Leaf spots on cotton, prevalence and pressure: scenarios and lessons learned from the 2019/20 harvest

Brazil closed the 2019/2020 harvest with 2,9 million tons of cotton lint harvested in accordance with ABRAPA and this result is 5% higher than the previous harvest. The planted area of ​​1,66 million hectares according to CONAB was also higher (+2,7%) than the 2018/2019 harvest, driven by demand for fiber, mainly in foreign markets.

Considering the cotton harvest and second harvest cotton scenarios, ramularia (Ramularia areola) continues to be considered by producers as the main crop disease and preventing the increase in the production ceiling in several situations. Additionally, ramularia, other foliar diseases, especially leaf spots, have grown in importance and caught the attention of cotton farmers in recent harvests. Due to the soybean-cotton cultivation system, which prevails in much of the cotton planted area in Brazil, the target spot (Corynespora cassiicola) has stood out as a threat to productivity. Other diseases such as alternaria (Alternaria spp.) and myrothecium (Myrothecium roridum), which affect cotton crops, have also been identified at the field level in recent harvests and occur concomitantly (Figure 1) with other spots, creating difficulty in identification in the field and, as a consequence, difficulty in understanding the establishment of fungicide management programs by the cotton technical chain. 

Context of leaf spots and the 2019/2020 harvest

Specifically in the 2019/2020 harvest, the scenario of leaf spots in cotton was confirmed as differentiated mainly due to the simultaneous occurrence of leaf diseases and the difficulty in diagnosis.

It is known that the target spot (Corynespora cassiicola) requires favorable environmental conditions for it to progress, such as temperatures between 20 to 30º C and long periods of leaf wetness which, when accompanied by shading from the lower part of the plant canopy, causes premature leaf fall and, consequently, reduced yield and feather quality.

When we observed the ideal conditions for the development of alternaria leaf spot (Alternaria spp.) in cotton cultivation, we are faced with the need for climatic conditions similar to the target spot in terms of temperature (20 to 30º C) and symptoms begin to be seen in the field very early on (from 30 days onwards) and remain throughout the cycle, since the inoculum for subsequent infections is already present in the plant itself.

Regarding the occurrence of myrothecium (Myrothecium roridum), literature reports and field observations suggest a greater severity of occurrence, correlated with stressful situations throughout the entire cycle. However, it is a pathogen widely distributed in the tropical regions where cotton is produced in Brazil.

Keeping the necessary similarities and differences in relation to the etiology and ideal conditions for the development of these three leaf spots on cotton, the symptoms of these diseases are expressed in a similar way at the field level and can generate difficulties in diagnosis and, as a consequence, poor understanding of performance of products that control these pathogens.

In this sense, the objective of this work was to correctly diagnose the leaf spots that occurred in the 2019/2020 harvest and correlate them with the intrinsic activity of the main active ingredients that are on the market or close to being launched for cotton cultivation.

 

Disease identification methodologies

Throughout the 2019/2020 cotton harvest, symptomatic leaves were collected for leaf spots (Corynespora cassiicola, Alternaria spp. And Myrothecium roridum) in eleven representative municipalities for Brazilian cotton farming: Campo Novo do Parecis/MT, Sapezal/MT, Campos de Júlio/MT, Sorriso/MT, Primavera do Leste/MT, Campo Verde/MT, Rondonópolis/MT, Chapadão do Sul/ MS, Luis Eduardo Magalhães/BA, São Desidério/BA and Formosa/GO. Sampling was carried out considering the diversity of materials and technologies available on the market (TMG 44 RX, FM 985 GLTP, FM 975 WS, TMG 81 WS, IMA5801 B2RF, FM 954 GLT, IMA 8001 WS and FM 944 GL) in commercial and experimental trials (always in untreated control plots) that were sown between December 18, 2019 and February 01, 2020. Sampling was carried out 130 days after emergence and the diseases present were monitored throughout the cycle using the disease progress curve.

 

The sampled leaves were completely tracked after a symptomatological analysis in the field and then sent in exsiccate form for identification via electron microscopy in the laboratory (Figure 2).

In the laboratory, the leaves underwent re-evaluation via microscopic study for visual confirmation of the structures and symptoms and then, the species were isolated (Figure 3) from Alternaria spp. (more than one species), Corynespora cassiicola e Myrothecium roridum which were later confirmed through molecular analysis.

Disease Prevalence Results

After field collections and identification of lesions by microscopy, isolation of pathogens from each lesion and confirmation via molecular analysis, it was possible to conclude that the predominance of symptoms occurring at field level in the 2019/2020 harvest was related to alternaria species ( approximately 94% of lesions in all evaluations), followed by target stain (Corynespora cassiicola) and myrothecium (Myrothecium roridum) according to figure 4.

It is worth mentioning that, regardless of the region (Parecis, BR163, Southern MT, Chapadões or Bahia) the predominance of leaf spots caused by alternaria was consistently higher than the other spots.

Another relevant point is related to the timing of the sampling, which was carried out 130 days after the emergence of the culture. In this sense, pathogen interaction may occur and the environment may provide prevalence of a certain species, therefore, in the current harvest (2020/2021) new work is underway with leaf sampling starting 60 days after the emergence of the crop.

Based on the high predominance of alternaria species in the diagnosis methodology, the present work is focused on understanding the active ingredients on Alternaria spp. through the assessment of mycelial growth.

 

Methodology for evaluating the sensitivity of alternaria to active ingredients

Aiming to obtain a standardized response of the different fungicides for the species of Alternaria spp. isolated, the test was carried out under controlled conditions, in petri dishes, with the main active ingredients used in cotton cultivation, as well as the main mixtures. The fungicides were incorporated into the PDA culture medium and 4 different isolates of Alternaria spp. were placed to grow in these culture media with different fungicides. The growth analysis was carried out 10 days after the start of growth, the tests were kept at 28º C. The percentage of control was calculated based on the difference in growth in the culture medium with fungicide and culture medium without fungicide.

 

Results of active ingredients on mycelial growth

In figure 5, it is possible to observe the performance of active ingredients belonging to the chemical group of Carboxamides (ADEPIDYNTM and Fluxapyroxad) in comparison to the market standard for alternaria control, which is Mertin 400 (fentin hydroxide) on four alternaria isolates from laboratory isolation. According to the results, it is possible to identify that there are differences in performance, with ADEPIDYNTM being superior to Fluxapyroxad and both active alone, inferior to the Mertin 400 standard. In figure 6, it is possible to visually confirm the suppression of mycelial growth of alternaria isolates for the same active ingredients against the standard and also a control that did not receive treatment.

 

In figure 7, in line with the treatments exposed above, active ingredients were tested in isolation, but from the chemical group of Triazoles (Difenoconazole, Tetraconazole and Mefentriconazole) maintaining the comparison with Mertin 400 (as market standard) with Difenoconazole being superior to Tetraconazole which in turn was superior to Mefentriconazole. Only Difenoconazole showed control over alternaria similar to the standard (Mertin 400). In figure 8, it is possible to visually confirm the suppression of mycelial growth of alternaria isolates for the same active ingredients against the standard and also a control that did not receive treatment.

In figure 9, two commercial products used by cotton producers to control diseases were tested on alternaria isolates (Fluxapyroxad + Pyraclostrobin and Azoxystrobin + Difenoconazole) maintaining the comparison with Mertin 400 (as market standard). Based on the results Azoxystrobin + Difenoconazole had similar performance to the Mertin 400 standard and both were superior to the combination of Fluxapyroxad + Pyraclostrobin. In figure 10, it is possible to visually confirm the suppression of mycelial growth of alternaria isolates for the same active ingredients against the standard and also a control that did not receive treatment.

Considering the effectiveness of ADEPIDYNTM on alternaria isolates (Figure 5), two formulations were tested where this active ingredient is combined in ready-made mixtures with difenoconazole and chlorothalonil (Figure 11). In the two formulations tested (ADEPIDYNTM + Difenoconazole and ADEPIDYNTM + Chlorothalonil), alternaria control was superior in relation to the other treatments, which was confirmed (Figure 12) by the suppression of mycelial growth of alternaria isolates as better controls in comparison against standard and also a witness who did not receive treatment.

How to Structure a Leaf Spot Management Program

Table 1 highlights the main modes of action that control the main leaf spots that occur in all cotton growing regions in Brazil.

 

Considering that strobilurins have a significant contribution to the control of initial diseases and also of myrothecium, it is essential that they are present within management programs focused – in addition to ramulosis – on patches in general, as evidenced in the results above on alternations of Azoxystrobin + Difenoconazole .

Equally important, triazoles play a fundamental role in the control of several cotton diseases, such as ramularia and in the case of leaf spots it is no different. For cotton producers, the adoption of Difenoconazole adds to the control of alternaria, as indicated by the product's performance in isolation compared to other triazoles.

When multi-site fungicides (such as Chlorothalonil) are added to the system, they significantly improve the control of leaf spots and ramularia and bring sustainability and longevity to site-specific products.

In the case of using Mertin 400 – which represents the class of tinned fungicides in a unique way – it is recognized as a fundamental tool by cotton producers throughout Brazil in rotation with other products based on Carboxamides, mainly up to 100 days after harvesting. emergency.

For Carboxamides, according to the results obtained, in addition to the excellent contribution to ramularia and target spot, the contribution to the control of alternaria is clear, with special emphasis on ADEPIDYNTM (both for proving its effectiveness in isolation compared to other active ingredients from the same group) as in the comparison of ADEPIDYNTM in its mixtures with Difenoconazole and Chlorothalonil, which are formulations that are reaching the market to control diseases in cotton cultivation.

In short, when structuring management programs for the cotton disease complex, it is essential to take into account the pillars of Conscious Management (Figure 13), as well as the search for control of the spectrum of pathogens that attack the cotton plant.

Final considerations

The 2019/2020 harvest takes us to a scenario where alternaria are more present (in more than 90% of situations) in cultivation, causing economic damage in the sampled regions. This brings us attention to an agronomic scenario that remains complex, increasingly requiring an effective fungicide program, which considers the entire dynamics of cotton cultivation and the complex of diseases. Using the best available technology, consciously and precisely, to guarantee high levels of productivity with sustainability.

It is also necessary to deepen the diagnosis of diseases at the field level so that a conscious management program can be established for the spectrum of diseases as a whole.

There are products currently on the market that are effective in controlling a complex of diseases, especially alternaria. Mertin 400 remains a technical standard in alternaria control. Among the carboxamides, ADEPIDYNTM (and its combinations with Difenoconazole or Chlorothalonil) proves to be an excellent tool with a broad spectrum. In the comparison between triazoles, the performance of difenoconazole was superior to the other active ingredients tested. Regarding formulated products, Priori Top showed excellent performance over alternatives.

 

 

Lucio Zabot1, Daniel Rosa2 & Tiago Batista3, Alfredo Riciere Dias4

1 R&D Manager - Fungicides. Doctor in Plant Physiology. Syngenta Crop Protection

2 R&D Manager - Resistance Management. Doctor in Plant Protection. Syngenta Crop Protection

3 R&D Researcher - Fungicides. Plant Protection Specialist. Syngenta Crop Protection

4 Phytopathology Researcher. Master in Agronomy. Agro Challenges