Helicobasidium brebissonii

The fungus is characterized by the production of a dark purple or reddish-violet mycelium that covers the root system of host plants, providing the visual diagnostic feature for the infection.

It acts as a polyphagous parasite, meaning it can survive and thrive on various plant species, including carrots, beets, potatoes, and various woody perennials like asparagus and fruit trees.

The pathogen is highly persistent, relying on sclerotia and long-lived mycelial mats in the soil to survive adverse conditions for several years without a host plant present.

Biological studies suggest that the fungus prefers humid environments and can spread through contaminated water, soil particles moved by machinery, or infested plant material.

This pathogen causes severe damage by destroying the vascular system and cortical tissues of roots, leading to the rapid decay of the plant's main structural components.

For root crops such as carrots and sugar beets, the damage results in total crop failure, as infected roots turn into a mushy, violet-covered mass that is completely unmarketable.

In woody plants, the fungus attacks the root collar and primary roots, causing the death of the plant over one or several seasons as the vascular supply is cut off.

Losses are often exacerbated during storage, where the fungus spreads from infected roots to healthy produce, potentially ruining an entire harvest within a short period.

The economic impact involves not only the direct loss of yield but also the long-term quarantine of the land, as the pathogen remains active in the soil for extended periods.

Infection cycles are most aggressive during periods of high soil moisture and moderate temperatures, typically occurring in late summer and autumn in many temperate regions.

The fungus remains dormant during winter as hardy sclerotia within the soil profile, resisting freezing temperatures and becoming active as soon as conditions improve in spring.

Secondary infections often spread through mycelial growth through the soil matrix, especially in heavy or poorly drained fields that retain water for long periods.

Increased disease incidence is frequently observed following heavy rainfall or irrigation practices that favor moisture accumulation around the plant's crown and root zones.

Field spread is most common during tillage, where contaminated soil is moved from infested to clean parts of the field, effectively widening the scope of the disease.

Initial symptoms are often subtle, presenting as a slow decline in vigor, leaf yellowing, and temporary wilting during peak daylight hours, often misidentified as drought stress.

Upon closer inspection of the root, a distinct purple or violet fungal mat becomes visible, covering the surface of the infected tissue and sometimes extending to the surrounding soil particles.

Affected tissues undergo rapid softening and necrosis, with the outer skin often peeling away to reveal a rotting, discolored interior that emits a damp, earthy, or foul odor.

• Purple/violet fungal mycelium on roots
• Softened, necrotic tissue decay
• Wilting of above-ground foliage
• Stunted plant development

In mature trees and shrubs, the first signs may include bark cracking near the base, followed by a gradual thinning of the canopy and eventually the death of the entire plant.

Long-term rotation is the most effective management strategy; crops susceptible to Helicobasidium brebissonii should not be planted on the same plot for at least 5 to 7 years.

Soil management, including the improvement of drainage and the addition of lime to reduce soil acidity, is recommended as the fungus prefers lower pH environments.

Rigorous sanitation measures, such as the removal and destruction of all infested plant debris and affected soil, help to reduce the primary inoculum load in the field.

Fungicidal seed treatments and soil drenching before planting can provide a protective barrier for young, vulnerable seedlings, though these are often supplemental to good cultural practices.

Farmers should focus on preventing the movement of contaminated soil between fields by cleaning equipment thoroughly before moving it from one production site to another.