Phoma leaf spot of beet

The disease known as Phoma leaf spot or heart rot in beets is caused by the fungus Pleospora betae (anamorph: Phoma betae). As a member of the Ascomycota phylum, this pathogen is a significant threat to sugar beet, fodder beet, and red beet production worldwide. It acts as both a seed-borne and soil-borne pathogen, complicating management strategies for growers.

Symptoms typically manifest as necrotic spots with concentric zones on foliage and dry rot on the crown or root of the plant. In storage, infected roots develop black, hard rot lesions covered in small, black pycnidia. Seedling infection often leads to pre-emergence damping-off, causing significant stand reduction in commercial beet fields.

The life cycle of Pleospora betae involves surviving as mycelium or pycnidia in crop residues, soil, and infected seed clusters. Primary inoculum is spread by splashing rain and wind-blown spores during the growing season. The pathogen requires moisture to germinate and penetrate the host tissue, often gaining entry through damaged leaves or root tissues.

Environmental conditions such as high humidity, frequent rainfall, and moderate temperatures (15°C to 25°C) are highly conducive to disease spread. Nutrient deficiencies, particularly a lack of boron, are known to predispose beet plants to infection, often resulting in symptoms of heart rot, which significantly lowers the plant's resistance to fungal invasion.

The economic impact of this disease is substantial, leading to reduced root weight, lower sugar content, and high spoilage rates in storage facilities. To manage and control the pathogen, integrated pest management programs are essential:

• Use of high-quality, fungicide-treated seed lots.
• Implementation of a 3-year or longer crop rotation cycle.
• Adequate soil fertilization and boron supplementation to boost plant health.
• Sanitation practices, including the removal or deep burial of crop debris.
• Application of preventative foliar fungicides when weather conditions favor disease development.