Bonsai pine disease – and some tips to help

19th July 2015 - 5 minutes read

In recent years a disease affecting pines has become much more common in the wild in the UK, it is well known to foresters but does not seem to have been recognized by Bonsai growers. At a recent exhibition it was obvious that many trees were infected with the disease and that the owners were not doing anything to mitigate and prevent the spread.

The disease is Dothistroma Needle Blight (commonly called Red Band due to the characteristic symptoms ) and is caused by a fungus. It can affect all pines but due to the [revalence of only a few that are widely planted for forestry purposes it is most commonly recorded on Corsican (Pinus nigra) Lodgepole (Pinus contorta) and to a lesser extent Scots (Pinus sylvestris). However where it comes into contact with other pines it also regularly infects them. I have not seen any science on the affects this disease has on the bonsai favourites of Japanese White, Red or Black pines but, as I witnessed at the exhibition these trees were displaying symptoms as well.

rbnb_symptoms02An infected tree will display the red banding which has a three tone banding, starting yellow as the needle starts to die, with a central red band, then when emitting spores and very fine black central strip (photo courtesy of Forest Research). A good image to remember is that the needle looks as if it has been pinched and died back at that point as a result. The needle then dies from this point outwards to the tip. It will quickly infect needles immediately adjacent and it is not uncommon for two needle pairs to show symptoms on both needles.

rbnb_symptoms04

Infected needles display the infection most fully in June and July, after which the needle will fully die back and fall. This reduces foliage to the freshest needles at the tips of twigs giving a Lion’s Tail appearance (see photo right of a tree in the wild) – totally ruining those well crafted foliage pads on the bonsai. The loss of needles rapidly slows the growth potential of the tree and can very easily lead to an owner over-watering the tree and rotting the roots (less needles = less need for water). In the worst cases the tree will die if left.

So what can we do? The fungus spores are now prevalent across the UK and unless you can totally protect your tree from the external environment (say in side a greenhouse) then attempting to treat the spores with a fungicide will have limited benefit unless repeated at an almost daily intervals as once the spores are inside the needle treatment will be pointless.The first thing I would advise is to very carefully check any trees you might consider buying for signs of ill health. In forestry trees the native pines show much more resilience that exotic ones (i.e. Scots vs Corsican) and this may be the case for continental imports that are not so ideally suited to our cool damp climate (which sadly is perfect for the fungus to develop and spread!)

However what I have found seems to contain the disease is just to regularly pick off needles showing signs of infection before they can sporulate and would then rapidly infect the rest of the tree. Sure you lose a few needles but we needle pluck anyway. By leaving a few more needles than I normally would, the removal of the infect ones balances out over the season. Only the show trees get a full ‘tidy’ the others, for their continued health, look a little more unkempt with a few out of place needles. Regular vigilance is needed from May onwards and a thorough search once a week seems to be adequate – besides I have found that I have really had to keep watch for aphids on the pines this year as following the mild winter they were out in force …and anyway who doesn’t need another excuse to spend more time with their trees?

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