Gordon Johnson, Retired Extension Specialist; email@example.com
There have been several articles in the WCU discussing the use of biofumigant mustards for soil disease control. Unfortunately, the ideal window for fall planting has passed. Another option that has been researched extensively has been the use of overwintered rapeseed which can be planted up to the middle of September.
Rapeseed (Brassica napus) has been used as a biofumigant in new and replant orchard sites. It has a deep root system, is good at reducing surface compaction, scavenges significant soil nitrogen, and suppresses weeds.
More importantly, rapeseed is in the mustard family and produce chemicals called glucosinolates in plant tissue (roots and foliage). These glucosinolates are released from plant tissue when it is cut or chopped and then are further broken down by enzymes to form chemicals that behave like fumigants. The most common of these breakdown products are isothiocyanates. These are the same chemicals that are released from metam-sodium and metam-potassium, commonly used as chemical fumigants. Rapeseed green manure can be an effective treatment to reduce dagger nematodes and Tomato Ringspot virus (TmRSV) in replanted orchards.
It is important to note that success with rapeseed biofumigant crops depends on several factors. The following are some suggestions to achieve the best results:
- Use only winter rapeseed varieties. A recommended variety is ‘Dwarf Essex’. The crop is planted in September and incorporated the following spring.
- Produce as much biomass of the biofumigant rapeseed crop as possible. This requires that you have a good stand, fertility, and sufficient growing time. The more biomass that is produced and that is incorporated, the more chemical is released. However, as plants mature, they will reach a point where levels of these active chemicals will decline, and you should not let the plants go past full flower. For optimum nematode control rapeseed should be incorporated there is adequate soil moisture and a soil temperature above 50 °F in the spring.
- Plant material must be thoroughly damaged so that enzymes can convert glucosinolates into isothiocynates. This means that you need to chop the material as much as possible and work it into the soil as quickly as possible to not lose the active compounds to the air. A delay of several hours can cause significant reductions in biofumigant activity. The finer the chop, the more biofumigant is released. A flail mower is the best tool for achieving this.
- The chopped material should be incorporated immediately after chopping. Tillage operations should be performed immediately behind the flail mower.
- The chopped material should be incorporated as thoroughly as practical to release the biofumigant chemical throughout the root zone of the area that is to be later planted to vegetables. Poor distribution of the biofumigant crop pieces in the soil will lead to reduced effectiveness. A tractor mounted rotary tiller or power spader is the best tool for this.
- Sealing with water (irrigating) after incorporation will improve the efficacy by reducing gas loss from the decomposing rapeseed (the active fumigant released). Soil conditions should not be overly dry or excessively wet. Packing the soil will also help this sealing process.
Dwarf Essex Rapeseed used for orchard site biofumigation.