Bipolaris maydis

the fungus Bipolaris maydis belongs to the Dothideomycetes class and the Pleosporaceae family. It is an agricultural pathogen of great relevance, especially for corn crops (zea mays).

This fungus is known to cause one of the most devastating diseases affecting corn: bipolaris leaf spot or brown spot. Its ability to cause significant losses in agricultural productivity makes it essential to study its biology, etiology and economic impact in detail.

Morphological characteristics and taxonomy

Understanding the biology of Bipolaris maydis begins with its morphological structure. This fungus has multicellular, fusiform or cylindrical conidia, with thick walls and dark brown coloration.

These conidia are produced in structures called conidiophores, which emerge from septate (with transverse divisions) and branched hyphae. The dark pigmentation of the conidia confers resistance to adverse conditions, such as ultraviolet radiation and desiccation, allowing them to survive for a long time in the environment.

Initially classified under the genus Helminthosporium, Bipolaris maydis It was later reclassified after advances in phylogenetic studies.

Currently, it belongs to the genus Bipolaris, which includes other phytopathogenic fungi known to cause diseases in various agricultural crops.

• Kingdom: Fungi

• Phylum: Ascomycota

• Class: Dothideomycetes

• Order: Pleosporales

• Family: Pleosporaceae

• Genre: Bipolaris

• Species: Bipolaris maydis

Its life cycle is predominantly asexual, although there are rare records of sexual reproduction involving the formation of ascocarps and ascospores.

Some other economically relevant Bipolaris species and their hosts

• Bipolaris cynodontis - grasses (Bermudagrass, for example)

• Bipolaris oryzae - rice (Oryza sativa)

• Bipolaris sacchari - sugar cane (Saccharum officinarum)

• Bipolaris setariae - grasses (setaria italica, for example)

• Bipolaris sorokiniana - wheat (Triticum aestivum), barley (vulgar hordeum) and other cereals.

Etiology and pathogenic mechanisms

The etiology of Bipolaris maydis is linked to environmental conditions. Temperatures between 25°C and 30°C, combined with high relative humidity (>85%), create ideal conditions for conidial germination and plant infection.

Frequent rainfall, overhead irrigation and winds favor the spread of spores, while infected plant residues in the soil serve as the primary source of inoculum during the off-season.

One of the most striking aspects of the pathogenicity of Bipolaris maydis is its ability to produce toxins and enzymes that severely compromise the host's metabolism.

The T race of this fungus, responsible for the devastating epidemic in the United States in the 1970s, produces T-toxin, a lipophilic molecule highly specific to corn varieties that carry the Tx gene. This toxin acts as an inhibitor of the mitochondrial respiratory chain, leading to cell death and energy collapse in corn cells.

We also pack any Bipolaris maydis secretes hydrolytic enzymes, such as cellulases and pectinases, which degrade plant cell walls, facilitating penetration and colonization of plant tissue. These combined mechanisms make the fungus highly efficient in causing irreparable damage to the host.

Life cycle and dispersion

The life cycle of Bipolaris maydis begins with the survival of conidia on infected plant residues in the soil.

Under adverse conditions, such as drought or extreme temperatures, conidia remain dormant until environmental conditions improve.

When exposed to mild temperatures and high humidity, the conidia germinate, producing infectious hyphae.

Germinated conidia penetrate directly into the epidermis of corn leaves or through stomata. Once inside the plant tissue, the fungus colonizes the cells, degrading them with hydrolytic enzymes and toxins. After successful colonization, the fungus begins to produce new conidia on the infected structures, restarting the life cycle.

Visual symptoms on leaves:

• Shape: Lesions are usually elongated or oval, with well-defined edges.

• Color: Initially, the lesions may be yellowish or light green, but they evolve to shades of brown, dark brown or black as the disease progresses.

• Size: They can range from a few millimeters to several centimeters in length, depending on the host species and the severity of the infection.

• Distribution: Lesions tend to be concentrated on older leaves, but may spread to younger leaves in advanced stages.

Genetic interactions and pathogenic races

The interaction between Bipolaris maydis and corn is a classic example of host-pathogen coevolution.

The Tx gene in corn confers susceptibility to T-toxin, while absence of this gene (tx-tx genotype) results in resistance. This genetic specificity highlights the importance of genetic improvement to develop resistant cultivars.

There are several breeds of Bipolaris maydis, each with different levels of virulence and specificity for corn genotypes. Race T, associated with the production of T-toxin, is the best known and most aggressive.

Other breeds do not produce T-toxin, but can still cause disease in sensitive cultivars.

Economic and agronomic impact

Bipolaris leaf spot has significant economic impacts. Symptoms include elongated necrotic lesions on leaves, premature drying, and stalk and ear rot. These damages directly compromise photosynthesis, growth, and grain yield, resulting in substantial losses for farmers.

One of the most notorious manifestations of the devastation caused by Bipolaris maydis occurred in the United States in the 1970s, when race T of the fungus decimated large areas of susceptible corn crops. This epidemic led to the development of resistant corn varieties, highlighting the importance of agronomic research and genetic improvement.

Integrated control and management

The development of corn cultivars resistant to different races of Bipolaris maydis is one of the most effective and sustainable approaches to controlling the disease. Breeding programs have been instrumental in identifying resistance genes and incorporating them into new varieties.

Cultural practices, such as crop rotation, elimination of infected plant residues and planting at appropriate times, help to reduce the presence of inoculum in the soil and avoid climatic conditions favorable to infection.

The application of specific fungicides, such as triazoles or strobilurins, at critical moments in the crop cycle can be effective in controlling the disease. (Click here to see pesticides registered for the control of "Bipolaris maydis")