Care for managing white spot in corn

Corn is the most produced cereal in the world. Brazil is currently the third largest producer, behind only the United States and China. The majority of Brazilian production comes from so-called safrinha or second-crop corn, which already has the largest planted area in relation to harvest corn. For both production systems, there is a constant search for increased productivity and, within this context, concerns about diseases become very important, due to the damage they can cause to the crop.

Among the most important diseases in corn crops, white spot has been highlighted, with high severity and reduced productivity. The causal agent of the disease for many years was considered to be the fungus Phaeosphaeria maydis, but more recent studies argue that the disease is caused by the bacterium Pantoea ananatis. Many researchers report that the cause of the disease is related to the interaction between the fungus and bacteria, but there is still no unanimity.

Producers have become increasingly concerned about the increase in the severity of white spot in the field and its identification is very important for decision-making regarding better management. In this sense, one of the main factors for diagnosis is the identification of signs in the lesions. Signals are developmental or reproductive structures of the pathogen that allow it to be identified. For example, in more advanced lesions, the presence of pycnidia (black dots) in the center of the lesions is characterized as signs of Phaeosphaeria maydis.

In the field, it is very common among producers and technicians that it is difficult to correctly differentiate the symptoms caused by the disease from injuries caused by the phytotoxicity of herbicides such as paraquat. Symptoms resulting from the drift of paraquat-based herbicides are characterized by chlorotic, whitish areas with a dry appearance and brown margins. Therefore, lesions produced by drift have often been confused with white spot.

In general, white spot begins with small symptoms on the leaves and evolves, taking on greater proportions. On the other hand, symptoms of phytotoxicity occur suddenly, hours or days after plants are exposed to the herbicide. White spot is typically associated with humid periods, high rainfall, cloudy days, leaf wetness and very moist soil.

Epidemiology and symptoms

Some conditions have made the cultivation environment favorable to the occurrence of the disease, such as the succession of crops and the increase in areas of safrinha corn. Corn always grown in the same area (season-season-season) causes the pathogen to find a green bridge and the inoculum to increase in the cultivation area.

Some environmental conditions are more favorable to the occurrence of the fungus, such as relative humidity above 60%, high precipitation and milder night temperatures (14°C). Productivity can be reduced by more than 60% in favorable environments, combined with the use of more susceptible cultivars. Normally, the greatest period of occurrence and severity of the disease occurs from the first half of November and lasts until March.

The primary inoculum of the fungus P. maydis It is associated with cultural remains, as it is a necrotrophic fungus. The pathogen is spread through wind and rain splashes on the soil, which subsequently come into contact with plant leaves. The flowering of the crop is the period when the plant is most sensitive to pathogen attack and, consequently, the symptoms are more severe. At the beginning of the disease incidence, symptoms are observed on the basal leaves, which quickly progress to the upper part of the plant. The lesions are chlorotic and oblong in shape, which evolve over time and become whitish. They later turn dark brown in color. The most severe occurrence of the fungus causes problems with grain filling (size and weight), a reduction in the cycle, which can lead to premature drying of the plant. The leaves in the lower third are those most affected by the disease.

White spot management

With the increasing productivity of new hybrids and the need to maintain their entire leaf area until the end of the cycle so that they express their full productive capacity, the challenge of managing diseases to protect productive potential arises. More care must be taken when producing second-crop corn, a time when the climate is more humid and conducive to the development of white spot. Periods with favorable climatic conditions, especially after the crop has flowered, make plants more susceptible to pathogen attack.

Great emphasis has been placed on the application of fungicide as a white spot control strategy. However, factors such as the intensity of the disease and the time of its occurrence must be considered when making a decision about the first application. Better results are observed with preventive applications of fungicides, which help to keep the plant green for longer in the field and allow for greater grain filling. Furthermore, the use of fungicides with mixtures of triazoles, strobilurins and carboxamides associated with multisites presents more significant results in controlling the disease, resulting in increased production (Figure 1).

One management strategy for the disease is to use the genetic tolerance of each hybrid. However, it is known that hybrids commercially distributed on the market present differences in relation to susceptibility to white spot (Figure 2). The higher the AACPD values, the more susceptible the corn hybrid is. Therefore, it is important to consider the planting time for the correct positioning of fungicide applications according to each hybrid used on the property.

Therefore, the main measures recommended for managing white spot are crop rotation, the use of resistant cultivars, the application of fungicides and the appropriate positioning of fungicides depending on these three factors. In areas where there is a history of the disease occurring, crop rotation with non-susceptible species becomes a widely recommended practice, as it aims to reduce the initial inoculum in the cultivation area, as it is a pathogen that survives in crop residues.

Nédio Tormen, Phytus Group Centro-Oeste, Elevagro; Marlon Stefanello and Daiane Dalla Nora, Elevagro