Reducing Verticillium with Cover Crops

Posted: November 23, 2011

Soil-borne diseases can be devastating to crops. Unseen they may persist in the soil for years. Harold Weaver, from Meadow Gate Vista Farm in Bowers, Pennsylvania, tried a new strategy for combating soil-borne disease this year: cover crops.

Weaver has one field with a history of verticillium wilt. Verticillium is a fungal disease that can attack over 200 plant species;especially tomatoes, potatoes,eggplant, strawberries and black raspberries. It is common in the cooler, wetter soils here in the Northeast. Infected tomatoes might not actually wilt, as the name implies. Instead they generally develop blotches on lower leaves that eventually turn into brown dead spots. If the plants are drought stressed, they wilt. 

The affected field had been rotated out of tomatoes for a number of years.But, Weaver knew that other crops or weeds could have been hosting the disease and was interested in trying cover crops to reduce disease pressure further. Certain cover crops, including mustards, rapeseed and sudangrass contain a chemical and an enzyme in the plant tissue. When these cover crops are chopped into small pieces with a flail mower, the chemical and enzyme come into contact and create a chemical reaction. The chemical and the enzyme are not toxic by themselves, but when they react, they create a compound that is toxic to soil-borne pathogens and even weeds seeds.

Harold decided to try using mustard and sudangrass cover crops to try to reduce the verticilliun in his field. He divided his field into three strips. On May 26 and June 16, 2010, he planted one strip to “Caliente 119” mustard at the normal rate of 16 pounds per acre, walking it on with a spinner spreader. He planted another strip to the “Caliente 119” mustard at twice the normal rate due to a problem with the spreader. He planted the third (middle) strip to buckwheat as a control. He fertilized (6-1 with sulfur) for a target rate of 120 N in a split application. There was rainfall of about an inch a week (or irrigation) and so the cover crops came up and grew well. Six weeks after planting, there was 1 to 1.2 tons per acre of mustard biomass (dry weight).

immature seed pods
By the first week in July, the mustard fields were in full flower and full of buzzing bees. The earlier planted section was starting to put on seedheads signaling that it was time to kill the cover crop. The trick is to wait as long as you can so that the plant has grown as much as possible, but not wait too long and have hard seed start to form. To incorporate the cover crop, first Harold did one pass with the flail mower. Flail Mowing Mustard Cover Crop Meadow View Farm

This chopped the mustard into nice small pieces, crushing the leaves to cause the chemical reaction.

Chopped Mustard Cover Crop Meadow Gate Vista Farm

Then he quickly disked the field to incorporate the fresh plant material.

Disking Mustard Cover Crop Meadow Gate Vista Farm

It was incorporated within ten minutes of chopping to make sure that the volatiles were released into the soil instead of the air. Walking behind the tractor, the spicy mustard pricked his eyes and nose – the chemical reaction was happening!

The last step was to set up sprinklers so that the water sealed off the soil surface.

He followed the mustards with sudangrass to give a double biofumigation to the plots. Sudangrass also has volatile compounds shown to kill soil-borne disease and weeds.

That was a lot of work and Weaver had to wait a whole year in order to see if it was worth the effort. This spring he transplanted tomatoes into the entire field, some of which had been treated with cover crop “biofumigation” and some of which had just had regular cover crops. We decided to monitor Green Zebra and Black Plum varieties which tend to be very susceptible to verticillium. Once a week, during the first three weeks of August (August 6th,15th and 24th) we harvested from plants that had received the mustard biofumigation and those that had not for both Green Zebra (one row and two rows per bed) and Black Plum. To make sure we were getting a good representation of what was going on, we randomly chose three sets of three plants in each variety to monitor.

The result: the yield in areas that had the mustard and sudangrass cover crop treatment were twice as high as those that did not for Green Zebra. Interestingly, the per plant yield was also almost twice as high where there was only one row of plants per bed versus two.

Tomato Yield

So was it worth it? To answer that question we would have had to track the time and money spent on growing and managing the cover crops. We have learned a lot from this and other experimentation with growing mustard cover crops.

These cover crops are picky – they need enough moisture, enough nitrogen, sulfur to create the chemical compounds, and have to be managed in a timely manner. Also, this was a field demonstration, without multiple reps, so we cannot say for sure that the cover crop biofumigation is what caused the larger yield. Though, based on other research, it seems likely. I think that mustards and sudangrass are another tool in the tool box for combating soil-borne disease. But it is a tool that has to be wielded precisely. And it might not always work depending on the season. These are living plants which react to growing conditions. Since you need large plants to have enough of the compound in the plant to kill the pathogens, you have to grow them well for it to work.

You also have to have the time to manage them correctly. For example, if you just grow the cover crop and don’t mow it or otherwise chop it into small pieces the chemical reaction will not happen sufficiently. Or if you don’t work the cover crop in right away (they say within 15 minutes) a lot of the compound will have escaped, volatizing into the air instead of the soil where the problem is.

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