14. November 2019

Late blight part 2: Late blight and composting

Can you really use bokashi to get rid of late blight? Part 2 of my article series about blight goes through everything you need to know about late blight and composting.

En skottkärra fullt med bladmögeldrabbade växtdelar från tomat. Late blight and composting, a wheelbarrow filled with affected plant parts.

You might want to use tomato and potato leaves for your bokashi composting, but does the fermentation process really get rid of the aggressive disease? There is a lot to consider when dealing with late blight and composting.


A lot of people think that late blight and composting (specifically bokashi) is a match made in heaven, since it's supposed to get rid of the disease in the affected plant parts. But, there is no research out there supporting the notion that late blight disappears if you compost the affected plants with bokashi. Actually, if you compost diseased plants and then use the compost in your garden, chances are that the disease will spread to the soil and then your nightshades. Does that sound like a good idea?


More about bokashi: Fertilize with leaves and bokashi


Late blight and composting

This subject has been a bit of a hot potato (pun intended) in several gardening-themed Facebook groups. A lot of people have been wondering if you can use the affected plant parts for bokashi composting. There are a lot of different opinions out there. Some assume that it works and others assume that it doesn't. A few want it to work and are adamant to give it a try. And others say that they just know that it works, based on previous experiences.

The reason why there's even a discussion about this is that late blight is such a difficult disease. It wreaks havoc in so many gardens and it specifically affects some of our favorite vegetables, potatoes, and tomatoes. When this happens, the leaves wither and die, and the plant won't survive for much longer. Already developed potatoes are usually OK, the situation is a lot worse for the tomatoes though. They just start rotting, one after one. All of my tomatoes in the beds outside were affected this year. I only got a handful of healthy tomatoes from that batch actually. So annoying!


Read more: Late blight part 1. What causes late blight?


The best way to prevent the disease from spreading further is to burn any plant parts affected by late blight. The plants simply need to go. Composting diseased plant parts are not recommended. Not even hot composting. The disease won't be affected by any heat in the compost pile.

Late blight is a lot more aggressive than for example mildew. The disease spreads incredibly quickly, and not only by air. It can go on living in the soil too, for up to a few years!


Facts: EM-1, EM-A, bokashi, and leachate

  • EM-1: Effective microorganisms, dormant. Found in bokashi and EM-A.
  • EM-A: Active effective microorganisms, fluid. Can be used in many different ways in the garden.
  • Bokashi: Active effective microorganisms, dry organic material often used in composts.
  • Bokashi leachate: A fluid extracted from the bokashi compost, some effective microorganisms as well as other materials.

Anyone interested in bokashi will at some point hear about EM-A and how it can prevent plant diseases in combination with bokashi. There is plenty of information out there about how bokashi strengthens the microbes in the soil. And bokashi will of course also be of interest when we talk about late blight and how to fight it. I have seen this subject discussed in so many bokashi-themed groups on Facebook. Many of these threads are riddled with advice about composting blight-affected plant parts. Don't!

The composting process will probably go as planned, and the plant material will turn into soil just like everything else. But what about the disease then? Does it disappear in the fermentation process when you do bokashi composting?


Expert opinion

I'm no expert in late blight and composting, but my intuition tells me that it's not a good combination. I decided to ask a few actual experts to see if my hunch turned out to be true:


Malin Hultberg

I started by calling the Swedish University of Agricultural Sciences and got a hold of Malin Hultberg, associate professor at the Department of Biosystems and Technology. So, should you really use plant parts affected by blight in your composting?

– I don't think this is a very good idea, that's my spontaneous reaction. But it does, of course, depend on whether or not you are going to use it for plants that are susceptible to late blight.

Malin continues to tell me about late blight. It's caused by an oomycete that can spread to the soil through the water. It can stay in the soil for up to 3-4 years.

– Blight is a real menace, and I really doubt that you can get rid of it through fermentation alone.

Malin Hultberg also tells me that bokashi has become very common just these past few years, so there's not enough research into the subject quite yet. As of now, there is no scientific evidence supporting the notion that bokashi can deactivate the disease and stop it from spreading further.

– We need more research to know for sure, we can't just trust our own experiences in the garden. There are so many different factors at play here, like weather, soil quality and more.


Lena Israelsson

Lena Israelsson is a Swedish garden writer who knows a lot about late blight. According to her, late blight is becoming more and more common. I wanted to ask her about late blight and composting, does bokashi composting really remove the disease?

– There is absolutely no research supporting the claim that bokashi composting works this way. It's difficult to say exactly what happens when we bury diseased material though since blight first and foremost spreads through the air. But all in all, I don't see why we should risk it.


The bokashi companies

I also decided to get an opinion from three of the largest bokashi producers online and asked them about what they tell their customers and followers on social media. Two of the companies completely discourage their customers from composting diseased plant parts with bokashi, since the disease can spread in the garden even after the fermentation process. The last company is officially more positive to the idea, but the company hasn't replied to my questions.

Claims about the benefits of composting blight-affected plant parts seem to be unfounded. Or at most, based on personal experience. The general idea is to compost the plant parts in buckets (like we usually do with bokashi.) According to some comments on Facebook, the blight spores will decrease during this process.

But how do you know for sure that this is the case if there are no studies proving it? You can't see the disease itself and it really doesn't matter if the spores are few, they will still multiply and spread to new host plants if they get the chance. That's just the way it is.


Burn it!

So, there doesn't seem to exist any proof that the fermentation process kills the spores. The blight on the plant parts will most likely still be there after the fermentation process too. Even despite the fact that the plant parts are also covered in the finest little microbes, you can give them. This is what the experts seem to have concluded at least.

If you get late blight in your garden, an entire season's worth of work can be completely ruined in a day or two. Anyone can write about their experiences on social media, but if we don't know for sure, it's not a good idea to start experimenting. There are plenty of things we can be slightly careless about in the garden, without too many consequences. Experimenting with late blight is not one of them. Composting is great of course, but we don't need to use every single plant. Especially the blight-affected ones.

So, what to do then? Well, my advice is to burn them!


More research ahead?

Last but not least, Malin Hultberg at the Swedish University of Agricultural Sciences became very curious about this discussion and said that she wants one of her students to do a dissertation on the subject. They have late blight in their lab at the university, and it would be extremely interesting to know more about what happens when the affected plant parts are fermented. So exciting! If the results show that bokashi is the answer, then I promise that I'm going to make a huge bokashi project with diseased plant parts. Until then, it's not going to happen.

The series continues, my next article about late blight is going to be about what you can do if your plants have this disease.
/Sara Bäckmo

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