Phoma leaf spot

Initial symptoms of the disease usually appear as small, light-colored or grayish spots on the basal part of the stem. As the disease progresses, these spots darken and become covered with small black dots known as pycnidia.

Leaves of infected plants may show irregular spots with a distinct margin. Under high humidity conditions, a grayish mycelial mat can often be observed on the surface of these lesions.

Sunken ulcers frequently form on the stems, which impede the normal transport of nutrients and water. This leads to premature wilting and subsequent lodging of the plants.

When the root system is affected, the plant begins to yellow and shows stunted growth. In severe cases, total root rot occurs, resulting in plant death during the active flowering period.

A characteristic sign is the cracking of stem tissues at the site of lesions, which creates an entry point for secondary bacterial infections.

The disease is caused by the ascomycete fungus Phoma bismarckii, which belongs to the Deuteromycetes group. It is characterized by its ability to persist for a long time in soil and on plant debris as pycnidia and mycelium.

The pathogen's life cycle includes the formation of pycnidia, which are specialized fruiting bodies that produce conidia. These spores are dispersed by rain splashes, wind, and insects, infecting healthy plant tissues throughout the growing season.

The fungus exhibits high ecological plasticity, allowing it to adapt to various environmental conditions. During winter, the parasite's mycelium can survive in stubble, making it highly resistant to harsh environmental stressors.

The primary survival strategy of Phoma bismarckii is a saprotrophic phase, where it feeds on decaying plant organic matter until a new host crop appears. This makes full eradication from infested fields difficult without deep tillage.

Inoculum is also maintained via seeds, which can carry the pathogen both externally and internally, ensuring primary infection of seedlings at an early stage.

The development of Phoma bismarckii is favored by moderately warm weather combined with high air humidity. The optimal temperature range for spore germination is +18...+25°C.

Frequent rainfall and dew during the summer months trigger mass production and dispersal of conidia. Thickly sown crops, where aeration is poor and a high-humidity microclimate prevails, are most susceptible to infection.

Failure to implement proper crop rotation, where susceptible host crops are returned to the same field too soon, leads to the accumulation of the pathogen in the soil.

Deficiencies in certain micronutrients, particularly boron, can lower the plant's immune system, making tissues more vulnerable to fungal hyphae penetration.

Mechanical damage to stems, caused by pests or agricultural machinery, acts as an entry gate for the pathogen, significantly increasing the severity of the disease.

The main impact of Phoma infection is a reduction in total crop yield due to impaired metabolism in damaged tissues, which leads to lower seed weight and decreased quality.

Stem damage causes fragility, leading to widespread lodging of crops before harvest. Consequently, harvest losses increase significantly during mechanical collection.

The disease can cause premature plant death, creating gaps in the field and reducing plant density, which provides space for weeds to thrive.

In commercial production, harvests from infested fields lose grade due to shriveled seeds and the presence of fungal toxins, negatively affecting the market value of the output.

Long-term persistence of the pathogen in the soil makes it impossible to cultivate susceptible crops in the area for several years without intensive protection measures.

The key management strategy is strict adherence to crop rotation. It is necessary to maintain an interval of at least 3–4 years before returning susceptible crops to the same field.

Deep plowing to bury plant debris is essential to accelerate decomposition and pathogen death in the soil. Using healthy, fungicide-treated seeds is strongly recommended.

• Application of preventive fungicides during active growth phases.
• Balanced fertilization, especially potassium and boron, to strengthen cell walls.
• Timely pest management to prevent stem damage.
• Removal of volunteer plants and weeds that may act as reservoirs for the infection.

Utilizing crop varieties with genetic resistance to Phoma is the most economically viable and environmentally friendly control method.