As mentioned above, natural product pesticides have long been a research hotspot for pesticide workers and are widely used to control bacterial and fungal diseases [
99]. Wang and coworkers devoted their research to natural products containing hydrazone structures for many years and discovered a batch of indole alkaloids, toad alkaloids, and echinin alkaloid derivatives with antifungal and antiviral activity (The antiviral activity is described in detail in
Section 4) [
100-
105]. For example, they took indole as the leader and introduced hydrazone structures to synthesize hundreds of hydrazone derivatives with broad-spectrum biological activities (Fig. S9 in Supporting information) and applied them to antifungal, antiviral, and insecticidal aspects. Among them, derivative
58 (Fig. S9) had good antifungal activities against 14 kinds of fungi with inhibitory activity ranging from 62.2% to 97.0% at a concentration of 50 mg/L [
100]. The authors then constructed derivatives
59, 60 and
61 (Fig. S9) based on derivative
58, although their spectrum of antifungal activity was not greatly improved. However, its structure was novel, providing ideas for developing new fungicides (more details for structure-activity relationship analysis of compounds
59-
61 see Section S2 in Supporting information). The aromatic aldehyde-containing compound
62 (Fig. S9) showed a strong inhibitory effect against both
A. solani (67%, 50 mg/L) and
S. sclerotiorum (87%, 50 mg/L). The antifungal activity was better than that of the control chlorothalonil (<50%, 50 mg/L) [
103]. They tried to change indole alkaloids into toad alkaloids and echinopsine with important biological activities and designed and synthesized toad alkaloid derivatives
63 and echinopsine derivatives
64 (Fig. S9) with hydrazone structures. Derivative
63 had good antifungal activity against
S. scltiorum,
R. ceralis, and
B. cinerea (100%, 98%, 71%, 50 mg/L) and was superior to the control chlorothalonil (93%, 45%, 19%, 50 mg/L) [
104]. Compound
64 had high antifungal activities against
P. piricola and
S. sclerotiorum, and the antifungal activity of compound
65 (Fig. S9) containing a substituted phenyl ring was relatively higher than that of the compound containing a heterocyclic ring [
105]. In addition, by integrating the lindenone and hydrazone structures into one molecule, a series of
N-aminomaleimide derivatives
66 (Fig. S9) with good antifungal activity can also be obtained [
106]. The antifungal activity of most compounds against phytopathogenic fungi was much higher than that of precursor lindanone. Among them, compound
67 (Fig. S9) showed broad-spectrum antifungal activities, and its inhibition rate against 11 plant pathogenic fungi, such as
R. cerealis, was more than 50% [
106]. The new hydrazone derivative
68 (Fig. S9, SAR analysis see Section S2) containing the natural bioactive sesquiterpene carabrone structure also has a good control effect on
B. cinerea and
C. arachidicola [
107]. In general, introducing chlorine or bromine atoms into the substituents could enhance the antifungal activity of carrageenan ketone hydrazone derivatives
in vitro and
in vivo [
107]. Compound
69 (Fig. S9), containing an indole group, had good bactericidal activities against three tested bacteria (
Xanthomonas axonopodis pv.
citri, (
Xac);
Xanthomonas oryzae pv.
oryzae, (
Xoo);
Ralstonia solanacearum, (
Rs)) and was superior to the commercial bactericides bismerthiazol and thiodiazole copper [
108]. Recently, the small molecular compound
70 (Fig. S9) designed by Yu with the natural product chromone as the lead compound could be regarded as an effective candidate for bactericide [
109]. It had excellent inhibitory activities against
Xoo,
Xoc and
Xac (half maximal effective concentration (EC
50) values were 8.0, 12.0 and 10.0 mg/L, respectively), which was significantly better than that of commercial bactericide (EC
50 values were 84.0, 151.0 and 145.0 mg/L, respectively) [
109].