Correct management of ramularia stain on cotton

Considered little expressive over a long time, the ramularia spot holds currently the status of the main disease in cotton crops. With control mainly based on the use of fungicides, this disease requires management correct, with the establishment of programs that favor more than one mode of action and use of all available chemical groups in order to avoid the selection of isolated less sensitive or resistant fungi.

A Cotton fiber is the main product from the cotton plant, but by-products originating from processing (pit, bran, cake and peel), have economic value and are used as a source of nutrients for animal feeding (PAIM et al., 2010).

At the Brazil, since the 2014 agricultural year, the area cultivated with cotton has remained stagnant and at times there was even a reduction. The expectation for the next harvest is one of growth, reaching an average of 15,4% more than the previous harvest 2016/17 (CONAB, 2017). This period of reduction was influenced by several factors such as the low price paid for fiber and high production costs, especially due to the large amount of use of insecticides and fungicides to control pests and diseases.

O The current cotton production system in the central region of Brazil is characterized by cultivation in extensive areas combined with crop succession (soy and cotton). This monoculture can lead to the worsening of illnesses before considered not very expressive, or certain diseases that are common to two cultures, such as White Mold and Target Spot. The ramularia stain, caused by fungus Ramularia areola, is an example of a disease that was considered not very significant for several years and which has currently become the main ingredient in cotton.

A northeastern region of Mato Grosso do Sul is responsible for a large portion of state production and uses a crop rotation cultivation system. In the next harvest, approximately 87% of the area will be sown in December, favoring crop rotation. This is one of the main reasons why the producer has managed to live with some diseases such as Target Spot (Graph 01) causing a small economic impact.

To the climatic conditions of low precipitation occurring in the months of April and May have favored less rotting of apples. However, the rain that occurred In these months, one of the strongest epidemics of Ramularia was triggered. (Graph 02) in the region. This finding was possible through monitoring carried out by Fundação Chapadão in its experimental area.

Several Diseases affect the cotton plant and can cause considerable damage to production. Your importance depends on each location, with some considered little expressive in certain regions, can significantly reduce the productivity in another producing region.

The appearance and development of diseases are the result of the interaction between a susceptible plant, an pathogenic and favorable environmental factors. The environment, therefore, is a component relevant in this interaction, and may even prevent the occurrence of the disease even in the presence of the host and pathogen. Environmental factors can determine the degree of predisposition of the host, influencing from the establishment of the disease in a culture until the outbreak of an epidemic (BERGAMIM FILHO et al., 1996).

Monitoring and knowledge of Diseases are essential for any culture, this being the first factor to be considered when thinking about integrated disease control, so that the From there, appropriate control practices can be determined.

The measures of control for ramularia spot adopted in the Chapadões region, as well as as in the main cotton producing regions of Brazil, are based mainly in the use of fungicides. This fact is due to the rapid action that these products promote the suppression of the disease and if used at the correct time and with rotation promote good efficiency for disease control.

In search of solutions and to better understand the chemical stain control strategy of ramulária in the region, the Chapadão Foundation carries out various field work since 2010. Among the various research in this line of control, it is worth highlighting the test carried out each year, in two locations (Chapadão do Sul and Costa Rica, Mato Grosso do Sul), to compare the efficiency of registered fungicides, enabling monitoring of the effectiveness of these products over the years (Graph 03).

You results from the last three harvests have turned on a “red light” on the effectiveness of these fungicides. A reduction in efficiency was found on the ramularia. An example is the treatment with Tetraconazole, used in a isolate, which was 91% effective in the 2013/2014 harvest and recorded a reduction to 23% control in the 2016/2017 harvest. However, when the same triazole was used in a commercial product associated with a strobilurin (Azoxystrobin+Tetraconazole), less loss of efficacy was noted, demonstrating better stability over the years compared to the use of triazole alone, according to data presented in Graph 03.

Result similar is being observed with another isolated formulation triazole, the Difenoconazole, which also in the 2013/2014 harvest was 95% effective and recently achieved an average of 61% in controlling the disease. But when the registered fungicide is used, in association with another chemical group (Azoxystrobin+Difenocozole) again showed better stability in its effectiveness (Graph 04).

Carboxamides were recently introduced into culture, so little is known about the issue of durability of this technology over the years. However, it is possible to state its importance in composing a fungicide program used to control of ramularia stain. In the experimental area of ​​Fundação Chapadão, this group of fungicides has been monitored since the 2012/13 harvest as presented in Graph 05. The commercial mixture of strobilurin and triazole (Pyraclostrobin+Metconazole) compared to strobilurin and carboxamide (Pyraclostrobin+Fluxapyroxad).

It is worth highlighting the importance of regional work, to individually evaluate fungicides, in sequential applications, to determine efficiency. These information should be used in determining control programs, prioritizing Always rotate fungicides with different modes of action.

Therefore, for a fungicide program in the currently it is necessary to use commercial products that have two modes of action. It is also important, due to the large number of spraying depending on the crop cycle, adopt programs with all available chemical groups (Triazoles, Strobilurins, Carboxamides and Multi-site), in order to avoid the selection of less sensitive fungal isolates or resistant. 

At the Ministry of Agriculture, Livestock and Supply (Map) is available at listing of various chemicals registered and recommended for the crop of the cotton plant, with detailed information on the active ingredient and dosages, in Agrofit.

Article published in issue 228 of Cultivar Grandes Culturas, May, 2018.