Alternatives for chemical control of ramularia stain on cotton

The main disease in cotton crops, ramularia spot has been more prevalent and more severe over the last few years in Brazilian crops. Increase in cultivated area, monoculture, susceptible cultivars and failure to destroy ratoon crops are among the explanations for this worsening. Without neglecting other management measures, the search for new chemical control alternatives is important to assist in the rotation of fungicides and prevention of resistance, in addition to increasing the tools available to producers.

Currently, ramularia spot, caused by the fungus Ramularia areola (Atk.), is undoubtedly the main disease of cotton crops, a situation that has worsened since 1998 due to the increase in cultivated area, monoculture and the use of genotypes of Australian and American origin that were quite susceptible to the disease.

The disease has demonstrated greater aggressiveness with each harvest, so that in some regions of Mato Grosso approximately 12 fungicide applications are already carried out during the crop cycle, with a state average of around eight applications. Another fact that has drawn attention is the emergence of symptoms of the disease at an increasingly earlier age, which occurs mainly due to the poor destruction of ratoon crops that has been carried out, which increases the initial inoculum of the disease, which remains active in these undestroyed plants. . In the last harvest, 2014/15, symptoms of ramularia were observed as early as 20 days after crop emergence, which has led to fungicide applications being started earlier and earlier.

The disease causes whitish, powdery-looking, bluish-white spots between the leaf veins on both leaf surfaces. Subsequently, the lesions become necrotic and can aggregate and cause defoliation. Ramularia is favored by temperatures in the range of 25ºC to 30ºC, relative humidity above 80% and humid nights followed by dry days, without prolonged periods of leaf wetness, a common situation in most producing regions of Mato Grosso, which has already been reported. losses of up to 75% of production due to the presence of the disease in susceptible genotypes cultivated in the state.

Currently, the main strategy for ramularia management has been chemical control, fundamentally based on the use of triazoles and organotin, also including strobilurins when these are commercially formulated with triazole. Therefore, in the present study, new control alternatives for the disease were sought, considering in addition to the traditionally used fungicides, also evaluating multisite fungicide (dithiocarbamate) and commercial mixtures of carboxamide + strobilurin, as described in the treatments in Table 1.

In order to compare the different fungicides in a clear and objective way, with crop protection for the period necessary to obtain high productivity, eight applications of each fungicide were carried out, starting 30 days after crop emergence and repeated at intervals. 14 days. Using eight applications of the same fungicide is a strategy valid only for research purposes, as this is the best way to achieve a reliable comparison between products and thus find better chemical control alternatives, with the aim of developing management programs. with rotation of active ingredients, covering the greatest possible number of mechanisms of action, in order to enhance disease control, as well as prevent the emergence of pathogen resistance to fungicides.

The experiment was conducted in the municipality of Primavera do Leste (MT), at the Ceres Consultoria Agronômica Experimental Station, using the FiberMax 975 WS cultivar sown on 15/12/2014. The experimental design was randomized blocks, with four replications and the applications were started preventively, that is, no symptoms of ramularia had yet been observed in the area. The correct time to start applications is a key factor in the success of chemical control, so late applications, after full establishment of the disease, result in deficiencies in ramularia control.

Applications were carried out with a CO pressurized backpack sprayer.₂ equipped with six 110.015 fan tips, with a spray volume of 150L/ha and the severity of ramularia was assessed weekly with the aid of a diagrammatic scale. With the severity data in hand, the Area Below the Disease Progress Curve (AACPD) was created. The results were subjected to analysis of variance and the means compared using the Skott knott test at a 5% error probability level.

The best control of the disease was obtained by treatments with fentin hydroxide and fentin hydroxide + carbendazim, which did not differ from each other. The third lowest AACPD occurred for the commercial mixture of pyraclostrobin + fluxapyroxad, which did not differ significantly from the protective fungicide mancozeb (Graph 1). These results are very promising regarding the future management of this disease, since protective fungicides, as well as mixtures of strobilurin + carboxamide, have not been used frequently for the control of ramularia and can be inserted in the management of the disease in the face of the use massive use of triazoles that has been occurring in recent years, with reports of a drop in efficiency. Next came the AACPDs of all triazoles, mixtures of azoxystrobin + cyproconazole and azoxystrobin + difenoconazole, in addition to azoxystrobin + benzovinflupyr, which did not differ statistically. The other treatments did not differ from each other, but they all differed from the control without application.

Fentin hydroxide is a product already recognized as having high efficiency for ramularia control, however in commercial crops, a maximum of three applications of this product are commonly carried out, due to the phytotoxicity caused to the cotton crop, especially in later applications. The use of carbendazim in addition to fentin hydroxide generated little increase in terms of ramularia control, but it is a product with good control action on other diseases.

Although it is a preliminary study, this experiment shows us the way forward in the next harvests, as good results were obtained with formulated mixtures of strobilurins + carboxamides and dithiocarbamate, constituting molecules that can be introduced into management programs, achieving a better rotation of active ingredients and mechanisms of action. At this point, the multisite action fungicide deserves to be highlighted, constituting an important tool in the management of fungus resistance to fungicides, however it is worth noting that this is a pioneering trial and new studies must be carried out with such fungicides, in order to seek the better adjustment of dose, moment of application and need to use an adjuvant, for example.

Ceres Consultoria Agronômica emphasizes that the present study strictly sought new alternatives to enable the chemical management of the main cotton disease, in order to collaborate in the rotation of fungicides with different mechanisms of action as a strategy to avoid the emergence of resistance. However, chemical control must be complemented by other control methods, mainly with the adequate destruction of ratoons and respect for sanitary conditions, in addition to the use of cultivars with a high degree of tolerance to the disease, such as RX cultivars. Only with the integration of control methods will it be possible to halt the momentum of cotton ramularia.

Cotton in Brazil and incidence of diseases

Cotton (Gossypium hirsutum L.) is cultivated in several Brazilian states, but the largest planted area, approximately 85%, is concentrated in Mato Grosso and Bahia. Mato Grosso is the largest national producer of cotton, cultivating more than 560 thousand hectares, with production of around 870 thousand tons of lint, corresponding to 57% of the country's production.

Cotton migrated to the Center-West as an alternative for crop rotation and after the efforts of the precursors of this cultivation in the Cerrado, technologies adapted to the region were developed which, associated with the extensive areas of flat topography and well-defined rainy season, made it possible to obtain high crop productivity. However, due to the large rainfall, high relative humidity and nights with mild temperatures, especially in higher altitude regions, pathogens are also favored, leading to epidemic outbreaks of diseases that were not observed when cotton cultivation was carried out. predominantly in the Southeast and South regions of Brazil.

Maurício Silva Stefanelo, Guilherme Almeida Ohl and Evaldo Kazushi Takizawa, Ceres Consultoria Agronomica 

Article published in issue 199 of Cultivar Grandes Culturas.