Didymella leaf spot of sorrel

The primary symptom of the disease is the appearance of distinct spots on the sorrel leaves. Initially, these appear as small brown or dark brown flecks on the leaf blades.

As the infection progresses, these spots enlarge, taking on a round or irregular shape with a darker margin. In the center of these lesions, small black dots known as pycnidia develop.

Severe infection leads to premature yellowing, wilting, and necrosis of the leaves, which significantly degrades the market value of the produce.

Under high humidity conditions, a fungal growth consisting of pathogen conidia may be visible on the surface of the infected tissue.

When the petioles are infected, the leaves lose their turgor, collapse, and become unsuitable for consumption due to significant loss of quality.

The causative agent of this disease is the ascomycete fungus Didymella rumicicola (anamorph Phoma rumicicola). This pathogen belongs to the class Dothideomycetes and specifically targets species within the Polygonaceae family, including cultivated sorrel.

The fungal life cycle involves the formation of pycnidia, which serve as the primary source of plant infection. The pathogen typically overwinters as mycelium and spore-producing structures on plant debris left in the field after harvest.

Spores are easily disseminated by wind, rain splashes, or irrigation, enabling rapid spread of the disease across the crop area.

The biology of this pathogen allows it to adapt to various growing conditions, maintaining viability for extended periods even under unfavorable environmental stressors.

The progression of the fungus is closely linked to the physiological state of the host plant, with weakened or over-mature sorrel stands being particularly susceptible to infection.

Favorable conditions for the development of Didymella leaf spot include frequent rainfall and high relative humidity throughout the growing season. The pathogen thrives at temperatures between +15 and +25 degrees Celsius.

Overcrowded plantings that prevent proper air circulation and slow leaf drying create a perfect environment for massive outbreaks of the infection.

Failure to rotate crops and returning sorrel to the same soil for more than 3-4 years significantly increases the inoculum pressure in the soil.

Overhead irrigation performed in the evening hours promotes prolonged moisture on the leaf surface, which acts as a trigger for fungal spore germination.

Imbalanced fertilization, particularly excessive nitrogen, makes plant tissues softer and more prone to fungal penetration.

The economic impact of the disease lies in the substantial loss of marketable biomass. Affected leaves rapidly deteriorate, becoming discolored and limp, rendering them unsellable.

In epidemic conditions, crop yields can be reduced by 30–50% or more, depending on the intensity of the initial infection.

The disease impairs photosynthetic activity, causing roots to accumulate fewer nutrients, which negatively impacts the overwintering ability and regrowth of the plant.

Reduced produce quality precludes storage and transportation, as damaged tissues are highly susceptible to secondary rot infection.

Financial losses stem not only from the direct reduction in yield but also from the added costs of implementing remedial control measures.

The cornerstone of control is the implementation of a proper crop rotation system. It is strongly advised not to grow sorrel in the same location for more than 5 years or following other Polygonaceae crops.

Crucial agrotechnical practices include the thorough removal and destruction of crop residues, which act as a reservoir for the pathogen during the off-season.

Producers must avoid excessive planting density, ensuring optimal spacing for proper aeration of the plant canopy.

Application of potash and phosphorus fertilizers helps improve the overall immunity of the sorrel plant against fungal pathogens.

• Use of certified pathogen-free planting material
• Systematic weed control to eliminate alternative hosts
• Timely harvesting to prevent leaf over-maturation
• Use of mulching in inter-rows to manage soil humidity
• Selective use of fungicides according to label directions and pre-harvest intervals