Phoma stem canker

A key symptom of infection is the development of necrotic lesions on leaves, which appear grey or brown with a dark border. Over time, small black dots known as pycnidia become visible within the center of these lesions.

When stems are affected, particularly at the root collar, deep cankers or dry rots develop. These lesions often girdle the stem base, leading to tissue softening and increasing the risk of plant lodging during windy weather.

The root system is also severely impacted by Boeremia, manifesting as the darkening and necrosis of smaller roots, which drastically reduces the plant's water and nutrient absorption capacity.

On pods of cruciferous crops, the disease appears as sunken, irregularly shaped spots. As the pods mature, the seeds inside often become shriveled or infected by fungal mycelium, rendering them unsuitable for future planting.

• Yellowing and wilting of lower leaves.
• Presence of grey, dry necrotic spots on vegetative organs.
• Black dot-like fruiting bodies (pycnidia) on lesions.
• Cracking of the root collar and stem at the base.

The disease is caused by fungi of the genus Boeremia, most notably Boeremia lingam (formerly classified as Phoma lingam). It is a highly specialized pathogen belonging to the ascomycetes, which primarily affects plants within the Brassicaceae family.

The fungus survives in the soil on crop residues and can be transmitted via infected seeds. The life cycle of the pathogen includes both an asexual stage (pycnidia containing spores) and a sexual stage (pseudothecia containing ascospores), which contributes to its resilience in varying environmental conditions.

The infection process begins when pycnidiospores are dispersed by rain splash or wind onto susceptible plant tissues. Ascospores, produced on overwintered debris, can be carried over long distances, initiating primary infection foci in young fields.

The fungal mycelium penetrates plant tissues through stomata or mechanical wounds. Once inside, the pathogen thrives, destroying host cells and vascular tissues, which disrupts the plant's metabolic processes and nutrient uptake.

The fungus can persist in the soil for up to 3-4 years, making strict crop rotation essential. Identification of the pathogen is often confirmed by the presence of characteristic black dots — pycnidia — on the necrotic tissue lesions.

Boeremia development is favored by high humidity and moderate temperatures ranging from +15°C to +25°C. Frequent rainfall and the presence of leaf surface wetness are critical factors that facilitate the germination of fungal spores.

Dense crop stands, which restrict airflow and maintain high humidity, provide an ideal environment for rapid infection spread. Insufficient light and excessive nitrogen fertilization may also increase plant susceptibility to the disease.

Mechanical damage to stems and leaves, caused by pests or field operations, creates entry points for the fungal infection. Timely control of insect populations is essential to reduce the risk of secondary infections.

Temperature fluctuations during the autumn often trigger pathogen activation in winter crops. Optimal conditions for the infection of cotyledons and young leaves persist during periods of prolonged fog and dew.

Agronomic stresses, such as micronutrient deficiencies or improper sowing dates, impair the plant's natural immune system, leaving them more vulnerable to the effects of Boeremia.

Phoma stem canker is one of the most devastating diseases of rapeseed and other brassicas, capable of causing total crop failure in favorable years. The primary damage involves the destruction of the root collar, which cripples the plant's vascular system.

Infection leads to premature plant senescence, resulting in a sharp decline in thousand-grain weight and oil content. In cases of severe disease progression, widespread lodging occurs, significantly complicating mechanical harvesting operations.

Beyond direct yield loss, the pathogen reduces seed quality, turning the harvest into a primary source of infection for subsequent seasons. Infected seeds exhibit poor germination energy and low overall viability.

Internal seed infection often results in pre-emergence damping-off, which thins out the crop stand and reduces plant density. This necessitates additional costs for reseeding and increased investment in agrochemicals.

Economic losses are compounded by the costs of fungicide applications, reduced market value of the produce, and the requirement for deep tillage practices to manage the soil-borne inoculum.

Strict crop rotation, ensuring a gap of at least 3-4 years before returning brassica crops to the same field, is the primary preventative measure. This interval allows the pathogen population in the soil to decline due to the absence of a susceptible host.

The use of high-quality, fungicide-treated seeds is the foundation of effective crop protection. Seed treatment with systemic fungicides eradicates external and internal infections, protecting the plant during critical early growth stages.

Timely deep incorporation of crop residues into the soil is a vital management practice. The decomposition of plant debris accelerates the destruction of the fungal mycelium and reduces the spore bank in the field.

Foliar fungicide applications should be guided by phytosanitary monitoring and weather forecasting. Treatments are most effective during the rosette stage and the onset of the budding phase.

Cultivars with high genetic resistance to Phoma are crucial for reducing reliance on chemical inputs. Breeding for immunity to Boeremia remains a top priority in modern agricultural science.