Biological and Chemical Resistance Inducers Approaches for Controlling Foliar Diseases of Some Vegetables under Protected Cultivation System

Greenhouse operations have been viewed by many as a means to diversify and improve farm income. These new growers are trying to take advantage of vegetable crop production which, in general, has higher return per unit area than agronomic crops. In addition, greenhouse vegetable growers have recently been able to benefit from the increased demand for specialty horticultural crops, a category which encompasses greenhouse vegetables [1,2]. Vegetables, i.e. Sweet pepper, Tomato, Cucumber, Cantaloupe which grown under protected cultivation system are represent Egypt's largest national crops export. Plant diseases represent an important role of growing vegetables under protected cultivation system and considered a serious problem and limiting factor for their production. Vegetables production in Egypt, is threatened by a large number of fungal, bacterial and viral diseases as well as by pests and insects vectors viral diseases which cause considerable loss in Egyptian quantitative and qualitative vegetables yield. Overall, most vegetable crops under protected cultivation system suffered losses due to the favorable conditions for most diseases as high humidity and temperature. The most prevalent fungal and bacterial diseases on vegetables continue to plague growers because they are difficult to prevent, and remedial disease management tools are generally suppressive at best. Moreover, plant diseases continue to cause severe damage to most agricultural crops during different stages of plant growth resulting in heavy losses of both yield and quality. Considering the regularity with which most serious diseases of crop plants appear in an area year after year, the rapid spread of most plant disease, and the difficulty of curing a disease after it has begun to develop.


Introduction
Greenhouse operations have been viewed by many as a means to diversify and improve farm income. These new growers are trying to take advantage of vegetable crop production which, in general, has higher return per unit area than agronomic crops. In addition, greenhouse vegetable growers have recently been able to benefit from the increased demand for specialty horticultural crops, a category which encompasses greenhouse vegetables [1,2]. Vegetables, i.e. Sweet pepper, Tomato, Cucumber, Cantaloupe which grown under protected cultivation system are represent Egypt's largest national crops export. Plant diseases represent an important role of growing vegetables under protected cultivation system and considered a serious problem and limiting factor for their production. Vegetables production in Egypt, is threatened by a large number of fungal, bacterial and viral diseases as well as by pests and insects vectors viral diseases which cause considerable loss in Egyptian quantitative and qualitative vegetables yield. Overall, most vegetable crops under protected cultivation system suffered losses due to the favorable conditions for most diseases as high humidity and temperature. The most prevalent fungal and bacterial diseases on vegetables continue to plague growers because they are difficult to prevent, and remedial disease management tools are generally suppressive at best. Moreover, plant diseases continue to cause severe damage to most agricultural crops during different stages of plant growth resulting in heavy losses of both yield and quality. Considering the regularity with which most serious diseases of crop plants appear in an area year after year, the rapid spread of most plant disease, and the difficulty of curing a disease after it has begun to develop.
So, it is easy to understand why almost all control methods are aimed at protection plant from becoming diseased rather than at curing them after they have become diseased. As a matter of fact, few infectious plant diseases can be satisfactory controlled in the field by therapeutic means, although certain diseases can be cured under managed environmental conditions. Furthermore, some of environmental factors i.e. direct sun, ultraviolet light, and the constant changes in temperature also play an important role in overall natural pest control often obtained under open conditions. The greenhouse literally could protects its plants and consequently their respective pests from these environmental conditions [1,3,4].
The regional problem being addressed is the development of a scientifically and environmentally sound scheme for the sustainable production of pathogen-free vegetables to meet domestic requirements and export market opportunities for Egypt. From an historical perspective, to date, production of pathogen-free vegetable seedlings and commercial vegetable crops have been heavily dependent upon the use (often overuse) of pesticides and other inputs which cause important adverse effects on the environment. Experts agree that this is especially true in the case of vegetables. On the other hand, there are serious legal and liability situations that a greenhouse grower could be faced with when using toxic substances in enclosed areas. Under greenhouse conditions the use of pesticides could cause highly volatile toxic vapors, as well as respond differently in numerous other chemical and physical fashions, and thus are considered a potential hazard to the grower [5][6][7].
Vegetables growing under protected cultivation system occupied about 24.000 Hectares in Egypt [8]. The most recorded serious foliar diseases attacked vegetables growing in plastic houses are Powdery and Downy mildews as well as Early and Late blights causing large yield losses in a number of crops [9][10][11].
The exceptionally long survival of vegetables disease pathogens along with the susceptibility of different vegetables cultivars rendered the issue of control of a paramount importance. The wide and indiscriminate use of chemical fungicides has been the cause of the appearance of resistant plant pathogens, leading to the occurrence of serious diseases. Therefore, there is an increasing interest to obtain alternative antimicrobial agents for using in plant disease control systems.
As control measures, some cultural practices, i.e. crop rotation, sowing date, fertilizers and irrigation were tried by many investigators but they failed to provide satisfactory control against plant pathogens. Regarding chemical control, the phytopathogens are not sensitive to selective pesticides as most pesticides have its own mode of action. While many plant diseases have been controlled, in part, by use of chemical pesticides, therefore alternatives to the use of chemicals would be of value. Plant associated microbes used as bio-control agents can play a role in reducing losses to such diseases. Biological control should play a major role in reducing the population of plant pathogens with management systems. Biological control of plant pathogens with antagonists has been the subject of numerous reviews and several books. Promising antagonists that may be useful for controlling soilborne plant pathogens were also reported. Researchers from all over the world paid more attention towards the development of alternative methods which are, by definition, safe in the environment, non-toxic to humans and animals and are rapidly biodegradable, one such strategy is the use of bio-control agents to control fungal plant diseases. There are several antagonistic microorganisms recorded to be promising candidate and effective for controlling wide range of plant pathogens. Among these bio-agents, species of the genus Trichoderma [12], strains Pseudomonas fluorescens PCL1751 and P. putida PCL1760 [13], and Bacillus spp. [14].
A successful disease-control program could involve generally requires the application of several control measures [15]. Generally, IPM is regarded as the use of environmentally safe practices to reduce the disease incidence and development or use of multiple control tactics integrated into a single pest control strategy [16]. The use of different approach in plant disease control, i.e., bio-control agents, plant extracts and natural compounds were used as an IPM program to control powdery mildew of greenhouse crops [15,17].
Among other control measures the use of compound that induce a systemic plant resistance which were successfully used against several plant diseases incidence affected either plant root or shoot systems. There are several fungicides alternatives commercially used for induction of plant resistance against viruses, bacteria, and fungal infections. In this concern, salts have been previously studied as foliar applied control agents for powdery mildews on various horticultural crops, e.g. cucumber, grape, nectarine, mango, and rose can be reduced through foliar applications of phosphate and potassium salts [18][19][20][21]. Also, reducing the severity of powdery mildew on E. japonica and pumpkin is achieved by application with potassium bicarbonate [22,23]. Sodium bicarbonate or calcium chloride significantly reduced the early blight incidence and severity of tomato plants in pot experiment under artificial infestation with pathogenic fungus [24]. They added that Calcium chloride proved higher efficacy for reducing both disease incidence and severity than that of sodium bicarbonate when applied either alone or combined with Saccharomyces cerevisiae. Induction of local and systemic resistance to powdery mildew in cucumber using phosphate foliar applications [21,25]. Moreover, using calcium administered as plant nutrient has been reported to be important for resistance to bacterial wilt [26] resistance in tomato.
The use of pesticides alternatives such as induces resistance compounds and bio-agent microbes to control plant disease and enhance crop production are desirable for the following reasons: 1) chemical pesticides are being severely restricted; 2) the public is demanding reduced pesticide use; and 3) pesticides alternatives are effectively and more safely used.

Biological Control Approaches
A greenhouse study [27] was conducted to evaluate the efficacy of plant spraying with different bio-control agents against incidence of some vegetables foliar diseases. Powdery and Downy mildews of Cucumber, Cantaloupe, Pepper and Early, Late blights of Tomato are subject for this study. Foliar spraying with tested bio-agents, i.e. T. harzianum, T. viride, B. subtilis, P. fluorescens and S. cerevisiae were applied twice with two weeks intervals starting one week from transplanting. Foliar artificial infestations with Sphaerotheca fuliginea and Peronoplasmopara cubensis the causal fungi of Cucumber, Cantaloupe and Pepper Powdery and Downy mildews, respectively as well as Alternaria solani the causal fungi of Tomato Late blight was carried out as spraying of pathogen suspension, one week after the second antagonistic foliar application. Phytophthora infestanse the causal of Tomato Early blight was applied as soil drench through soil irrigation with fungal suspension.
As a result of this experiment [27] all applied bio-agents significantly reduced the recorded foliar diseases comparing with untreated control. Moreover, application with either T. harzianum or B. subtilis had superior reduction effect on diseases incidence comparing with the other applied bio-agents. Moderate reduction in Powdery mildew incidence was achieved by applied other bio-agent treatments, T. viride, P. fluorescens and S. cerevisiae. Similar trend was also observed for Downy mildew, Early and Late blights.
In this concern several works which conducted with biocontrol applications against plant diseases control reported the efficacy of biological control of vegetable foliar diseases by different microorganisms. Biological control using natural products or antagonistic microorganisms in order to reduce the pesticides use and decline of their residues on agriculture products proved to be successful for controlling various plant pathogens in many countries [28]. It is still not expensive and is easy in application however it can serve as the best control measure under restricted conditions.
In addition, its application is safe, un-hazardous for human and avoids environmental pollution [29]. In this regard, Paulitz and Bélanger [30] reviewed that during the past ten years, over 80 biocontrol products have been marketed worldwide. A large percentage of interesting alternative to fungicide application for plant disease control involves the use of some organic and inorganic salts with antimicrobial properties generally used in food processing and preservation. Selected organic and inorganic salts are active antimicrobial agents and have been widely used in the food industry.
Many of these salts are effective against a range of micro-organisms; most of them have low mammalian toxicity and therefore have potential for postharvest disease control. Salt treatments can inhibit plant pathogens or suppress mycotoxin production [42,43]. Also, Ashour [44] found that from field experiments, that spraying cantaloupe plants three times with fungicides in alternation with another three sprays with any of calcium chloride or salicylic acid resulted in significant reduction in the disease severity with significant increase in the fruit yield when compared with unsprayed (check) plants. Furthermore, Sodium and ammonium bicarbonate were shown to inhibit fungal pathogens of fruits, field crops, vegetables, and ornamentals [22]. Also, sodium bicar-bonate applied at room temperature at 2 to 4% reduced blue mold caused by Penicillium italicum in Citrus fruits [45].
Moreover, beneficial effect of two food additives, ammonium molybdate and sodium bicarbonate on antagonistic yeasts for control of brown rot in sweet cherry was evaluated [46]. They found that application of additives improved bio-control of brown rot on sweet cherry fruit under various storage conditions. It is postulated that the enhancement of disease control is directly because of the inhibitory effects of additives on pathogen growth, and indirectly because of the relatively little influence of additives on the growth of antagonistic yeasts.
The efficacy of foliar spray with some plant resistance inducers and/ or bio-agent (S. cerevisiae) treatments under open greenhouse conditions was studied [47]. They reported that all applied spray treatments of tested plant resistance inducers individually or in combination with S. cerevisiae were able to reduce foliar diseases incidence of artificially infested with the diseases causal agents grown Cucumber, Cantaloupe, Tomato and Pepper grown in pots under open greenhouse conditions comparing with untreated check control. Also, data revealed that the most significant reduction in diseases incidence of Cucumber, Cantaloupe and Pepper Powdery, Downy mildews and Tomato Early and Late blights in combined treatments of chemical inducers and S. cerevisiae were (Chitosan+Thyme oil); (Chitosan+Saccharin); (Chitosan+Calcium chloride+S. cerevisiae); (Chitosan+Potassium monohydrogen phosphate); (Saccharin+Potassium monohydrogen phosphate); (Humic & folic+Thyme oil) and (Chitosan+S. cerevisiae) comparing with other applied treatments as well as untreated control.
Under plastic houses conditions, in another study [48] announced highly significant effect was observed at combined treatments, [Potassium bicarbonate+Thyme oil] and [Chitosan+Thyme oil] followed by treatments, [Calcium chloride+S. cerevisiae] and individual treatment [Potassium monohydrogen phosphate], respectively. Moreover, these applied treatments, could suppress both downy and powdery mildews incidence up to 60 days of growing cucumber plants under natural infestation comparing with untreated control which recorded diseases incidence.
In another study [49] under plastic houses conditions, they recorded that the applied treatments caused significant effect on foliar diseases resulted in high reduction in both Early and Late blights diseases incidence comparing with check control. They added that the highest reduction in disease incidence calculated as 100, 100, 85.7% and 100, 100, 76.7% for Early and Late blights at the combined treatment, these have been developed for greenhouse crops. Products containing Trichoderma, Ampelomyces quisqualis, Bacillus, Ulocladium and Pseudomonas are being developed to control the primary foliar diseases, Botrytis and powdery mildew in greenhouses could predominate over chemical pesticides.
In the study of Abdel-Kader et al. [27] spraying vegetables, Cucumber, Cantaloupe, tomato and Pepper with the bio-agents, T. harzianum, T. viride, B. subtilis, P. fluorescens and S. cerevisiae was effectively able to reduce the foliar diseases comparing with untreated control. These results were in agreement with several previous reports. A field experiment [31] was conducted at Rahuri, Maharashtra, India to investigate the efficacy of the culture filtrates of T. viride, T. harzianum, T. hamatum, T. longiflorum and T. koningii and the fungicide carbendazim against the powdery mildew (Leveillula taurica) of guar. They found that all treatments recorded beneficial effects on growth parameters and disease control. Also, Elad [32] recorded that Trichoderma harzianum which can be regarded as a model to demonstrate bio-control under commercial conditions and the mechanisms involved. He added that this bio-control agent controls the foliar pathogens, Botrytis cinerea (gray mold), Pseudoperonospora cubensis (downy mildew), Sclerotinia sclerotiorum (foliar blight) and Sphaerotheca fusca (powdery mildew) in cucumber under commercial greenhouse conditions. Similarly, Deore et al. [33] evaluated culture filtrates of T. viride, T. harzianum, T. hamatum, T. Iongiforum and T. lconlngll for the management of powdery mildew of Cluster bean plants caused by LevelIlula taurlca. They found that culture filtrates of Trichoderma spp. either alone or in combination were found effective against powdery mildew. Also, Trichoderma harzianum T39 (TRICHODEX) spray reduced powdery mildew severity caused by Sphaerotheca fusca on greenhouse cucumber by up to 97% [34,35]. Furthermore, Deore et al. [36] stated that foliar application of Pseudomonas fluorescens combined with a half of the recommended dose of azoxystrobin is of practical significance, since an application of fungicide alone requires three to four further following sprays for an effective control of downy and powdery mildews of cucumber. Also, Yazici et al. [37] suggest that Paenibacillus macerans, Serratia plymuthica, Bacillus coagulans, Serratia marcescens-GC, Bacillus pumilis and Pantoea agglomerans the bacterial isolates as foliar spray reduced the disease severity of early blight significantly when compared with control and have a good potential to be used as biocontrol agents of A. solani in tomato.
The mode of action of biocontrol mechanisms was explained by many investigators. The potential of Bacillus sp. to synthesize a wide variety of metabolites with antifungal activity is known and in recent years it has been a subject of experiments [38,39]. Most of these substances belong to lipopeptides, especially from surfactin, iturin and fengicin classes. Not so much is known about the mechanism of antifungal activity of these substances produced by Bacillus sp. Some of them (iturin and surfactin) are able to modify bacterial surface hydrophobicity and, consequently, microbial adhesion to surfaces to mycelium [38]. Also, antibiotics of the iturin group were found to act upon the sterol present in the cytoplasmic membrane of the fungi [39,40]. A local isolate of S. cerevisiae was reported to have a reduction potential against radial growth of pathogenic fungi Macrophomina phaseolina and Fusarium solani, the cause of root rot diseases in tomatoes and eggplants [41].

Integrated Fungicidal Alternatives and Yeast (S. cerevisiae) Approaches
The use of plant resistance inducers in combination with bioagents was subjected to evaluation in many reports. In this regards, an [Potassium bicarbonate+Thyme oil] followed by treatment, [Chitosan+ Thyme oil] at tomato growth periods of 60, 90 and 120 day, respectively. The other applied treatments, [Calcium chloride+S. cerevisiae] followed by individual treatment [Potassium monohydrogen phosphate] showed similar effect on the diseases incidence throughout the growth period in a lesser effect that at 120 days of plant growth they recorded the lowest reduction in diseases incidence calculated as (68.9, 50.8%); (70.9, 49.6%) for Early and Late blights, in respective order.
In the reviewed studies [27,[47][48][49][50] it was reported that application of different formula of biological and/or chemical plant resistance inducers as foliar spray resulted in reduction of Powdery, Downy mildews of Cucumber, Cantaloupe and Pepper as well as tomato Early blight and Late blight diseases incidence and severity which reflected positively in plant stand and its healthy growth as well as its produced yield. The obtained results showed high efficacy of application plant resistance inducers Calcium chloride, Potassium bicarbonate, Potassium monohydrogen phosphate, Thyme oil, Chitosan, S. cerevisiae as foliar spray treatment during the growing growth season against foliar diseases of tomato plants. In this regards, many investigators studied the influence of various salts on microorganisms. These studies focuses on finding compounds that are safe to human and environment. An alternative to pesticide application is that, it may be possible to utilize a scheme of inducible plant defense which may provide protection against a broad spectrum of disease-causing pathogenic microorganisms. Biological control using microbial antagonists has shown potential as an alternative for natural control of plant pathogens instead of synthetic chemical fungicides [51]. Another alternative control method is given by enhancing natural resistance of plants towards the pathogen. Compounds which are triggering plant's own defense mechanisms are termed elicitors.
Similar reports conducted the efficacy of chemical inducers application individual or combined with bio-agents against plant pathogens were cited in literature.
It was reported [52] that in vitro activity of sodium carbonate salt against germinated or un-germinated spores of P. italicum and P. digitatum was higher than that of bicarbonate salt, although their efficacy was similar in vivo. They suggested that the original toxicity of the treating sodium bicarbonate solution to the spores is altered by inter-actions in wounds with constituents of the rind and the relatively high pH of these solutions has been proposed to be the mode of action. The use of sodium bicarbonate alone to control postharvest decays of fruit has its limitations [45], but it can be combined with other alternative treatments to synthetic fungicides, resulting in the control that is superior to individual treatments alone. For example, sodium bicarbonate was successfully used in combination with bacterial and yeasts bio-control agents to enhance control of postharvest decays on citrus, pome, and stone fruits [52][53][54]. These reports are clearly demonstrated in the present study and show that the application of S. cerevisiae enhanced the control of foliar vegetables diseases when combined with either sodium bicarbonate or calcium chloride spray. In this concern inoculums used in the present work was more effective where it reached (2×10 4 cfu/mL). Many researchers have shown that calcium plays an important role in the inhibition of postharvest decay of fruits [55,56], and in enhancing the efficacy of postharvest bio-control agents [57,58]. Postharvest calcium treatment of apples provided broadspectrum protection against the postharvest pathogens of Penicillium expansum and Botrytis cinerea [59]. The addition of CaCl 2 (2% w/v) to the formulation of the yeast bio-control agent, Candida oleophila, enhanced the ability of this yeast to protect apples against postharvest decay [58]. The efficacy of controlling grey mould and blue mould rots in apples was enhanced when Trichosporon sp. was applied in the presence of CaCl 2 (2% w/v) in an aqueous suspension [60]. In a study of Abdel-Kader et al. [48] it was found that calcium chloride application either alone or combined with other plant resistance inducers and/or S. cerevisiae was able to reduce foliar disease incidence. Applications of Ca contributed to reducing the intensity of white mold on dry beans, but did not affect bean yield in a soil with high amounts of this nutrient. In the USA, found that foliar-applied Ca was found to enhance both disease control and dry bean yield [61]. Gulya and Miller [62] suggested that Ca may be a nutritional supplement that increases plant resistance to white mold. Nutritional effect is particularly noticeable in the case of Ca compounds with high water solubility, like CaCl 2 . It has been suggested that plants resistant to S. sclerotiorum have higher Ca levels than susceptible ones. Also, Paula Júnior et al. [63] recorded that incidence and severity of white mold on dry bean were significantly reduced with application of calcium chloride and calcium silicate.
Saccharin and Chitosan [48] had shown effective influence for reducing foliar diseases when sprayed either alone or in combined treatment. These records were confirmed with previous reports. Saccharin is a metabolite of probenazole and has also been found to induce resistance in a number of host-pathogen systems, including tobacco mosaic virus [64], barley Blumeria graminis f. sp. hordei [65] and broad bean Uromyces fabae [66]. In barley, saccharin did not induce defenses directly, but rather primed plants against infection by the powdery mildew fungus [65]. Also, Saccharin treatment provided systemic protection of barley against the leaf scald pathogen R. secalis. This agrees with previous work which showed protection of barley against powdery mildew [65], broad bean against rust [66] and tobacco against TMV [64]. These data also confirm earlier studies which showed that saccharin treatment primed CAD activity in barley against subsequent challenge with powdery mildew [65]. As for Chiosan, in recent years, the importance of chitosaccha-rides as plant growth promoting and disease control agents has been emphasized [67,68]. CHN (β-1-4 linked N-gluco-samine) has been shown to induce defense responses in different plants [69,70]. Chitosan oligomers was found to induce defense responses in grapevine leaves, as evidenced by an accumulation of stilbene phytoalexins, trans-and cis-resveratrol, ε-viniferins, and piceids, and a stimulation of chitinase and β-1,3glucanase activities [68]. They added that the combination of Chitosan and CuSO 4 increased phytoalexin production. This elicitor capacity of Chitosan and/or CuSO 4 appeared to be associated with an induced protection of grapevine leaves against gray mold and downy mildew diseases. Furthermore, Hadwiger and McBride [71] recorded that chitosan/copper complex retained a presence on the potato leaf surface late and early blights where infection by either A. solani or P. infestans occurs. Also, Chitosan enhanced the accumulation of pathogenesis related-proteins such as ss-1,3-glucanase, chninase and PR14 in treated and upper untreated tomato leaves [72]. Their studies with chitosan against tomato late blight suggested thatchitosan displays dual effects: (a) direct interference in developmental stages of P. infestans and (b) by lesion formation, leading to disease resistance mechanisms. Moreover, several workers suggested two different mechanisms of chitosan molecule and target microorganism interaction: the first is the adsorption of chitosans to cell walls leading to the cell wall covering, membrane disruption and cell leakage; the second is the penetration of chitosans into living cells leading to the inhibition of various enzymes and interference with the synthesis of mRNA and proteins [73][74][75].
On the other hand, Humic acid is a suspension, based on potassium humates, which can be applied successfully in many areas of plant production as a plant growth stimulant or soil conditioner for enhancing natural resistance against plant diseases and pests, stimulation plant growth through increased cell division, as well as optimized uptake of nutrients and water [76]. Furthermore, Abd-El-Kareem [77] reported that bean plants treated with humic acid induced resistance against root rot and Alternaria leaf spot. The role of Humic acid in overcoming the harmful effects of chocolate spot and rust diseases in faba bean plant may be due to the increase in chitinase activity [77] and stimulation plant growth through increased cell division, as well as optimized uptake of nutrients and water [78,79] also, regulate hormone level, improve plant growth and enhance stress tolerance [80].
Humic acid is a suspension, based on potassium humates, which can be applied successfully in many areas of plant production as a plant growth stimulant or soil conditioner for enhancing natural resistance against plant diseases and pests [76] which consequently increase yield of plant. Foliar application of Humic acid consistently enhanced antioxidants such as á-tocopherol, â-carotene, superoxide dis-mutases, and ascorbic acid concentrations in turf grass species [59]. These antioxidant may play a role in the regulation of plant development, flowering and chilling of disease resistance [8,82,83]. The effect of spray application with Ascorbic acid as antioxidant against foliar diseases was also observed in the study of Abdel-Kader et al. [48]. They reported that under artificial infestation with Powdery and Downy mildews pathogens, Cucumber; Cantaloupe and Pepper plants expressed disease incidence as 54.4 and 46.6% in medium, meanwhile when sprayed with Ascorbic acid only or combined with S. cerevisiae revealed diseases infection estimated as 28.8, 25.5%, and 23.3 and 20.0%, in respective order. Similar effect was also recorded concerning Tomato Early and Late blight which recorded as 18.3 and 14.9% in Ascorbic acid only or combined with S. cerevisiae treatments comparing with 33.3% in control treatment, respectively.
Essential oils as natural alternatives that are user friendly and demonstrate low toxicity to humans are desirable to be tested either alone or in combination in the present work. Thyme oil applied alone or in combination showed effective reduction in foliar diseases incidence more than 50%, moreover complete reduction in Tomato Early and Late blights was recorded in Thyme oil combined with either S. cerevisiae or Humic & Folic acid (a mixture). In this regards, several investigators reported the antifungal effect of essential oils. Thyme and Egyptian geranium oils are considered antimycotic natural compounds may be useful for inhibition of mold fungi on wood in service or during storage of building materials [84]. Moreover, Momol et al. [85] had the first report on the use of Thymol for controlling a plant disease under field conditions, which indicated that this compound pro-vided effective control of bacterial wilt on susceptible tomato cultivars. Also, Thymol has been reported to have fungicidal activities and fumigation with thymol has been used for control of postharvest fungal diseases [86,87]. Modes of action of the antibacterial property of thymol appeared to include disruption of bacterial cell membrane integrity by altering protein reactions [88,89].
In general, biological control using microbial antagonists has shown potential as an alternative for natural control of plant pathogens instead of synthetic chemical fungicides [51]. The mode of action of antagonistic yeasts may be competition for space and nutrients [51,90], production of cell-wall lytic enzymes [91], and induction of host resistance [92,93]. In large-scale tests, the use of biological control often needed to be combined with low doses of synthetic fungicides to obtain a level of disease control equivalent to synthetic fungicides [94]. In order to completely eliminate the use of synthetic fungicides, more environmentally friendly and harmless compound (s) should be explored to improve the activity of the antagonist. Selected chemicals such as calcium chloride [58,60], chitosan [95], in combination with biological control agents have been demonstrated to give beneficial effects on control of fruit decay. The control is not total, but in combination with bioactive additives it is possible to obtain the efficacy of the chemical standard [96,97]. Another alternative control method is given by enhancing natural resistance of plants towards the pathogen. Compounds which are triggering plant's own defense mechanisms are termed elicitors. Moreover, the bio-control activity of S. cerevisiae against pathogenic fungi might have possibly resulted from mycoparasitism [98], secretion of lytic enzymes such as β-1,3 glucanase [99] and production of antibiotics [100].

Conclusion
The preliminarily recorded results in the present review showed that foliar spray with bio-agents can have a considerable activity against Powdery and Downy mildews of Cucumber, Cantaloupe, Pepper and Early, Late blights of Tomato under open greenhouse conditions. Their non chemical properties suggest potentials for commercial formulation and application which could suggested as a broad spectrum use against foliar pathogens under plastic greenhouses conditions.
The present findings demonstrate that plant resistance inducers in combination with bio-agents may have important implications for the future use of antagonistic microorganisms on a commercial scale for controlling such diseases especially under protected cultivation regime.
Hence, the objective of this study was to determine if plant resistance inducers and essential oils could provide enhancement effect against downy and powdery mildews as well as early and late blights. Considering their attribute and broad-spectrum activities, successful development of such compounds as antifungal would not only provide a potent tool for control of vegetables foliar diseases, but also could promise success in multipurpose biorational alternatives to conventional fungicides for the management of other plant diseases.