Measures for managing ratoon stunting in sugarcane

Sugarcane cultivation is one of the most dynamic in Brazilian agribusiness and, possibly, Brazil's most important contribution to the global effort to mitigate the greenhouse effect, whether by replacing fossil fuels in the automotive industry or by generating bioenergy. electricity is especially important, as the beginning of the sugarcane harvest coincides with the beginning of the drought and the need to save water from hydroelectric plants.

The importance of sugarcane within Brazilian agriculture can be seen by the growing increase in its production, which in 2005-06 was 431,4 million, with a jump to 615,8 million in 2018-19. However, the crop has a huge challenge ahead, as a more detailed analysis reveals that the increase in production has been achieved by the growth in the planting area, 5,8 million hectares in 2005-06 and 8,6 million in 2018 -19; productivity has remained practically stable, around 70ton/ha, varying from 74,3ton/ha in 2005-06 to 71,3ton/ha in 2018-19.

Many biotic and abiotic factors influence sugarcane productivity, and identifying each of these aspects is not an easy task, as it depends on the management system of each production unit. Regardless of this difficulty, however, it is possible to infer that ratoon rickets, caused by the bacteria Leifsonia xyli subsp. xyli, is one of those responsible for the stagnation of productivity in the production sector as a whole, a productivity that has remained stable for more than 20 years, despite the constant incorporation of the most modern technologies and the launch of varieties with increasingly greater productive potential. Some of the reasons that lead us to believe in the importance of the disease for the national sugarcane sector include scientific data obtained over several years by different countries, as little information is available in Brazil: in South Africa, a drop of up to 15% in production, in Australia up to 14%, in the United States it was up to 32%, in Fiji it was 29% and in Ethiopia up to 28%. One of the few scientific data available for Brazil showed a reduction of 26,22% in the RB867515 variety when comparing the average weight in three cuts between canes from the portion infected with the bacteria and the healthy one. A study of three varieties that are no longer cultivated showed an average yield of 22% higher than canes treated heat-treated to control rickets.

Another worrying fact about ratoon stunting in Brazil was revealed by the 2012-13 survey, which showed that 10% of the 1.154 plots of the 50 plants evaluated in the Center-South region were positive and that 9,4% of the sugarcane plots -plant were infected, showing that contaminated seedlings were used in the formation of the sugarcane field.

One of the most important recommendations for controlling the disease is exclusion, which is nothing more than preventing the bacteria from entering a sugarcane plot. This recommendation is extremely important because the bacteria is not transmitted by insect vectors, does not survive in the soil and has no other host other than sugarcane. Therefore, the only way for the disease to be introduced into a renovation area is by planting propagative materials contaminated by the bacteria.

Although the use of pre-sprouted seedlings is becoming more widespread as a means of producing raw materials for sugarcane plantation renovation, the use of proprietary propagative materials is still a very common practice. For the latter, the recommendation is that a health examination for rickets be carried out to ensure that the propagative materials to be used in the renovation of the sugarcane field are healthy.

This article aims to make the sugarcane production sector aware of new data on the incidence of ratoon stunting in plots selected to provide seedlings for sugarcane reform. These results are a compilation of diagnostic tests for the disease carried out using the “dot blot” serological method by the Molecular Genetics Laboratory of the Federal University of São Carlos, over a period of six years (2013-2018) and which were recently published in the journal Crop Protection scientific journal, where a more detailed discussion of this study can be found.  

The first important fact to be highlighted is that ratoon stunting was identified in 518 of the 1.431 plots evaluated during 2013 to 2018, that is, 40,2% of the plots analyzed were contaminated with the disease (Table 1). A more detailed analysis was carried out using the incidence of the disease over the years, as the number of plots sampled varied greatly over the period, therefore the percentage of plots affected by ratoon stunting was the appropriate parameter. When this assessment was carried out, it was found that there was an increasing increase in the incidence of rickets throughout the period evaluated, 2013 having the lowest incidence, with 20,5% of plots, and 2018 with an explosion, reaching 86,7%. This data is frightening, as it shows a constant increase in the disease and that in 2018, almost all of the renovated plots would already have rickets, if the plot reform was carried out with propagative material collected from the plots sent for diagnostic examination in the laboratory. It is important to highlight here how difficult it is to identify this disease in the field, as it does not present characteristic symptoms, which can only be done safely through laboratory examination. The absence of symptoms is one of the characteristics that most difficult its control and one of the main causes for silent spread in production units.

Another analysis focused on the distribution of the bacteria that causes ratoon stunting in each of the five varieties most used in sugarcane field renovation (Table 2). The total number of plots analyzed was 1.431, with the varieties RB867515 and RB966928 corresponding to 65,8% of the total, demonstrating the popularity of these two varieties for sugarcane renovation areas. The incidence level of ratoon stunting in these two varieties was 35%, very close to the general average for the period. As the best plots of a production unit are those chosen to provide propagative materials and, therefore, are the ones that were analyzed, these numbers suggest that the incidence of stunting on the property may be even higher if unselected plots are considered. In relation to the other varieties, RB855156 stood out for presenting the lowest level of disease among all, with 27,3% of the plots, while CTC4 presented the highest level, with 49,3%.

After this more general overview, a more detailed study was carried out by observing the incidence of rickets in each variety over the years and also by the internal distribution of the disease within each plot. This is because the fact that a plot is positive for rickets does not allow us to elucidate how many setts within that plot were positive, whether one or 99. As the “dot blot” diagnostic test is carried out using saps from 100 setts, the distribution of rickets -of ratoons within the plot was determined by the percentage of positive setts within the plot. The smaller the internal contamination, the longer it will take to transmit the disease to all clumps in the plot, as a contaminated stalk can transmit the disease up to 13m away via the harvester blades; The more sick setts there are, the more sources of disease inoculum there will be and the faster the entire plot will become sick. Therefore, the drop in productivity will be more pronounced the more that ratoon stunting is widespread within a plot.

The analysis of the incidence of ratoon stunting in each variety and also within each diseased plot was carried out over the six years (Table 3). The first fact that caught our attention was the frightening increase in the incidence of ratoon stunting, which had already been observed in the general compilation (Table 1), but was also repeated in the varieties, that is, the highest incidence of the disease occurred in last year of the study (2018) in four of the five varieties, with percentages varying between 91,8% (RB966928) and 72,7% (CTC4). The exception was CTC4, which showed a high percentage of ratoon stunting in all years of the period; RB92579 also had this worrying characteristic. CTC4 was one of the varieties that grew most in preference for sugarcane renovation, it had a 7,7% overall share in 2015 and jumped to 14,3% in 2018-19. This data is worrying, as the recommendation to discard the entire plot for use in sugarcane reform when a positive sett is detected may not have been followed due to the need for a large quantity of propagative material required in this expansion. Unfortunately, the use of heat treatment within plants to eliminate bacteria in contaminated billets has not been adopted.

Data on the percentage incidence of ratoon stunting within a plot may suggest that the planting of diseased propagative material was used in many situations. This is if the recommendation to use sugarcane, and not other cuts, has been followed. It is recommended that propagative materials for sugarcane renovation be plant cane, as the use of harvesters spreads the disease. Thus, if a plot of sugarcane plants shows stunting before the first harvest, it can be deduced that contaminated propagative material was used in the renovation of the sugarcane field. In terms of the management strategy for ratoon stunting, diagnostic analysis in sugarcane plots evaluates the health of the material used in renovation areas and the examination of that same plot in the last cut before renovation shows how much the disease is spreading. spread during the production process of a plant, that is, how efficient the disease control is.

The number of plants/production units that adopt the diagnostic test of the materials to be propagated is not known, many may not yet be aware of the importance of using healthy propagative material to control ratoon stunting. Furthermore, many of the plants that already use this control strategy may not be aware of this upward trend in the incidence of rickets observed in the period studied.

Another study was also initiated at UFSCar, Araras campus, to complement the contamination data of propagative materials presented here. The research uses three important Brazilian varieties (RB867515, RB92579, RB966928), healthy and sick in relation to ratoon rickets and subjected to different conditions (with or without thermotherapy). The objective is to examine the effect of each of the treatments on sugarcane production, the effectiveness of heat treatment and the progress of the disease as a function of the number of cuts (or harvests). A point that has already drawn attention in the field is that it is not possible to identify diseased plants during vegetative development. An illustrative example can be seen in Figure 1, which shows the canes two months after transplanting to the field; the only visible difference occurred in the development of the varieties. Figure 2 shows sugarcane with 100 days in the field, where the same pattern was repeated, that is, the impossibility of differentiating sugarcane from sick, healthy or heat-treated plots, if they are of the same variety. The impossibility of identifying diseased plants in the field seen in this study confirms once again how treacherous this disease is. The consequence of using diseased vegetative material will only be verified (and quantified) at the time of harvest, which is expected to be achieved in a near future.

The objective of this article was to provide the production sector with the most recent data on the sanitary quality of plots in relation to ratoon stunting and the potential danger of this disease. This seeks to help prevent the spread of the disease to new sugarcane fields and increase the sector's productivity.

Alfredo Seiiti Urashima, UFSCar Campus Araras

Cultivating Great Crops September 2020

With each new edition, Cultivar Grandes Culturas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in the cultivation of large crops, the Magazine helps farmers in the search for management solutions that increase their profitability. 

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