Special Issue “Sustainable Control Strategies of Plant Pathogens in Horticulture”

1. Introduction

European Regulation No. 1107/2009 [1] recommends the adoption of alternatives to synthetic products among plant protection products, thereby repealing Council Directives 79/117/EEC [2] and 91/414/EEC [3]. This recommendation is driven by the potential adverse effects of synthetic products on the environment, as well as on human and animal health [4,5]. Furthermore, the European “Green Deal” has introduced various initiatives that aim to facilitate a green transition to counteract climate change and safeguard the environment.

One of the key objectives outlined in the “Green Deal” is to substantially reduce the use of chemical pesticides in agriculture by 2030 [6,7], to reduce the associated risks and to address potential challenges in the management of plant pathogens. This strategy also outlines a set of measures to be achieved by 2030, including the promotion of 25% of organic agriculture. This ambitious target highlights the need for innovative research to identify alternative solutions to chemical pesticides. Over the past ten years, most of the research has focused on the use of beneficial microorganisms such as fungi and bacteria [8,9,10], natural substances [11,12], resistant varieties [13,14], RNAi gene silencing that targets specific pathogens [15,16] and organic cultivation systems [17].

2. Special Issue Contents

The Special Issue on “Sustainable Control Strategies of Plant Pathogens in Horticulture” features research articles and two reviews. These contributions present potential alternatives to synthetic pesticides, showing innovative results related to the use of natural substances, beneficial microorganisms and resistant varieties in the management of several pathogens affecting plants.

The majority of the articles deal with the antifungal activity of natural substances derived from plants (

Table 1. Main results obtained with natural substances in this Special Issue.

Natural Substances	Plant/Pathogen/Method	Activity	Ctrb.
Essential oils of thyme, common juniper, hyssop	Carrot (C), tomato (T) and onion (O) seeds naturally infected by Alternaria spp. Agar plate assay amended with essential oils.	Thyme and common juniper: 40–100% long-lasting antifungal activity for C, T and O. Hyssop: no activity for C; 20–60% long-lasting antifungal activity for T and O.	1
Giant reed extract	Zucchini. Plant growth substrate inoculated with Pythium ultimum and treated with the extract.	Disease reduction up to 73% and pathogen growth (colony forming units) in the substrate by 90%.	2
Forty extracts from 20 Argentinian plant species	Penicillium digitatum, P. italicum and Geotrichum citri-aurantii. Agar plate diffusion assay.	Inhibition of G. citri-aurantii growth of more than 50% by most of the extracts. Inhibition of P. digitatum and P. talicum by some extracts.	3
Saponins from Medicago species and oat grains and homogenates from sprouts of Brassica species	Verticillium dahliae. Agar plate assay amended with saponins and homogenates. Maize and tomato seeds treated and sown on filter paper.	Reduction in mycelium growth and conidium formation. No phytotoxic effect on seed germination.	4
Water-soluble polysaccharides from Jania adhaerens	Tomato. Seeds treated with polysaccharides. Plant growth substrate inoculated with Rhizoctonia solani and P. ultimum before seeding or with Fusarium oxysporum before transplant.	Disease reduction of R. solani, P. ultimum and F. oxysporum up to 58%, 53% and 29%, respectively. Increase in seedling emergence and plant development. Up-regulation of HQT, HCT, PR1 PAL and PR2 genes. Increase in β-1,3-glucanase activity.	5
Gelidium sesquipedale by-product (alkaline residue)	Tomato. Greenhouse experiments. Grapevine. Plasmopara viticola in field trials.	Increase in peroxidase and PAL activities and up-regulation of PR9 genes in tomato plants. Reduction in downy mildew symptoms in grapewine.	6
Guava wood vinegar by-product of charcoal production	Potato. Colletotrichum coccodes. Agar plate assay amended with the by-product. Pot experiments—stem/soil inoculation with the pathogen.	Inhibition of pathogen mycelial growth. Black dot disease reduction by an average of 23% (stem colonization), 20% (roots covered with sclerotia) and 30% (wilted plants) in the two seasons of experiments.	7
Bio-composts from aromatic plant residues	Garden cress. R. solani, Sclerotinia sclerotiorum.	Reduction in S. sclerotiorum damping-off by all of the raw composts. Reduction in R. solani damping-off by 7 composts. Overall, 2 composts showed suppression levels up to 60%.	8

), such as essential oils from thyme, common juniper and hyssop (1); giant reed extract (Arundo donax) (2) against Alternaria spp. and Phytium ultimum; and extracts from Argentinian plant species against Penicillium spp. and Geotrichum citri-aurantii (3). On the same topic, two articles demonstrate the in vitro antifungal activity of saponins from Medicago species; oat grains and homogenates from sprouts of Brassica species against Verticillium dahliae (4); and the in planta disease reduction of Rhizoctonia solani, P. ultimum and Fusarium oxysporum following tomato seed treatment with water-soluble polysaccharides from Jania adhaerens (5). Three more articles focus on the potential of by-products and bio-composts to reduce disease symptoms and increase plant resistance to pathogens. An alkaline residue from Gelidium sesquipedale agar production elicits resistance in tomato and reduces Plasmopara viticola symptoms in the vineyard (6), and the guava wood vinegar by-product of charcoal production effectively reduces potato black dot disease by Colletotrichum coccodes (7). Several bio-composts from aromatic plant residues controlled damping-off by R. solani and Sclerotinia sclerotiorum on garden cress (8).

Three articles deal with beneficial microorganisms to control fungal and viral diseases (

Table 2. Main results obtained with beneficial microorganisms and resistant varieties in this Special Issue.

	Plant/Pathogen/Method	Activity	Ctrb.
Microorganisms
Bacillus amyloliquefaciens QSB-6	Apple replant disease. Soil amendment. Field conditions.	Increase in plant growth parameters (i.e., plant height), soil bacteria population (i.e., Actinomycetes) and soil enzymatic activity. Reduction in soil phenolic acid content and Fusarium spp. population.	9
Bacillus subtilis HA1 culture filtrate	Tomato. TMV. Foliar treatment. Pot experiments.	Increase in plant growth (root and shoot parameters). Increase in total phenolic and flavonoid content up to 27 and 50%, respectively, and in the activity of ROS-scavenging enzymes. Reduction in TMV accumulation up to 91%. Up-regulation of PR1, PAL, CHS and HQT genes.	10
Trichoderma asperellum T. hamatum T. harzianum T. koningiopsis	Fusarium solani. Dual plate assay on agar medium not amended or amended with the fungicides captan, chlorothalonil and mancozeb.	Trichoderma species inhibited F. solani up to 67%. High compatibility of T. asperellum with captan and mancozeb. No compatibility of Trichoderma species with chlorothalonil.	11
Resistant varieties
Fourteen Tunisian melon landraces	Podosphaera xanthii, 3 races (2, 3.5 and 5). Artificial infection in a growth chamber. Natural infection in a greenhouse.	Susceptibility of all landraces to the 3.5 and 5 races and resistance of several landraces to race 2, in the growth chamber. The resistance of three landraces to P. xanthii race 2 was confirmed under natural conditions.	12

). Among these, Bacillus amyloliquefaciens as a soil amendment and B. subltilis culture filtrate that was sprayed on leaves were effective in reducing apple replant disease (9) and TMV accumulation in tomato (10). One contribution showed the mycoparasitic activity of Trichoderma species against Fusarium solani, the compatibility of T. asperellum with captan and mancozeb and the incompatibility of all Trichoderma species with chlorothalonil in vitro (11).

Table 2 also reports an article on the possibility of using melon landraces to counteract powdery mildew caused by three races of Podosphaera xanthii (2, 3.5 and 5) under both artificial and natural infection, showing that the resistance of the three melon landraces to race 2 of the pathogen was confirmed under natural conditions (12).

The topic of this Special issue is complemented by two reviews (13, 14). One review (13) focuses on sustainable options for the management of diseases in horticulture, such as the use of biocontrol agents, natural products, forecasting models, precision farming, nanotechnology, endotherapy, systemic resistance inducers and gene silencing. The second review (14) deals with the use of basic substances against several seed-borne pathogens, fungi, oomycetes, phytoplasma, bacteria and viruses. The basic substances are active, non-toxic substances which fulfil the criteria of a “foodstuff” as defined in Article 2 of Regulation (EC) No 178/2002 [18]. For their use as plant protection products, basic substances are regulated in the EU according to criteria presented in Article 23 of Regulation (EC) No 1107/2009 [1]. The basic substances examined in this review (14) are those already approved in Europe and some of those that are still under evaluation.

3. Conclusions

This Special Issue comprises articles that aimed to identify alternative, sustainable and effective strategies for the management of important plant diseases. The extensive use of synthetic fungicides in managing plant diseases has led to the development of resistance in fungi and oomycetes. Additionally, in recent years, there has been growing consumer demand for food devoid of residues and produced in an environmentally friendly manner. Following this trend, the potential strategies outlined in this Special Issue offer viable alternatives for adoption in large-scale trials.

All the articles contribute valuable insights that enhance understanding in these research fields and have the potential to facilitate future sustainable practical solutions for plant disease management.

The Guest Editors express their gratitude to all authors for delivering interesting research findings in this Special Issue and for sharing their expertise. Furthermore, they extend appreciation to the Journal “Horticulturae” for its helpful support, enabling the realization of this Special Issue.