Rational management of orange rust in sugarcane

Orange rust is a serious disease, capable of reducing the productivity, quality and also the useful life of sugarcane fields. A better way to deal with this problem involves rational management, with diversification of variety cultivation, genetic improvement and, when necessary, the use of fungicides based on strobilurins and triazoles.  

Among the problems affecting sugarcane cultivation is orange rust, capable of reducing agricultural productivity by between 20% and 40%, depending on the susceptibility of the variety used. The causal agent of this disease is the fungus Puccinia kuehni (W. Krüger), which grows and reproduces exclusively in living culture tissues.

The characteristic symptom of this disease is the formation of leaf lesions that evolve in the form of orange pustules, as urediniospores (resistance structures) are produced and mature, being easily dispersed to other leaves through the wind. These pustules can occur on the entire leaf surface, however, they tend to cluster in points close to the insertion of the leaves on the stems, causing necrosis in most cases.

 The drop in productivity is a consequence of the reduction in the active photosynthetic area resulting from the formation of these pustules, and thus, the varieties used end up not expressing their full productive potential, as the development of sugarcane plants is seriously affected. This reduction may be more pronounced after successive cuts, when the disease may become more aggressive due to the weakening of the sugarcane field, which qualifies it as one of the most problematic diseases for the sugar and alcohol sector.

This disease has existed for some time in Asian countries and Oceania, being detected on the American continent only in recent years, initially in South Florida (USA) in June 2007 (Comstock et al., 2008), later in Guatemala (Ovalle et ., 2008), Costa Rica, Nicaragua (Chavarría et al., 2009), Panama, El Salvador, Mexico (Flores et al., 2009).

At that time there was an exchange between Brazilian researchers and countries where the disease was already a reality, with the aim of exchanging experiences and selecting resistant clones even before their entry into Brazil. However, at the end of 2009, orange rust was reported for the first time on a property close to the municipalities of Rincão and Araraquara in the interior of the State of São Paulo (Barbasso et al., 2010), being disseminated to other sugarcane producing regions. de-sugar in the country, where we have been living with the disease throughout the country since then.

In field conditions, orange rust can be confused with brown rust, caused by the fungus Puccinia melanocephala, which is already well-known in Brazilian sugarcane fields, but causes less economic damage as it is considered less aggressive. Differentiating these rusts in the field solely by the appearance of the pustules requires some training, as their symptoms and resistance structures are very similar. In some situations, confirmation of the disease can be carried out through simple laboratory analyzes, where the color, size and shape of the urediniospores are observed, and in others molecular techniques can also be used, which is very rare.

This differentiation between urediniospores is possible because P. kuehnii present larger urediniospores, measuring 35 µm to 68 µm in length, while those of P. melanocephala they are generally smaller, between 24 µm to 43 µm (Dixon et al, 2010). Another important characteristic is the shape of the urediniospros, which for P. kuehnii They are mainly obovoid or pyriform, with orange or brown walls in cinnamon tones, as for P. melanocephala, urediniospores of obovoid and ellipsoidal shapes are observed, with a cinnamon brown to dark brown color (Virtudazo et al., 2001).

Orange rust generally develops at the end of the crop cycle, and is favored by humid environments with mild temperatures (21°C) and alternation between heat and cold in autumn. However, there are reports of the onset of symptoms in plants naturally infected in the field, approximately 80 days after cutting, mainly in susceptible varieties grown in environments favorable to the disease. The central and eastern regions of São Paulo are those with the most favorable climatic conditions for development, followed by the western region of the state (Santos, 2013).

Some varieties, such as RB72454 and SP89-1115, are susceptible to orange rust, and have been replaced in sugarcane fields in São Paulo by others that are resistant or tolerant to the disease, which has yielded good results. The use of resistant varieties is an efficient and economically viable control method for orange rust. However, resistant varieties do not always present desirable characteristics for the sugarcane agroindustry.

There are established sugarcane genetic improvement programs in the country that periodically select varieties with good agronomic and industrial characteristics and that present resistance, or tolerance, to the disease. This selection process is generally carried out in areas with high inoculum pressure (high concentration of viable urediniospores in the environment) and optimal soil and climate conditions for the development of the disease. But only after a few years of selection are some interesting varieties released onto the market that meet the needs of the sugar and alcohol sector.

Given this, in some cases it is justifiable to use fungicides to control the disease and maintain agricultural productivity in the short term, or until the ideal moment to replace a susceptible variety with a resistant or more tolerant one.

For these situations, there are some products on the market registered by the Ministry of Agriculture, Livestock and Supply (Mapa), which are generally mixtures based on strobilurins and triazoles, capable of providing good efficiency in controlling orange rust and increasing productivity, especially when used in sugarcane fields that present low disease severity.

This severity can be quantified in the field through the use of a visual rating scale and can provide good guidance as to the appropriate time to use these products. However, it is worth highlighting that in these cases the cost/benefit ratio must be taken into consideration and that varietal management, that is, varieties, in most cases is still the best option.

Although there are already a reasonable number of sugarcane varieties resistant or tolerant to orange rust available to producers, one must take into account the fact that P. kuehnii  it is a fungus and therefore a living organism  that can adapt to the production environment, and in some cases, manage to establish itself and develop symptoms of the disease in varieties considered tolerant, especially when grown in places with high inoculum pressure.  

Therefore, the constant development of new varieties through genetic improvement programs is fundamental, as well as the rational management and diversification of variety cultivation in large areas, as this way the chances of a drop in productivity, as a result of the development of orange rust, will be reduced. lows and in this way it will be possible to live in the best possible way with the disease in Brazilian sugarcane fields.

Sugarcane in Brazil

Brazil has around 9 million hectares cultivated with sugar cane (Saccharum officinarum), with the state of São Paulo being the largest producer, followed by Goiás, Minas Gerais, Mato Grosso do Sul, Paraná, Alagoas, Pernambuco and Mato Grosso, mainly responsible for supplying raw materials for the production of ethanol, sugar and energy in the country. 

Taking into account that ethanol is the fuel that presents one of the best alternatives for reducing greenhouse gas emissions and that sugar produced in Brazil supplies 50% of the international market, it is easy to understand the significant economic importance of culture.

Brazilian sugarcane production is around 654 million tons, with productivity estimated at 73,21 tons per hectare (CONAB, 2015). After successive cuts (harvests), this productivity tends to decrease due to the lesser response of the varieties to the management used in the culture, where it becomes necessary to reform the sugarcane fields, which generates costs for the producer and can be even more problematic when it needs to be anticipated in advance. resulting from phytosanitary problems.  

Juliana Cristina Sodário Cruz, São Paulo Agribusiness Technology Agency (APTA) - Centro Oeste Center

Article published in issue 201 of Cultivar Grandes Culturas.