Management of alternaria stain on cotton

Genetic resistance emerges as the cheapest, safest and most rational among the alternatives for dealing with alternaria leaf spot, a secondary disease that has demanded attention in cotton crops in Brazil, favored by the agricultural model and environmental conditions.

Foliar diseases caused by fungi have been reported for a long time in cotton farming in the southern region of Brazil. However, except for localized and intense occurrences of ramulosis, and a severe episodic outbreak of spot, stemphylium In the years 1994 to 1996 – in both cases due to the incidence in very susceptible cultivars – such diseases were never significant in the region. Even with frequent incidences, they have always been considered secondary in their effects.

This perception, however, has undergone considerable change since the first five years of this century, due to the combination of two main factors. On the one hand, there is a movement of the crop to regions of the Brazilian Center-West, especially Mato Grosso, where climatic conditions prevail that are highly favorable to the pathogens responsible for these diseases. On the other hand, in the wake of this geographic displacement, there has been a notable technological change in the conduct of culture, now carried out in large areas and with a high level of mechanization in its practices. This second factor would require cultivars adapted to the new conditions, especially regarding the high percentage of fiber, size and compactness of the plants, resistance to lodging, an earlier and more determined cycle, greater retention of the plume in the capsules and adaptation to mechanical harvesting. Due to the non-existence, in Brazil, of cultivars with these properties, when the new production systems were implemented, the use of introduced material in crops began and became the rule, which proved to be susceptible to most of the pathogens that occur here. occur. And this has been happening not only for the most destructive diseases, but also for those once considered secondary, including Alternaria spot.

Known and described for more than a century, this disease is caused in Brazil by the fungus Alternaria macrospora Zimm. Its importance has been highlighted in other countries, through reports of incidence mainly in cotton cultivars of the species Gossypium barbadense, practically not cultivated commercially in Brazil. These reports indicate losses of up to 20% in production due to this pathogen. The circumstances already mentioned may occur in current Brazilian cotton farming, both due to environmental conditions favorable to the fungus and the use of highly susceptible cultivars.

 The progress of the disease is favored by high relative humidity, mainly at temperatures in the range of 20ºC to 30ºC.^ow. The infection already occurs in the cotyledons, then manifesting itself in the true leaves of the plants, when they reach 20 days to 30 days of age. Under the same conditions, the development of the disease is much greater in the cotyledons than in the leaves, which makes them the main reservoir of inoculum to infect the leaves that appear next.

The initial symptoms are areas of light green color and undefined shape, which evolve into circular brown spots with blackish, well-defined edges. Dark-colored concentric rings can be seen in them, where the spores of the pathogen are found. In the center of the spots there is a yellowish halo. In cases of more severe incidence, coalescence of the lesions may occur, leading to the formation of irregular necrotic areas, which cause tears in the leaves and even their fall.

The pathogen in question is transmissible through seeds and survives in crop residues. These facts already indicate necessary measures to prevent the occurrence of the disease, especially if a susceptible cultivar is used in crops. Among them, the use of exempt or treated seed, destruction of cultural remains and crop rotation. However, once the disease is established, it becomes necessary to control it by chemical means. For this, tin fungicides are the most recommended, with several of them, alongside other chemical groups, registered for the disease, at the Ministry of Agriculture, Livestock and Supply (Mapa). In areas where fungicides are already expected to be used to control other diseases, such as ramulosis and ramularia, it is possible to consider the use of products that, in addition to these, also control Alternaria leaf spot.

Obviously, the safest, most rational and economical control of this and other diseases is through resistant cultivars. This possibility can be seen in tables 1 and 2, which contain results obtained by the Campinas Agronomic Institute (IAC), in field experiments carried out in locations in São Paulo. In them, cultivars and lines recently developed by the main entities that carry out genetic improvement of cotton in Brazil were evaluated for the disease, under natural infestation by this pathogen. The performance of each cultivar is characterized by means of average scores, on a scale that ranges from 1 (maximum resistance) to 5 (maximum susceptibility). There is a notable difference between cultivars, ranging from highly resistant to highly susceptible, and others, of intermediate performance. It should be noted that among the most susceptible, losses of up to 12% in production were estimated, due to the incidence of the disease. It should also be noted that some cultivars have inconsistent performance, proving resistant under conditions of low or medium infestation and susceptible under more severe incidences of the disease. This fact must be taken into account so that mistakes do not occur when choosing the cultivar to be used. 

Edivaldo Cia and Milton Geraldo Fuzatto, Agronomic Institute (IAC)

Article published in issue 211 of Cultivar Grandes Culturas.