Bipolaris sorokiniana

Bipolaris sorokiniana (Sacc.) Shoemaker is a phytopathogenic fungus of great relevance to Brazilian and global agriculture, responsible for causing economically important diseases in winter cereals, mainly wheat and barley.

This pathogen stands out for its ability to infect different organs of the plant, causing multiple diseases such as helminthosporiosis (brown spot), common root rot and black tip of grains.

Taxonomy and morphological characteristics

From a taxonomic point of view, B. sorokiniana belongs to the Kingdom Fungi, Phylum Ascomycota, Class Dothideomycetes, Order Pleosporales and Family Pleosporaceae.

This fungus has been known by several scientific names, including Helminthosporium sativum, Helminthosporium sorokinianum e Drechslera sorokiniana, reflecting taxonomic revisions that have occurred over time. Its sexual phase, although rarely observed in nature, is called Cochliobolus sativus.

Morphologically, the pathogen is characterized by the production of fusiform to ellipsoid conidia, dark brown in color, with 3-10 transverse septa, measuring approximately 60-120 μm × 15-28 μm.

Its mycelium is septate and dematiaceous (dark), producing erect conidiophores that emerge through the stomata or directly from the colonized plant tissue.

A distinctive feature of the genre Bipolaris is the bipolar germination of conidia, with the emission of germ tubes from the ends of the spore.

Etiology and pathogenic mechanisms

The pathogenicity of B. sorokiniana is associated with several virulence mechanisms that facilitate its infection, colonization and reproduction in host tissues.

The infectious process begins with the germination of conidia under favorable conditions of humidity (>80%) and temperature (18-28°C). Penetration can occur directly through the cuticle, using mechanical pressure and lytic enzymes, or indirectly via natural openings and wounds.

Among the main virulence factors, the production of cell wall degrading enzymes (cellulases, pectinases and proteases), specific phytotoxins (prehelminthosporol, helminthosporol and sorokinianin) and melanin stand out.

The toxins produced induce tissue necrosis, interfere with photosynthesis and alter the permeability of the host's cell membranes. Melanin, in turn, provides protection to fungal structures against adverse environmental conditions and plant defense mechanisms.

The pathogen also has mechanisms to suppress host defenses by producing compounds that inhibit or delay defense responses, altering cellular metabolism and interfering with the production of phenolic compounds and phytoalexins. This sophisticated ability to manipulate host defense mechanisms contributes significantly to the success of the infectious process.

Life cycle and dispersion

The life cycle of B. sorokiniana is complex and adapted to ensure its survival in different environmental conditions.

The pathogen survives primarily as dormant mycelium and conidia on crop debris, where it can remain viable for 1–3 years. It can also survive on infected seeds, in soil associated with organic matter, and on volunteer plants or alternative hosts such as wild grasses.

The dispersion of B. sorokiniana occurs through several mechanisms. Infected seeds are the main means of introducing the pathogen into new areas, allowing it to be transported over long distances. Conidia can be dispersed by wind (aerial dispersal), by water splashes (rain or irrigation) and by surface runoff. Contaminated agricultural implements also contribute to the spread of the pathogen between different cultivation areas.

Environmental factors and agricultural practices significantly influence the spread and establishment of this pathogen.

Dry periods with winds favor aerial dispersion, while rainy periods facilitate dispersion by splashes.

The no-till system, predominant in Brazil, can increase the initial inoculum by maintaining infected crop residues on the soil surface, especially when not associated with adequate crop rotation practices.

Economic importance in the Brazilian context

In the Brazilian agricultural context, B. sorokiniana assumes great economic importance, mainly in the South and Central-West regions, where winter cereal production is concentrated. States such as Paraná, Rio Grande do Sul, Santa Catarina, Mato Grosso do Sul and Goiás are particularly affected, as they have climatic conditions favorable to the development of the pathogen and extensively cultivate host species.

The damage caused by this fungus manifests itself in different ways: reduction of the photosynthetic area due to leaf lesions; impairment of water and nutrient absorption due to root rot; and reduction in the quality and commercial value of the grains due to the occurrence of black tip.

In situations of high incidence and severity, productivity losses can reach 30-50%, depending on the susceptibility of the cultivar and environmental conditions.

In addition to direct damage to production, the occurrence of B. sorokiniana imposes additional costs related to control measures, such as fungicide applications and seed treatment. The pathogen can also affect the industrial quality of the final product, especially in the case of barley intended for malt production, where grains with black tips are undesirable.

Management strategies

The effective management of B. sorokiniana requires an integrated approach, combining different strategies to reduce the initial inoculum, prevent new infections and minimize crop damage. 

Management: cultural practices

Crop rotation with non-host species such as soybeans, beans, canola or sunflower for 2-3 years is essential to reduce the inoculum present in the soil and crop residues. In no-till systems, this practice becomes even more important to interrupt the pathogen cycle.

Proper management of crop residues, including incorporation into the soil to accelerate decomposition in areas with high incidence of the disease, helps reduce pathogen survival. Likewise, the elimination of volunteer plants that serve as a green bridge between crops is essential to reduce inoculum.

Choosing the right sowing time, avoiding periods that coincide with climatic conditions that are highly favorable to the pathogen, and adopting balanced fertilization are also important practices in the cultural management of the disease.

Management: genetic control

The use of cultivars with genetic resistance is one of the most sustainable strategies for managing B. sorokiniana. In Brazil, institutions such as Embrapa and IAPAR have developed varieties with different levels of resistance to diseases caused by this pathogen.

Genetic resistance to B. sorokiniana It is generally quantitative, conferring partial tolerance. Although complete resistance is rare, the use of less susceptible cultivars, associated with other management practices, contributes significantly to reducing the damage caused by the disease.

Management: chemical control

Seed treatment with fungicides such as carboxin+thiram, difenoconazole, iprodione and triadimenol is essential to reduce seed-to-seedling transmission and protect plants in the early stages of development. This practice is particularly important to prevent common root rot and seedling damping-off.

Foliar application of fungicides also plays a fundamental role in the management of helminthosporiosis. Products from the groups of triazoles, strobilurins, carboxamides and their mixtures are widely used. The timing of application is crucial and should be carried out preventively or in the early stages of the disease, based on constant monitoring of the crop.

To avoid the selection of resistant populations, it is recommended to alternate chemical groups and use ready-made mixtures with different modes of action, following the principles of integrated fungicide resistance management.

Management: biological control

The use of biological control agents, such as Trichoderma spp., Bacillus spp. And Pseudomonas spp., has shown promising results in the management of B. sorokiniana. These antagonistic microorganisms can be applied to seeds or soil, helping to reduce pathogen inoculum and promote plant development.

The strategic integration of cultural practices, genetic resistance, chemical and biological control, adapted to the specific conditions of each region and production system, constitutes the most effective approach for the sustainable management of B. sorokiniana. Regular crop monitoring and the use of forecasting systems based on climate conditions allow for the optimization of control strategies, reducing costs and environmental impacts.