Management of ratoon stunting in sugarcane

Ratchet rickets is a silent and insidious disease, capable of causing serious damage without being noticed, whose incidence tends to explode with the mechanization of sugarcane harvesting. In a survey carried out over three harvests, with just the most planted cultivar, RB867515, the estimated loss could exceed one million dollars. Urgent control measures, such as the use of healthy propagative material, are necessary to tackle the problem.

The mechanization of sugarcane harvesting has been practiced by all mills and their suppliers in the Center-South region, the main producer in Brazil. This change from manual harvesting of burned sugarcane to mechanized harvesting resulted in crop residues remaining in the field, since the harvest is done using raw sugarcane, that is, without burning. These remains have many beneficial effects, such as increasing soil organic matter and moisture, decreasing the temperature of surface layers, erosion and the emission of carbon dioxide from the soil, among others. However, little information reaches producers regarding the consequences of this mechanization on crop diseases.

Among the numerous diseases that attack sugarcane, ratoon rickets, caused by the fastidious bacteria Leifsonia xyli subsp. xyli, is the disease that will increase the most with the mechanization of harvesting. A study carried out in the United States showed that harvester blades were capable of spreading the bacteria at a distance ranging from 3,9 meters to 7,3 meters from a diseased clump. Added to this fact is the difficulty of disinfecting the blades when changing plots. Thus, the existence of a single plot with ratoon stunting in a plant can cause the disease to spread to other fields.

Other factors that reinforce the importance of this disease include: a) All varieties are attacked by the bacteria; b) Difficulty in identifying sick planes in the field, due to the absence of characteristic symptoms; c) Damage to productivity with increased cuts, justifying its name: ratoon rickets. These characteristics make the disease considered the most treacherous and dangerous among all in culture, that is, its ability to cause damage without being noticed. Data on productivity losses due to the disease in other countries showed a drop of 41% in South Africa, 37% in Australia, 33% in the United States and up to 50% in India. In Brazil, there is a study that shows 26% damage to the productivity of RB867515.

Despite all this information, the real importance of the disease for the national sugarcane industry could not yet be assessed due to the lack of data indicating how widespread ratoon stunting was in Brazilian commercial fields. It is worth remembering that surveys carried out previously focused only on plots destined for the multiplication of vegetative material, therefore not showing a real vision of the problem. Therefore, to overcome this deficiency, the laboratory (LAGEM/UFSCar) formed a partnership with the company Syngenta to study the degree of spread of the disease in plants in the Center-South of Brazil. To this end, 92.114 samples were collected, covering 13.173 hectares in 1.154 plots of 50 plants in five states (São Paulo, Minas Gerais, Goiás, Paraná, Mato Grosso do Sul) during the years 2012 and 2013. In this work, the size of the area and the number of plots sampled followed the importance of each variety in the current scenario. This was the reason why RB867515 was the sugarcane with the largest area sampled (3.001 hectares and 242 plots), followed by SP81-3250 (2.466 hectares and 213 plots) and RB855453 (2.037 hectares and 162 plots); RB855536, RB855156, RB966928, RB835054 and RB92579 completed the list of varieties examined.

In a first analysis of the data, 121 plots (10,5%) of a total of 1154 samples were contaminated with the bacteria that causes ratoon rickets, representing 11,3% of the total area assessed. When these values ​​were distributed by variety, RB92579 stood out for presenting 17,3% of plots with the disease, followed by RB855536 (13,2%), RB966928 (12,8%) and SP81-3250 (11,3% ) (Table 1). But the data that caught the most attention was that 58% of the plants sampled (29 out of a total of 50) had at least one plot with the presence of the bacteria. This is a very important aspect as the presence of a single infected plot in a  plant will cause the disease to spread to all varieties during harvest.

It was also possible to study the incidence of the bacteria according to the sugarcane cuts (Table 2). Of all the data in this table, the most important refers to sugarcane, as it was not possible to examine the incidence of the disease depending on the cuts as the plots evaluated were not the same in the two years of the study. Thus, 9,4% of sugarcane plots were contaminated with the disease, corresponding to 10,4% of the total sugarcane area sampled. What draws attention to these data is that 32% of all the plants studied used seedlings contaminated with the rickets bacteria in the renovation of sugarcane fields. The logic for this conclusion is based on the fact that the pathogen cannot survive in cultural remains and does not have an insect vector for its dissemination, making propagative material the only means of entry of the disease into a new area. Therefore, the use of healthy propagative material is the main measure to control the disease.

For sugarcane production, the Center-South region is the most important in Brazil. In this study, this region was subdivided into four sub-regions so that the spread of the rickets bacteria could be better assessed (Table 3). The cerrado (the study covered Goiás and Minas Gerais) was the region that showed the highest percentage of infected plots (16%) and also the largest area (16,7%); however, the most impressive thing is that 81,8% (9 of the 11 plants) had at least one plot infected with the bacteria. The South region, which encompassed the south of the state of São Paulo and the states of Paraná and Mato Grosso do Sul, also showed a very high percentage (70%) of plants with at least one sick plot. On the other hand, half of the plants analyzed in the Central and Western regions of the state of São Paulo had plots with ratoon stunting, demonstrating that the disease is a concern throughout the Central-South region.

The fact that this survey considered the main varieties, division into sub-regions, sugarcane cutting cycle and large number of mills provided a comprehensive, robust and very realistic view of the incidence of ratoon stunting in commercial fields in the Center-South Brazilian. These precautions also made it possible to calculate, for the first time and safely, the economic loss that the disease is causing. For this, information was used regarding the RB867515 variety, which is the most cultivated in the country. According to data from the National Supply Company (Conab), the area planted with this variety in the 2012-2013 harvest was 1,5 million hectares or 29,1 .5,5% of the entire sugarcane area. According to the survey, 85,5% of the areas evaluated with this variety presented the bacteria. Assuming that this proportion of contamination also remains throughout the Center-South, an area of ​​0,2 thousand hectares would be affected by ratoon stunting, with an infection rate of 47% as not all tussocks would be contaminated in that area. (in the present study, 20.870 of the 2009 analyzed samples of this variety were diseased). In research carried out by Gagliardi & Camargo (26,2), which compared healthy plots with diseased plots, the difference in productivity between them in three harvest years was 100%. Considering that the average productivity is 867515 tons per hectare for RB0,2 and the average infection rate is 78.600%, it is estimated that there will be damage to productivity of 100 tons of sugarcane per year due to ratoon stunting (1,5 tons average productivity X 0,2 million hectares planted with the variety X 26,2% average infection rate found in the study X 2015% drop in productivity). Based on the average price per ton of sugarcane paid in 52,8 of R$13,2 (US$78.600), this loss of 1 tons of sugarcane caused by ratoon stunting resulted in losses of more than XNUMX million dollars. It is worth noting that this loss only considered RB867515 and that all varieties can be attacked if control measures are not adopted. Another important factor lies in the fact that these values ​​can be even higher, as sampling was only six samples per hectare, collected randomly. This quantity was established due to the magnitude, scope, cost and logistics of the survey. In the analyzes carried out by the laboratory (LAGEM/UFSCar), the recommendation is that 100 samples be collected per plot, directed to weaker plants. Although the size of the plot varies greatly, surveys in 2009, 2010 and 2011 with samples collected following this guideline, showed an average incidence of 30,4% in RB867515, corroborating the suspicion that contamination by ratoon stunting may be even bigger.

It is hoped that the release of this data will show the sugarcane industry the current importance of ratoon stunting and the invisible damage that this disease is causing. The sector urgently needs to adopt control measures, using exclusively healthy propagative materials. To achieve this, it is essential before planting to carry out laboratory analysis and heat treatment of propagative materials, when this is the system adopted. If the option is to plant pre-sprouted seedlings, an available alternative is Plene products, which, as they come from tissue cultures, guarantee the health of these materials. 

Alfredo Seiiti Urashima, Federal University of São Carlos

Article published in issue 214 of Cultivar Grandes Culturas.