Screening for the identification of potential biological control agents that induce systemic acquired resistance in sugar beet

A pool of 11 randomly selected, uncharacterized Bacillus pumilus isolates from sugar beet were evaluated using a high-throughput screen that utilized laboratory-based tests for 2 pathogenesis-related proteins, chitinase and β-1,3-glucanase, and biphasic hydrogen peroxide production. The screen was followed by a glasshouse test for induction of systemic acquired resistance for control of Cercospora leaf spot in sugar beet. These isolates were compared to the known biological control agent, Bacillus mycoides isolate Bac J, and a chemical inducer of resistance, acibenzolar-S-methyl. All laboratory-based screens identified B. pumilus isolates 203-6 and 203-7, that reduced Cercospora leaf spot symptoms by approximately 70%, even when spatially separated from the casual agent, Cercospora beticola. This level of control was similar to B. mycoides isolate Bac J and acibenzolar-S-methyl. In all cases, systemic resistance elicitation was marked by an increase in 2 pathogenesis-related proteins, chitinase and β-1,3-glucanase, and was preceded by biphasic hydrogen peroxide production, also found in incompatible plant–pathogen interactions in which systemic resistance is induced. A combination of glycol chitin and aniline blue plate assays correctly identified all in planta inducers of systemic resistance as measured by control of Cercospora leaf spot in classical challenge assays for systemic acquired resistance without the inclusion of false positive identifications, reducing the workload in subsequent disease challenge assays by nearly 70%.