Leaf spots (fungal)

Spots vary in shape from circular to angular, often delimited by leaf veins, and in color, ranging from light beige and grey to dark brown, almost black, or with a reddish halo.

Often, in the center of the spots, pathogen sporulation develops, which can be seen upon close inspection as black dots (pycnidia) or velvety coatings of different colors.

As the disease progresses, spots tend to increase in size and coalesce, leading to the destruction of significant leaf surface areas.

The final stage of infection is premature yellowing, drying, and leaf drop, which critically reduces the overall photosynthetic capacity of the plant.

Leaf spots are a collective group of infectious diseases most commonly caused by pathogenic fungi from various classes, including Ascomycetes and Deuteromycetes.

Bacteria can also act as causal agents (bacterial leaf spots), causing water-soaked or oily lesions, which require different diagnostic and management approaches.

In rare cases, specific leaf spots are induced by viruses, where the spots often take the form of chlorotic mosaics or necrotic rings.

Pathogens persist in the soil, on crop residues, in seeds, and some species can overwinter as mycelium on perennial plant parts.

During the growing season, the infection spreads via spores (conidia) transported by wind, rain splashes, or insect vectors.

The key factor for the development of most leaf spots is high humidity and the presence of free water on leaf surfaces for extended periods.

Optimal temperature ranges for the germination of spores of most fungal pathogens are between +18 and +25 degrees Celsius.

Densely planted crops, poor ventilation within the agro-ecosystem, and excessive nitrogen fertilization, which reduces tissue resistance, create ideal conditions for epiphytotics.

Poor crop rotation practices, where susceptible crops are grown in the same field too frequently, contribute to the accumulation of inoculum in the soil.

Weakened plants due to nutrient deficiencies or pest damage have significantly higher susceptibility to pathogen entry through stomata or micro-wounds.

The primary damage is a sharp reduction in green surface area, which blocks photosynthesis and disrupts the carbohydrate metabolism of the plant.

The disruption of physiological processes leads to stunted growth, reduced plant immunity, and an overall decrease in yield for the current season.

Affected crops show reduced quality, loss of market value for vegetables and fruits, and lower seed weight and viability.

Severe infestations can trigger premature shoot death and total plant loss, especially under stress-inducing weather conditions.

Economic losses arise not only from reduced yields but also from high costs associated with chemical control applications and field sanitation.

Protection strategies are based on cultural methods, including crop rotation and spatial isolation from fields with related crops.

Important steps include deep plowing of crop residues in the autumn and using only healthy, certified seeds for planting.

The use of modern fungicides (both contact and systemic) allows for the suppression of disease spread in the early stages, supported by phytosanitary monitoring.

• Utilizing disease-resistant cultivars and hybrids;
• Timely elimination of weeds that serve as pathogen reservoirs;
• Optimizing plant density to improve aeration;
• Balanced application of mineral fertilizers, especially potassium and phosphorus.

In cases of widespread infection, preventive fungicide applications must be conducted according to the local regulations and label requirements for specific products.