Influence of straw maintenance on diseases that affect sugarcane

Sugarcane is an important source of raw material for fuel and energy and its production has increased in Brazil in the last decade, with the harvested area estimated at 8,38 million hectares in the 2019/2020 harvest (Conab, 2019 ). Brazil, India and China are the largest producers in the world. The sugarcane harvesting method in Brazil has recently changed from manual to mechanized, mainly in response to social and environmental calls. And changes in the production system invariably lead to changes in the epidemiology of diseases. Manual harvesting involves burning the straw beforehand, while the mechanized technique leaves up to 20 tons of straw on the ground after harvest. The absence of fire and the consequent maintenance of straw in the system favor populations of necrotrophic microorganisms (those that multiply in decomposing plant tissues) and also provides a more favorable environment for life in the soil, through the contribution of carbon, maintaining humidity and reducing temperature and light incidence on the soil.

The plant remains of plants are degraded through the action of microorganisms (fungi, bacteria, etc.) and there is a great diversity of them in the environment, some of which are pathogenic to plant species, that is, capable of causing disease. It is expected that maintaining sugarcane straw on crops will favor the multiplication of fungi that cause leaf spots, such as ring spot, eye spot and brown spot, leading to an increase in the severity of these diseases. On the other hand, it is possible that maintaining straw reduces the occurrence of diseases caused by soil fungi, such as pineapple rot and “pokkah boeng”, since straw improves conditions for the soil microbiota, thus facilitating the processes of biological control and natural suppressiveness to pathogens.

Ringspot caused by the fungus Leptosphaeria sacchari It has been reported as a widely distributed disease in sugarcane, but considered a secondary problem restricted to old leaves. Different varietal responses have already been reported for this disease, indicating its importance in breeding programs. The selection of lines with absence or reduced severity of leaf spots during genetic improvement is pointed out as one of the main reasons why this disease remains unimportant in crops. However, an increase in this disease has been observed in Brazil: previously restricted to old leaves, it is now common in young leaves of susceptible cultivars, and the maintenance of unburned plant remains in the field favors the multiplication of the fungus that causes the disease.

Experiments conducted in Dourados, Mato Grosso do Sul, showed that maintaining straw in the crop promoted an increase in the severity of ring spot. This result was already expected, due to the necrotrophic characteristic of the fungus. In other words, the fungus remains in the environment, along with the straw, leading to an increase in the disease inoculum for new plants. In this same experiment, two other diseases were evaluated: red rot and “pokkah boeng”.

The incidence of red rot, caused by the fungus Colletotrichum falcatum, was practically not influenced by the maintenance of straw on the farm. It is known that red rot is normally associated with the incidence of borers and/or the presence of wounds on the stems, which serve as a gateway for the fungus. Although the fungus is not considered a true soil fungus, spores dispersed in the soil can produce infection in clumps.

Another disease evaluated was “pokkah boeng”, caused by the fungus Fusarium moniliforme. The number of diseased stems was lower where the straw was kept in the field. F. moniliforme It is a typical soil fungus, and is therefore subject to interaction with other soil microorganisms. In this case, it was already expected that the maintenance of straw would reduce the incidence of the disease, as the beneficial effects on the soil due to the maintenance of straw, such as carbon input, water retention, temperature reduction, etc., favor microbial activity in the soil. and, consequently, the spontaneous biological control promoted by microorganisms that inhabit the soil.

Therefore, there is a need to observe the incidence and severity of sugarcane diseases from a new perspective, that is, knowing that the environment now favors fungi that cause leaf spots, the severity of which must be more carefully evaluated , giving preference to planting those with less stains, at the same time that genetic improvement continues with its mission (among many others) of providing producers with varieties resistant to the greatest number of diseases possible. On the other hand, the change in the environment promoted by the maintenance of straw favors the soil microbiota, which can facilitate the biological control (natural or induced) of diseases caused by soil fungi. 

Alexandre Dinnys Roese and Cesar José da Silva, Embrapa Agropecuária Oeste, Dourados (MS)