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Moso bamboo (Phyllostachys heterocycla f. pubescens) is an abundant but underused biomass, for which measures are required to promote its utilization. To examine whether Moso bamboo extracts can be utilized as natural pesticides for plant protection, the inhibitory effects of various extracts on phytopathogenic bacterial and fungal growth were investigated by in vitro assays. Some ethanol extracts from leaf, branch, outer culm, inner culm, and knot exhibited significant but weak effects of inhibiting the growth of two Gram-negative bacteria (Pseudomonas syringae pv. actinidiae biovar 3 and Erwinia chrysanthemi) and two fungi (Glomerella cingulata and Trichoderma harzianum), whereas no hot water extract exhibited antimicrobial activities. Super-heated steam (SHS) extracts, particularly extracted from the culm at 210 °C, markedly inhibited the growth of two Gram-negative bacteria (P. syringae pv. actinidiae biovar 3 and E. chrysanthemi) and three fungi (Botrytis cinerea, G. cingulata, and T. harzianum) compared with ethanol extracts. Compared with bamboo vinegar, SHS extracts exhibited equal antibacterial and strong antifungal activities. Gas chromatography–mass spectrometry analysis revealed that the SHS extract contained acetic acid, propanoic acid, furfural, and small amounts of phenolic compounds (phenol, guaiacol, etc.). An acetic acid dose (approximately 120,000 ppm) equivalent to its concentration (113,327 ppm) in SHS extracts inhibited phytopathogen growth at a level comparable with that of SHS extracts, suggesting that acetic acid is mainly responsible for the antimicrobial activity of SHS extracts. Additionally, pH-increased SHS extracts did not greatly inhibit fungal growth, indicating that the antifungal effects are pH dependent. Thus, it is more effective to use SHS for culm extraction for utilizing Moso bamboo biomass as a natural pesticide.