Brown streak

Alternaria brown spot, caused by the fungus

Fr. (Keissler) f. sp. citri, was found in the 2002/2003 harvest by researchers Eduardo Feichtenberger from the Sorocaba/APTA Research and Development Unit and Marcel Spósito from Fundecitrus, in fruits of 'Ponkan' and 'África do Sul' mandarin trees and 'Murcott' tangor, in Minas Gerais, municipality of Campanha, in Rio Grande do Sul, municipality of Montenegro, and in 7 municipalities in the State of São Paulo. The first report of this disease in the country was made in 2001 by Prof. Antonio de Góes from UNESP/Jaboticabal, affecting 'Dancy' mandarin trees in the State of Rio de Janeiro. This disease has already been reported in Australia, the United States, South Africa, Israel, Turkey, Spain and Colombia, where it has been causing serious damage to the crop.

the fungus

f. sp. citri has two forms: the “rough lemon pathotype”, which is specific to the rough lemon (

Lush) and 'Cravo' lemon (

Osbeck), and causes lesions on leaves of these two species, and the “tangerine pathotype”, which causes disease in tangerine trees and their hybrids (tangores and tangelos). The symptoms of the “tangerine pathotype” have been observed for some time on leaves and branches in Brazilian orchards, however, until now, they had never been observed in fruits. The varieties most seriously affected by the “tangerine pathotype” of this fungus are the 'Dancy' mandarin (

Hort.); the 'Orlando', 'Nova' and 'Minneola' tangelos; and the 'Murcott' tangor. The fungus also causes lesions on 'Ponkan' mandarin, however, the disease appears to be less severe in this variety.

The fungus causes lesions on young leaves, fruits and branches. The period of susceptibility varies depending on the tissue. In leaves, the tissue remains susceptible until the leaf reaches its maximum expansion. Once infected, symptoms on the leaves are observed within 48 hours in the form of small necrotic spots, brown to black, surrounded by a yellowish halo. These lesions expand, becoming circular or irregular of variable size, and can occupy large areas of the leaf surface and reach the veins. On young branches, symptoms are similar to those observed on leaves, with lesions measuring 1 to 10 mm in diameter. In fruits, infection can occur from the time the petals fall until they develop, and they are susceptible up to 4 months after flowering. The lesions on the fruits are small dark necrotic spots, which may vary in size depending on the age of the fruit. In some cases, lesions can be observed whose center becomes corky and prominent, forming a pustule. Depending on the intensity of the disease, the lesions can cause plant defoliation, branch death and premature fruit drop. The fruits that remain attached to the plant lose their value on the fresh market due to injuries caused by the fungus.

Alternaria alternata f. sp. citri is a saprophytic fungus that survives in the tissues of the citrus plant. Asexual spores of the fungus (conidia) are produced both on leaves and branches on the plant, and on decaying leaves in the soil, but are not produced in fruits. Conidia have a thick wall and are resistant to drying and other adverse conditions. These conidia are released by the action of rain or tissue wetting by dew and are spread over long distances by the wind. During germination, the conidia of Alternaria alternata f. sp. citri release specific toxins into their hosts. These toxins kill plant cells to allow infection and colonization of tissues by the fungus, playing a fundamental role in the expression of symptoms and the occurrence of the disease in the host plant.

Regarding control, an important aspect is knowing the period of release and infection of the fungus during the year. In Spain, studies have shown that the release of Alternaria alternata f. sp. citri can occur throughout the year, but infection occurs when the wet period is 8 to 10 hours and the temperature is between 20 and 30oC. Therefore, the occurrence of alternaria brown spot is highly dependent on favorable climatic conditions. Studies of the same type should be conducted in Brazilian conditions to better understand the disease and determine control strategies.

For adequate control of this disease, there is a need to adopt strategies involving orchard management and fungicide treatments. As a general rule, to avoid disease, the coincidence of high relative humidity in the orchard with the presence of young, highly susceptible plant tissue must be avoided. Cultural practices, such as heavy nitrogen fertilization, excess irrigation, as well as severe pruning carried out at inappropriate times, which lead to a large vegetative flow, and, therefore, the formation of tissue susceptible to the fungus, should be avoided. Protective products are recommended as chemical control, such as cuprics and dithiocarbamates, and, among systemic ones, strubilurins, dicarboximides and, with less efficacy, triazoles. It is important to know that benzimidazoles have no effect on the fungus and may even worsen the disease situation in the orchard.

As a general rule for chemical control, a period of approximately four months should be prioritized, starting in pre-flowering. This early start, compared to other sprays commonly carried out on the crop to control diseases, aims to protect the first shoots that begin after winter. In countries such as the United States, South Africa and Colombia, spraying with protective products is carried out at 15-day intervals. With systemic products, the interval can be 30 days, always remembering that systemic products from the same chemical group must be used at most twice per harvest to prevent the fungus' resistance to these products. These sprays also control other fungal diseases, such as warts (

spp.) and black spot (

), and can be added to treatments for alternaria, mineral or vegetable oil, aiming for better control of black spots. In regions where there are several vegetative flows throughout the year, special care must be taken to avoid an increase in inoculum that leads to high disease severity and, consequently, losses to the producer. In these cases, the producer must be alert to the occurrence of symptoms in the orchard and apply fungicides in time to prevent serious damage.

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