Efficiency of fungicides against ramularia and spots on cotton in the 2022-23 harvest

Ramularia spot, widely known as ramularia by producers and professionals who work in cotton cultivation, is the main fungal foliar disease of cotton in the Brazilian cerrado. In this biome, the disease is caused mainly by the fungus R. pseudoglycines. Similarly, the target spot (Corynespora cassiicola), as well as other spots of secondary occurrence or with an etiology not yet well defined, henceforth treated as leaf spots, have shown high incidences in the main producing regions, with increasingly greater levels of severity over the last few harvests. Among the production regions that present this problem, the states of MT, BA, GO, MS, PI and MA stand out. Given the high progress of leaf spots and the need for improvements regarding control efficiency, the National Research Cooperation Network: Rede Ramulária was expanded to act on leaf spots, starting at the end of 2022.

Thus, in the 2022/2023 harvest, 52 experiments were carried out in the main cotton producing regions of the country, through cooperation between research institutions and companies obtaining registration. This cooperation for applied research included the participation of 17 researchers, with the main purpose of contributing to increasing knowledge regarding the efficiency of controlling the two main diseases that currently affect the aerial part of the cotton plant. The objective of this publication is to briefly present the summarized results of the four cooperative trials for the control of ramularia leaf spot and leaf spots, carried out during the 2022/23 cotton harvest.

The management of ramularia, as well as cotton leaf spots, must be done through the integration of control measures, which includes chemical control. Therefore, detailed knowledge about the control efficiency of the agents responsible for these diseases is fundamental to the adequate dimensioning of a chemical control program.

The planning and implementation of a control program, which includes the positioning of products according to their control efficiency, through the use of site-specific and multi-site fungicides in prefabricated or associated mixtures, taking into account the alternation of modes of action and active ingredients according to the phases of the cotton cycle, the period of disease occurrence, inoculum pressure and the history of disease occurrence in the cultivation area, are essential aspects for the successful management of these diseases . While the appropriate choice and positioning of a fungicide program are decisive for the success in controlling cotton aerial diseases, inadequate design of the chemical control program can result in failure to control ramularia, as well as leaf spots.

This management will be impactful both in the short, medium and long term, ensuring that the control program does not incur in the selection of resistant inoculums in the planting areas, is effective in reducing inoculum pressure in the area and, also, ensures the lowest possible impact of the disease on the productivity and quality of the cotton fiber produced. Results on the efficiency of fungicides that could be part of the fungicide program for the control of ramularia and leaf spots can be obtained from the Cooperative Testing Network (Rede Ramulária, 2023). The results are updated in November of each year and are readily available on the Rede Ramulária website (rederamularia.com.br) and the Fitossanidade Tropical network (fitossanidadetropical.org.br), in addition to publications from the Embrapa series.

The Cooperative Testing Network for the evaluation and monitoring of the effectiveness of fungicides for the control of ramularia and leaf spots carries out experiments that follow standardized methodologies. During the 2022/23 harvest, the network conducted 52 experiments in 13 different locations, with four test modalities, covering the regions of the States of Mato Grosso, Bahia, Mato Grosso do Sul and Goiás. The objectives were to evaluate the efficiency of local fungicides -specific for isolated formulation or ready-made mixture with multisites (Test 1); evaluate the efficiency of multisite fungicides applied alone or in combination with site-specific fungicides in controlling ramularia (Trial 2); evaluate the efficiency of registered fungicides in controlling leaf spots (Trial 3); monitor the control efficiency of fungicides for ramularia throughout the harvests (Trial 4), as well as evaluate the effect of fungicides on cotton productivity.

The experimental design used in the four experiments was randomized blocks, with four replications per treatment for the experiments with site-specific fungicides in isolated formulation or in ready mixes with multisites (Trial 1); leaf spots (Trial 3) and monitoring (Trial 4); and with five replications for the multisite fungicide experiments (Trial 2). In all experiments, eight fungicide sprayings were carried out. Ramularia leaf spot severity assessments were carried out before each spray and at seven, 14 and 21 days after the last application. Leaf spot severity assessments were carried out weekly and continued until seven, 14 and 21 days after the last application.

The results presented in Table 1 are based on the control averages achieved at the sites that were grouped in the joint analysis for the test of site-specific fungicides in isolated formulation or in ready mixes with multisites (Test 1). The highest percentage of ramularia control was obtained in treatment with applications of pydiflumetofen/chlorotalonil (T10 - 75,1%), followed by treatment with fentin hydroxide (T8 - 73,6%), measured by the reduction in the area under the curve of disease progress (AACPD) of each treatment in relation to the control without applications. In the other treatments, the control efficiency was between 63,5% and 68,2% (Table 1).

Seed cotton productivity in treatments that received fungicide application ranged from 307,2 to 323 arrobas per hectare (@/ha), which were higher than that obtained in the control, which did not receive fungicide application (T1 – 282 @/ha ). This result demonstrates the importance of using fungicides within the integrated management of this disease. The average reduction in control productivity in relation to the treatment that presented the highest productivity was 12,4% (Table 1).

The incorporation of multisite fungicides into the ramularia chemical control program is important to reduce the risk of emergence of resistant pathogen populations in the field and increase control efficiency.

In the multisite fungicide trials (Trial 2 - Table 2), the site-specific fungicide pyraclostrobin/mefentrifluconazole/fluxapyroxad and the multisite chlorothalonil were used, applied alone or in combination. The association of the site-specific fungicide with the multisite resulted in lower intensity of ramularia throughout the cotton cycle (lower AACPD) and, consequently, in greater control of the disease, as seen in T4, with 70% control (Table 2). For fungicides applied alone, control was 64,4% with specific site (T2) and 61,3% with multisite (T3). Results similar to this, with an improvement in disease control performance with the association of a multisite with a site-specific fungicide, were obtained in network trials carried out in previous harvests, which demonstrates the importance of using multisite fungicides, aiming to increase control and reduction of the risk of selection of resistant biotypes/isolates of fungi that are the etiological agents of cotton ramularia spot.

In fungicide trials for the control of leaf spots on cotton (Table 3), the highest percentages of control, measured by the ability to reduce AACPD, were obtained in treatments with applications of prothioconazole / mancozeb (T4 - 42,8%), azoxystrobin / mancozeb / prothioconazole (T5 - 41,1%), fluxapyroxad / prothioconazole (T2 - 39,1%). Next, in order of efficiency and control, treatment with trifloxystrobin / prothioconazole / bixafen (T3 - 33,8%), followed by treatment with metominostrobin / tebuconazole + chlorothalonil (T6 - 24,8%).

With regard to seed cotton productivity in this set of experiments, with the exception of T6 (metominostrobin / tebuconazole + chlorothalonil), all treatments that received fungicide applications showed higher productivity than the control, with seed cotton productivity varying between 338,4 .349,7 and 7,4 @/ha. The reduction in the average productivity of the control in relation to the treatment that presented the highest productivity was 3% (Table XNUMX).

Table 4 shows the results obtained in the experiment monitoring the sensitivity of R. pseudoglycines to site-specific fungicides conducted in the 2022/23 harvest (Trial 4). In this set of tests, the highest percentage of control was observed when the fungicides pyraclostrobin + fluxapyroxad were used (T6 - 64,3%). Then, treatments with the fungicides azoxystrobin + difenoconazole (T4 - 47,3%) and the fungicide difenoconazole alone (T2 - 47,2%) were presented, followed by the fungicide consisting of the mixture of azoxystrobin/flutriafol (T5 – 42,7 .3%), which surpassed treatment with tetraconazole alone (T35,9 - XNUMX%).

Regarding the stability of control of the severity levels of ramularia spot throughout the harvests (Figure 1), a trend towards lower control was observed by the majority of fungicides under monitoring, evaluated in the 2022/23 harvest, when compared to the previous harvest (2021/22), considering that the majority of fungicides evaluated presented a lower control percentage than the values ​​obtained in the 2021/22 harvest.

On the other hand, it is observed that some of the active ingredients of fungicides presented values ​​similar or very close to those obtained in the 2019/20 harvest (Figure 1), an example of this behavior can be seen for treatments with tetraconazole and azoxystrobin + flutriafol . This behavior demonstrates that although some fungicides demonstrate a low percentage of control, such control levels, under field conditions, have not demonstrated a considerable reduction in the last three harvests.

Selection pressure, resulting from the high number of applications used throughout the cotton cycle in the field, has favored the risk of emergence of isolates or biotypes of fungi resistant to the fungicides used, therefore, it is emphasized that ramularia spot cannot be underestimated due to the low incidence that occurred in some locations in recent harvests. Regarding target spot and other insurgent leaf spots, with possible different etiological agents, it is necessary to increase knowledge of the efficiency of fungicides for the effective control of spots that affect the aerial part of the cotton plant.

Likewise, it is of crucial importance that efforts are driven by research and innovation actions, capable of making new tools and methods available, which contribute to the control of these diseases, while preventing the emergence of isolates/biotypes resistant to fungicides. In this scenario, the use of effective measures to destroy crop remains, in order to ensure the sanitary void, as well as knowledge about the level of resistance of cultivars, the history of the region and production areas and the implementation of an appropriate program monitoring, is a measure that contributes to the effective control of ramularia and leaf spots on cotton.

In this way, the union of efforts carried out through the Ramulária Network, which will henceforth be called the Ramulária and Manchas Network, is extremely important in order to increase knowledge, stimulate and boost applied and basic research actions, capable of contribute to the advancement of knowledge and management of these diseases in the field. Ultimately, these actions seek to cooperate in favor of the Brazilian cotton producer and the sustainability of cotton cultivation in the country.

The results briefly presented come from a union of efforts between public and private research institutions, companies that produce phytosanitary products and the production sector, through the support of the Brazilian Association of Cotton Producers (Abrapa). The authors would like to thank everyone who, directly or indirectly, makes up this work, and works in cotton farming in Brazil.

By Fabiano José Perina (Embrapa Algodão); Monica Cagnin Martins (Go Search); Luiz Gonzaga Chitarra e Alderi Emídio de Araújo (Embrapa Algodão); Lucas Henrique Fantin (Chapadão Foundation); José Wellington dos Santos (Embrapa Algodão); Alfredo Ricieri Dias (Agro Challenges); Rafael Galbieri (IMAmt); Nédio Rodrigo Tormen (Instituto Phytus / Staphyt); Emerson Cappellesso (Círculo Verde Pesquisas); Fabiano Andrei Bender da Cruz (Go Research); Jairo dos Santos (Agrodynamics); João Paulo Ascari (MT Foundation); Márcio Marcos Goussain Júnior (Assist); Maurício Silva Stefanelo (Ceres); Monica Anghinoni Müller (MT Foundation); It is Tiago Fernando Konagesky (Rural Técnica Querência).

Article published in issue 295 of Cultivar Grandes Culturas Magazine