Fundecitrus researcher points out reasons for the increase in leprosis in orchards and management actions

Citrus leprosis has always been an important disease in Brazilian citrus farming, both due to the damage caused to production, cosmetic quality of the fruit and plant decline, as well as the high cost of control. However, in the last five years, the disease has drawn more attention from citrus growers in the citrus belt of São Paulo and Triângulo/Southwest of Minas Gerais due to the increase in premature fruit drop. In the 2016/2017 harvest, the leprosy drop rate was 0,25%, corresponding to 700 thousand boxes of 40,8 kg. In the 2020/2021 harvest, leprosis prematurely dropped 5,8 million boxes (drop rate of 1,7%) (see chart below). Furthermore, there has been a lot of talk about the difficulty in controlling the leprosy mite, observed by the reduction in the control period obtained after the application of the acaricide, resulting in a greater number of applications per harvest.

Until the early 2000s, control of the leprosis mite was based on its population density and the history of the presence of the leprosis virus in plots. The level of action, or mite infestation for the application of acaricide, varied from 1 to 10% of the organs sampled with the presence of the mite, depending on the citrus grower's aversion to risk. To estimate the level of mite infestation in the plot, monitoring was carried out every 7 or 10 days, on 1 to 2% of the plants in the plot, observing in each plant the presence of the mite in 3 to 5 fruits or, in absence of fruits, in the first 30 cm of branches. Once the action level was reached, the application of acaricides was carried out quickly, observing the rotation of products with different modes of action. Control periods of more than 300 days were thus achieved between the application of the acaricide and the moment when the mite infestation reached the established action level again. With this management, with less than two applications per year, except in years extremely favorable to the mite, cases of leprosis epidemics in orchards were rare.

Since then, some changes in citrus farming have made it more difficult to control the leprosy mite and contributed to the increase in the disease in orchards. Here are some common mistakes, guidelines, and new research findings:

Mite monitoring

Monitoring the leprosy mite population is essential to support control action. Sampling, as recommended above, incurs estimation errors of up to 70%, and it is common for different inspectors, sampling the same plot on the same day, to arrive at different estimates of infestation. Furthermore, in this sampling the same number of fruits and/or branches are sampled per plant regardless of the quantity of fruits on the plant. To reduce this sampling error, the percentage of plants sampled per plot and the number of fruits and/or branches sampled per plant must be increased considering the fruit load on the plant. Also, the interval between samplings must be reduced or the number of inspectors in each sampling increased. However, with the increase in labor costs for pest inspection in the field, monitoring is becoming less and less intense. It is common to observe intervals between inspections of more than 15 days, and not uncommonly 30 days. Also, a reduction in the sample size per plot is observed, with less than 1% of the plants sampled and with the observation of fewer fruits and/or branches per plant. All of this leads to greater errors in estimating the mite population density and to detection often well above the stipulated action level.

Time between action level detection and acaricide application

Once the action level is detected in the field, the acaricide application should be carried out immediately. However, in an attempt to reduce production costs, the range of equipment and sprayers is increasingly lean and, often, there are no machines available to apply the acaricide when the action level is detected. Therefore, it is common for the acaricide to be applied more than two weeks after detecting the level of action, which means that, at the time of application of the acaricide, the mite population is at a density well above the level of action. action and, even if the acaricide has the same mite control efficiency (causes 90% mortality), the mite population surviving the application will be greater and, consequently, it will take less time to reach the action level again, resulting in a shorter period of mite control by the acaricide.

Acaricide application technology

For good control of the leprosy mite, the acaricide must be applied in such a way as to have good deposition and coverage above 50% on the fruits, branches and leaves located inside the plant and throughout its crown. Application of spray volumes between 8 and 10 thousand L/ha in orchards with adult plants was common, generating high costs and waste of water and product. In the 2000s, by increasing the number of nozzles in the spray branch and using nozzles that produce fine drops (100 to 200 µm in volumetric median diameter), for greater penetration of the drops inside the plant canopy, it was possible to reduce the volume of spray solution to 2 to 3 thousand L/ha in adult orchards (100 to 150 mL/m3 of canopy), obtaining the same mite control efficiency and without the need to correct the acaricide dose. However, it is not uncommon to see citrus growers using poorly adjusted equipment and reducing the volume of acaricide syrup by simply increasing the speed of the turbopulverizer or changing the working pressure without adopting the technology correctly. Another common problem, especially in orchards with very tall plants (>3,5 m), is the positioning of the center of the spray turbine below the average height of the plant canopy, which hinders the application of the acaricide to the upper part of the plant.

Densification of orchards

The density of orchards in the planting line makes it difficult for pest inspectors to move from one street to another, hindering mite inspection, and increases contact between plants, which facilitates the mite's spread from one plant to another. The density between planting lines, with the plant canopy very close to the spray nozzles, often even touching the nozzles, prevents the good formation of the spray fan and leads to a poor distribution of coverage and deposition of the applied solution, having areas without acaricide, reducing control efficiency. Ideally, the spray nozzles should be at least 40 cm from the edge of the plant's crown.

Mixes in the spray tank

As a way to reduce costs, product mixtures are made in the spray tank and some of them can affect the effectiveness of the acaricides, due to changes in the pH of the solution and electrical conductivity, as well as incompatibility between the compounds. The mixture of foliar fertilizers potassium phosphite, magnesium sulfate and the mixture of zinc and manganese chlorides with magnesium sulfate resulted in a decrease in the effectiveness of the acaricides propargite and acrinathrin on the leprosy mite. Under laboratory conditions, it was observed that the mixture of the insecticides imidacloprid, bifenthrin, cypermethrin and phosmet with the acaricide spirodiclofen, although it does not present physical and chemical incompatibility, reduced leprosy mite mortality from 10 to 22%. No reduction in mortality was observed in mixtures of the acaricides propargite and cyflumetofem with the insecticides. A lower efficiency of the acaricide in leprosy mite mortality will reflect a shorter period of control of the applied acaricide.

Mite resistance to acaricides

Successive applications of the same acaricide molecule causes the selection of mite populations resistant to the applied acaricide, which, consequently, loses its control efficiency. There are reports of leprosy mite resistance to the acaricides hexithiazox, dicofol, propargite, flufenoxuron and, more recently, to spirodiclofen. Therefore, rotation of acaricides from different chemical groups and mode of action is recommended for leprosy mite management. However, as there are currently few acaricides available with good leprosis mite control efficiency, the rotation of acaricides is quite difficult. The need for new acaricides is urgent.

Management of other pests and diseases in leprosy mite control

The presence of early blight and citrus canker in the citrus belt considerably increased the use of fungicides or copper-based products in orchards, as well as the control of the greening psyllid, from 2004 onwards, increased the use and frequency of insecticides. This ended up favoring the leprosy mite, since many insecticides and fungicides affect their natural enemies (entomopathogenic fungi, mites and predatory insects). Furthermore, the effect of insecticides, mainly pyrethroids and neonicotinoids, is widely reported, stimulating the reproduction or reducing the mortality of mites in several crops. Although not yet proven for the leprosy mite, it is believed that this effect may be occurring due to successive applications of insecticides to control the psyllid. The doses of insecticides used to control the psyllid do not cause mortality in the leprosy mite, but could stimulate its oviposition, increase the viability of the eggs or their longevity, which would result in an increase in the rate of reinfestation of the orchard after the application of the acaricide.

Climate

Climatic conditions favorable to the leprosy mite and unfavorable to its control have been observed in the citrus belt in recent years. Periods with long days, high temperature, low rainfall, low relative air humidity and low water availability in the soil favor the increase in the leprosy mite population. Furthermore, these same conditions favor the evaporation of fine and very fine droplets during the application of the acaricide, reducing coverage and deposition of the spray on the plants.

In conclusion, there are several reasons responsible for the increase in the difficulty of controlling the mite and leprosy damage in orchards. All must be taken into account and corrected to better control the mite and the disease.