In Belgium, lettuce and chicory production has largely shifted from soil-based to hydroponic systems due to the presence severe soil-borne diseases. While hydroponics improves efficiency, water recirculation facilitates rapid dissemination of oomycete pathogens. Since 2017, Phytophthora cryptogea has emerged as a major threat in Belgian hydroponics, causing root rot and annual losses of up to €50,000 and €380,000 per hectare in lettuce and chicory, respectively. Co-occurring Pythium spp. further complicate disease dynamics. Given the limited efficacy of chemical control, sustainable alternatives such as biological control organisms (BCOs) within integrated pest management (IPM) are urgently needed. This dissertation first presents a meta-analysis of 49 studies (355 BCO treatments) evaluating factors affecting BCO efficacy against Phytophthora root rot in annual crops. Multivariate analyses (FAMD, HCPC) showed that the BCO application method was the primary determinant of efficacy: root and soil applications outperformed seed treatments, and bacterial BCOs were more effective when applied with their specialized metabolites. Subsequently, commercial BCOs were evaluated against P. cryptogea in hydroponic chicory forcing systems. Trichoderma- and Bacillus-based products were tested under laboratory and small-scale water recirculating conditions. Bacillus velezensis QST713 (Serenade®ASO) provided significant disease suppression, but only when water circulation was temporarily halted post-application, highlighting the importance of system-adapted application protocols. In hydroponic lettuce cultivation, Serenade®ASO again showed the highest efficacy. Comparative assays with B. velezensis QST713 and GA1 demonstrated that disease suppression depended on the activity of specialized metabolites rather than living cells. qPCR analyses revealed that these metabolites enhanced bacterial root colonization while suppressing pathogen establishment. Interactions between Pythium sp. and P. cryptogea were also investigated. Pythium became pathogenic only at high inoculum densities. At intermediate, non-pathogenic concentrations, Pythium completely suppressed P. cryptogea-induced disease via rapid root colonization and spatial competition, rather than direct antagonism or metabolite-mediated inhibition. Studies using B. velezensis GA1 mutants revealed that bacilysin mediated in vitro mycelial inhibition of P. cryptogea, whereas cyclic lipopeptides were essential for in planta protection, likely by inhibiting cyst germination. BCO efficacy was also influenced by preculture conditions. Finally, Pseudomonas spp. were evaluated as potential BCOs, with strong antagonism against oomycetes driven by specialized metabolites such as sessilin and tolaasin. Overall, this work demonstrates that effective biocontrol of P. cryptogea in hydroponic lettuce and chicory is metabolite-driven and highly dependent on optimized application strategies and shows that the use of biological control organisms represent a promising control tool within an IPM framework.
Kilian Van Loocke (Wed,) studied this question.