Anthracnose Risk Establishment Based on Age-Related Susceptibility of Grape Leaves, Flowers, and Berries to Infection by Elsinoë ampelina.

Anthracnose is an important disease of grapevines caused by the fungus Elsinoë ampelina. In recent years, there have been regular outbreaks in humid grape-growing regions around the world. Young leaves and berries are reported to be highly susceptible to E. ampelina, but detailed and seasonal development of age-related susceptibility remains unclear. Experiments were conducted under greenhouse and vineyard conditions by inoculating 1- to 19-day-old leaves, flowers, and berries at different phenological stages of three grapevine cultivars (Vandal-Cliche, Marquette, and Vidal). Leaf susceptibility was highest when inoculated at 1 to 2 days old, and inoculated leaves were moderately susceptible at 3 to 6 days old and almost resistant when older than 6 days. The influence of leaf age on anthracnose relative severity was adequately described by an exponential decay model. The susceptibility of the inflorescences was high when inoculated from their initiation to the full flowering (50% fall of the caps), and the inflorescences/flowers were moderately susceptible until veraison, after which the berries were practically resistant. The flower/berry susceptibility as a function of degree-days accumulated since 1 April was modeled using a sigmoid model. Based on this model, 50% disease incidence is reached when 656, 543, and 550 degree days are accumulated for the cultivars Vandal-Cliche, Marquette, and Vidal, respectively. These results suggest that the risk of anthracnose development is high from bud-break to fruit set, and on newly emerged leaves either early in the season or following pruning. More knowledge on anthracnose epidemiology is needed, but these results could be used to improve timing of fungicide applications and pruning activities.

Influence of Leaf Wetness Duration and Temperature on Infection of Grape Leaves by Elsinoë ampelina under Controlled and Vineyard Conditions.

On susceptible varieties, indirect damage to vines infected by Elsinoë ampelina range from reduced vigor to complete defoliation while, on berries, damage ranges from reduced quality to complete yield loss. Limited knowledge about the relationship between weather conditions and infection makes anthracnose management difficult and favors routine application of fungicides. The influence of leaf wetness duration and temperature on infection of grape leaves by E. ampelina was studied under both controlled and vineyard conditions. For the controlled conditions experiments, the five youngest leaves of potted vines (Vidal) were inoculated with a conidia suspension and exposed to combinations of six leaf wetness durations (from 0 to 24 h) and six constant temperatures (from 5 to 30°C). A week after each preset infection period, the percent leaf area diseased (PLAD) was assessed. At 5°C, regardless of the leaf wetness duration, no disease developed. At 10 and at 15 to 30°C, the minimum leaf wetness durations were 4 and 6 h, respectively. Above the minimum wetness duration, at temperatures from 10 to 30°C, PLAD increased linearly, with increasing leaf wetness up to 12 h, and then at a lower rate from 12 to 24 h. The optimal temperature for infection was 25°C. Relative infection was modeled as a function of both temperature and wetness duration using a Richards model (R2 = 0.93). The predictive capacity of the model was evaluated with data collected in experimental vineyard plots exposed to natural wetness durations or artificial wetness durations created using sprinklers. In total, 264 vineyard infection events were used to validate the controlled experiments model. There was a linear relationship between the risk of infection estimated with the model and the observed severity of anthracnose (R2 = 90); however, the model underestimated disease severity. A risk chart was constructed using the model corrected for vineyard observations and three levels of risk, with light, moderate, and severe risks corresponding to ≤5, >5% to ≤25, and >25% leaf area diseased, respectively. Overall, 93.9% of 132 independent observations were correctly classified, with 100, 29.4, and 9.4% of the light, moderate, and severe risks, respectively.