Biocontrol potential of stem blight in macadamia by Bacillus spp. in plantlets with different scion-rootstock combinations

Fungi from the Botryosphaeriaceae family are causal agents of stem blight and affect macadamia at all developmental stages, being particularly lethal to plantlets. This study evaluated the effectiveness of Bacillus spp. isolates in controlling the disease in macadamia plantlets and the susceptibility of macadamia varieties to Lasiodiplodia pseudotheobromae (Botryosphaeriales: Botryosphaeriaceae), considering rootstock-scion combinations. Three macadamia scion varieties (IAC 4-12B, HAES 333, and HAES 741) were inoculated with three L. pseudotheobromae isolates. The rootstock (HAES 10–14) remained the same. Bacillus spp. isolates were selected for their antagonistic properties and tested for disease control in plantlets. Disease severity varied depending on the rootstock-scion combination and the L. pseudotheobromae isolate. Bacillus velezensis (CMAA1935 and CMAA1936) and B. subtilis (CMAA1934) (Bacillales: Bacillaceae) reduced disease severity, but their efficacy depended on the scion variety. Our findings indicate that resistance to stem blight in grafted plantlets varies with the L. pseudotheobromae isolate and the scion genotype, also influencing disease severity in the rootstock. These results suggest that Bacillus spp. may serve as a complementary tool for managing stem blight, particularly when resistance varies with scion-rootstock combinations.