In the Northeast alone 1,687,080 tons of fresh market and processing vegetables on 264,490 acres, worth $701,377,000 suffer 10-15% losses from soil borne diseases (NASS Crop Profiles, 2007). Disease suppressive cover crop rotations may provide an additional tool for managing soil borne disease. Researchers have documented significant increases in yield after sudangrass, brassica, millet and other cover crops. Here we describe recent results of a two season on-farm case study using cover crops to suppress Verticillium wilt in tomato.
Mustards, rapeseed and sudangrass contain a chemical and an enzyme in the plant cell wall. When these cover crops are chopped into small pieces with a flail mower and then quickly incorporated and sealed into the soil using a cultipacker or water, the chemical comes into contact with the enzyme and it breaks down into a chemical that behaves like a fumigant. The chemicals and the enzymes are not toxic by themselves, but when they come in contact with each other, the chemical is broken down by the enzyme into compounds that are toxic to soil-borne pathogens and even weeds seeds. Cover crops can also improve soil organic matter and related soil water holding capacity, infiltration and microbial activity which positively impact yields over time.
From 2010 to 2013 we worked with Harold Weaver from Meadow Gate Vista farm. Weaver planted strips of cover crops in fields that had a history of Verticillium wilt on tomato. We worked with mustard cv. Caliente 119 and sudangrass cover crops known to have "biofumigant" properties and compared them to buckwheat as a control. In 2011 tomatoes after a rotation of mustard and sudangrass used as a biofumigant yielded twice as much as tomatoes grown after the buckwheat cover crop control. Although growing and incorporating the cover crops was a lot of additional work, Weaver felt the yield boost made it worth the effort.
In 2013, we did not observe any differences in the yield between tomatoes planted after buckwheat or the sudangrass that was chopped and incorporated as a biofumigant. However, keep in mind that 2013 was a wet year. Verticillium impacts tomatoes by clogging up the vascular system of the plant making it difficult for the plant to move water from its roots to the rest of the plant. Tomato plants in 2013 at this site did not appear to be as stressed by Verticillium wilt like they would in a dry year, which might explain why there were no differences. These results are not surprising and success in only one out of two years also reminds us that working with living cover crops to suppress soil-borne disease is likely to produce variable results and requires a long-term integrated approach. Success also depends on our ability as managers to successfully grow and incorporate the cover crop, as well as the variable whims of Mother Nature.
Successful suppression of soil-borne disease with cover crops
Grow a large lush cover crop
Mustard cover crops are picky - they need enough moisture, enough nitrogen, a good seedbed and sulfur to create the bio-active compounds. The nitrogen recommendation for caliente mustards is 120 units of nitrogen and 20 of sulfur/A. This nitrogen will be recycled for the following cash crop. Phosphorus and potassium should be at sufficient levels for your following cash crop. Most mustard cover crops including 'Caliente 119' are particularly susceptible to water stress which will cause them to mature and flower when they are still small. Spring or mid-August plantings tend to do best. Planting in droughty soils is not recommended. Sudangrass has similar nitrogen requirements, but it is does well in warm summer conditions and can withstand drier soils. Rapeseed is fall planted and will take advantage of fall and spring moisture. Remember the more cover crop biomass you grow, the more bio-active compound you will have to suppress soil-borne disease. You want to grow large cover crops.
Choose appropriate varieties
Some cover crop varieties have been selected for increased levels of bio-active compounds. Mustard cvs. Caliente 199 and 119, rapeseed cv. Dwarf Essex, sudangrass cv. Trudan 8, sorghum sudangrass cvs. 79, SS-222 and SS-333 are among those that have done well in research trials. In addition, the mustard is selected for reduced seed viability to reduce potential problems with becoming weedy.
Plant at appropriate rates
Sudangrass should be drilled at 30-50 lb/A. Mustards and rapeseed are planted at 8-10 lb/A.
Chop the cover crop into small pieces
A flail mower does a good job of crushing plant tissue and breaking it into small pieces. The more the tissue is crushed the greater the chemical reaction of the bio-active compounds.
Incorporate the cover crop immediately after mowing
These bio-active compounds are volatile. As much as 80% can be lost if the cover crop is not incorporated within 15 minutes of mowing.
Irrigate or cultipack to trap compounds
Sealing the surface of the soil with water or a cultipacker can trap volatile bio-active compounds giving them more time to work.