Problems with soil-borne pathogens in various crops have been increasing in recent years. With the elimination of chemical soil disinfectants or the reduction of their dose, soil-borne diseases are increasingly difficult to control. Therefore, in the AltChem project, the Belgium Proefstation voor de Groenteteelt sought which alternative soil disinfection techniques could best be applied to reduce disease pressure in the crops lettuce, corn salad, and chicory. The AltChem project ran from 1/06/2020 to 31/05/2024.
Within the project, some crops and pathogens were taken as model crops to research:
- Rhizoctonia in lettuce
- Pythium (yellowing disease) in lamb's lettuce
- Sclerotinia in ground chicory
Using these model crops, four sustainable alternatives to chemical soil fumigation were tested:
1. Soil treatments
1.1. Steaming
For good pathogen killing, one hour at 70°C (75°C for Fusarium oxysporum f.sp. lactucae) is assumed. The most difficult thing here is to achieve these high temperatures even at depth.
Since soil tillage, soil dryness, and soil type can affect the penetration of the steam into the soil, it is advisable to insert sensors in the soil to monitor the temperature course. These sensors are available at various practice centers.
High-pressure steaming leads to a good killing of pathogens and after steaming, planting can be done quickly. Steaming is best done in dry soil that has been loosened but not prepared too finely. Research has shown that breaking up the soil and digging too deep at 40cm gave the best results.
A point of attention, however, is the possible exemption of manganese, which may make sampling after steaming advisable. Growing after steaming in warm soil can also bring challenges. Also, keep in mind that steaming is not selective and also kills off non-pathogenic microorganisms.
- Sail steaming
A tarpaulin is placed on the ground under which the steam is introduced. The steam then has to find its own way into the depths. This method of steaming is the most common for the time being. Surely, count on six to eight hours of sailing sphere. In practice, the higher the pressure applied, the better.
- Steaming under pressure (= steam drainage)
Here, too, a tarpaulin is placed on the ground under which the steam is introduced. A fan is also placed on the existing drainage. In this way, the steam is drawn through the soil and reaches the depth more quickly. This makes it possible to limit the steam duration (about 4 hours sailing sphere). However, this technique requires the presence of the necessary drainage.
- Low-pressure steaming
It is also possible to apply steam at lower pressure. To test this, various techniques such as steam breaking, steam freezing, and plate steaming were used. Here, the steam was always introduced into the ground very locally and for a short time. However, this often led to a heterogeneous distribution of the temperature in the ground, and also no high temperatures were measured at depth. This method could be used to treat weeds but is insufficient to tackle pathogens.
1.2. Anaerobic soil disinfection
Mechanism of action
Anaerobic soil fumigation is based on the fact that due to lack of oxygen in the soil, anaerobic bacteria incite fermentation (fermentation) and certain substances formed are lethal to a lot of organisms. In our regions, anaerobic conditions are mostly obtained by adding organic material to the soil and then covering it airtight by deeply fixing a TIF film. This organic material should contain a high C content, here it is best to assume a carbon-nitrogen ratio of 20 to 1. This carbon serves as a food source for the aerobic bacteria present in the soil, which in this way use up the oxygen so that afterward the anaerobic bacteria can get to work. Abroad, anaerobic conditions are sometimes caused by flooding the soil.
Implementation
This is a method that does take some time (6 or so weeks) and the amount of product to be spread is not to be underestimated. The deep incorporation of the film is also fairly labor-intensive.
The nitrogen release after treatment can increase sharply in subsequent crops. Especially for a first application, it is advisable not to start with too high a dose to monitor the behavior of the product in the soil.
There is a commercial preparation (Herbie) on the market that can be used for this purpose, but other organic materials (such as wheat bran) could also possibly be used. Here it is always important to check that the product has gone through pre-treatment processes that rule out the presence of pathogens.
Sufficient watering is necessary to give the bacteria a chance to do their job.
Direct killing on entrenched pathogens in smaller field trials was good, but in larger field trials the method still proved insufficient to suppress pathogens present at the grower. For instance, there was an insufficient effect on Phytophthora in chicory, yellowing disease in corn salad, and Fusarium oxysporum f.sp. lactucae in head lettuce. However, nice results were seen against weeds in a greenhouse with high weed pressure.
1.3. Biofumigation
Mechanism of action
Biofumigation is based on the natural defense mechanism of certain plants (mainly Brassica species). When leaves are damaged (by eating or fungi), various substances come into contact with each other, leading to the formation of the gas AITC. This gas is chemically related to MITC, which is formed during chemical disinfection with metam products or dazomet.
Implementation
Biofumigation can be done on the one hand by using fresh material. This involves setting up a complete crop of crops with the potential to produce a lot of AITC (e.g. mustard, fodder radish, ...). After miniaturization and incorporation, the crops should then be quickly covered to allow the gas to do its job. This of course requires that time and space be set aside for this. Making the crops small enough and working in and covering them quickly is often a practical objection.
Another alternative is to use dried material. In this case, the chemical reaction for the formation of the AITC starts only after the addition of water. Spreading and incorporation of this product is easy to carry out.
According to the literature, the duration of coverage could be limited (a few days to a week).
Tests have shown that good killing is possible, but there are large differences between soils and the pathogens targeted. Further research is needed before this method is ready for practice. There is also no legal authorization for the use of the dried material in Belgium yet. We are reviewing this further.
(Bio)solarisation
Solarisation uses the sun's energy to raise the temperature in the soil. Translucent foil is best used for this purpose (creating a greenhouse within a greenhouse). Optionally, organic material can also be added to generate additional heat by composting this material (= bio solarisation). This is a method that also takes some time (6 or so weeks) and there is a strong dependence on weather conditions. Very high temperatures cannot be obtained via these methods (max. 45-50°C), but literature indicates that the effect is partly caused by the length of time the temperature is maintained. This method could not control high disease pressure in growers.
2. Biological control organisms
The project also investigated the operation and monitoring of biological control organisms (BCOs). Microorganisms also have recognition as crop protection agents. Within this project, we looked at two middle based on Trichoderma and one based on Pythium oligandrum.
ILVO worked with the manufacturers to develop qPCR techniques to monitor BCO levels in the soil and on the roots.
Once the agents were applied (including historically), they were also always found in the soils, even when soil fumigation methods were applied. However, little correlation could be established between the quantities recovered in the soil and the roots.
Within the project, no significant action of the tested BCOs against the pathogens present in the soil could be demonstrated in any trial, the disease symptoms on the plants were in other words no better than the control.