Sustainable Strategies for the Control of Crop Diseases and Pests to Reduce Pesticides

Crop diseases and pests may cause considerable yield losses and threaten food supply and security. Moreover, there is clear evidence that the geographical distribution of several pathogens and pests is expanding due to climate change and increasing global trade [1,2].

Regular ways of pest and disease control based only on the use of synthetic pesticides are no longer effective or sustainable because their intensive application negatively affects the environment, human health, and biodiversity. Moreover, one of the challenges is the development of resistance in pests and pathogens towards these pesticides, making them less efficient or completely inefficient. Thus, to ensure better control and ecological sustainability, it is crucial to reduce pesticides by adopting original alternative strategies to maintain the pest/pathogen populations below the economic injury level and move towards achieving the European Green Deal.

The European Green Deal aims to boost the efficient use of resources by moving to a clean, circular economy and stopping climate change, reverting biodiversity loss, and cutting pollution.

The Farm to Fork Strategy, one of this agreement’s pillars, lays down a new approach to ensure that agriculture and the food value chain contribute appropriately to the objective of a climate-neutral Union in 2050, aiming to make food systems fair, healthy, and environmentally friendly [3].

This Special Issue gathers together twenty-two original research articles and reviews that show a viable alternative to the use of pesticides or reduction in them in different countries from Europe (Spain, Portugal, Italy, France, Greece, Serbia, Slovenia, Denmark, and Russia), Asia (China), Africa (Kenya, Republic of the Congo); and the USA and Latin America (Mexico and Colombia). These papers cover a wide range of crops and involve diverse research organizations, some of them showing cooperative work in important crops such as maize, coffee, fruit production (chestnut, grapes, pears), brassicas, tomatoes, and potatoes.

In this summary, we put the contributions together according to the different control strategies proposed to minimize the impacts and risks of pesticides and achieve resource-efficient and competitive agriculture.

1. In the case of the use of biological control, Ahedo-Quero et al. [4] showed under laboratory conditions that an alternative to mitigate the environmental impact associated with agrochemicals for the control of the weevil insect Scyphophorus acupunctatus could be the use of the plant’s oil formulations of Salvia hispanica, Triticum vulgare, and Olea europea combined with the entomopathogenic nematodes Steinernema glaseri and Heterorhabditis bacteriophora. Formulations with nematodes could be a viable and environmentally friendly alternative for the control and management of S. acupunctatus, thereby helping to reduce the use of chemical pesticides. Munywoki et al. [5] evaluated the effect of the entomopathogen Metarhizium anisopliae ICIPE 41 to control invasive fall armyworm (FAW). Three oil formulations, including canola, corn, and olive oils and an aqueous formulation of ICIPE 41, were used in laboratory and field experiments. The fungal-based biopesticides exhibited high mortality and mycosis rates on FAW with no negative impacts on the parasitoid Cotesia icipe compared to pesticides. ICIPE 41, formulated with corn oil, could be used for sustainable FAW management in maize cropping systems. Wakil et al. [6] evaluated the effects of the fungi Beauveria bassiana and Metarhizium robertsii controlling Tetranychus urticae, an important pest of tomato crops that affects plant yield and growth. They used two isolates (WG-12 and WG-19) of B. bassiana and one isolate of M. robertsii (WG-02) when colonizing different organs (leaves, stems, and roots) and assessed their influence on plant growth and their acaricidal activity against T. urticae (female adults). They recommended the use of the M. robertsii isolate as it offered complete protection and promoted the tomato’s growth. Hatem et al. [7] assessed the ability of dry mycelial biomass from the Penicillium chrysogenum VKM F-4876D strain (DMP), either alone or in combination with fungicides based on difenoconazole, tebuconazole, and fludioxonil, to inhibit the growth and development of a strain of Botrytis cinerea (gray mold) isolated from fresh grape berries. In this study, a combination of DMP with fungicides showed high antifungal activity on B. cinerea, indicating its potential for reducing the fungicide dosages required to control gray mold and other plant pathogenic microorganisms.

2. Other alternatives to the use of pesticides included in this issue are plant extracts from leaves and fruits or plant phytochemical and secondary metabolites such as a caffeine oleate formulation, plant resistance inducers—PRIs (which can be chemical compounds as well as microbial or plant extracts), and plant growth regulators. Fontana et al. [8] used Smilax aspera, known as sarsaparilla, to develop leaf and fruit hydromethanolic extracts with phytoconstituents. The extracts demonstrated high antibacterial activity against three important phytopathogens under in vitro conditions, inhibiting Pseudomonas syringae pv. actinidiae and Xanthomonas campestris pv. campestris. The permeability assays showed noticeable alterations in bacterial membrane permeability. In addition, in vivo trials conducted on Pyrus communis trees using the combined extract from the aerial parts have shown significant protection against the bacterium Erwinia amylovora. Góngora et al. [9] developed a caffeine-based formulation with oleic acid, Tween 80, and kaolin, as an insecticide to control coffee berry borer (CBB), Hypothenemus hampei, and other coffee pests, such as Monalonion velezangeli. CBB is the pest that causes the most economic damage to coffee crops in Colombia. The caffeine oleate formulation showed laboratory efficacy by causing mortality in more than 90% of CBB adults in preventive tests. In tests in which spraying occurred after CBB infested the fruits, the formulation caused 77% mortality of the insects. The insecticide was also effective against M. velezangeli, causing 100% mortality. This caffeine oleate formulation increases compounds’ permeation through the cuticle and facilitates the flow of blockers through a lipoidal membrane. The effects of caffeine and caffeine oleate inside the insect could be related to the role of caffeine as a neurotransmitter inhibitor. At the muscle cell level, caffeine affects the ryanodine receptor channels that regulate calcium release and are essential for feeding, flight, locomotion, and oviposition. Monteiro et al. [10] examined the expression of stress-responsive genes linked to various plant functions in two grapevine cultivars, Alicante Bouschet and Trincadeira, under organic and integrated-production farming systems. This study revealed that specific products used in organic farming, such as plant resistance inducers, significantly impacted gene expression, leading to the upregulation of multiple genes compared to integrated production farming. These results underscore the importance of exploring the effects of these substances on plant gene expression and developing new compounds to enhance the productivity and resilience of organic farming practices. The study of Grabber et al. [11] aimed to examine foliar and plant density responses of interseeded alfalfa (Medicago sativa) to factorial combinations of foliar applications of prohexadione (PHD) followed by fungicide plus insecticide (FI) and to determine the lowest effective rate of PHD to apply before FI to facilitate good establishment of interseeded alfalfa under corn (Zea mays). Applying the PHD growth retardant decreased etiolation, while FI reduced foliar damage and enhanced the vigor of alfalfa growing under corn. The results obtained alongside other research suggest that excellent alfalfa establishment under moderately competitive corn can be achieved with FI treatment during normal to dry growing conditions. However, a combination of PHD and FI treatments is necessary to achieve reasonably good stands during wet growing conditions. The authors consider that developing more effective agrochemical treatments, improved alfalfa varieties, and better farming methods will help overcome challenging growing conditions and make interseeding successful.

3. The third type of alternative to the use of toxic pesticides are chemical agents (calcium cyanamid, dazomet, and ammonium bicarbonate), phosphonate salts, or physical barrels-inert dusts (diatomaceous earth, quartz sand, Norway spruce wood ash, zeolite, and tree of heaven leaf dust), and electrolyzed oxidizing water. Zhang et al. [12] carried out greenhouse and field trials to assess the effects of plastic film covering and three chemical agents (calcium cyanamide, dazomet, and ammonium bicarbonate) on the control of clubroot, a disease caused by Plasmodiophora brassicae that affects the production of Brassica crops, specifically the Chinese cabbage cultivar ‘Granaat’. Although an individual application of plastic film failed to effectively control the disease, calcium cyanamide, dazomet, and ammonium bicarbonate proved to be significantly effective in managing it. These three agents significantly reduced the density of resting spores in the soil. Additionally, calcium cyanamide and ammonium bicarbonate increased the soil’s alkalinity. Ammonium bicarbonate also promoted lateral root development in the cultivar ‘Granaat’, helping infected plants to access adequate water and nutrients. Therefore, sustained lateral root development is crucial for effectively resisting pathogen invasion. Bastianelli et al. [13] assessed the effectiveness of phosphonate salts in controlling chestnut brown rot, a fungal disease caused by Gnomoniopsis castaneae, in chestnut (Castanea sativa) orchards. Their research demonstrated that phosphonate-based salts, particularly Zn-phosphonate, reduced the disease’s impact and the pathogen inoculum in fruits with efficacy comparable to the fungicide Tebuconazole. Integrating Zn-phosphonate into orchard management can enhance post-harvest fruit processing through water-based heat treatments that efficiently inactivate G. castaneae. These salts are considered optimal fungicides for chestnut orchards due to their low environmental impact at recommended doses, high translocability and stability, and multiple mechanisms of action. Batistič et al. [14] tested various inert dusts (diatomaceous earth, quartz sand, Norway spruce wood ash, zeolite, and tree of heaven leaf dust) to control Colorado potato beetles (CPB) at different developmental stages. The results indicated that wood ash and diatomaceous earth were the most effective, particularly against adult CPBs. Wood ash has a smaller particle size and is hydrophilic, while diatomaceous earth destroys the insect’s cuticle and absorbs its protective wax layer, enhancing their effectiveness. Ground leaves of the tree of heaven (Ailanthus altissima) generated low defoliation, possibly due to an antifeedant effect attributed to catechin hydrate. This compound, identified as the most abundant polyphenol in A. altissima, is known to influence the feeding behavior of other pests. Mexicano et al. [15] applied electrolyzed water to tomato crops to assess its effectiveness in controlling the bacterium Pseudomonas syringae pv. tomato. They also examined whether this treatment affected the quality of the harvested fruits. The lowest severity was observed in plants treated with oxidizing water, showing results comparable to the commercial bactericide Kasumin but without affecting the crop’s growth and development and being environmentally friendly.

4. Another control strategy we consider in this issue is identifying and using resistant varieties and better techniques for phytopathogen identification. Genetic resistance is one of the best methods of controlling pests and diseases, making it easier for farmers to use than other interventions. It is compatible with other Integrated Pest and Disease Management (IPDM) options. Knowledge of the mechanisms involved in plant resistance is essential for selecting genotypes with this characteristic to increase the efficiency of the breeding program and pest and disease control strategies. Jiménez-Galindo et al. [16] investigated new sources of resistance to the corn weevil (Sitophilus zeamais) in various accessions of maize (Zea mays) from Mexico. The novelty of the work is that it focuses on almost all stages of the biological cycle of S. zeamais and explores the different resistance mechanisms (antixenosis, antibiosis, and tolerance) to gain a more detailed picture of the performance of maize varieties against pest attack. The most resistant accession identified showed high adult mortality and fewer 1st-generation adults. The 1st-generation adults are an optimum type for evaluating resistance to S. zeamais in corn. The type of endosperm influenced the resistance/tolerance to the S. zeamais. The flint varieties showed significantly higher mortality of adults before entering the seed, lesser emerged adults of the first generation in the obligate-antibiosis test, and lower net consumption. Two tolerant accessions, Cristalino-079 (flint) and Azul (floury), revealed a lower level of consumption by S. zeamais and could be used in breeding programs to reduce losses caused by this pest. Nuambote-Yobila et al. [17] focus on the fall armyworm (FAW), Spodoptera frugiperda, a pest of maize native to America, first reported in West and Central Africa in 2016, that severely threatens maize production and food security in Sub-Saharan Africa. An intensive screening for FAW resistance was conducted through artificial infestation in greenhouse conditions in Kenya on approximately 3,000 inbred lines from the International Maize and Wheat Improvement Center (CIMMYT) germplasm collection. Among these lines, four showed resistance to FAW. The information on the resistance mechanism of these inbred lines is relevant to the program initiated by CIMMYT-Kenya to develop FAW-resistant tropical maize hybrids, and local and exotic lines can contribute to that. Alworah et al. [18] identified the resistance response to Fusarium stilboides of three commercial cultivars (Ruiru 11, Batian, and SL 28) of Coffea arabica, plus two other coffee species, C. canephora and C. liberica. This pathogen causes Fusarium bark disease (FBD) in coffee, leading to yield losses and plant death in coffee-growing countries such as Kenya. The coffee plants were inoculated with different concentrations of three F. stilboides isolates (NRF 930/18, TN002B (I), BS008B (A). Seedlings inoculated with F. stilboides exhibited symptoms like wilting, stunted growth, and defoliation. Among the isolates, TN002B (I) was the most virulent. The varying responses to F. stilboides infection among the coffee genotypes provide valuable insights for breeding programs focused on disease resistance. The study performed by Tavares et al. [19] revealed that the transcription patterns of the Hypersensitive Induced Reaction (HIR) gene family, Phytoalexin Deficient 4 (PAD4), and Enhanced Disease Susceptibility 1 (EDS1) genes did not differ in a coffee genotype designated by Timor Hybrid-HDT (natural C. arabica × C. canephora hybrid) inoculated with the rust fungi Hemileia vastatrix and Uromyces vignae (host vs. nonhost resistance). To date, HDTs are some of the most widely used sources of resistance to H. vastatrix, the causal agent of coffee leaf rust. HIR4, PAD4, and EDS1 genes showed the highest sequence diversity in C. arabica, C. canephora, and C. eugenioides genomes. The predicted proteins of PAD4 in C. arabica and EDS1 in C. canephora are atypical proteins within the PAD4/EDS1 protein family; the first lacks all the conserved domains, and the second has a protein domain named formin. These differences in the amino acid sequence could lead to changes in protein function, potentially impacting the host’s resistance response to the disease. Quiroga-Cardona et al. [20] employed the detached leaf inoculation method to select Coffea arabica genotypes resistant to the biotrophic fungus Hemileia vastatrix, the causal agent of coffee leaf rust (CLR). They performed experiments under laboratory and field conditions using three hybrid populations (F1) of C. arabica, derived from sources carrying the S_H1 resistance gene and the CX.2385 line, obtained from the cross between the variety Caturra and Timor Hybrid CIFC-1343. The authors concluded that the plants with higher levels of resistance to H. vastatrix identified in this study can be utilized when looking for new sources of genetic resistance to CLR in Colombia. Furthermore, inoculating detached leaves reduces the number of plants required for field testing, significantly contributing to a cost-effective allocation of resources in selecting and developing coffee varieties.

5. Other papers focused on various strategies to reduce the amount of pesticides applied by exploring key stages of pathogen and pest invasion using field and in vitro inoculation tests. Wang et al. [21] used an Effect and Less Spraying Control (ELSC) method for applying fungicides to control gray mold, a destructive disease in grapes caused by Botrytis cinerea. The spraying schedule was determined by exploring key stages of B. cinerea invasion using field and in vitro inoculation tests. The disease incidence/severity in the full-bloom stage was higher than in the pre-bloom. To evaluate the ELSC method, a field trial in China compared its performance with four kinds of fungicides. Fludioxonil was the most effective fungicide, followed by the bio-fungicide Pythium oligandrum, difenoconazole + azoxystrobin, and pyraclostrobin. The ELSC method was more effective against B. cinerea than the traditional control schedule, as evidenced by lower disease severity. Additionally, the average yield per hectare with ELSC demonstrated that spraying during flowering does not negatively impact grape yield. In their study, Ivezić et al. [22] investigated the European Corn Borer (ECB) (Ostrinia nubilalis), a major agricultural pest in southeastern Europe, including Serbia. They monitored data on the flight dynamics of ECB adults for the development of a predictive model of adult numbers within the growing season. This approach could notably decrease both insecticide usage and the frequency of chemical treatments. Moreover, it holds potential for integrated pest management (IPM) and organic farming and is entirely compatible with biological control methods.

6. Rounding out this Special Issue are three review papers. One, by Krndija et al. [23], compiled the literature data on available natural enemies of pear psyllas, Cacopsylla pyri (Hemiptera: Psyllidae), which has emerged as a major pest in pear orchards across many European countries, including those in the Western Balkans. This paper further underscores the potential of natural predators for the biological control of this pest. Ferrucho et al. [24] brought together different strategies for controlling the main coffee diseases in Colombia that enable the design of integrated disease management, ensuring a consistent supply of high-quality and safe coffee. Góngora et al. [25] compiled information on sustainable control of the principal coffee pests in Colombia. The control strategies include a deep knowledge of the biology of insects and coffee plants, their relationship with weather and habitats, and natural enemies. These control strategies do not require the application of insecticides, are environmentally sustainable and innovative, and have the potential to be implemented in other coffee-growing countries.