What are the effects of cultivations on pest and disease?
The traditional view of birds following a tractor is that they are eating pests. Come to think of it, if you see a tractor in a field these days you blink and it’s gone! The birds may well be eating harmful ‘grubs’ such as wireworms, cutworms, swift moth larvae, millipedes and all, but they are also eating beneficial predators such as centipedes and ground beetles. They are also eating worms.
Even in my lifetime students have been taught that cultivations control pests. Winter diggers tell me that exposing the soil to the elements reduces pests!
When I read on some of those fascinating vegetable blogs written by diggers, I often see concerns expressed about soil-living pests. I cannot help thinking that constantly cultivating the ground and perhaps working in organic materials and expensive garden centre potions actually creates the excessively aerated conditions where grubs thrive. I have no evidence for this and it might be a foolish notion. Perhaps it’s just that I don’t dig and don’t see them! My instincts tell me that where the natural balance in the soil is undisturbed by cultivation, pests like symphilids, springtails, millipedes and woodlice are less of a problem. A complication to my argument is that sometimes springtails, millipedes and earwigs are sometimes beneficial predators!
Don’t get me wrong, I have my fair share of sick plants. Only yesterday I cut down my plum tree as it had acquired silver leaf disease (I was actually pleased to have an excuse to give Brenda for me to get rid of it). My delphiniums this year are pathetic (all that leaching of nutrients with all that rain last year). Shame on me, I even have some glyphosate damage, my favourite Aster amellus and Aster nove-anglae are so sensitive to drift.
|Silver leaf on plum, early signs. The silvering is due to fungal toxins ascending from the trunk.
|Silver leaf fungal infection in the trunk eventually kills the tree.
|Glyphosate damage on phlox. I hang my head in shame, I have been very careless with the ‘roundup’ in my cemetery garden.
Never-the less when I survey my gardens I see thousands of healthy plants. I am of course completely blind to minor blemishes, cuckoo spit and small aphid infections. I know my brassicas will be covered in cabbage white fly in August and I might have to abandon my carrots to carrot fly later in the season. None of my vegetables would pass supermarket standards but they do taste good!
But yes, I do claim that most of my plants are healthy. I believe plant health to be the norm if plants are well grown. I have used no insecticides or fungicides so far this year. I do confess to a few slug pellets under protective fleece which is there to prevent the collared doves and pigeons from decimating my young brassicas.
(My post on slugs suggests my minimum cultivation methods which involve leaving plant debris on the soil surface contribute to minimizing the damage of slugs and snails. Contrary to the popular view).
So why do I think that minimum cultivation helps to grow healthy plants?
The essential thesis is that if plants are grown well they will be less likely to succumb to pest and disease. To me this is a firmly accepted principle in pest control. Of course there are many situations where either because of pathogen virulence and/or plant susceptibility, no matter how well you look after your plants they might become infected with pest or disease.
For over a year now I have hammered away at why minimum cultivation grows healthy plants. All these matters are inter-related
- Soil organic matter is elevated
- Soil density is reduced by spaces and pores. Soil structure is improved.
- Soil aeration is improved.
- Drainage is improved and rainfall penetrates into the soil rapidly.
- Roots grow more deeply.
- Walking on wet soil is less damaging than feet compacting cultivated ground.
- Plant roots and dormant vegetative structures are undisturbed by cultivation.
- It easier to control weeds. Some weeds may be hosts for pest and disease, on the other hand they might support beneficial predators and parasites!
- There is more biological activity in the soil.
- Worm populations are enhanced. Apart from contributing to the fertility listed above harmful organisms are digested in the worms’ gut, worms’ coelomic secretions have anti bacterial and nematicidal properties and worm-casts enhance nutrient availability. My post on worms
- Beneficial fungi are increased. Apart from mycorrhiza that aid nutrient uptake, nutrient sharing and drought tolerance, other species of fungi kill nematode pests (eelworms). It his fascinating how some fungi ‘lasso’ and digest their prey.
I would have liked to show you some pictures of soil grubs from my garden but could not find any!
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