Phoma leaf spot of beet

Phoma betae (teleomorph Pleospora betae) is a significant fungal pathogen classified within the Ascomycota division. It is recognized primarily as the causal agent of phoma leaf spot, damping-off of seedlings, and dry rot in sugar beets, mangels, and table beets, representing a major threat to beet production worldwide.

The fungus manifests through various symptoms depending on the plant growth stage. On young seedlings, it causes damping-off, leading to poor crop establishment. In mature plants, the disease is identified by characteristic light-brown, circular leaf spots marked with black pycnidia arranged in concentric rings, which eventually degrade the photosynthetic capacity of the foliage.

The life cycle of the pathogen is highly adaptable. It survives as mycelium or pycnidia in infested crop debris, soil, and contaminated seeds. Primary infection occurs through infected seed or soil-borne inoculum. During the growing season, conidia are dispersed by rain splash and wind, facilitating the spread of the pathogen to healthy leaves and roots.

Environmental factors, particularly high humidity and temperatures between 18°C and 22°C, are optimal for disease outbreaks. The pathogen thrives in fields with poor drainage and in instances where plants are stressed due to nutrient deficiencies, especially boron deficiency, which renders the beet root tissue more susceptible to dry rot infection during storage.

Economic damage is substantial, as the infection leads to reduced root yield, lower sucrose content, and increased susceptibility to secondary rots. Management strategies focus on an integrated approach to mitigate risk:

• Planting high-quality, fungicide-treated seeds.
• Implementation of a 3-to-4-year crop rotation to break the infection cycle.
• Proper fertilization programs, focusing on boron and potassium levels.
• Deep plowing to accelerate the decomposition of infected plant debris.
• Application of foliar fungicides during the vegetative growth period when conditions are favorable for disease spread.