Colletotrichum simmondsii

This pathogen was previously misclassified under the Colletotrichum acutatum species complex. However, recent taxonomic revisions have established it as a distinct species. It is characterized by its high aggressiveness and ability to persist in diverse environmental conditions.

Microscopically, it is identified by the production of cylindrical conidia within acervuli, which are specialized asexual fruiting structures. These structures are the primary source of secondary infection spread during the growing season.

The fungus is highly adaptable and can survive in the soil or on infected plant debris as mycelium, waiting for optimal conditions to produce new spores. Its genetic diversity contributes to its ability to overcome traditional control measures.

Understanding the life cycle of C. simmondsii is critical for growers, as it can propagate both sexually and asexually, allowing it to adapt to changing environments and spread efficiently throughout plantation areas.

The most prominent host for Colletotrichum simmondsii is the strawberry. It causes devastating losses by infecting flowers, fruits, petioles, and crowns, often resulting in complete crop failure if left unmanaged.

Beyond strawberries, the pathogen has a wide host range, including blueberries, raspberries, and several vegetable species such as peppers and tomatoes. It can survive on alternative weed hosts, making complete eradication challenging.

The economic impact is profound, as the infection renders fruits unmarketable due to the development of necrotic lesions. Furthermore, crown infections can lead to the sudden collapse of entire plants, requiring replanting and additional labor costs.

Infected nursery stock is a major vector for the global spread of this pathogen. Once established in a field, the fungus can remain in the soil for several seasons, creating a long-term threat to production on that site.

The pathogen's ability to infect all parts of the plant makes it particularly dangerous, as even asymptomatic nursery stock can carry the fungus, leading to outbreaks long after the plants have been introduced to the field.

Active development of the fungus typically occurs during the flowering and fruiting stages of the host plants. High humidity and rainfall are the primary drivers of spore dispersal and infection establishment.

The optimal temperature range for the development of C. simmondsii is between 20°C and 28°C. In such conditions, the fungus can cycle from infection to new spore production in as little as 3-4 days.

Summer months, characterized by intermittent rain and warm weather, represent the peak risk period. Rain splashes from the soil surface transport spores onto susceptible plant tissues, initiating new cycles of infection.

During the dormant winter season, the pathogen survives as mycelium within plant debris, infected crowns, or in the soil. It remains quiescent until temperatures and moisture levels become favorable again in the spring.

The rapid infection cycle enables C. simmondsii to cause significant outbreaks in a very short time frame, meaning that delayed intervention often results in severe economic losses for growers.

Initial signs of anthracnose caused by C. simmondsii include small, dark brown to black sunken spots on flower stalks and young leaves. These lesions quickly expand, leading to the collapse of the affected tissue.

On berries, the infection manifests as sunken, necrotic lesions. These spots often develop a salmon-pink or orange sticky spore mass in the center, which is highly visible under high humidity and serves as an inoculum source.

Key visual symptoms include:

• Sunken, dark lesions on green and ripening fruit.
• Blighting of flowers and flower stalks (peduncles).
• Development of necrotic spots on leaves, often leading to leaf tissue death.
• Sudden wilting of the entire plant when the crown tissue is severely infected.

As the disease progresses, the berries often become firm, dry, and shriveled (mummified). These mummified fruits act as major reservoirs for the fungus, continuing to release spores throughout the season.

Effective management of Colletotrichum simmondsii requires an integrated approach that prioritizes prevention and sanitation to limit the initial inoculum density in the field.

Essential control measures include:

• Sourcing certified pathogen-free planting material from reliable nurseries.
• Implementing drip irrigation to avoid wet foliage, which significantly reduces spore dispersal.
• Promptly roguing and removing all symptomatic plants and fruits to prevent further spread.
• Maintaining proper plant spacing to improve air circulation and reduce humidity around the canopy.

Chemical control should rely on a preventative spray program using fungicides with different modes of action to prevent resistance development. Applications are most effective when timed before bloom and during high-risk weather events.

Biological control agents, such as those containing Bacillus subtilis or Trichoderma species, can be integrated into the program, particularly during harvest periods when chemical restrictions limit the use of conventional fungicides.