Introduction

Plants produce more than 400,000 kinds of bioactive components, such as alkaloids, organic acids, flavonoids, phenols, and plant essential oils, most of which have anthelmintic, insecticidal, antifungal, or antibacterial activities (Yao et al., 2017). These bioactive compounds have a broad spectrum of target organisms, are safe for non-target organisms, and are characterized by low toxicity, low residue, easy degradation, and no target resistance (He et al., 2006; Yoon et al., 2013; Zhang et al., 2013). Plant bioactive components are a key research topic for controlling many plant diseases and developing new botanical pesticides, with prospects of wide exploitation and utilization (Li et al., 2017; Tang et al., 2018). Potato glycoalkaloids are steroids produced as secondary metabolites by potatoes (Guo et al., 2017). The main components of potato glycoalkaloids are α-solanine and α-chaconine (Zhao et al., 2013), which represent more than 95% of the total glycoalkaloids (Kodamatani et al., 2005). Previous studies have shown that potato glycoalkaloids have a wide range of biological activities (Qiao, 2017) and can inhibit infection or growth of fungi, bacteria, viruses, and other plant pathogenic microorganisms, prevent insects from feeding or harming plants (Friedman, 2004; Liang et al., 2017), exert protective effects on plants, and possess important medicinal value (Zhao et al., 2013). Fewell et al. (1993) reported that α-solanine and α-chaconine in potatoes can inhibit the growth of fungi such as Alternaria brassicicola , Ascobolus crenulatus ,Rhizoctonia solani , and Phoma medicaginis (Fewell and Roddick, 1993). Furthermore, Zhao et al. (2013) showed that potato glycoalkaloids have high inhibitory effects on Alternaria porriand Cercosporella brassicae , and Ombra et al. (2014) found that potato extract had antibacterial activity against Bacillus cereus , Escherichia coli , and Pseudomonas aeruginosaunder in vitro conditions.
Fusarium is one of the most important fungi in nature, and can survive in soil and plants during winter and summer, with wide distribution, diverse hosts, strong resistance, and rapid growth and reproduction (Du, 2017). Fusarium solani is one of the most common Fusarium spp., which infects the host vascular tissue, destroys its conducting tissue, and produces toxins that harm crops during growth, development, and metabolism. To date, F. solani is one of the most difficult plant pathogens to control in production, and can cause root diseases in various economic crops worldwide, such as white mulberry (Zhang, 2013), Chinese angelica (Zhao et al., 2012), walnut (Zheng et al., 2016), medlar (Chen et al., 2017), wild pepper (Li et al., 2016), and pear (Tang et al., 2017). Fusarium solani can also cause fruit rot (Ramdial and Rampersad, 2010) and deformity (Zhao et al., 2018), resulting in wilting and death of crops, affecting yield and quality, and producing huge economic losses. Currently, F. solani control depends on chemical fungicides such as carbendazim, mancozeb, thiram, cymoxanil–mancozeb, Xinjunan acetate, or pyrazole-kresoxim-methyl (Chen et al., 2017; Tang et al., 2017; Wang et al., 2014). However, long-term usage of chemical fungicides can result in pesticide residues, environmental pollution, and disease resistance, leading to a series of adverse effects (Yao et al., 2017). Therefore, developing alternatives for chemical fungicides using active substances in plants, such as botanical pesticides, is urgently needed.
Currently, studies on control of F. solani disease using plant bioactive compounds as well as the antifungal mechanism of these compounds are limited. Zhao et al. (2012) showed that the growth inhibitory rate of aqueous extracts of berberine (root), licorice (root), and burdock (root) at a concentration of 0.2 g∙mL−1 on F. solani causing root rot inAngelica sinensis exceeded 50%, also inhibiting spore germination to some extent. Zhang et al. (2013) demonstrated that the volatiles and extracts of onions, scallions, garlics, and leeks had obvious inhibitory effects on F. solani . Furthermore, Zhao et al. (2009) revealed that the water extracts of nine plants – including wheat (whole plant) and corn (root, stem, and leaf) – had a high inhibitory effect on F. solani growth.
Potato glycoalkaloid extract has been noted to have some inhibitory effect on three economic forest pathogenic fungi: F. solani ,Capnodium leaophilum , and Marssonina juglandis . Notably, the inhibitory effect on F. solani was the strongest (Duo et al., 2017). Therefore, in the present study, the effects of potato glycoalkaloid extract on the ultrastructure, cell membrane permeability, and contents of reducing sugar, soluble sugar, soluble protein, and mycelial fat of F. solani were examined, and the inhibition mechanism was preliminarily determined. The results could provide a theoretical basis for prevention and control of economic forest diseases as well as for the development and utilization of plant-derived fungicides.