Bipolaris spot: damage in the second crop

A disease from the 1970s that reduced U.S. corn production by 16 percent, equivalent to about $8 billion in today's terms, has been causing concern in Brazil. Known as Bipolaris leaf spot, it is caused by both the fungus Bipolaris maydis as for Bipolaris zeicola, affects corn crops in several regions of the world, but mainly in countries with hot and humid conditions (White, 1999).

In recent years, the complex of Bipolaris has expanded significantly in Brazil, affecting several regions and grain production environments, both in the summer harvest and mainly in the second harvest, with losses that can exceed 70% in production (Embrapa, 2014). This increase in incidence reflects the adaptation of the pathogen to different climatic conditions and agricultural practices, becoming a growing concern for producers.

The disease manifests itself with straw-colored spots across the entire leaf and chlorosis in the infected tissues, limiting photosynthesis by reducing leaf area and leading to reduced productivity. The high saprophytic capacity of bipolaris leaf spot contributes to a higher primary inoculum level, especially in areas where the disease frequently occurs.

No case of Bipolaris maydis, there are three races of the pathogen: T, O and C. This first race “T” was responsible for the epidemic in the United States, when it was noted that approximately 85% of the area with corn was planted with cultivars that contained the T cytoplasm (Texas), which confers male sterility to the plants (Embrapa, 2014). While corn with normal cytoplasm showed resistance to the disease. The fungus Bipolaris zeicola, presents race 1 and 3.

Both fungi (B. zeicola e B. maydis) survive on crop residues from the previous year's corn crop. As the weather warms, spores (fungal reproductive structures) produced on corn residues are dispersed by wind or rain splash to plant leaves at an early stage of crop development. Typically, the earlier the disease infection occurs, the greater its potential to reduce corn yield.

Under favorable environmental conditions (25-32°C and relative humidity below 90%), the disease develops rapidly, and its cycle occurs between 60 and 72 hours. Bipolaris spot normally occurs from the initial stages of crop development until the physiological maturity of the corn.

Symptoms of bipolaris leaf spot vary depending on the races of the fungus present in the area and infecting the plant, as well as the genotype planted. Race “O”, the most prevalent in the world, attacks only the leaves and causes lesions that are initially small and oval (Embrapa, 2014). In susceptible cultivars, race “O” causes lesions on the leaves, which are initially small and oval, but which become elongated when mature, straw-colored and delimited by the veins. When the disease is caused by race T, the leaf lesions are larger and lesions may occur in other aerial organs, such as ears.

In general, the symptoms caused by the complex of Bipolaris resemble the symptoms of spotting cercospora and even bacterial streaks, which requires careful examination of the signs for correct diagnosis.

Cercospora leaf spot lesions are rectangular, have edges delimited by the vein and are narrower and more elongated than those caused by B. maydis. Bacterial streak presents initial symptoms with small dots on the leaves, surrounded by a yellowish halo, and as it progresses, it can present narrow and elongated lesions that vary in length.

Management and prevention

Farmers have several options for managing bipolaris leaf spot, including hybrid selection, cultural practices, and fungicides. The combination of these crop protection practices provides good security for increasing yield and sustainably managing diseases in corn crops.

The first tip is to choose hybrids with good genetic tolerance, which offers an effective defense against the problem. Although no hybrid offers resistance to all diseases, even partial resistance helps a lot in protecting production.

The second step is early and frequent monitoring, as early detection helps minimize economic damage. Generally, the first symptoms are observed on the lower leaves of the plants at the beginning of crop development.

The third point is continuous crop rotation, which is a very solid practice to help reduce threats, breaking the disease cycle in the field, in addition to acting effectively against all races of the fungus.

In addition to these measures, the use of registered foliar fungicides, such as Fox Xpro, which contains Bixafem, Protioconazole and Trifloxystrobin, is an efficient option for controlling bipolaris leaf spot. Early applications, at V4, tend to be more effective. It is important to emphasize that preventive chemical management is quite effective, as it controls the fungus before it penetrates the plant tissues, providing a longer residual period for the crop. In addition, this approach is crucial to avoid outbreaks of the disease that can significantly compromise crop productivity, especially in regions with high pathogen pressure. Always seek guidance from an agricultural engineer.

* By William Zancan (left) e Bruno Carvalho (right), Bayer