The MGS Forum
Gardening in mediterranean climates => Pests and diseases => Topic started by: Marilyn on November 09, 2011, 12:02:32 PM
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I took part in an olive picking yesterday. It was the first time I had really looked up close at these wonderful fruits on the tree - the sheer variety of colour and shape, the feel of them, the abundance, the combination of textures between fruit and leaf. Divine!
This was a young olive grove with heavy "life support" - almost constant irrigation and fertilizing from March to October, and regular chemical treatments to get rid of pests. BUT - I know that this is not necessarily needed; I have seen people here with trees in poor soils that receive no water and no feeding, beyond an occasional manuring, and these trees still produce very decent fruit.
Our trees suffer with the fly and the moth caterpillar and goodness knows what else, so we don't see much of a harvest at all. My question is: how can one treat these plagues organically? Is it possible? Will I really need to do a chemical blast first? If anyone could recommend a good book on the subject I would be most grateful, have not had much luck looking online. I am sure people must be farming organically but the message I have been getting back from agronomists and the internet so far is, CHEMICALS CHEMICALS CHEMICALS!
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'The Garden', sep11, p.27 (the main journal of the RHS) mentions olive scab, caused by Fusicladium oleagineum. It is worth being aware of.
A few points to note:
- the fungus overwinters on fallen leaves - so fallen leaves should be disposed of (and not by composting)
- infected leaves should be pruned out
- it also affects the fruit
- the spores germinate only after prolonged surface wetness (on the leaves)
- pruning trees to open up their canopies allows quicker drying of the leaves and therefore helps reduce germination of the spores (we have discussed opening the centre of the crown earlier on this thread)
It is called peacock spot in Australia (and maybe elsewhere); there are some very good photos here -
http://www.olivediseases.com/olives/peacockspot/peacockspot.html (http://www.olivediseases.com/olives/peacockspot/peacockspot.html). To see the images, you need to use the arrow button to the right of the first image, which could be obscured by the RH edge of your browser window.
That's also a great site for olive disorders, diseases and pests (though the last is somewhat specific to Australia). I didn't know, for instance, that the rounded lumps on olive trunks are called sphaeroblasts (also spelled spheroblasts).
Technical description - http://www.mycobank.org/MycoTaxo.aspx?Link=T&Rec=486814#Descriptions (http://www.mycobank.org/MycoTaxo.aspx?Link=T&Rec=486814#Descriptions)
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Mike, I looked at the link from Australia and saw something I have on one of my olive trees, olive knots. I had no idea that they were caused by the bacterium Pseudomonas savastanoi pv. savastanoi (or Pseudomonas syringae subsp. savastonoi). The farmer who looks after my olive trees called it cancer (καρκίνος), he said it was caused by frost and too much water, the tree gets grey-water from the kitchen, but I see now this is making the tree susceptible to the bacterium.
What do the experienced olive growers out there recommend, should I try to spray with copper (at least twice, in the autumn and especially in spring), or is the tree in that position doomed and should be removed? None of the other olive trees are affected.
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Though I'm not experienced and we don't even try to crop our parched olive trees, my own experience of the knots suggests that you shouldn't worry about them too much, Jorun. We were told by local growers that (as so often with plants and animals) the bacteria invade trees that are in poor health anyway, thus causing the knots, and that when conditions improve so that the general health of the tree improves, although the knots obviously remain the bacterial infection will reduce or disappear. That does seem to have happened with two trees that seemed quite badly affected but that are now clear of new infection though the old knot remain.