Pyrenochaeta leaf spot of soybean

The fungus produces pycnidia — small, dark-colored, flask-shaped fruiting bodies, often featuring bristle-like structures. Inside these bodies, pycnidiospores are formed, which serve as the primary inoculum for spreading the infection during the growing season.

The pathogen survives in soybean crop residues left on the soil surface or incorporated into the top layer. When favorable conditions return, the fungus resumes activity, initiating the infection cycle on new plant tissues.

The fungus mycelium penetrates the leaf tissue, stems, and petioles, destroying cells and disrupting essential photosynthetic processes. Tissue damage is accompanied by pigment changes, which makes visual diagnosis of the disease relatively straightforward.

The taxonomy of this fungus places it within the genus Pyrenochaeta, known for causing various lesions in a wide range of agricultural crops, although P. glycines is exclusively specialized for soybeans.

The primary host is soybean (Glycine max). The fungus attacks all aerial parts of the plant, including leaves, stems, petioles, and occasionally pods, which directly impacts the commercial quality of the harvested crop.

The harmfulness of the disease manifests in premature leaf senescence, which significantly reduces the leaf area available for photosynthesis. This leads to weakened plants and a substantial reduction in overall biomass.

In cases of severe infection, nutrient translocation to the seeds is impaired. Consequently, pods become underdeveloped, and the 1000-seed weight decreases significantly, leading to direct yield losses.

Stem infections can cause plants to lodge during high winds, complicating mechanical harvesting and leading to additional grain losses in the field.

The disease can also reduce seed germination rates if the pathogen moves from the pod valves to the seeds, making control measures vital for seed production farms.

The first signs of the disease typically appear during the active growth phase, most commonly in the second half of the summer when high air humidity and moderately warm temperatures prevail.

Epiphytotic development is favored by heavy rainfall accompanied by frequent fog. Water droplets serve as the main vector for spore dispersal from the lower infected leaves to the upper parts of the plant.

The optimal temperature for mycelial growth and sporulation ranges from +20 to +25 degrees Celsius. If temperatures fluctuate significantly or drought conditions occur, the disease progression slows down temporarily.

The life cycle of the fungus includes multiple secondary infection cycles during a single growing season; therefore, in years with frequent rainfall, the severity of the infection increases rapidly.

As the crop reaches maturity, the pathogen enters a resting phase, forming pycnidia on dying plant parts, which serve as the primary source of inoculum for the following season.

The initial symptom of the disease is the appearance of small brown or reddish-brown spots on the leaves, which may eventually coalesce to form large necrotic zones.

• Development of irregular dark brown spots on the leaf surface.
• Gradual yellowing and necrosis of the leaf tissue surrounding the spots.
• Formation of tiny black dots (pycnidia) within the necrotic leaf areas.
• Infection of the petioles, leading to premature leaf drop.
• Elongated dark lesions appearing on stems, sometimes causing bark cracking.

Under high humidity conditions, a faint fungal growth consisting of conidiophores and spores of the pathogen can often be observed on the surface of the lesions.

Severe infection makes the plant appear scorched; leaves curl and fall prematurely, exposing the pods and making them more susceptible to secondary infections.

Visually, the disease is often confused with bacterial blight of soybean, but the presence of clear pycnidia (tiny black dots) in the center of the spot is a reliable diagnostic feature of Pyrenochaeta leaf spot.

The fundamental control method is practicing crop rotation, ensuring soybeans are not planted in the same field for at least 3–4 years, which allows the pathogen to die out in the soil due to the absence of a host.

An important agronomic practice is the deep incorporation of crop residues into the soil (plowing with soil inversion), as this accelerates the decomposition process and ensures the death of the fungal mycelium.

Using certified seed of resistant or tolerant soybean varieties is the most cost-effective way to protect crops from this disease.

Fungicide application during the growing season is recommended if an epidemic is forecasted. Triazole and strobilurin-based products have demonstrated high efficacy against leaf spot diseases.

It is necessary to conduct thorough phytosanitary field cleaning, removing soybean volunteers and weeds that may act as additional reservoirs of infection during the off-season.