This study explored the chemical compositions of garlic essential oil, the inhibitory activity of garlic essential oil and diallyl disulfide (DADS) against Phytophthora nicotianae, and the effects on mycelial plasma membrane permeability and P. nicotianae inhibition. In total, 29 compounds were detected in garlic essential oil, of which 26 were detected by gas chromatography‒mass spectrometry (GC-MS) and 21 by headspace solid-phase microextraction (HS-SPME) GC-MS. DADS (60.12% and 19.09%) and trisulfide di-2-propenyl (14.18% and 17.98%) were the major components identified by HS-SPME GC-MS and GC-MS analysis, respectively. Half-inhibitory concentration (Ec50, antagonism) and minimum inhibitory concentration (MIC, fumigation) of DADS against P. nicotianae were 150.83 μL/L and 20 μL/L, respectively, while Ec50 of garlic essential oil was 1108.25 μL/L. Mycelial membrane permeability gradually increased in a concentration-dependent manner, and cell death increased at 450 μL/L DADS. Furthermore, DADS treatment significantly reduced the incidence of tobacco black shank and the number of P. nicotianae pathogens in rhizosphere soil. DADS also promoted root development of tobacco seedlings at low concentrations, which was inhibited at high concentrations. Therefore, DADS may play an important role in the antifungal effect against P. nicotianae by destroying mycelial cell membrane integrity, causing an increase in cell membrane permeability, and leading to cell death.

The composition and allelopathy to Phytophthora nicotianae (the causal agent of tobacco black shank disease) of root exudates from a resistant tobacco (Nicotiana tabacum L.) cultivar Gexin 3, a susceptible cultivar Xiaohuangjin 1025 and their reciprocal grafts were investigated. Grafting with disease-resistant rootstock could improve resistance to black shank; this is closely related to the allelopathy of root exudates. The root exudates from the resistant cultivar inhibited the growth of P. nicotianae, while those from the susceptible cultivar promoted the growth; the grafting varieties had intermediate properties. The root exudate composition differed among cultivars. Gexin 3 was rich in esters and fatty acids, while Xiaohuangjin 1025 contained more hydrocarbons and phenolic acids. The composition of root exudates of grafted cultivars as well as their allelopathy to P. nicotianae were altered, and tended to be close to the composition of cultivar used as rootstock. Eugenol, 4-tert-butylphenol, mono (2-ethylhexyl) phthalate, 4-hydroxybenzoic acid, 2,6-di-tert-butylphenol, dipropyl phthalate, and methyl myristate were identified as the main compounds contributing to inhibitory properties of root exudates. Sorbitol was suggested to play a role in disease induction. Overall, rootstock–scion interaction affected the composition of tobacco root exudates, which may be attributed to the different disease resistance among grafted plants, rootstock and scion.

The present study evaluated the fumigant effect and potential mechanisms of Chinese prickly ash seed kernel (PSK) against tobacco black shank caused by Phytophthora nicotianae Breda de Haan. The results showed that PSK fumigant increased the soil organic matter, hydrolysable nitrogen, available potassium, and total phosphorus significantly, while the pH was decreased. Application of 2% PSK promoted plant growth, with higher plant height, root length, and dry weight compared to those in the treatment with no PSK. Both in vitro and in vivo tests indicated that PSK fumigant effectively inhibited mycelial growth of P. nicotianae, and significantly reduced the severity of tobacco black shank. Based on the analysis of volatiles in PSK by headspace solid-phase microextraction (HS-SPME) gas chromatography-mass spectrometry (GC-MS), 28 components were identified, among which monoterpenes were the major components (accounting for 79.46% of peak area). Among the six top components, linalool, piperitone, sabinene, and limonene exhibited strong mycelial inhibition of P. nicotianae. Linalool was the most effective with an IC₅₀ value of 18.03 μL/L. Results of GC-MS revealed that the main components of PSK extract were cis-Vaccenic acid (40.81%), n-Hexadecanoic acid (15.67%), 9,12-Octadecadienoic acid (Z,Z)- (15.25%), Ethanone-1-(2-hydroxy-4,6-dimethoxyphenyl)- (10.32%), cis-9-Hexadecenoic acid (6.21%), and Psilocin (11.75%). Among them, Ethanone-1-(2-hydroxy-4,6-dimethoxyphenyl)- and 9,12-Octadecadienoic acid (Z,Z)- have an obvious inhibitory effect on P. nicotianae. Additionally, changes in soil bacterial and fungal communities were observed. The relative abundance of some microbes, such as Pseudomonas, Azospirillum, Thermomonas, Glutamicibacter, Dyella, and Trichoderma, increased, which have many beneficial microbes in these genera, indicating that microbial community shift could be one of the important outcomes of the PSK fumigant effect. In conclusion, PSK could be a new bio-fumigation material against plant soilborne pathogens, which can function as both a fungicide and fertilizer.