How to control citrus leprosy

The leprosis mite, Brevipalpus phoenicis (Geijskes, 1939) (Acari: Tenuipalpidae), also called the flat mite, became important in Brazil after the discovery of its association with the virus that causes citrus leprosis (Citrus leprosis virus - CiLV) in 1963.

Until now, the direct damage caused by the mite to citrus trees was insignificant, due to its low population density. It is estimated that the damage caused by CiLV, transmitted by the mite, is 35 to 100% with the premature fall of fruits, the depreciation of damaged fruits for the fresh fruit market, the defoliation of the plant and the drying of affected branches, compromising the next flowering.

This mite has a cosmopolitan distribution and can be found in several citrus-producing countries, including several where the disease is not yet present.

It has a large number of hosts, with more than 200 species of fruit, ornamental and invasive plants, including citrus as the main one. Invasive plants in citrus orchards and others normally used as hedges and windbreaks can also host and multiply the leprosis mite, serving as its reservoir (Table 08).

The place of occurrence on the plant can also influence the development of the mite. Fruits with wart lesions are preferred by the mite, followed by fruits without warts, branches and finally leaves (Table 5). The wart provides an excellent shelter or refuge for the mite. Mites normally occur in greater intensity on fruits than on leaves, with an average of 95,2% of mites on fruits, 4,3% on old leaves and only 0,5% on new leaves.

Citrus varieties can also affect spider mite development. The fruits of the 'Valencia' and 'Murcote' varieties are more favorable to the development of the leprosy mite, while those of the 'Persian' lime, the 'Tahiti' acid lime and the 'Sicilian' lemon are less favorable. Similarly, the greatest number of mites per leaf is found in the varieties 'Lima', 'Hamlin', 'Lima Verde', 'Baía', 'Valência' and 'Barão' than in the varieties lima da 'Persia', 'Pera Rio', 'Seleta' and 'Natal'.

The leprosy mite can be found throughout the year, but population levels rise from March to April, a period in which rainfall normally begins to decrease and the fruits begin to ripen. They reach higher population levels from July onwards and their maximum peak is generally in the months of September to October, before gradually decreasing with the rains and fruit harvest.

If the mites are infected with the virus, after 17 to 60 days of inoculation with the viral agent, through the mite's feeding process, the symptoms of leprosis appear. All stages of the mite, except the eggs, are capable of transmitting CiLV after feeding on a typical leprosy lesion or even in an area where contaminated mites previously fed, and maintains its ability to transmit the virus for most of its life. even when transferred to other healthy plant organs.

Symptoms of leprosy appear on the leaves, branches and fruits. On the fruits, leprosy lesions begin to appear when the oranges measure about five cm in diameter. Initially, the spots are pale green in color, with yellowish halos, which stand out clearly from the dark green color of the healthy part of the fruits.

Later, the center darkens, becoming straw-brown, brown or dark brown, and as the fruit ripens, the yellow halo begins to blend in with the color of the rest of the fruit, with symptoms similar to those that occur in ripe fruits. These appear as round, depressed brown spots, measuring 0,12 to 1,2 cm in diameter, clearly limited, since the virus is not systemic. These spots generally occur in groups, with there almost always being at least five or six spots on the same fruit, although many more spots can be found on a single fruit.

When the attack is intense, the fruits fall, which can be in large quantities. The leaves are left with yellowish spots, and as the disease progresses, brown spots appear in the center, which later become necrotic. Infected branches have brown spots that dry out and detach over time, causing them to die.

The main strategy used to control leprosis is the elimination of the vector, the mite B. phoenicis. By eliminating the mite, we seek to prevent contamination of plants and its spread in orchards. To prevent contamination, the incidence of the mite on the plants must be assessed and its control carried out when it reaches the action or control level, which has varied from 2 to 10% of the sampled organs with the presence of the mite, depending on the size of the number of sprayers, agility in acquiring the acaricide and depending on the incidence of the disease in the orchards.

For greater efficiency and optimization of mite control, plots with approximately 2 plants should be adopted, and those with a greater number of orange trees can be subdivided, with control directed at those subplots that present a mite population at the control level. In these plots or subplots, the number of plants to be sampled should be at least 1%.

In orchards in formation, inspection should be carried out on the branches, mainly where there is a branch bifurcation and in places where there are protrusions and indentations. In orchards in production, inspection should be carried out on the fruits, those that are ripe and that are inside the canopy of the plant, preferably with warty lesions. If the plant does not have fruits or they are green, inspections can be carried out on the branches. In both new orchards and those in production, the number of samples per plant should be three fruits or branches.

The sampling frequency must be a maximum of 15 days.

In recent years, the annual expenditure on acaricides to control the leprosy mite has been approximately US$ 60 to 70 million, which indicates the importance of controlling the disease and its vector in the cost of citrus production and the need for better apply integrated management strategies for this pest.

* By Pedro Takao Yamamoto, Renato Beozzo Bassanezi, Marcel Bellato Spósito, Jose Belasque Junior e Waldir C. de Jesus Junior (Fundecitrus)

Article published in the magazine Cultivar Hortaliças e Frutas 30 (February 2005)