*2.4. VOCs and the Inhibition of Plants Diseases and Insect Pests* 2.4.1. Inhibition of Plants Disease

VOCs not only have an inhibitory effect on plants, but also on pathogenic bacteria. A number of experimental trials showed that leaves' VOCs inhibited germination and the growth of plant pathogens and had stronger activity than commercial fungicides [92–94]. The essential oil of oregano triggers the expression of hundreds of genes involved in the grapevine immune system, so it can prevent *Plasmopara viticola* infection in grapevine (*Vitis Vinifera*) and primes plant immunity mechanisms [95]. The inhibition rate of volatile allelochemicals from leaves of *Ocimum adscendens* to 29 different kinds of mycelium was over 98%, and the inhibition rate of mycelium growth of Aspergillus reached 100%; especially, the activity of *A. lavus* was 10~100 times higher than another five commercial fungicides [96]. Chaturvedi et al. found that volatiles released by *Adenocalymma allicea* could effectively control leaf spot in rice and kill the pathogenic fungus *Drechslera oryzae*. When volatile oil obtained from *A.alicea* plants compared with the activity of the synthetic fungicides blitox-50 and m-45, the activity of volatile oil to *D. oryzae* was up to four times higher than those synthetic fungicides [97]. In particular, this volatile oil had no harmful effect on the growth and development of rice seedlings. The volatile oil released from the leaves of the same genus *Ocimum basilicum* also inhibited the growth of other fungal mycelia by more than 85%, and the dosage was only 1/4 of the commercial fungicide [98]. Therefore, using VOCs to replace commercial fungicide is no longer a dream. Phenols are a kind of allelochemicals that have been concerned and studied. Eugenol is an important class of phenolic allelochemicals. Cloves have a long history of use, and the main volatile component eugenol has a strong inhibitory effect on fungi and bacteria [99,100].

In a recent study, Quintana-Rodriguez et al. performed a screening on the efficacy of 22 VOCs, which were known to be volatilized from infected plants leaves, against the fungal pathogens *Colletotrichum lindemuthianum*, *Fusarium oxysporum*, and *Botrytis cinerea*. The work results showed that nonanal, (+)-carvone, citral, trans-2-decenal, L-linalool, nerolidol, and eugenol significantly inhibited the growth of the three fungal species, and eugenol had the most active among them. Therefore, the VOCs of plants have the disease resistance function [101].

In addition to eugenol, there are many VOCs that can also inhibit pathogens. DADS is a volatile organosulfur compound derived from garlic (*Allium sativum* L.) bulbs, and it is known as an allelochemical because of the potential allelopathy of garlic. A large number of study results show that it has a strong inhibitory effect on a variety of pathogenic bacteria [102]. In a recent study, Yang et al. demonstrated that the VOCs (DADS) from green garlic (*Allium sativum* L.) increased the accumulation of H2O2 and the disease resistance of cucumber [103]. Sekine et al. reported that other VOCs such as cuminaldehyde and *p*-cymene also have been demonstrated to possession antifungal activity against *B. cinerea*, *F. oxysporum*, *Verticillium dahliae*, and *Alternaria mali* [104]. According to the work of Mandal and Mandal, linalool, a substance with antifungal and antioxidant potential, was found in the volatile oil of coriander (*Coriandrum sativum* L.) [105].

## 2.4.2. Inhibition of Plants Insect Pests

Numerous studies showed that plants not only produce toxins and hormones directly in response to insect feeding, but also release VOCs to attract predators [106,107]. A class of VOCs produced as a response to herbivore attacking are mainly terpenoids, predominantly monoterpenoids, and sesquiterpenoids [107], and they benefited the host plant by interacting with herbivores. For example, when the larva of *Spodoptera exigua* Hübner. were feeding on corn, the corn released volatile terpenoids to attract the parasitic wasp, which was the natural enemy of *S. exigua*. If it was not mechanically damaged, the corn did not release VOCs to attract the natural enemy. Further studies found that the oral saliva of the beet moth contained volicitin, when the beet moth bit corn, its messenger jasmonic acid derivatives were activated by volicitin to release terpenoids to lure *Cotesia marginiventris* Cresson [108]. Kessler and Baldwin found that wild tobacco could release VOCs to attract mealoptera, the natural predators of caterpillars, when caterpillars ate them. Additionally, the VOCs released by wild tobacco also could prevent caterpillars from laying eggs on the leaves [21].

Under natural conditions, plants can also achieve effective control of related pests by releasing volatile substances to attract natural enemies. It is confirmed that the indirect chemical defense of plants by releasing VOCs to attract natural enemies is a chemical relationship in nature. *Ageratum conyzoides* L. released volatile terpenoid into the air by stalks, leaves, and flowers to attract predatory mites (*Amblyseius* SPP.) and maintain their population density [26]. Predatory mites are the most effective natural enemies of red mites (*Panonychus citri*). So, the population density of red mites was reduced [26]. Degenhardt et al. found that VOCs from corn root (E)-b-caryophyllene attracted insectkilling nematodes to control a major pest [109]. Therefore, the sustainable control of pests and diseases can be achieved through the natural chemical mechanisms that exist in the field to regulate plant–organism interactions. As a natural fungicide, VOCs have no harmful effect on plant growth and development, and using VOCs is a more environmentally friendly and economical way to kill bacteria.
