Management strategies against ramularia spot on cotton

Considered not very expressive over several years, ramularia spot, caused by the fungus Ramularia areola, has grown in importance over the last few harvests and currently already holds the status in main disease of cotton. The use of fungicides is an important tool control whenever associated with other management strategies. AND It is essential to direct efforts to prevent the selection of fungal isolates less sensitive or resistant.

The current system of cotton production in the central region of Brazil is characterized by the cultivation in large areas combined with crop succession (soy and cotton). This one monoculture can lead to the worsening of diseases previously considered little expressive diseases, or a certain disease that is common to both cultures, such as white mold and target stain. Ramularia spot, caused by the fungus Ramularia areola, is an example of a disease that was considered little significant for several years and is currently the main disease of cotton.

The northeast region of Mato Grosso do Sul is responsible for a large portion of the state's production. It uses a crop rotation cultivation system in which approximately 80% of the area is sown in December. This is one of the reasons why the producer has managed to live with some diseases such as target spot, causing small economic impact. Low precipitation weather conditions occurring in the months of April and May favored less apple rot. However, the rain that occurred in these months triggered one of the strongest epidemics from Mancha de ramularia (Ramularia areola) in the region. This fact was verified through monitoring carried out by Chapadão Foundation in its experimental area.

The ramularia stain It is considered the main disease that affects cotton crops. The reduction productivity caused by this disease reaches 30%, in the region of Chapadões.

Symptoms of the disease begin with lesions on the leaves, located on the lower part of the plant, with a bluish-white color on the lower surface of the leaf. At this stage, detection is difficult, due to lesion has little shine. Soon after, sporulation of the fungus is observed with white or yellowish in color and with a powdery appearance, characterized by cause angular necrosis (Figure 1). When high severity of disease, combined with favorable environmental conditions, causes extensive defoliation early in cotton plants, impacting productivity.

Since the 2009 harvest, several works have been developed in the field by Fundação Chapadão and Federal University of Mato Grosso do Sul (UFMS) in the search for alternatives for integrated control of Ramularia spot on cotton. Monitoring and quantifying the severity of the disease each year is one of the strategies that help in making control decisions in the region.

In the last eight harvests it has been observed that the incidence of the disease in the experimental area has been occurring at the development stage of the “B2” cotton plant, and that the severities verified in the last evaluations (Stage C6), vary between 27% and 80% in severity (Figure 2). This fluctuation in the leaf area damaged by the pathogen is related to environmental conditions that each year provides, making it possible to state through of this monitoring that the conditions observed in the months of March and April each year determine the potential of this disease. It is still possible to conclude that In this period, the years 2013, 2016 and 2017(with the lowest rainfall) are the ones with the highest importance of the Ramularia patch in the Chapadões region (Figure 2).   

Among the combat strategies diseases, cultural control is important in cotton management. One classic example lies in crop rotation and destruction of cultural remains. Another aspect being studied in the region is the height of cotton plants, which has also impacted on the severity of Ramularia spot. Smaller crops (1,0 m to 1,20 m) has seen an increase in the severity of the disease. This fact may be linked to the greater amount of light that falls inside the plant canopy, favoring the increase in temperature and shorter period of wetness during the day. Posteriorly, at night, the humidity provided by dew conditions a extremely favorable to the occurrence of the disease. It is worth highlighting that in a A quick search in the literature reveals several authors stating that the pathogen Ramularia areola It is favored by a few hours of wetness.

It is necessary to take some precautions when handling plants, as the lack of containment of their growth can also favor leaf shading that predisposes the formation of a microclimate favorable to the development of fungi, providing a suitable environment for the emergence of diseases in cotton, such as the rotting of apples, white spot and white mold.

O use of resistant cultivars represents a pillar among the strategies used for disease control (Figure 3). The use of this technology is the method of most desirable control, as it is low cost, highly effective and causes lower environmental impact. However, it is extremely important to associate with other control measures such as cultural management and chemical control, given the possibility of genetic variability of pathogens. Another questionable point Another challenge to genetic improvement is the durability of the technology. He has loss of resistance was observed as cultivars were exposed to over time. On average, technology is lost every three years in the region of Chapadões.

Control measures for ramularia patch adopted in the Chapadões region, as well as in the main cotton producing areas of Brazil, is based mainly on the use of fungicides. This occurs due to the rapid action that fungicides promote to the suppression of the disease and, in addition, if used at the correct time and with Product rotation promotes good efficiency in disease control.

Some precautions must be adopted when using chemical control, It is essential that the use of fungicides begins as soon as the first lesions are identified on older leaves. The monitoring constant cropping is extremely important, since the first injuries are difficult to identify.

At the chemical control it is essential to know the mode and type of action of the fungicide on the plant, to make the decision about which product to use and also its timing. application. It is also essential that your employment is carried out alternately with different active ingredients, as this strategy is effective in avoiding increased frequency of resistant isolates.

In the search for solutions and better understand the chemical control strategy for ramularia leaf spot in region, the Chapadão Foundation has carried out various field work since 2010. Among the various researches in this line of control, an essay worth highlighting held 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 (Figure 4).

The results of the last three harvests, a “red light” shined on the effectiveness of these fungicides. He was a reduction in efficiency was found on Ramularia spot. An example lies in the treatment with Tetraconazole used alone, which presented 91% effectiveness in the 2013/2014 harvest and reduced to 23% control in this harvest (2016/2017). However, when the same triazole was used in a commercial product associated to strobilurin (Azoxystrobin+Tetraconazole), less loss of effectiveness, demonstrating better stability over the years unlike a triazole alone, as shown in Figure 4.

Result similar is being observed with another triazole with an isolated formulation, Difenoconazole, which also in the 2013/2014 harvest was 95% effective and recently obtained average of 61% on disease control. When fungicide is used registered, in association with another chemical group (Azoxystrobin+Difenocozole) again presents better stability in its effectiveness (Figure 05).

It is worth highlighting the importance of work regions to individually evaluate fungicides, in sequential applications and thus determine control efficiency. This information must be used in determining control programs, always prioritizing the rotation of fungicides with different modes of action.

A fungicide program in the cotton currently requires the use of commercial pesticides that contain two modes of action. Due to the large number of sprays depending on the cycle of culture it is recommended to use programs with all chemical groups available (Triazoles, Strobilurins, Carboxamides and Multi-sites), in order to avoid selection of less sensitive or resistant fungal isolates. 

Article published in issue 219 of Cultivar Grandes Culturas, August, 2017.