Briosia leaf spot

Initial symptoms of the disease appear on leaves as small, round, or angular spots. Initially, the spots have a brown or dark-brown shade, often surrounded by a light or yellowish halo, which is characteristic of many fungal infections on grapes.

As the disease progresses, the spots may increase in size and merge, covering a significant area of the leaf blade. Tissue in the affected areas gradually dies, becomes brittle, and may fall out, forming characteristic perforations.

A grayish-olive coating often forms on the underside of the leaf in the area of the spots. This coating consists of conidiophores and conidia, which are easily carried by air currents to neighboring grapevines.

Leaf damage leads to premature drying and shedding. Heavily affected vines lose the ability to properly ripen their shoots, which negatively affects their cold hardiness and productivity for the following season.

Diagnosis is complicated by its visual similarity to other leaf spots; therefore, when identifying the disease, it is important to look for the specific sporulation of the fungus, which is clearly visible under a magnifying glass.

The disease is caused by the imperfect fungus Briosia ampelophaga, which belongs to the Hyphomycetes group. This pathogen primarily affects the common grapevine, causing a specific leaf spotting that can significantly weaken the plant's foliage if left unchecked.

The fungus specializes in attacking the tissues of the vine's vegetative organs. The pathogen's mycelium develops in the intercellular spaces of the leaf tissue, gradually destroying epidermal and parenchyma cells, which disrupts photosynthesis and the plant's overall metabolism.

The pathogen's life cycle is closely linked to plant debris, where it can survive during unfavorable periods. In spring, when favorable conditions arise, fungal spores are dispersed by wind or rain droplets onto young leaves.

Infectious material persists on affected shoots and fallen grape leaves. This makes sanitary cleaning of the vineyard a critically important step in reducing the overall inoculum levels on the site.

The biology of Briosia ampelophaga requires increased attention from agronomists in high-humidity regions, as these conditions are ideal for the most active sporulation and spread of the pathogen.

The development of Briosia leaf spot is promoted by periods of high air humidity and frequent rainfall. The optimal temperature range for spore germination and active mycelium growth is between +18 and +25 degrees Celsius.

Densely planted vineyards with poor air circulation create a microclimate that is perfectly suited for the rapid spread of the infection. A poorly ventilated bush retains moisture on the leaves for longer, facilitating the penetration of spores into the tissues.

Outbreaks of the disease are most likely during prolonged rainy periods in the first half of the growing season. At such times, primary infection spreads rapidly, affecting new layers of grapevine leaves.

Imbalanced nutrition, especially excess nitrogen, promotes the growth of soft, loose tissue that is more easily attacked by the pathogen. Proper agrotechnical practices are a crucial factor in suppressing epiphytotics.

The presence of free moisture on the leaf surface is a mandatory condition for successful conidia germination. Dew and fog also contribute significantly to the dynamics of fungal population development.

The main harmfulness of the disease lies in the premature disruption of the leaves' photosynthetic activity. Massive infection reduces sugar accumulation in the berries and impairs the overall quality of the grape harvest.

Due to the loss of a significant portion of leaf area, the plant weakens, leading to poor shoot ripening. Immature vines are extremely sensitive to low temperatures and often die during the winter period.

The disease can lead to a decrease in the overall immune system of the vine. Weakened plants become more susceptible to secondary infections like powdery mildew or downy mildew, which complicates the overall protection scheme.

In cases of severe infection, yields may drop by 20–40%. If no measures are taken, the inoculum on the site will increase annually, making grape cultivation economically unviable.

The deterioration of the appearance and general condition of the plants requires additional costly treatment measures, increasing production costs and labor hours in the vineyard.

The foundation of protection is timely sanitary measures. It is important to carefully collect and destroy fallen leaves, as this is where the majority of the pathogen's infection survives the winter.

Agrotechnical practices are aimed at improving vineyard ventilation: timely disbudding, shoot thinning, and tying allow for significantly reduced humidity within the canopy.

Chemical control includes the application of broad-spectrum fungicides. Copper-based preparations and contact fungicides, applied preventively before the appearance of massive signs of infection, prove effective.

• Preventive spraying with Bordeaux mixture in the spring.
• Systematic removal of affected leaves at the first signs of symptoms.
• Ensuring an optimal nutrient regime with an emphasis on potassium fertilizers.
• Adhering to planting density schemes to prevent canopy overcrowding.

When planning sprayings, weather conditions should be taken into account. In years with frequent rainfall, intervals between sprayings are reduced to maintain a protective layer on the surface of young leaves.