Chocolate spot, caused by Botrytis cinerea and B. fabae is a major disease of faba bean. Disease epidemics can cause significant yield losses, which can result in complete crop loss. To reduce grain yield losses growers are currently advised to adopt integrated disease management strategies that combine paddock rotation, avoiding highly susceptible varieties, sowing clean seed and applying foliar fungicides prior to rain events. To improve disease management for faba bean growers this study investigated variety selection and seven fungicide strategies for chocolate spot control.
Field experiments were conducted during 2020 in five locations in Victoria (Horsham, Gymbowen, Lake Linlithgow, Lake Bolac and Yarrawonga). Experiments were designed as a split plot with four replicate blocks. Fungicide treatments were randomised to main plots and cultivars to subplots.. Four varieties with differing resistance; PBA Amberley (moderately resistant to moderately susceptible (MRMS)), PBA Samira (moderately susceptible (MS)), PBA Bendoc (susceptible (S)) and Fiesta (S) were compared against seven fungicide strategies. Fungicide strategies were; 1) no control, 2) a complete control (regular fungicides to minimise disease), 3) Carbendazim at canopy closure, 4) Procymidone at early flowering, 5) Tebuconazole + Azoxystrobin at early flowering, 6) Bixafen + Prothioconazole at early flowering, and 7) Fludioxonil + Pydiflumetofen at early flowering. All treatments except no fungicides received a 4-node Tebuconazole application. At Lake Linlithgow and Lake Bolac disease pressure was high, so an additional three fungicides were applied (Carbendazim and Procymidone in rotation) to all treatments except the no control. Disease severity was visually assessed as a percentage leaf area affected (% LAA) in season and grain yield was measured at crop maturity.
At Horsham, Gymbowen and Yarrawonga, minimal to no disease detection was recorded. At Lake Bolac and Lake Linlithgow, the maximum disease severities were 75% and 100% LAA, respectively in the no control treatment. As hypothesised, based on resistance ratings in both experiments the susceptible variety PBA Bendoc showed the greatest disease severity followed by Fiesta, PBA Samira and then PBA Amberley. All fungicide treatments significantly reduced disease severity compared to the ‘no control’ treatment at both locations (P<0.05). A significant interaction (P<0.05) between fungicide treatment and variety was recorded at Lake Linlithgow. At both sites, only the Fludioxonil + Pydiflumetofen and Carbendazim treatments provided disease control (both disease severity and grain yield) that was not significantly (P>0.05) different to the ‘complete control’ treatment. The improved resistance in PBA Amberley compared to PBA Bendoc led to reduced grain yield losses in 'no control' treatments. At Lake Bolac and Lake Linlithgow, PBA Amberley experienced 32% and 51% grain yield losses, respectively, versus 43% and 79% for PBA Bendoc.
These results highlight the environmental dependence of crop response to fungicides and emphasise the importance of integrated disease management, particularly varietal resistance, to prevent grain yield losses due to Chocolate spot in faba bean. These fungicide treatments also demonstrated the benefits of newer dual active fungicides such as Fludioxonil + Pydiflumetofen. However, older single active chemistries like Carbendazim, when applied at canopy closure, were just as effective.