1.1. Inorganic Fungicides
The most known and used inorganic fungicides are copper and sulfur. Copper and sulfur are presented in numerous forms and are applied in various crops to control important foliar fruit diseases. Various copper and sulfur formulations for organic farming have been authorized by the European Union. Following their biogeochemical cycle, inorganic fungicides can run off to surface water and be adsorbed in the soil and sediments. Copper and sulfur are important trace metals for organisms’ fundamental functions, yet large concentrations can be harmful
[1].
When copper oxide is dissolved in water, the dominant and biocidal oxidation ion is Cu
2+ [2]. Acidity and salinity play an important role in aquatic biota toxicity. High pH values result in a reduction of available hydrogen ions, which leads to copper toxicity. Therefore, copper ions can be attached at aquatic organisms’ cells. Other physicochemical factors that influence the toxicity levels are the dissolved organic matter and dissolved water organic carbon content
[1][3]. As Beck and Saundo-Wilhelmy
[4] have reported, the tendency of sediments to adsorb heavy metals is high, and thus sediments can facilitate the availability of toxic chemicals in the water and aquatic organisms. Some species have a high level of sensitivity to copper, whereas others can efficiently overcome it. Copper is bioaccumulated in fish, decapod crustaceans, and algae and stored in bivalves, barnacles, and aquatic insects. The most sensitive species to copper exposure are cyanobacteria, while coccolithophores and dinoflagellates have a lower sensitivity to copper, and diatoms present resistance to copper
[5].
Copper cannot be degraded in soil but can be accumulated through copper-based degradation compounds occurring in different forms. Copper’s mobility in the soil profile is characterized as medium to low. It was reported that the high concentrations of copper in vineyards soils and groundwater was caused mainly by copper-based fungicide use, negatively affecting water quality and food safety
[6]. Copper residues in soil could cause toxic effects on macro- and microorganisms, adversely influencing the various beneficial interactions in soil, such as pesticide biodegradation, soil structure, nutrition availability for plants, and pathogen resistance
[7]. Element copper is able to cause toxicity to beneficial bacteria and fungi in the environment
[8]. Diaz-Ravina et al.
[9] reported that the microbiocidal activity in vineyard soil can be significantly reduced by high application rates and prolonged use of copper-based pesticides. For instance, high-dose application of Cu-based pesticides can have negative effects on arbuscular mycorrhiza fungi (AMF)
[10]. Schoffer et al.
[11] reported that copper soil pollution is more common in countries or regions that have not enacted regulations for copper-based pesticides applications, which consequently follow only commercial formulation guidelines.
Sulfur can be found in various forms in the environment, such as gas (for example, SO
3) and salt (for example, MgSO
4), which are created through bacterial physiological processes. In sediments and soils, sulfur can be found as a trace element or in an inorganic form. Sulfur can cause toxicity to bacteria and fungi that are beneficial to the environment, which are not considered as crop pests. In addition, it was indicated that sulfur can be phytotoxic to some plants, such as cucurbits, apricots, and raspberries
[12][13]. Kuklinska et al.
[8] also reported that
Vibrio fischeri is sensitive to sulfur exposure. The available information regarding sulfur interaction with organisms, its toxicity threshold, and its environmental fate is limited compared to copper.
1.2. Organic Fungicides
The environmental fate of organic fungicides depends on various physicochemical parameters, such as ionization (pk
a), water solubility, volatility, K
ow, and half-life in soil and water (DT
50). Soil texture, organic carbon content, pH, clay mineral type, dissolved organic matter, and cation exchange capacity also play an essential role in the environmental fate of fungicides, defining processes such as run-off to surface water, adsorption, or leaching. In addition, rainfall, irrigation, microbiological degradation, hydrolysis, photolysis, and application rate could affect fungicide fate
[14][15].
Fungicide residues in surface water (for example, streams, lakes, rivers) and groundwater have been detected by many monitoring studies worldwide. The majority of these studies were focused on a few fungicides of local importance. The extensive (multiple applications and high doses) use of fungicides in specific crops (for example, vineyards, horticulture, orchards and so on) can lead to pollution of nearby natural waters from fungicide residues. Hence, the spatial and temporal distribution of fungicide residues in surface waters varies throughout the year and amongst agroecosystem compartments. Usually, the highest concentrations of curative fungicides are detected during growing or preharvest seasons, whereas preventative fungicides are found at early plant growth stages and during the winter period. Regions planted with grape and tree crops have received high application doses of fungicides, resulting in high detection frequency and high concentrations of fungicides in the ecosystems
[16][17].
The presence of various pesticides has been investigated in vineyard groundwater bodies in northern Italy. The environmental quality standard set by the EU (0.1 μg/L) was exceeded by five fungicides (metalaxyl–M, fluopicolide, penconazole, tetraconazole, and dimetomorph), presenting significantly high concentrations. Τhe maximum concentrations of metalaxyl–M and penconazole were 8.015 μg/L and 18.72 μg/L, respectively
[15]. In addition, a similar monitoring study was conducted in Spanish vineyards, where the detection frequency of metalaxyl, dimethomorph, and penconazole reached 50%. Moreover, the highest concentration was observed for same fungicides (metalaxyl and penconazole)
[18]. These results indicate that the extensive use of fungicides in vineyards can cause an essential surface and groundwater pollution.
Papadakis et al.
[14] conducted a pesticide monitoring study in two river basins in North Greece, with corn, cotton and cereals as the main crops, over a two-year period. Twenty-nine fungicides were detected at least once, while multiple detections (7 to 10 times) of boscalid, diphenylamine, etridiazole, and hexachlorobenzene were also observed. Extremely high concentrations for seven fungicides (azoxystrobin, diphenylamine, etridiazole, propiconazole, tebuconazole, quintozene, and difenoconazole), ranging from 0.153 to 0.819 μg/L, were identified. In the worst-case scenario, the risk quotient index was higher than one for four fungicides. It was showed that fungicides contribute to ecotoxicological risk for river basins.
The presence of 24 fungicides was investigated in the surface water and sediments of a horticulture area in Australia. The agricultural activity of the studied area included tree fruits, bulbs, vineyards, vegetables, and herbs. Although the authors reported that the individual fungicide residues did not pose environmental risks, due to low ecotoxicological endpoints, several fungicides were detected in concentrations above 0.2 μg/L (iprodione, myclobutanil, pyrimethanil, cyproconazole, trifloxystrobin, and fenarimol) and others had a detection frequency ranging between 18 and 36% (myclobutanil, trifloxystrobin, pyrimethanil, difenoconazole, and metalaxyl). The temporal distribution of residues was affected by the chemical class of the fungicides. Preventing fungicides were detected across the whole season, and curative fungicides mostly in March or October
[19]. Although agricultural activity is the main source of pollution, urban and industrial activities can pollute the environment as well. Merel et al.
[20] confirmed that the presence of carbendazim in the Rhine river (west Germany) originated from industrial wastewaters.
Recently, new ecotoxicological endpoints have been introduced for many fungicides due to their secondary side effects. The majority of studies for toxicological effects on non-target organisms have been conducted on a laboratory scale, using model organisms such as
Lemna spp.,
Daphnia spp., and
Dario spp.
[16].
Dario rerio is an essential organism for toxicological studies, as
Dario is sensitive to the exposure of toxic compounds. Endocrine dysfunction, oxidative stress, and immune system disorders were observed when zebrafish were exposed to carbendazim during larval and fetal stages at concentrations above 4000 ng/L
[21]. Apart from carbendazim, tebuconazole caused adverse effects on the congenital system of Zebrafish, limiting locomotion at concentrations 4 and 6 mg/L
[22]. The acute and chronic toxicity of strobilurins kresoxim-methyl, pyraclostrobin, and trifloxystrobin were investigated in
D. magna neonates and embryos by Cui et al.
[23]. The results showed that Daphnia embryos are more sensitive to fungicide exposure than neonates, presenting 157.3 µg/L, 3.9 µg/L, and 1.7 µg/L 48-h EC
50 for kresoxim-methyl, pyraclostrobin, and trifloxystrobin, respectively. In addition, the lowest-observed-effect concentrations were similar to the environmental concentrations, and thus the authors reported that the studied fungicides were very toxic for
D. magna.
1.3. Chiral Fungicides
Many fungicides have an asymmetric center, which can provide two types of stereoisomers: enantiomers and diastereomers. Enantiomers have identical physicochemical properties but behave differently in asymmetric environments, such as in their biochemical processes. Enantiomers also show different biological activity, environmental fate, and toxicological profile. Diastereomers may not have identical physicochemical properties, and their biological activity usually varies
[24].
Stereoselective fungicides differ in terms of toxicity, bioaccumulation, and bioavailability on non-target organisms [24]. Deng et al. [25] investigated the toxicity of four stereoisomers of metconazole to the aquatic algae Chorella pyrenoidosa. The results showed that the 96 h EC50 values were different, following the pattern cis-1S,5R-Z > trans-1S,5S > trans-1R,5R > cis-1R. In addition, the photosynthesis dysfunction, the generation of reactive oxygen species (ROS), and the antioxidant response were induced more drastically by 1S,5S. In a similar study, three enantiomers of epoxiconazole were tested for their toxicological impact on the green alga Scenedesmus obliquus. The EC50 values followed the order (+)-epoxiconazole > (−)-epoxiconazole > rac-epoxiconazole, whilst different effects on the determined chlorophyll contents, malondialdehyde contents, and antioxidant enzyme activities of algae cells were observed [26].
The environmental behavior of chiral fungicides has mainly been studied by evaluating their half-lives in crops and soils. The half-life values can provide interesting information about the potential biodegradation and the persistence of the studied compounds
[27]. While fungicide enantiomers may present different half-life values from the racemic mixture, similar values were observed in other cases. The half-life of penconazole enantiomers in plant tissues and soil was determined in a field experiment. The results showed that the penconazole enantiomer (−) was degraded significantly faster than its (+) isomer in grapes and soil
[28]. On the contrary, propiconazole stereoisomers were studied under aerobic, anaerobic, and sterile conditions by incubating the stereoisomers in three different types of soil, with the study investigating the dissipation process. The results showed no significant stereoselectivity under anaerobic and sterile conditions in all tested soil after 200 days of incubation, which is in contrast with the aerobic conditions where significant stereoselectivity was identified
[29].
1.4. Nanofungicides
A fungicide is classed as nanofungicide if the size of the active ingredients ranges between 10 and 100 nm. The use of nanoparticles in fungi disease management can be divided in two categories: nanoparticles as protectants (alone) and as carriers for organic fungicides
[30]. The main advantages of nanoparticles as carriers for organic fungicides are the improvement of persistence and activity of the active ingredient, the increased ability for translocation within plants, the confrontation of the low-water-solubility problems, and the achievement of slow release.
Preventative nanofungicides can be applied directly to roots, leaves or seeds. Copper, silver, zinc oxide, and titanium dioxide are amongst the most studied nanofungicides. The effectiveness of ZnO, Ag, CuO, and Cu nanoparticles was compared with a commercial formulation containing Cu(OH)
2 against common fungi strains such as
Bacillus cinerea, Alternaria alternata and so on. The comparison evaluated mycelial growth, colony formation, seed germination, and the hyphal and spore morphology of the fungi. Mycelial growth of fungi strains was inhibited in vitro by all the nanoparticles, but the most effective were Cu and ZnO. In addition, the nanoparticles were more lethal at the spore germination stage
[31]. Shyla et al.
[32] investigated the antifungal activity of zinc oxide, silver, and titanium dioxide particles against
Macrophomina phaseolina, which can infest oilseed and pulse crops. At lower concentrations, Ag nanoparticles were more effective than ZnO and TiO
2 against target fungi. In general, silver presented the highest antifungal activity from the other metals. Silver ions can cause dysfunction in thiol groups of fungal cell walls. As a result, the electron transport chain, energy metabolism, and transmembrane function are disrupted. Furthermore, fungal DNA can be mutated, respiratory chain dissociated, membrane permeability decreased, and cell lysis affected by silver-based fungicides.
Another popular nanoparticle fungicide with low toxicity risk to human health and the environment is chitosan. Chitosan can block nutrient supply, prevent the biosynthesis of mRNA and proteins, disrupt the cell membrane, and inhibit H+-ATPase of fungi. Some of the fungi that can be managed by chitosan are Fusarium crown rot, root rot in tomato, gray mold of grapes, and rice blast disease in rice
[33]. Chitosan–lactide copolymer nanoparticles were used as carriers for pyraclostrobin, a low-water-soluble fungicide. The carrier was tested against
Colletotrichum gossypii at different concentrations and compared to a commercial formulation of pyraclostrobin. The results showed that nanofungicide effectiveness did not exceed the commercial one at three and five days post-application. Nevertheless, the nanofungicide antifungal activity was increased at day 7 post-application
[34]. Janatova et al.
[35] achieved a successful
Aspergillus niger control by formulating five individual essential oils with silica material MCM-41 in nanocapsules. Their effectiveness was reported to be higher than commercial oils at 14 days post-
Aspergillus niger infection.
Despite the advantages of nanofungicides, these compounds can enter natural waters through leaching, run-off, or spray-drift. Soil properties such as surface charge, cation species, and the type of soil can define the mobility of nanoparticles in the soil. In addition, the nanoparticles can modify the soil sorption capacity of pesticides, resulting in the fluctuations of their toxicity severity. As a result, toxicity effects have been observed in humans, plants, microorganisms, and vertebrates due to their exposure to nanofungicides
[36]. According to Ameen et al.
[37], the exposure of nanopesticides can have a different impact on plant growth depending on application conditions such as application rate and size and type of nanomaterial. Nanoparticles can be taken up and cross the plasma membrane through various processes, such as endocytosis, pore formation, and carrier proteins
[38]. Plants can activate defense systems and overcome stress parameters (including nanoparticles). However, there is the possibility for plants to fail to overcome toxicity effects, showing symptoms such as damaged DNA, reduced rate of transpiration, and others
[39]. For instance, decreased content of leaf photosynthetic pigment (chlorophyll a and b) and reduced biomass (17–20%) have been observed in maize after application of the nanofungicide Cu(OH)
2 [40]. In addition, nanofungicides have the potential to harm beneficial soil bacteria and fungus. Abd-alla et al.
[41] reported that high concentrations of Ag-nanoparticles reduced mycorrhizal colonization, glomalin content, and mycorrhizal responsiveness of AMF
Glomus aggregatum. Hence, the nitrogen-fixing
Azotobacter vinelandii presented various toxicity symptoms under Ag-nanoparticle presence, such as deduced cell number, apoptosis structural damage, inhibition of biological nitrogen fixation, and ROS generation
[42].
Furthermore, various aquatic organisms have been investigated for their responses to nanoparticle exposure. Mortality, hatching delays, and various developmental malformations were shown when zebrafish embryos were exposed to nanoparticles
[43].
1.5. Chemical Plant Defense Activators
Chemical plant defense activators constitute another group of fungicides with novel mode of action. Acibenzolar s-methyl (ASM) is a compound that can induce plants’ defense mechanisms, such as systemic acquired resistance, with salicylic acid taking part in the process. ASM can be used as an alternative solution to common bactericides and fungicides. Many researchers report that ASM is able to effectively manage various diseases in different crops, for example, onion and tomato
[44]. In addition, ASM induces the production of enzymes and phytoalexins when the plant is chemically, biologically, or physically stressed. However, ASM has been identified as phytotoxic, has been linked to production losses, and in some situations, may exacerbate other pest assaults
[45]. Potassium phosphate (PP) is a salt that is applied in cultures as a foliar fertilizer. Plant defense function is also stimulated by PP use. PP is characterized by high mobility and solubility. As a result, harmful oomycetes are successfully controlled in different cultures, such as papaya, tomato, and potato
[45][46]. Another salt, fosetyl-Al, is a worldwide broad-spectrum fungicide that is commonly used in horticulture. Its action is based on preventing the development of fungi spores and mycelium, inhibiting the pathogen penetrating into the plant. It was also documented that fosetyl-Al plays a role in plant defense mechanism activation
[47].
The environmental fate and toxicological effects of chemical plant activators to non-target organisms have not been extensively studied. The effects of fosetyl-Al on model species
Danio rerio in water and
Enchytraeus crypticus in soil were evaluated at a laboratory scale. The ecotoxicological assessment showed that fosetyl-Al is not considered safe for
D. rerio and
E. crypticus for concentrations higher than the PECs, which are 1.067 mg/kg for soil and 0.06496 mg/L for surface water
[48]. In the case of ASM, Guziejewski et al.
[49] reported that it is moderately to highly toxic to fish, moderately toxic to invertebrates. and highly toxic to aquatic plants.