Does Compost Reduce Plant Disease?

Plant disease reduction is a common benefit attributed to compost—but is it true. Will compost, added to the garden, reduce diseases in the garden? This is a very complex question that leads to some very interesting discussions about plants, and their diseases.

Does Compost Reduce Plant Diseases?

The science of plant diseases and in particular soil borne plant diseases are still a frontier of science. We don’t really understand what goes on in the soil. What are the interactions between plants and microbes? How do microbes interact with each other? How does organic material affect such interactions? For the most part these are still mysteries of science, although some facts are starting to emerge.

Does compost reduce diseases? Probably not in a direct fashion. Compost is not like a drug we might take to fight a sickness. You don’t add compost and suddenly see a plant get better.

Compost does not eliminate diseases organisms or stop infections from occurring. However, compost does change the microbe environment in the soil and this change will affect the number and type of microbes in the soil. This can influence the interaction of microbes and their ability to infect plants. And in this indirect way compost can affect the diseases of plants.

If this sounds a bit vague  you are correct. Scientists are still not sure of the details and can’t say for sure what the role of compost is. Let’s have a look at what we do know.

Plant Health Reduces Diseases

There is clear evidence that a healthy plant can resist diseases better than an unhealthy plant. With the right amounts of nutrients, soil moisture, and light a plant is better able to produce the chemicals it needs to fight disease.

In the presence of disease causing fungus, plants will produce chemicals that reduce or prevent the infection by the fungus. In effect the plant produces its own disease fighting drugs. A healthy plant is better able to detect a potential problem, and to fight the disease.

Compost provides a wide range of nutrients, including all of the micro-nutrients. Compost also improves soil structure which makes it easier for a plant to grow a good root system. Both of these contribute to a healthier plant, and indirectly help reduce diseases.

Good Microbes vs Bad Microbes

Soil contains thousands of different types of microbes. Most of these are either beneficial to plants or at least they do no harm to plants. A few are disease causing. So we have the good guys and the bad guys. Of course this is an over simplification, but it is a practical way of thinking about them.

If we have a close look at soil it consists of a huge multidimensional community of microbes. They compete with one another for resources. They kill and eat one another. They each have their own preferred environmental conditions, and some can even change the environment in their immediate vicinity. Populations are constantly shifting and changing. The populations are different in summer and in winter because of temperatures. They change with food sources, so adding compost will affect the community. We know very little about this community so far.

We do know that plants also affect the community, especially right around their roots—an area called the rhizosphere. Plant roots produce a range of chemicals, exudates, and exude them into this rhizosphere and by doing so, they affect the microbe community living there.

Plant exudates feed some of the good guys so that they live next to the roots. These good guys, eat the bad guys keeping their numbers low, reducing the chance of disease.

Plants also exude chemicals that specifically fight the bad guys, a type of chemical warfare.

Compost helps plants stay healthy and able to deal with the microbes in the rhizosphere.

Suppressive Soils

A suppressive soil is a soil where a known existing disease organism does not seem to affect the plants. The disease is present in the soil, but for some reason it is suppressed—it does not infect the plants. Scientists are starting to understand these systems.

Suppressive soils go through phases. They will be suppressive for a given disease for a few years, and then plants suddenly get the disease again. A few years later the soil may become suppressive again. It is believed that this cycling is due to the changes in microbe populations.

If a field has a lot of the bad guys, plants and good guys start the war on them. Plants help increase the population of the good guys and over a couple of years there are more good guys than bade guys and the incidence of disease goes down. The soil is suppressive.

Once the number of disease microbes is low, plants and the good guys stop the fight—the community of microbes changes, and it makes room for disease microbes to start growing again. The cycle starts all over.

One reference reports that “sugar beets funnel about a fifth of their photosynthetically captured carbon through their roots into the soil to fuel the microbes”. That means that 20% of the chemicals a plant produces through photosynthesis is used to control the microbes in the rhizosphere–that is a lot.

Suppressive Soil Armies

What do the good guy armies look like? Is it one specific type of microbe that suppresses the disease? Are there many different species involved? Are diseases reduced by specific species or is it sheer numbers?

Scientists don’t agree on the answers. There are certain species that seem to be important for each type of disease, but recent work suggests that it could be more than just 1 or 2 species. It seems likely that quite a few species are important for fighting any particular disease organism. Scientists also found that some important species don’t actually kill the bad guys. No one knows what they do, but their presence is important for fighting disease. It’s also possible that it is a numbers game. When there are lots of good guys around, the bad guys have too much competition and die out.

Does Compost Reduce Plant Disease?

I started this post by saying that compost does help in the fight of diseases by making plants healthier and I think that is still true.

A recent review of some 2500 case studies found that it is not clear if compost actually reduces diseases. Half the studies did not show a reduction of disease after adding compost. These studies were looking at specific diseases, and I suspect most were lab tests, not field tests. The following statements seem to summarize the current research.

• Compost may suppress some diseases, but not others.
• Disease suppression was low if less than 20% v/v compost was added.
• Sterilization of the compost resulted in no or little suppression indicating the cause is biological.
• Results are less consistent in field studies.
• Some diseases increase after adding compost.

Also interesting is that peat, also an organic soil amendment, had almost no disease suppression. Organic material in general (including compost, peat and plant residue) also increased diseases 20% of the time.

As you can see, the scientific community is not sure of the answer. There is certainly clear evidence that in some cases with certain diseases, compost does reduce disease. But there are many cases were it does not work, and in some cases it might even increase the incidence of disease. The whole system of soil chemistry and microbiology is still too complex for us to fully understand what is going on.

What does this mean for your garden?

Keep using compost. If nothing else, it feeds your plants and improves soil. I would not expect it to solve disease problems, and I would not worry about the fact that it might cause some diseases. You might have noticed the 20% value above. If this value is correct, it means that you need to add a lot of compost to see suppression of diseases. Compost at 20% is too much for your garden as I’ll discuss in another post. Don’t add a lot of compost to fight diseases.