more evident in winter, or in the driest and coldest periods of the year, which is why, even on properties with greening management, the number of eradicated plants is greater during this season. Despite the higher incidence of greening in winter, with greater elimination of plants, symptoms can be observed throughout the year, and in certain years, there has been a high incidence even in summer. This is probably due to the time of inoculation of the bacteria and/or conditions favorable to the manifestation of symptoms in that season. Therefore, the survey has to be routine and continuous throughout the year.

Analyzing the surveys carried out since 2004, it can be observed that both the incidence of affected plots and symptomatic plants have increased from year to year. This clearly demonstrates that the inoculum is increasingly larger and as a consequence, there is also a higher transmission rate. Therefore, even on properties with intensive management of the vector, the psyllid D. citri, sick and symptomatic plants are still found. Even with strict control, the psyllid still finds a moment when the plant or part of it is unprotected and manages to transmit the bacteria.

Transmission of bacteria by vector

In orchards, the bacteria are spread by the psyllid D. citrus, a small insect that sucks phloem sap, measuring 2mm to 3mm in length and which has transparent wings, with dark spots. Both the young and the adult phase can acquire the bacteria by feeding on symptomatic or asymptomatic branches of citrus plants and myrtle, an ornamental plant widespread in Brazil and the world, being used as a living fence in urban areas and even close to orchards Transmission will occur after a latency period, which can vary from one to two weeks. However, if the nymphs develop on diseased branches, when adults emerge, they are capable of transmitting the bacteria. Once the bacteria have been acquired, the adult maintains its transmission capacity throughout its life, because, just like in plants, bacteria also reproduce in insects.

Recent studies indicate that transovarian transmission of Approx. L. asiaticus, that is, the bacteria can be transferred from the female to her descendants, however, the transmission rate is low, approximately 3,6%. Another means of transmission is during mating,  in which infected males can transmit the bacteria to healthy females. However, this transmission rate is also low, ranging from 2,8% to 5,6%.

The acquisition of the bacteria occurs during the process of ingestion of phloem sap by the vector, during its feeding on the citrus plant. The inoculation of the bacteria takes place during the salivation process that the psyllid carries out before starting to ingest the sap.

Research results have shown that diseased and, especially, symptomatic plants are more attractive to adults in D. citri. This greater attractiveness may be related to the greater production of certain volatile compounds, the process of which is induced by the bacteria for greater attraction and subsequent dissemination of the pathogen. Therefore, one of the main strategies for managing the disease is the elimination of symptomatic plants, as their presence in the orchard can favor the transmission of Approx. Liberibacter spp.

Time of occurrence of the psyllid

The insect has the greatest population growth during the plant's budding period, where the females lay their eggs, and which favors better development of the young stages, the nymphs. During the period in which plants sprout, climatic conditions are more favorable to the development of the vector, as at higher temperatures and higher relative humidity, the life cycle is completed more quickly. However, adults can be observed throughout the year in the orchard, even when there are no sprouts, and they can even feed on mature leaves.

Due to the possibility of the vector occurring throughout the year, monitoring must be constant, intensifying from spring to mid-autumn.

Greening management

One of the recurring questions on the topic is whether it is possible to eliminate the disease from Brazilian orchards. That answer is relatively simple, and it is no. Due to the spread and incidence of the disease in orchards in São Paulo, its eradication is unfeasible. But reducing the incidence and especially the damage caused by greening is possible, through some measures such as:

1. Strictly eliminate plants with symptoms – if there are no plants where the psyllid can become contaminated, there will be no transmission of the bacteria. Compliance with current legislation for greening (IN No. 53, of October 16, 2008) is essential for the successful management of the disease and only by adopting the elimination of plants will it be possible to rationalize the use of insecticides to control the vector. .

2. Regional management of the disease (management in large areas) – as the psyllid moves a lot, controlling the vector in large areas is one of the alternatives to reduce this mobility and consequent transmission of bacteria.

3. Strict control of the insect vector – this topic is linked to the rigor in eliminating symptomatic plants, the lower the incidence of the disease, the lesser the efforts in terms of controlling the psyllid. However, the use of insecticides to control the vector can help reduce the incidence of greening, but it is not the solution to the problem. With this indiscriminate use, the incidence of pests previously considered secondary has increased. There is growing concern regarding agrochemicals due to problems arising from resistance, as well as the possibility of environmental and human contamination and elimination of natural enemies, such as the parasitoid tamarixia radiata.

4. Synchronized efforts and Cooperation – these are the basis for successful regional management. In order to be successful in managing the psyllid, control must be synchronized and jointly between neighboring owners, with control carried out at the same time.

However, successful greening management will only be achieved if all possible strategies are combined and adopted. Control focused solely on eliminating the psyllid will not be enough to reduce the incidence of the disease. Furthermore, all citrus growers must eliminate symptomatic plants on their properties in order to reduce the transmission of bacteria associated with greening.

Pedro Takao Yamamoto, Gustavo Rodrigues Alves, Vitor Hugo Beloti, Department of Entomology and Acarology, Esalq/Usp

Article published in issue 79 of Cultivar Hortaliças e Frutas