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New trends in soil and substrate disinfestation

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By Oded Achilea, PhD

Almeria, in Andalucía, southern Spain, is probably the largest global concentration of greenhouses, encompassing almost 33,000 hectares of "plastic sea", most of grow winter soil-grown tomatoes, cucumbers, bell peppers, courgettes, eggplants, strawberries, watermelons, melons and more.

The lion's share of this industry is organic-certified, and some 75 percent of these products are exported to other European countries, at a turnover of ~ €3 billion. One of the constrains of this vastly intensive operation is the damages provoked by a large variety of pathogenic soil-borne, air-borne, bacteria, fungi, nematodes, viruses and pests.

These facts were, undoubtedly the reasons that supported the International Society of Horticultural Science’s (ISHS) decision that the University of Almeria should host the 10th International Symposium on Soil and Substrate Disinfestation, in June 2023. This symposium attracted some 76 researchers, and industry stakeholders from Europe, Asia, Africa and the Americas, and was organized under the leadership of IFAPA’s researcher Dr. Miguel de Cara and his dedicated team.

Two main lines were discussed by the presenters: 1. New chemical disinfection materials and their application methods; and 2. Physical and biological concepts of restoring soil health by solarization and fumigation.

1. After phasing out of methyl-bromide (MB), other chemicals were adopted for soil disinfection. Chinese Dr. Dongdong Yan reported that since MB banning in China (only in 2019), it has been largely replaced by: chloropicrin, dazomet, metam sodium/ potassium, sulfuryl fluoride, dimethyl disulfide, allyl-isothiocyanate and 1,3-dichloropropene. Many application methods for these agents have been developed in China, including direct injection, chemigation, hot gas application and mechanical soil integration by rotary tillers. Soil fumigation produced significantly positive effects in greenhouses and in open fields, including more vigorous plants, larger and greener leaves and markedly higher yields.

But Dr. Dongdong showed that on top of the fumigants' pesticidal effect, they also inhibited the microbial nitrification activity (oxidizing ammonia to nitrate), thus radically reducing nitrogen (N) losses by leaching. This phenomenon has been evident both in the lab and in the field. As can be expected, heavier textured soils and lower pH soils show longer inhibition time, during which new planting will suffer from the biocidal effect of the fumigants. Fumigation also stimulates nitrogen mineralization processes, whereby ammonium is released to soil by degradation of organic nitrogenous compounds. Therefore, chemical soil fumigation increased soil's N availability, thanks to nitrification inhibition, and the promotion of N mineralization. Other reports by Chinese researchers showed that chemical fumigation improves the availability to plants of other soil nutrients, including, P, K, Fe, Mn, Cl and soluble organic carbon.

2. More holistic and organic methods apply physical and/or biological methods to improve soil health. Soil solarization, soil flooding and soil steaming are typical physical practices, that are more environmentally friendly than chemical ones, yet very effective too. Soil solarization is using solar energy during high solar radiation seasons, to increase soil temperature to kill or critically weaken soil-borne plant pathogens. It is carried out by covering moist soil with a tarp, usually a water-, and gas-impermeable plastic cover, that absorbs solar energy, and keeps it within the treated plot for 30-50 days, and is removed thereafter. The water content supports solar heat convection to deeper layers of the treated soil. It can be done with water alone, or in combination with chemicals (mixed solarization) or organic materials (bio-solarization). Solarization is currently used by more than 90 percent of the farms in southeastern Spain.

ASD (Anaerobic Soil Disinfestation) is carried out by irrigating the soil up to field capacity and amending it with a labile C source and organic N, and tarping by a black totally impermeable film (TIF). Professor F.D. Gioia (Penn State University) explained that waterlogging the soil produces anaerobic conditions, which kill most aerobic plant pathogens. And the organic amendments undergo anaerobic decomposition by facultative and obligate anaerobic microbes, which results in the production of pyruvic, acetic, lactic, propionic, butyric and formic acids. All these give rise to accumulation of toxic/suppressive products, volatilization of organic compounds, creation of N derivatives like NH3, N2O and N2, and to leaching of NO2- and NO3-.

Additionally, the flourishing anaerobic populations limit the growth, and control the pathogenic ones, by supporting their parasitic organisms, by competition for nutrients, and/or by production of antibiotics and toxic metabolites, and by inducing crop resistance. A large variety of organic amendments have been reported to produce excellent results, e.g., maltose, molasses, rice bran, wheat middling, dried distiller’s grain, residual strawberry extrudate, chicken manure, sheep manure, fish meal, green manure made of triticale and/or crimson clover, and Everlizer (a heat-processed chicken litter fertilizer).

Bio-disinfestation methods were reported to use the following organic amendments: fresh pepper and cucumber plant debris, fresh-, and semi-composted sheep manure, wheat husk, sunflower pellets. The application of fresh plant debris to the soil through bio-disinfestationn is suggested as an alternative for the management of crop residues, in accordance with the principles of the circular economy. Developing suppressive soils by controlling plant pathogens can be also done by enriching the soil with the following amendments: green composts and biocontrol agents (Trichoderma spp., Bacillus amyloliquefaciens, Pseudomonas sp.), frass from black soldier fly larvae, commercial chitin, sunflower seed husks, wheat bran, beer bagasse and rapeseed cake.

Bio-fumigation implies the cultivation of specific species, commonly belonging to the Brassicaceae family, in the pathogens affected field, followed by chopping the plants, and their rapid incorporation into the soil. Volatile inhibitory substances, in particular isothiocyanates, are produced after the hydrolysis of glucosinolates, contained in the plant tissues, by the enzyme myrosinase that is released due to crushing of the plant tissues. The use of the following species has been reported: rape (Brassica napus), B. juncea, B. carinata, also, white radish (Raphanus sativus), mustard (Sinapis alba), and Camelina sativa.

Read our June/July 2023 issue of New AG International here.

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