Grape tar spot

The disease is classified as a leaf spot fungus. It spreads primarily via ascospores, which are produced in fruiting bodies known as stromata that develop on overwintered, fallen grape leaves.

The biological cycle of the fungus is strictly seasonal. Overwintering occurs in the form of mycelium and fruiting structures on decomposing leaf debris, which serves as the primary inoculum source for the next spring season.

The infection process initiates when spores land on susceptible leaf surfaces during periods of high humidity. The mycelium penetrates the epidermal layers, eventually establishing a nutrient-absorbing relationship that leads to leaf tissue degradation.

This pathogen is relatively host-specific. While it is rarely the most destructive disease in commercial vineyards compared to downy mildew, it can become a significant issue in neglected or poorly managed vines.

The hallmark symptom of grape tar spot is the appearance of distinct spots on the leaf surface. Initially appearing as small chlorotic areas, they eventually transition into dark, raised, and shiny black lesions.

These spots are often described as having a «tar-like» appearance due to the dense, glossy black stroma formed by the fungus just beneath the leaf cuticle. This gives the lesion a physical thickness and texture.

A yellow or pale green halo often develops around the edge of each spot, indicating the border between the pathogen's toxic activity and the remaining healthy leaf tissue.

When environmental conditions favor heavy infection, these spots can coalesce, covering large portions of the leaf. This severe infection causes the leaf to curl, dry out, and eventually drop prematurely.

• Raised, shiny black circular spots on leaves.
• Characteristic yellow halo surrounding the black lesions.
• Potential leaf deformation and premature defoliation.
• Presence of black stromata on fallen leaves in late season.

The development of grape tar spot is heavily dependent on moisture. Prolonged rainy weather or frequent morning dew periods are essential for spore germination and successful fungal colonization.

Poor vineyard sanitation creates an environment conducive to the disease. If infected leaves are allowed to accumulate on the ground during winter, the pathogen remains close to the vine, ensuring infection in the following spring.

Dense vine canopies with poor airflow prevent leaves from drying quickly after rainfall. This persistent wetness significantly extends the window of opportunity for Rhytisma vitis to infect new growth.

Moderate temperatures ranging from 18°C to 25°C generally accelerate the pathogen's growth cycle. High humidity combined with these temperatures results in the fastest appearance of new symptoms.

Sites with poor drainage or those located in low-lying areas where humidity remains trapped are consistently at a higher risk of sustaining severe tar spot outbreaks compared to well-ventilated, sunny sites.

The primary impact of tar spot is the reduction of the leaf area available for photosynthesis. This decline in metabolic activity leads to lower sugar accumulation in the developing berries and poorer fruit quality.

Premature leaf drop caused by the disease weakens the vine heading into dormancy. This reduction in energy storage leaves the plant more susceptible to winter injury and cold-related stress.

Repeated yearly infections can impact the overall longevity of the vine. By reducing the plant's vigor, tar spot can decrease the number of fruitful buds formed for the subsequent growing season.

In nursery settings, tar spot can be particularly damaging to young vines, hindering their growth and reducing their overall quality before they are even planted in the vineyard.

While often considered a secondary disease, it contributes to increased management costs due to the necessity of additional sanitation measures and potential fungicide applications to protect vine health.

The most effective strategy for managing grape tar spot is rigorous sanitation. Removing and destroying fallen leaves from the vineyard floor significantly reduces the primary spore load for the following season.

Improving canopy management through proper pruning, shoot positioning, and leaf pulling is crucial. Increasing airflow helps keep leaves dry and limits the conditions necessary for fungal survival.

Chemical control is rarely required if sanitation is practiced correctly. However, in cases of severe infection, copper-based fungicides or other broad-spectrum vineyard fungicides can be used to inhibit the disease.

Maintaining vine balance through appropriate fertilization is beneficial. Excessive nitrogen should be avoided as it leads to succulent, overly lush growth that is inherently more prone to fungal pathogen penetration.

Regular monitoring of the vineyard during the early stages of leaf development allows for early intervention. If symptoms are spotted, applying protective fungicide sprays can effectively mitigate the spread across the block.