Seed Treatments to Control Seedborne Fungal Pathogens of Vegetable Crops

Introduction

  • Management of plant diseases is crucial for most crops, especially for high-quality seed production.
  • Plant pathogens can diminish the quantity and quality of harvested seeds; seedborne pathogens can be preserved within seed lots, leading to unintentional dissemination.
  • Seed treatment enhances seedling emergence, especially in seeds with low vigour or damaged seed coats.
  • Seed treatment is important for eradicating or reducing seedborne pathogens, particularly when seeds are grown for seed production, or high-quality seed with a low percentage of fungal infection is required.
  • Traditional seed treatments primarily involved fungicides.
  • New methods excluding fungicides are increasingly sought, especially in organic farming, where seeds must be produced under organic conditions (EU regulation EEC 2092/91).
  • Non-chemical methods like physical treatments and seed coating with plant extracts and biocontrol agents (BCAs) are being developed.
  • The success of seed treatment depends on:
    • Effectiveness of the applied compound.
    • Degree of internal seed infection.
    • Amount of inoculum in the seed lot.
    • Specificity and potential phytotoxicity of the treatment.1
  • Types of seed treatment, depend on pathogen location:
    • Seed disinfestation: Control of disease organisms on the seed surface.
    • Seed disinfection: Elimination of pathogens that have penetrated the seed's living cells.
    • Seed protection: Application to protect the seed from seedborne and soilborne organisms (e.g., Pythium, Fusarium, Rhizoctonia), preventing seed rot and damping-off.
  • Systemic fungicides can offer post-emergence protection against foliage diseases.

Seed Treatments

Fungicide Treatments

  • Early fungicides contained sulfur, copper, and mercury.
  • Due to toxicity issues, mercury compounds have been banned. 5^{5}
  • Newer systemic fungicides have replaced inorganic compounds due to their higher efficiency and lower risk to crops, animals, and the environment, as they are readily degraded by soil microorganisms.
  • Fungicides were once categorized as volatile and non-volatile (before mercurial ban); now, most approved seed fungicides are non-volatile. 6^{6}
  • Fungicides can be:
    • Broad-spectrum: Toxic to many kinds of fungi.
    • Narrow-spectrum: Effective against only a few species.
  • Contact fungicides:
    • Effective only against fungal spores on the seed surface.
    • Ineffective against internal fungal seed infections.
  • Translaminar or cytotropic fungicides:
    • Penetrate the superficial layers of seeds.
    • Counter shallow fungal infections.
  • Systemic fungicides:
    • Effective against fungal diseases deep within the seed.
    • Protect against early infection from airborne and soilborne diseases.

Physical Treatments

  • Include heat treatments like hot water, hot air, and electron treatments.
  • Thermotherapy inactivates or kills the pathogen without harming the host tissue. 7^{7}
  • Hot water treatment: Involves immersing plant material in agitated water at a predetermined temperature and time.
    • Historically used for sanitizing cereal seeds. 8^{8}
  • Aerated steam 11^{11} and electron seed treatments are modern treatments under intensive investigation. 3,12^{3, 12}
  • Aerated steam: Kills pathogens while leaving seeds unharmed.
  • Electron seed treatment: Electrons destroy the DNA of harmful organisms on the seed surface and coat within milliseconds, preserving the seed's interior.
  • Germination assays are necessary to determine the optimum treatment for a given seed batch. 13^{13}

Treatments with Biopesticides

  • Increasing chemical inputs lead to pathogen resistance and environmental impact. 14^{14}
  • Integrated pest management strategies offer environmentally sound and economically feasible alternatives. 14^{14}
  • High pesticide costs and consumer demand for pesticide-free food drive the search for substitutes. 15^{15}
  • Biological control is considered an alternative or supplement to synthetic chemicals. 16^{16}
Plant Extracts and Natural Compounds
  • Plant extracts contain antimicrobial compounds for seed disinfection, either alone or combined with physical treatments. 17^{17}
  • Essential oils (tea tree, clove, peppermint, rosemary, laurel, oregano, and thyme) have shown antifungal activities in vitro against pathogens like Ascochyta spp. and Alternaria spp. 18^{18}
  • Thyme oil, containing thymol, has shown general antimicrobial activity against seedborne bacteria and fungi. 19,20^{19, 20}
  • Plants from the genus Allium produce sulfur-containing compounds with antimicrobial effects. 21^{21}
  • Onion seed exudates have an inhibitory effect on pathogenic fungi. 22,23^{22, 23}
  • Chitosan is a biopolymer with antifungal properties that acts by chelating nutrients or inducing host defence responses. 24,25^{24, 25}
  • Chitosan induces broad-spectrum and long-lasting resistance. 26^{26}
  • Chitosan application increases germination index, reduces mean germination time, and enhances shoot height, root length, and root and shoot weights. Efficacy has been shown in several studies. 27−30^{27-30}
Biocontrol Agents
  • Biological control of fungal plant pathogens is a viable approach. 16^{16}
  • Numerous microorganisms (fungi and bacteria) have been identified as BCAs. BCAs have gained wide acceptance, especially fungal-based BCAs because they have a broader spectrum in terms of disease control and production yield. 31^{31}
  • BCAs might be applied during seed priming to improve emergence, particularly under wet or cold conditions. 32−34^{32-34}
  • BCAs are added as a suspension in water during hydration, followed by incubation and drying of the seeds.
  • Fungal-based BCAs survive better than bacterial ones on onion and carrot seeds. 32^{32}
  • For effective protection, an antagonist must colonize the rhizosphere 35^{35}
  • Colonization patterns differ depending on the antagonist microorganism. 39^{39}
  • The seed coat conformation (texture and ornamentation) influences spatial colonization patterns. 40,41^{40, 41}
  • Inoculation of seeds with BCAs does not alter the ecophysiological structure and physiological profiles of the rhizosphere bacterial community. 42^{42}
  • Survival and establishment of beneficial microorganisms is fundamental for continued promotion of plant growth and disease control. 42^{42}
  • Bacteria such as Pseudomonas chlororaphis and Pseudomonas fluorescens tend to decrease in number in the rhizosphere soil and on roots, whereas fungi such as C. rosea and T. harzianum remain constant or increase in number. 32^{32}
  • Many BCAs are difficult to formulate as commercial products. 43^{43}
  • To control different diseases that affect the same crop, the association of several microorganisms is needed. The combination of two or more BCAs means multiple registration processes, with increased costs and difficulties in providing all of the studies required according to strict legislation. A solution might be the labelling of already registered biofungicides, based on different antagonistic strains, as compatible with each other and proposed for joint use. 14^{14}
  • Trichoderma spp. act via mycoparasitism, competition, antibiosis, and induction of plant defenses. 44−46^{44-46}
  • Plant-growth-promoting rhizobacteria (PGPR) enhance plant growth and yield. 47−49^{47-49}
  • PGPR promote a reduction in the populations of a broad spectrum of root and foliar pathogens that are found in the rhizosphere
  • Mechanisms of action: antibiosis, competition, parasitism, and induction of systemic resistance. 50−53^{50-53}
  • The success of PGPR is influenced by a number of biotic and abiotic components that represent limiting factors for root colonisation. 51−53^{51-53}
  • The quality of PGPR formulations, in terms of viability and efficacy, determines their large-scale adoption at the end-user level. 54,55^{54, 55}
  • Pseudomonas spp. have positive features such as rapid growth and mass production, efficient use of seed and root exudates, rapid colonization, production of bioactive metabolites (antibiotics, siderophores, volatiles and growth-promoting substances), aggressive competition towards other microorganisms and adaptability to environmental stress. 56^{56}
  • Pseudomonads are responsible for the natural suppression of some soils towards soilborne pathogens. 57^{57}
  • The major weakness of pseudomonads as biocontrol agents is their inability to produce resting spores (as do many Bacillus spp.), which complicates the formulation of these bacteria for commercial use; indeed, they are formulated as frozen cell pellets that must be kept on dry ice until application. 58^{58}
  • Fluorescent pseudomonads are the most applied as biocontrol agents for horticultural seed production.
  • Bacillus spp. promote growth through phytohormone production, phosphate solubilization, siderophore production, antibiosis, inhibition of ethylene synthesis, and induction of systemic resistance. 59−63^{59-63}
  • Gram-positive bacteria such as streptomycetes are active in the rhizosphere and effective in biocontrol through antibiosis, lysis of the fungal cell wall, competition and hyperparasitism. 66,67^{66, 67}
  • Unlike gram-negative microorganisms, sporulating gram-positive microorganisms can be formulated readily into stable products such as a dry powder through their heat-resistant and desiccation-resistant spores. 68,69^{68, 69}
  • Piriformospora indica: Arbuscular mycorrhizal fungus that induces tolerance against salt stress and resistance against root and shoot pathogens. 70^{70}
  • The efficacy of biological treatments on vegetable seeds is often better in a greenhouse than in the field.

Treatment of Different Pathosystems

Daucus carota/Alternaria dauci, A. radicina

  • Alternaria leaf blight and Alternaria black rot are destructive diseases of carrot, caused by Alternaria dauci and Alternaria radicina, respectively. 71−79^{71-79}
  • Fungicide applications minimize seedborne infections, which results in fewer infections in the early season in production crops. Commonly applied fungicides include chlorothalonil, iprodione, pyraclostrobin, and azoxystrobin. 80−82^{80-82}
  • Other effective fungicides include thiram, boscalid, maneb, macozeb, benomyl, and thiofanate methyl. 83^{83}
  • In vitro trials show that trifloxystrobin and tebuconazole have sufficient inhibitory activity on Alternaria spp. colonies. 84^{84}
  • Infected seeds treated with fungicides can reduce, but not eliminate, contamination. 85−87^{85-87}
  • Hot water treatment (50–53 °C for 10–30 min) is >95% effective. 88^{88}
  • Hot air treatments have eradicating effects against A. dauci and A. radicina, which are as effective as chemical treatments, while electron treatments have shown low efficiency. 10^{10}
  • Thyme oil shows antimicrobial activity against A. dauci. 20^{20}
  • Thyme oil (0.1 to 1%) reduces A. dauci and A. radicina. 10^{10}
  • Manuka oil also exhibits antimicrobial activity against A. dauci. 20^{20}
  • Allicin demonstrates antimicrobial action against seedborne Alternaria spp. in carrot. 90^{90}
  • Pseudomonas (including P. fluorescens) protect against A. dauci and A. radicina. 10^{10}
  • Burkholderia cepacia's efficacy against carrot black rot is comparable to the chemical treatment with iprodione. 81^{81}
  • C. rosea increases plant stands. 10^{10}
  • Biopriming reduces A. radicina from 29 to <2.3% and A. dauci from 11 to <4.8%. 82^{82}

Brassica spp./Alternaria brassicicola, Leptosphaeria maculans

  • Alternaria brassicicola and Leptosphaeria maculans cause black spot and blackleg, respectively. 91−96^{91-96}
  • Thiram- and iprodione-based fungicides control A. brassicicola. 97,98^{97, 98}
  • Flutriafol- and fluquinconozole-based fungicides control L. maculans. 99,100^{99, 100}
  • Hot water treatments (50 °C for 25–30 min or 53 °C for 10 min) reduce L. maculans infections by 87–92% and A. brassicicola infections by 92–99%. 88^{88}
  • Temperatures lower than 53 °C or shorter treatment times are recommended. 88,102^{88, 102}
  • Aerated steam and electron treatment are reliable against A. brassicicola. 98^{98}
  • Thyme oil controls A. brassicicola. 98^{98}
  • B. subtilis increases healthy cabbage plants while reducing A. brassicicola infection. 98^{98}
  • S. griseoviridis reduces the inoculum of A. brassicicola. 98^{98}
  • Rhizobacteria Serratia plymuthica and P. chlororaphis reduce blackleg disease severity by 71.6 and 54%, respectively. 103^{103}
  • Combined treatments of S. plymuthica and Clonostachys rosea f. catenulata with metconazole reduce disease infestation. 104^{104}

Lycopersicon esculentum/Fusarium oxysporum f. sp. lycopersici, Alternaria solani

  • Fusarium oxysporum f. sp. lycopersici and Alternaria solani cause Fusarium wilt and early blight, respectively. 105−110^{105-110}
  • Commonly applied fungicides include chlorothalonil, mancozeb, mefenoxam, thiram, and azoxystrobin. 111^{111}
  • Neem leaf extracts suppress mycelial growth of A. solani and F. oxysporum f. sp. lycopersici. 112^{112}
  • Leaf extracts of zimmu inhibit mycelia growth of A. solani and reduce symptoms. 113^{113}
  • Methyl jasmonate reduces the development of early blight. 114^{114}
  • P. fluorescens increases seedling emergence and reduces Fusarium wilt incidence. 115^{115}
  • Brevibacillus brevis reduces tomato wilt. 116^{116}
  • Talc-based formulations of S. griseus applied with chitin control F. oxysporum f. sp. lycopersici. 117^{117}
  • A combination of fluorescent Pseudomonas, T. harzianum, and Glomus intraradices reduces Fusarium wilt incidence and severity and increases yield. 118^{118}
  • The application of BCAs along with plant extracts helps to overcome pathogen infections by increasing the levels of defense-related enzymes and phenolic substances. 113^{113}

Fabaceae/Ascochyta spp.

  • Ascochyta spp. cause Ascochyta blight on Fabaceae crops. 119^{119}
  • Species include Ascochyta pisi and Ascochyta pinodes (on pea), Ascochyta rabiei (on chickpea), Ascochyta lentis (on lentil), and Ascochyta fabae (on faba bean). 119^{119}
  • Fungicide seed coatings prevent spore germination and reduce mycelial growth. 122^{122}
  • Fungicides against A. rabiei include maneb, benomyl plus thiram or captan, tridemorph plus maneb, and thiabendazole. 122−126^{122-126}
  • Metalaxyl, thiabendazole, ipconazole, and azoxystrobin increase the percentages of healthy plants. 127^{127}
  • Thiabendazole and benomyl induce seedling emergence and increase yield. 128^{128}
  • Some fungicides exhibit phytotoxic actions. 130^{130}
  • Hot water, hot carbon tetrachloride, and steam/air mixtures fail to control Ascochyta infection on legumes. 123,125,128^{123, 125, 128}
  • The difficulty of controlling seedborne Ascochyta spp. with alternative treatments might be explained by the position of these pathogens in the seed. 131^{131}

Concluding Remarks

  • Seed treatments can reduce seedborne pathogens. 1^{1}
  • Environmentally friendly treatments can be supported to replace fungicides as part of integrated pest management practices. 2009/128/EC^{2009/128/EC}
  • Fungicide treatments reduce seedborne pathogens effectively (>80% compared to control), remaining more reliable than alternative treatments.
  • Alternative treatments are as effective as chemical treatments in some cases, especially for physical treatments against A. dauci, A. radicina, A. brassicicola, and L. maculans.
  • Plant extracts and essential oils are effective against all pathogens (50 to 80% compared with the control).
  • Thyme oil is frequently effective against various pathogens.
  • The combination of essential oils or plant extracts with BCAs ensures greater protection against pathogens.