Down to Earth – Episode 12: Soil borne diseases (1990)


[Music playing] (Kevin Handreck) Our garden soils and potting mixes absolutely team with countless billions of microorganisms, most of them are beneficial to our plants, but a few, a very few, given the right conditions, can attack the roots of our plants and absolutely destroy them. These tiny worms are parasitic nematodes. They attack the roots of many of our plants. In these tomato plants they’ve caused yellowing of the leaves and very stunted growth. As well as these lumps on the roots. This tree probably fell victim to root rotting fungi last winter. With a diseased root system it had little chance of surviving a dry summer. Have a look at these two fuchsias. They’re both the same variety, but this one is obviously much healthier than this one here. My guess is that there aren’t any roots on this one anymore, they’ve been killed by some sort of disease causing organism. Let’s have a look. Well you can see that there’s hardly any roots there at all. There’s just an odd one poking out, and they’re all brown anyway. Let’s compare that with what we’ve got in this pot here. Well there’s really no comparison, is there? Look at all those nice, fresh white roots there. That’s what a really healthy root system should look like. Well what do with this one? All we can do is throw it away. But we can in fact prevent it from happening again. Let me show you this here. I’ve got the same mix in this pot as there was in that diseased pot there. So I’m pouring water on the top. It’s barely dripping out the bottom. That’s a very tight mix. The drainage is very poor, and that means that there’s very little oxygen in that pot, and therefore there would be very little oxygen around the roots of a plant in there. It’s bound to get diseased. By comparison, under this plant here I’ve got a very open mix. Pouring the water on and see how it pours out the bottom? That’s what a potting mix should be like. And drainage is also important in our garden soils. If a patch of soil remains wet for a long time that really encourages root disease. What we have to do is to drain the water away from that area through pipes, or build up the area with more soil. If you’ve had problems with root disease in your garden, the simplest way of getting rid of the nasties is to solarise the soil. The first step is to dig the soil, then smooth it over, and then water very deeply. You need to have at least the top 300 millimetres of soil quite moist. The next step is to cover the wet soil with thin, clear plastic film. The edges of the plastic have to be covered with soil, like this, so that none of the moisture from underneath the plastic escapes. And then of course we have to weigh it down with bricks, like this, so that it doesn’t blow away in the wind. The hot summer sun beating down on this plastic will heat the soil to about 50, 55 degrees C, and that’s enough over a period of about four or six weeks to kill most – not all – most of the nasties in the topsoil. There are many chemicals that are very effective at killing soil borne diseases, but the problem is that they’re also toxic to humans. I think it’s far better to prevent disease in the first place, make sure that drainage through your soil and your potting mix is adequate, and then use such non-polluting methods as solarisation.

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