One of the most common causes of Phalenopsis mortality in cultivation is the presence of what are called endophytic bacteria. These live between the cells, and are normally tolerated if not benign. When the environment is not to the plant's liking, however, its immune system weakens / the cells leak nutrients/ slow growth means that sugars pile up in the cells. Whichever of these is predominant, the bacterial population explodes, dissolving the tissue and creating what is often called crown rot. Young, expanding tissue is most affected. In dry situations, this generates pocks and blotches, areas of dead leaf and so on. In wetter conditions, it eats the plant whole. Either way, once it starts it seems fatal unless treated.
Potting materials can become bacteria-saturated (something often associated with over watering and most of all, over-fertilising.) Normal brown garden soil contains about a billion microorganisms per gramme, but rotting potting material can be up to half living organism by weight. Under these conditions, defences are overwhelmed and the roots take up bacteria with the water that the plant needs. That of itself is not the end of the matter, as it is also other undesirable growing conditions are needed to turn the invasion into a conquest. In the case of Phalaenopsis, the chief of these seems to be temperature extremes: midday spikes in temperature, watering with an icy spray. I have no evidence for what follows, It has worked for me, but it may kill your plant stone dead. I cannot take responsibility if you choose to follow the steps that I describe and you find that the results are not as you would wish. Once the disease is recognised, one can halt it by first cutting off the affected parts. Second, by immersing the plant bare root in a suitable disinfectant, typically one of the bactericidal surfactants such as Physan and its many cousins. Third, by re-potting or mounting the plant. If potted, it needs a mix such as very coarse bark. One then drenches with the same disinfectant weekly until growth resumes. If growth resumes... Maybe... Frankly, this is only worth doing if the plant is in some way hard to replace, as - so far as my experience is any guide - they will never thrive. It is better to see the Phalaenopsis as the "pit canaries" of your collection, signaling that all is not well. If the potting medium looks good, and the roots tips are un-gnawed, suspect temperature extremes. The issue of root tip loss is also important. Root meristems do all sorts of things, including secrete many cytokinins that keep the plant non-senescent, so their loss matters a great deal to the plant. The chief culprit are usually slugs, tiny ones of which can live invisibly in the potting compost. Metaldehyde pellets are usually effective, and you can also drench with metaldehyde emulsion. (See safety notes below, However, Metaldehyde has been used as a human soporific, and is widely used as a fuel in camping stoves.) One pest which is frequently overlooked are wood lice - I think called 'sow bugs' in the US, These seemingly benign invertebrates often dine on Phalaenopsis root tips. There are granules which kill both them and slugs, but they can be hard to purchase. You may wish to consider the option of drenching the pots so affected in a spray-strength suspension of a synthetic pyrethroid insecticide in order to eliminate them. Please note the following in respect of all chemical mentioned in this note. . The decision to use such products is yours alone, and whilst is offer this advice in good faith, I also take no responsibility for any adverse consequences which you may encounter. You should use gloves and take reasonable precautions to avoid exposure. In respect of the pyrethroids, you may wish to note that the same chemical family is used for sheep dips, flea powders, insecticidal shampoo intended for human use, cat collars and cattle ear tags, even insecticidal carpet soaps. The toxicology as it applies to mammals like us is reasonably well understood. Useful names: bifenthrin, cypermethrin. ______________________________ Oliver Sparrow +44 (0)1628 823187 www.chforum.org _______________________________________________ the OrchidGuide Digest (OGD) orchids@orchidguide.com http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com
