Control of ramularia leaf spot on cotton

With well-known effects on productivity, ramularia leaf spot has also to interfere with the quality of cotton fiber. Therefore, keep disease under control until the apples reach maturity is measured essential to prevent losses.

The ramularia leaf spot of the cotton plant, caused by fungus Ramulariopsis gossypii (without. Ramularia areola), and the most important disease of cotton crops today. The illness can cause productivity reductions of up to 75%, depending on susceptibility cultivar, initial inoculum, environmental conditions and management practices used for control.

The symptoms of ramularia leaf spot are quite characteristic and appear on both sides of the leaf, initially as bluish-white spots on the adaxial face and presence of white mycelium on the face abaxial, due to abundant sporulation. The disease can cause damage to cells from the mesophyll, to the membranes, to the functioning of the enzymatic machinery, in opening and closing of stomata, affecting transpiration, the influx of CO2 and photosynthetic rate.

The beginning of the infectious process occurs from conidia or ascospores produced on cotton crop residues or in perennial cotton plants, constituting the primary inoculum. O secondary inoculum is formed from infection of the first leaves and then It spreads through rain, irrigation, wind, people and machines. Of that Thus, areas with a history of cotton cultivation tend to have greater severity of ramularia spot.

The effects of this disease on the cotton productivity are well known by cotton farmers and researchers. However, in addition to reducing the quantity, ramularia leaf spot can also harm the quality of cotton fiber, affecting technological characteristics such as micronaire, length and resistance. These characteristics, along with others determined through HVI analysis (High Volume Instrument), are used to define the quality of the cotton batch and, consequently, whether there will be a premium or discount on the sale value.

The analysis and classification of fibers are indispensable in the cotton market, helping producers and buyers in decision-making regarding the feather trade. Currently, the standard official classification of cotton in Brazil is determined by the Ministry of Agriculture, Livestock and Supply (Map), through Normative Instruction number 24, of July 14, 2016.

The fiber produced in crops needs meet the minimum standards established by the textile industry and required by the final consumer so that cotton can be commercialized and industrialized. Obtaining a quality fiber depends on several factors. biotic and abiotic, including the genetics of the cultivar, type soil, nutrition, environmental conditions and occurrence of pests, diseases and weed plants.

Experiments field

A field experiment was carried out with the cultivar BRS 336 in Cristalina, Goiás, with the aim of determining the damage caused by ramularia leaf spot on the productivity and quality of fiber cotton. Treatments with and without the application of fungicides and the variables measured were the severity of disease, seed cotton productivity and the technological characteristics of Micronaire fiber, length and resistance.

Based on the results obtained, it can be conclude that the severity of ramularia spot was greater in plots that did not received fungicide application, which consequently obtained lower productivity (Figure 1). The difference of productivity between the control and the treatment with fungicide application was 76 arrobas/ha (24,5%).

The quality of the fiber obtained, represented in the present work due to the micronaire technological characteristics, length and resistance, was negatively affected by ramularia leaf spot. The values obtained for each of these characteristics were lower in plots that did not received fungicide application (Figure two).

Cotton fiber is made up mainly by cellulose, which represents approximately 90% of its weight dry. The cellulose deposition process, responsible for fiber formation, is directly affected by the supply of carbohydrates produced through photosynthesis. Ramularia leaf spot affects the photosynthetic capacity of the plant and consequently the quality characteristics of the fiber.

These results alert to the fact that that it is important to maintain monitoring of cotton crops and the disease under control until the apples reach maturity, thus avoiding reduction in fiber quality. Disease control has been achieved through rotation of crops, balanced fertilization, genetic resistance of cultivars, timing of adequate sowing, cultural management and application of fungicides.

Nedio Rodrigo Tormen, Luiz Eduardo Bassay Blum, Ricardo Silveiro Balardin, Institute Phytus

Article published in issue 231 of Cultivar Grandes Culturas, August, 2018.