Translucent spots on leaves are due to the breakdown in cellular structure,
which is accomplished by microorganisms acting either alone or, more commonly,
as a collective. Thus fungi may liquefy cell walls to allow bacteria to enter,
break down the inner constituents and thrive. The fungi then (may) dissolve
the bacteria and absorb their inner ooze, and so on.
The question is why this happens in some cases and not all: why does
everything not rot? The answer lies in the shifting balance between defence
and attack. If a plant is weakened, or if the attack on it is intensified,
then it is plainly at risk. There is one common situation which alters both
side of this mortal balances for the worse, and that is a poor rooting medium.
Air and water bring hopeful spores to the upper part of plants, but the soil
is a continual battle field in which the level of challenge is vastly much
greater. Humid, dark soil may contain a billion microorganisms in a gram,
probably presenting as tens of thousands of species. The plant is capable of
restricting the entry of these into the roots under normal conditions. If it
is weakened, however, perhaps through poor light or incorrect temperature
regimes then these defences are also weakened. It becomes prone to invasion.
Equally, however, the application of fertiliser to the soil or rooting medium
leads to a boom in the microorganism community, which may increase in numbers
by a factor of a hundred or a thousand. The type of organisms that thrive will
change sharply when the medium is poorly aerated, which happens when the plant
is over-watered, for example, or when the medium breaks down. The
over-cosseted orchid, watered daily and fertilised to excess will exist in a
soup of bacteria that are busy breaking down the potting medium into an
airless slough that is saturated with organisms that its rots cannot
withstand. A downward cycle begins. .
The population of microorganisms begin to erode the roots. They penetrate to
the xylem, the system of tubing which takes water up to, amongst other
destinations, the leaves. Once inside these, they proliferate within the
intercellular spaces and set to dissolving cell walls. That is when the
translucent spot appears: far too late to do anything much that is useful.
To prevent this, grow dry. No plant should be permanently wet unless it is
specialised in such environments. If you have a plant that is showing the
signs mentioned, you may care to consider the following as a logical
development from what has just been discussed. I do not offer it as advice,
however, an will accept no responsibility for any loss which may be incurred
as a result of following this rationale.
= First, dispose of the current growing medium.
= Second, cut off the parts of the plant where the rot is focused.
= Third, immerse the plant completely for half an hour in clean
water treated with the manufacturer's recommended concentration
of a quaternary ammonium disinfectant such as Physan.
= Fourth, repot in the lightest granular medium you feel that you
can accept.
Phalaenopsis species, which are notorious martyrs to this kind of infection,
are for the most part happy when mature with 2 cm chunks of bark in an
open-mesh pot, or mounted dry on a slab. (Some are damp growers, of course,
and one has to "read the label" before acting.)
I have found that a weekly treatment with a systemic fungicide is also
extremely helpful in managing this and related problems. Once again, this is
not advice to the reader but merely a description of my own practice. If you
choose to follow this you do so at your own discretion and risk.
Some months ago, I asked whether anyone had experience in managing necrosis in
young Paph. leaves. The response was limited. Since then, I have found that
moving from monthly to weekly fungicidal treatment has completely removed the
problem. (Or the onset of the British winter has done so; but we shall see,)
There are many good systemic fungicides now available: I use propiconazole,
which is systemic, lasts well and has low mammalian toxicity (rat LD50 about
1.5 g/kg.) I use the lowest end of the recommended concentration, and apply
uniformly to leaves and roots, where exposed.
______________________________
Oliver Sparrow
+44 (0)1628 823187
www.chforum.org
_______________________________________________
the OrchidGuide Digest (OGD)
[email protected]
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