1. Causal Organism
Botrytis blight of marigold is caused by the necrotrophic fungus Botrytis cinerea, a ubiquitous pathogen with a broad host range. The fungus is especially notorious for its capacity to infect senescing tissues and for its prolific production of airborne conidia under humid conditions.
2. Etiology
Botrytis cinerea belongs to the phylum Ascomycota and exhibits both saprophytic and pathogenic lifestyles. The pathogen produces conidiophores bearing clusters of conidia that appear as grayish masses on infected tissues. Under favorable environmental conditions—cool temperatures (15–25°C) and high humidity (>90%)—the pathogen rapidly colonizes plant surfaces and enters through wounds, natural openings, or weakened tissues. The fungus may also form sclerotia, which serve as overwintering structures, enabling long-term survival in plant debris and soil.
3. Symptoms
Initial symptoms manifest as water-soaked, necrotic lesions on petals, leaves, and stems, especially near senescent or damaged tissues. These lesions expand rapidly, developing a characteristic grayish-brown moldy growth, often referred to as "gray mold." Affected floral tissues exhibit discoloration, blight, and collapse, leading to premature flower drop. In severe cases, systemic infection may occur, causing stem girdling and wilting. The presence of conidial masses is a diagnostic feature of the disease.
4. Disease Cycle
- B. cinerea survives as mycelium or sclerotia in infected plant debris, soil, or greenhouse structures.
- Under moist conditions, sclerotia germinate to produce conidiophores and conidia, which are disseminated by wind, water splash, or mechanical means.
- Conidia germinate on plant surfaces, especially in humid microclimates, and penetrate tissues via wounds or natural openings.
- The pathogen exhibits a polycyclic disease cycle, enabling multiple infection cycles within a single growing season.
- Environmental factors such as prolonged leaf wetness, poor air circulation, and dense canopies favor disease outbreaks.
5. Management
- Adopt cultural practices that promote air circulation, such as proper spacing, pruning, and avoidance of overhead irrigation.
- Remove and destroy infected plant parts and crop residues to reduce inoculum load.
- Avoid working with plants when they are wet to prevent mechanical dissemination of spores.
- Apply fungicides such as iprodione, chlorothalonil, or boscalid as protective measures, particularly during humid periods.
- Rotate with non-host crops and use resistant or tolerant marigold cultivars, where available.
- Implement sanitation practices in greenhouse settings, including regular disinfection of tools and benches.
Note: An integrated disease management strategy that combines environmental control, cultural practices, and judicious fungicide use is essential for sustainable control of Botrytis blight in marigold cultivation.