Epichloe bromicola

The fungus colonizes the intercellular spaces of the grass, growing systemically within the plant tissue. Its lifecycle includes an asexual (anamorphic) phase and a sexual (teleomorphic) phase that produces a stroma.

This pathogen is highly host-specific, primarily targeting grasses within the Bromeae tribe. Unlike many other plant pathogens, it does not immediately kill the plant but derives nutrition from its host's metabolic resources.

Transmission occurs through seed infection, where the fungal hyphae persist within the embryo. Once planted, the fungus grows along with the host, ensuring its survival and spread in the next generation of plants.

The sexual stage of Epichloe bromicola involves the formation of a dense, white-to-yellow stroma around the stems, which is essential for the production and dispersal of ascospores.

The most distinctive symptom is the development of a fungal stroma that forms a tight collar around the upper stems and internodes of the grass, often referred to as "choke disease."

As the stroma matures, it physically encases the emerging inflorescence, preventing the panicle or spike from emerging normally from the leaf sheath. This leads to severe stunting and distortion of the flower heads.

The affected tillers become sterile, as the fungal growth prevents successful pollination and grain set. The stroma eventually turns into a hard, thickened layer that can be white, cream, or yellowish in color.

Early infections may show few outward signs. The presence of the fungus is often only visually confirmed when the host reaches the reproductive stage and the stroma becomes clearly visible.

Plants heavily colonized by the fungus exhibit a general lack of vigor, premature leaf yellowing, and decreased biomass compared to non-infected, healthy plants in the same field.

Environmental conditions play a critical role in the development and spread of Epichloe bromicola. High humidity and mild temperatures favor the rapid expansion of the fungal stroma.

Free moisture, such as dew or rain, is required for the successful germination of ascospores. Wet weather during the flowering period of the host plant significantly increases the likelihood of widespread infection.

Dense grass stands create a microclimate with reduced air circulation, which helps maintain higher humidity levels around the stems, thereby facilitating the spread of spores between individual plants.

Agronomic factors, including excessive nitrogen fertilization, may promote vigorous host growth but can also increase the plant's susceptibility to the rapid colonization by the fungal endophyte.

The timing of spore release coincides with the anthesis of the host grass. Insects may also play a minor role in the transmission of spores from the stroma to the open florets of healthy plants.

The primary damage caused by Epichloe bromicola is the total loss of seed yield in infected tillers, as the fungus prevents the proper development of the inflorescence.

In forage production, the presence of the fungus reduces the nutritional quality of the hay. Some species of Epichloe are known to produce alkaloids that can be toxic to livestock.

Infected stands often show reduced longevity and vigor, leading to a faster decline of the grass population within a field or pasture, which necessitates more frequent reseeding.

For seed producers, this pathogen is a major concern because infected seeds serve as the primary inoculum for the next crop, making it difficult to maintain pure, disease-free seed stocks.

The economic impact involves not only direct yield losses but also the increased costs associated with management, monitoring, and the potential replacement of degraded forage crops.

Effective management begins with the use of certified, endophyte-free seeds. Ensuring that the seed source is free from Epichloe bromicola is the most reliable strategy to prevent initial outbreaks.

Crop rotation is a vital tool for interrupting the lifecycle of the pathogen. Rotating susceptible grass fields with non-host crops helps to eliminate the reservoir of the fungus in the soil and debris.

Proper field management, including the optimization of planting density, improves airflow throughout the stand, which helps lower humidity levels and discourages the growth of the fungal stroma.

Early mowing or harvesting of the grass stand before the fungus begins to form spores can significantly reduce the amount of inoculum present, preventing further spread to nearby areas.

• Application of systemic fungicides during the early flowering phase if infection is detected.
• Seed treatment with appropriate fungicides to eliminate internal hyphae before planting.
• Selection of grass cultivars known to be resistant or less susceptible to fungal endophytes.