Black rot of grapevine: towards the construction of new risk indicators based on primary inoculum, aerial spores, and modelling

Abstract: To better predict and control the risk of black rot, we set up a sentinel system for the 2024-25 seasons. This system was based on calculating infection and incubation index; scoring the infection of trap plants at an inoculum stock and at a distant plot; monitoring the maturation of the primary inoculum; trapping aerial spores. The trap plants and spore traps showed the release of inoculum over time, with variable peaks ranging from late April to mid-May, depending on the year. Consecutive infection events with an identical infection index can generate highly different infection levels, depending on cyclisation of inoculum release and/or production, as well as the location of the trap plant within the field. These results highlight the need to consider the Molitor index alongside the level of inoculum released. Our study of perithecium development revealed a sharp increase in the proportion of mature perithecia between early and mid-April 2024 and between late March and mid-April 2025. These periods presented a high risk of infection under favourable climatic conditions. Spore trap analysis with microscopy and primary inoculum maturation studies need to be improved for high-throughput use in risk prediction.
Epidemics can start on leaves very early, just after bud break, even with low ascospore
detection, if conditions for infection are favourable. Calculating the incubation period with
minimum and maximum thresholds of 7 and 26 °C, seems to be very robust. This enables us to accurately identify and predict the end of the incubation period on both leaves and bunches. Damage to bunches can be severe when the onset of leaf symptoms coincides with a sensitive phenological stage and rain, even with a low infection index. Calculating the timing of events and the end of the incubation period should facilitate epidemic management, particularly with regard to more sustainable inoculum control, e. g., diseased leaf removal. We demonstrated the efficacy of removing diseased leaves, even under favourable conditions for infection on bunches (IM 218) with a decrease of disease severity on bunches of 52 %. The role of secondary inoculum originating from leaves appears particularly critical for bunch damage.