Orange rust on sugar cane

Brazil has around nine 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, which are mainly responsible for supplying the raw material for ethanol production, 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 a certain cost to the producer, which can be even more complex when necessary. be anticipated due to some phytosanitary problems of the crop.

Among these problems was the orange rust disease, which can reduce 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 tissues of sugar cane.

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, but they tend to cluster at 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, therefore, the varieties used end up not expressing their full productive potential, as the development of the 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 southern Florida (USA) in June 2007 (Comstock et al, 2008), later in Guatemala (Ovalle et al, 2008), Costa Rica, Nicaragua (Chavarría et al, 2009), Panama, El Salvador, Mexico (Flores et al.

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 spread to the other sugarcane producing regions of the country, where the disease has 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 analysis, where the color, size and shape of the urediniospores are observed. 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 35µm to 68µm in length, while those of P. melanocephala are generally smaller, between 24µm and 43µm (Dixon et al, 2010). Another important characteristic is the shape of the urediniospores, which P. kuehnii They are mainly obovoid or pyriform, with orange or brown walls in cinnamon tones, as for P. melanocephala, obovoid and ellipsoidal shaped urediniospores are observed, with a cinnamon-brown to dark brown color (Virtudazo et al.

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 the state of São Paulo are those with the most favorable climatic conditions for its development, followed by the western region of this 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 in optimal edaphoclimatic conditions for the development of the disease, but only after a few years of selection are some interesting varieties launched on the market that correspond to to 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, 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 have 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, therefore a living organism that can adapt to the production environment and, in some cases, be able 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 increases the chances of a drop in productivity, due to the development of orange rust, will be low. Therefore, we are living as best we can with the disease in Brazilian sugarcane fields.

Dr. Juliana Cristina Sodário Cruz, Scientific researcher, Institution: São Paulo Agribusiness Technology Agency (APTA) - Central-West Hub

Article published in issue 199 of Cultivar Grandes Culturas.