Disease-suppressive crops work in multiple ways: by not being a crop host to the disease, by actively lowering pathogen levels, and by boosting soil health so the overall growing environment is more resilient to disease. In a recent episode of Potatoes in Canada’s Tuber Talk podcast series, Robert Larkin, a USDA-ARS research plant pathologist based in Maine, discussed the different ways crops can lower disease pressure and all about soil health more broadly.
Listen to the episode on the Potatoes in Canada website here, or download it for later wherever you listen to podcasts. Below is an except taken from the podcast transcript (courtesy Potatoes in Canada) – read the full transcript here.
Q: What is the correlation between soil health and disease?
Larkin: Well, that’s closely related. When you have poor soil health, you have conditions such as inadequate drainage, poor soil structure, low microbial activity, low organic matter – those are all things that can lead to more diseases.
Practices that help improve soil health will, in general, improve soilborne diseases. Just improving soil health may not remove soilborne diseases, but you’re going to have less soilborne disease problems in any soil that has that has a good level of soil health.
Q: You’ve spoken about disease-suppressive crops work in multiple ways: by not being a crop host to the disease, by actively lowering pathogen levels, and by boosting soil health so the overall growing environment is more resilient to disease. Can you expand?
Larkin: In general, we think of crop rotations as being a break from your main crop and [so rotation length makes a difference]. The longer you’re out of potatoes, the pathogen involved will decrease in quantity because there’s no host crop available. But other crops have the ability to more directly affect soilborne diseases and soil pathogens.
One mechanism is through this buildup of soil health properties – increasing organic matter, having a more extensive root system that leads to more extensive soil microbiology – all of these things can help stimulate microbial activity. Diversity leads to a general suppression of pathogens and diseases, and it may also help stimulate more beneficial organisms that are antagonistic to pathogens.
The third mechanism has to do with the plant crop itself directly inhibiting the pathogens and reducing those pathogens. In that case, the plant products themselves produce compounds, that when broken down, release volatile toxins or compounds that can kill or inhibit pathogen growth. They also can reduce nematodes and weed seeds and through a process that’s similar to using a chemical fumigate.
Plants in the Brassica family produce compounds called glucosinolates which breakdown to produce isothiocyanates. [Isothiocyanates] are a similar chemical product as are produced with compounds such as metam sodium, which is a chemical fumigant, but this is a biologic production of it. By chopping and incorporating the plant material, it releases these gases and you get a biofumigant effect that reduces populations of these pathogens.
In addition, many of these disease suppressive crops also alter the soil microbial communities in ways that may or may not be the same or different from these biofumigation properties. [The crops], in compounds that they produce, change the soil microbial composition to some degree, and those also can be directly inhibitory to pathogen compounds and reduction of soilborne diseases.
Q: Building soil organic matter is a long-term process. How long should you commit to trying out these practices before you switch to try another practice?
Larkin: A lot of these [practices], it’s not like adding a chemical fumigant where you immediately see a dramatic reduction in the first year. You should see a reduction, a significant reduction, but you’re not going to eliminate your problem in one or two years. And some of these other soil health building processes can take many years of building organic matter, that’s not going to happen in one or two years.
We’ve designed our trials so that we are looking at these [practices] over the long term. You’ve got a long-term cropping system study that has been put in place for 15 years now, where we’re able to look at what these longer-term consequences are. Some of these small incremental changes that may not look like they’re having a big effect in those first couple of years, but after five or six years, they can have more dramatic effect.
Source: Potatoes in Canada. Full article here
Photo: Blackleg starting to appear on a potato. Courtesy of Dr Eugenia Banks.