Saturday, 26 April 2014

Horse chestnut scale and azalea gall

Slightly Unusual Problems
Horse chestnut scale
      I was looking at my picture library and a couple of things stood out which may be worth a few words: the first one is horse chestnut scale (Pulvinaria regalis). The poor old conker tree, which is currently suffering attacks by a number of enemies, has lent its name to this one although lime, sycamore, cornus, elm, magnolia, acer and bay laurel are also prime targets. It is an unwelcome immigrant from Asia, probably introduced on plants, and is most common in urban areas. Apparently the reason for its prominence in these parts is down to the fact that gardeners and park-keepers sweep up leaves and, lodging on them, the pupae of a tiny parasitic wasp which influences the spread of scale - a case of babies and bathwater in the cause of tidiness. On occasion a whole trunk can be covered with the females shown, giving it a mottled appearance.

      The females shown in the picture breed by a process of parthenogenesis - don't mate with males- so that a close look shows them looking pretty bored. The white 'skirt' is where the eggs are, about 2,000 per scale, and mother dies when she's finished laying. The corpse remains attached to the egg sac, giving some protection to the contents but presumably not much satisfaction to mum.

      All scale are sap-suckers but usually the host plant is so large that there is little real damage apart from aesthetically. This is good because controlling the problem on a large tree, short of burning it to the ground, is next to impossible. This brings to mind the old Benny Hill sketch where he pulverises a tomato plant with a large lump hammer:

      "Don't do the tomato much good", he says with a cheeky smile and what I take to be a west country accent, "but it gets rid of the greenfly". For more about horse chestnut problems go to this.
Azalea gall
      The second thing worth a word or two is the Azalea gall (Exobasidium vaccinii). I first saw this many years ago on an evergreen azalea at Bodnant Gardens and, in my ignorance, thought it was a fruit. However it turns out to be a fungal problem and it fairly quickly becomes white with spores which are thought to be passed to other plants via rain splashes. From what I can make out, the one in the picture is fairly unusual in the regularity of its shape - usually it is far more lumpy and can look a bit like peach leaf curl.  Flowering can be reduced, so the galls are best removed before reaching the white, spore spreading stage. I have seen a recommendation that the infected plant should be sprayed with Bordeaux mixture early in the year, but have no experience of the efficacy of such treatment, so retain an open mind.

     The main thing my researches have come up with is the fact that no one seems to know a lot about the problem so, if you're really worried about it, try growing spuds.

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