Bipolaris maydis

Bipolaris maydis is a phytopathogenic fungus of great importance to maize (Zea mays L.) crops worldwide, especially in humid tropical and subtropical regions. In Brazil, where maize occupies vast areas in both the main and off-season crops, the pathogen causes helminthosporium leaf spot or southern leaf blight, a disease that can significantly reduce grain yield and quality when susceptible hybrids are grown under favorable temperature and humidity conditions.

The historic 1970 corn epidemic in the United States, which destroyed approximately 15% of the corn crop and generated losses estimated in the billions of dollars, illustrates the vulnerability of production systems based on genetic uniformity, in this case the widespread use of Texas sterile male cytoplasm (cms-T). Although the T race responsible for the epidemic has lost relevance after the replacement of susceptible cytoplasm, *Bipolaris maydis* remains a constant threat, with potential losses of up to 50-70% under extreme conditions of susceptibility and favorable climate. Integrated management, focused on genetic resistance, cultural practices, and rational fungicide application, is considered essential to minimize impacts on Brazilian productivity.

The correct scientific name is Bipolaris maydis (Nisikado & C. Miyake) Shoemaker. The teleomorph (sexual phase) corresponds to Cochliobolus heterostrophus (Drechsler) Drechsler. Among the common names, Southern corn leaf blight (SCLB) or maydis leaf blight stand out in English; in the Brazilian and Portuguese context, the disease is known as corn helminthosporium leaf spot, Bipolaris maydis leaf spot, or southern corn leaf blight. These names reflect the characteristic symptoms of leaf lesions that coalesce and cause widespread burning of the aerial part.

The taxonomy of *Bipolaris maydis* has undergone significant revisions over time. Originally described as *Helminthosporium maydis* by Nisikado and Miyake in 1926, the fungus was transferred to the genus *Bipolaris* by Shoemaker in 1959, based on distinctive morphological characteristics such as the bipolar germination of conidia from terminal cells. Molecular phylogenetic studies conducted by Manamgoda et al. (2014) reassessed the helminthosporioid fungi complex, confirming *Bipolaris maydis* as the type species of the genus and establishing its position within the family Pleosporaceae, order Pleosporales, class Dothideomycetes, and phylum Ascomycota. These authors also discussed synonymy with the teleomorph *Cochliobolus heterostrophus* and proposed the conservation of the name *Bipolaris* for practical use in phytopathology, as it is the most commonly used term in disease reports and applied literature. The current classification reflects both morphological data and ribosomal DNA sequences and other gene loci, providing greater taxonomic stability for diagnostic and research purposes.

In the biology of *Bipolaris maydis*, the anamorphic phase predominates, with abundant production of asexual conidia. The conidia are ellipsoid to fusoid, slightly curved, olive-brown in color, with approximate dimensions of 40-120 µm in length by 9-20 µm in width and 3 to 9 transverse septa. Germination occurs bipolarly, with the emission of germ tubes predominantly from apical and basal cells. The conidiophores are dark, geniculate, and produce conidia in succession.

The sexual phase, although possible in the laboratory under controlled conditions of temperature, light, and substrate, is rare in the field; Cochliobolus heterostrophus is heterothallic, with MAT1-1 and MAT1-2 sexual compatibility loci. The fungus is necrotrophic and secretes hydrolytic enzymes (cellulases, proteases) and toxins that accelerate the death of host tissue.

The T race produces T-toxin, a polyketide that specifically binds to the URF13 protein in the inner membrane of plant mitochondria with cms-T cytoplasm, causing uncoupling of oxidative phosphorylation, mitochondrial swelling, and rapid cell collapse.

Race O, the most common currently, does not produce T-toxin and causes rectangular lesions limited by the leaf veins. Race C is rare and reported mainly in China. Transcriptomic studies demonstrate that, during infection in susceptible non-CMS maize genotypes, Bipolaris maydis positively regulates genes involved in mitochondrial functions, cell wall and chitin synthesis, sugar transport, secondary metabolism, and the production of putative effectors, facilitating colonization and virulence.

The bionomics and ecology of Bipolaris maydis are closely linked to maize crop residues and environmental conditions.

The fungus survives the off-season mainly as mycelium and conidia in residues of infected leaves, stems, and ears, acting as a facultative saprophyte. The primary inoculum consists of conidia released by wind or rain splash, which reach young leaves of healthy plants. Infection requires free water on the leaf surface for at least 6-12 hours and temperatures between 18 and 30 °C, with an optimum near 25-28 °C. After direct penetration into the epidermis or via stomata, the pathogen rapidly colonizes the tissue, producing necrotic lesions that serve as a source for polycyclic secondary cycles, and can complete a cycle in a few days under ideal conditions. Long-distance dispersal occurs by wind, while short-distance dispersal is favored by rain. Natural populations exhibit high haplotype diversity and a balanced proportion of sexual compatibility types, suggesting a mixed reproductive strategy, with a predominance of clonal asexual reproduction and occasional sexual events that generate genetic variability.

In regions of Africa and Asia, Bipolaris maydis coexists with other maize foliar pathogens, such as Exserohilum turcicum and Cercospora zeina, sharing similar ecological niches and requiring integrated management strategies.

In Brazil, the pathogen thrives in areas with high rainfall and mild to warm temperatures during the vegetative and reproductive development of corn, especially in no-till systems that maintain crop residue on the surface.

Control of *Bipolaris maydis* relies on an integrated approach, emphasizing genetic resistance as the most effective and sustainable measure. Corn hybrids with quantitative (polygenic) resistance or specific genes, such as rhm for race O, reduce disease severity and limit secondary inoculum production. The near-total elimination of susceptible cms-T cytoplasm after 1970 removed specific vulnerability to race T. Cultural practices include crop rotation with non-host crops (soybean, wheat, beans), crop residue management through deep incorporation or stimulation of microbial decomposition, sowing during periods of lower climatic risk, and avoiding continuous corn monoculture in the same area.

Chemical control is recommended in situations of high inoculum pressure or moderately susceptible hybrids, with preventive applications or at the onset of symptoms, especially during the critical period between the VT and R2/R3 stages. Fungicides from the strobilurin (Qo inhibitors), triazole (IDM), SDHI groups, and commercial mixtures registered in Brazil provide effective control when applied promptly and in rotation to avoid resistance. Field studies demonstrate that chemical or integrated management modules outperform purely organic options in reducing severity and increasing productivity.

Biocontrol agents, such as isolates of Trichoderma harzianum and Pseudomonas fluorescens, exhibit inhibitory activity in vitro and complementary potential in the field, although they do not replace primary control measures. Constant monitoring of leaf lesions, combined with time-based forecasting models, allows for more precise decisions and reduces the unnecessary use of inputs.