Phoma leaf spot

The primary symptom of Phoma leaf spot is the appearance of characteristic spots on the stems, leaves, and petioles of affected plants. These spots have an irregular shape and are usually dark brown or grayish in color.

Over time, small black dots become clearly visible on the surface of the spots. These are the pycnidia of the fungus, in which spores develop for further spread of the infection across the field.

When stems are affected, cracking or constriction may occur in the area of the spot. This disrupts normal sap flow and nutrient uptake, leading to wilting and the subsequent death of individual organs.

In the early stages, signs of the disease may resemble other fungal infections, so laboratory analysis of plant material is often required for accurate diagnosis.

A characteristic sign of progressive Phoma disease is premature yellowing and leaf fall, which significantly weakens the overall condition of the plant during the yield formation period.

The causal agent of the disease is the fungus Phoma boerhaviae. This microorganism belongs to the class of deuteromycetes and leads a parasitic lifestyle on the vegetative parts of plants.

The fungus is capable of surviving in the soil and on plant debris in the form of pycnidia — specialized fruiting bodies. Conidia, which are the primary source of initial crop infection at the beginning of the growing season, are formed inside these pycnidia.

The pathogen's life cycle is closely linked to environmental humidity. Under favorable conditions, the fungus germinates actively, infecting host tissues through stomata or mechanical damage to the epidermis.

Inside the tissues, the mycelium spreads intercellularly, gradually destroying the plant's cell structure. The pathogen shows high specialization in its host selection, making it dangerous for specific groups of wild and cultivated plants.

Conidia dispersal occurs primarily through rain splashes, wind, and agricultural tools when proper agronomic practices are violated. The infectious background can accumulate in the field over several seasons if crop rotation is not practiced.

Phoma boerhaviae develops most actively in high humidity conditions. Prolonged rainfall and fog create an ideal environment for conidia germination and mass tissue infection.

The optimal temperature range for the pathogen's development is within moderate values, typical of the spring-summer period. Extreme heat and drought can temporarily halt mycelial development.

Dense crop stands, where air ventilation is poor, significantly facilitate the rapid spread of the disease. Moisture stagnation in the lower parts of plants creates a favorable microclimate for pycnidia activation.

Violations of agronomic rules, such as improper irrigation or lack of weed control, increase the risk of initial infection. The pathogen can maintain viability for a long time in infected residues.

The use of infected planting material or seeds is also a major factor contributing to the primary appearance of the infection in new areas or fields.

The damage caused by Phoma disease is primarily manifested in a significant reduction in both the volume and quality of the harvest. The impairment of photosynthesis due to foliage damage leads to slowed plant development.

Stem infection can lead to plant lodging, which makes mechanical harvesting impossible and increases losses during the harvest stage. This is particularly critical for industrial crops.

When generative organs are infected, the fungus can reduce seed germination rates and vigor. Affected fruits lose their marketability and become unsuitable for long-term storage.

Mass spread of the disease can destroy up to 30–50% of the harvest in the absence of timely intervention. This requires significant financial expenditure on protective measures.

The infection also exhausts the plant, making it more susceptible to other diseases and pests, which complicates the overall phytosanitary situation in the field.

The primary method of control is practicing crop rotation by excluding susceptible crops from the rotation for 2–3 years. This allows for breaking the pathogen's reproduction cycle in the soil.

It is important to deep-plow plant residues into the soil after harvest. This promotes their rapid decomposition and the destruction of the fungal structures contained within them.

Using high-quality, treated planting material significantly reduces the risk of infection in the early stages of crop growth. Fungicidal seed treatment is a mandatory step in protection.

During the growing season, it is recommended to apply authorized systemic or contact fungicides. Applications should start at the first sign of spotting for maximum efficacy.

Regular weeding and optimizing plant density help improve the microclimate, preventing the accumulation of excess moisture around the stems.