Wednesday 21 November 2012

Growing healthy plants

Aster novae-belgii is a martyr to powdery mildew. Why is this one so healthy?
More questions than answers

  • If you make a list of pests that can attack a single plant it will shock you. I have thousands of different plants and have only sprayed four times this year. Is plant health the norm?
  • Why do good growers consistently produce healthy plants, and, let’s face it, some gardens are somewhat disease prone? 
  • Why does spraying against one pest encourage a different one?
  • Why do some gardeners reject safe, synthesized pesticides but are happy to use poisons as long as they are natural?
  • When you have a sick plant why does one expert pontificate about virulent pests and disease when another advisor asks about how it was grown?
  • Could it be that most health problems are disorders not caused by pathogens?
You can almost hear the snail’s rasping radula. My Scarborough Lily is rather tatty.
An evolutionary arms race: coevolution
Pathogens and their hosts have often evolved together. There would seem to be a spectrum of relationships with virulent pathogens at one end and symbiosis at the other.
  • Why are some pathogens specific to a single host? Why does chocolate spot fungus only attack broad beans? Its spores are everywhere. Are other plants resistant?  What’s going on?
  • Is there ever an advantage for a plant in nature to be attacked by a pest? It has been suggested that a plant’s ‘spare’ sugar in an aphid’s honeydew drops to the ground and encourages valuable free-living nitrogen fixing bacteria.
This is the first post in a new series about sick plants
I want to suggest answers to some of the above questions and explore what a gardener can do about plant disorders. He will have in his armoury the following solutions
  • doing nothing at all
  • spraying
  • cultural  control - little tricks and sound growing practice 
  • biological controls - these can either be highly successful or a complete waste of time
  • natural control - giving a nudge to nature’s predators and parasites 
I want to explain how some health problems are multifactorial. How there can be a syndrome of related factors. These include growing conditions, climate, pathogen, age and species of plant. 

An example of cultural control
Most gardeners will be familiar with controlling blackfly on broad bean by pinching out infected shoots. The principle is that if the concentrated infection is removed, then nature’s predators and parasites will keep the other aphids in check. In my own garden I apply this idea to plants such as shrubs and herbaceous perennials: the secateurs are my weapon.

This brassica leaf is badly infected with mealy aphid, but the rest of the cauliflower plant was almost pest free. But look, in cutting out the aphids, I have also inadvertently removed a few swollen honey coloured aphids. They are parasitized by a wasp called aphidius, whose larvae have eaten the aphid’s innards. Nature’s not nice!

And just one answer
The aster in the opening picture is healthy because
  • it’s been wet this year
  • the plant is in an open, sunny position
  • although Aster novae-belgii is susceptible to powdery mildew, this particular variety is more resistant than most


  1. So how exactly do you run a "no dig" garden? The ere must some cultivation surely. Where can I find the information, I do not seem to be able to find it here, perhaps due to my ineptitude

    1. Use the search boxes at the top and bottom of this blog (They do different things). My bog is littered with 'reasons to dig' and 'why not to dig' and how to do it. 'Converting to a no dig system' might be useful. Put in the search box the shortest phrase possible 'Converting' is enough


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