Re: "damping off"


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Abgeschickt von Peter Corkhill am 14 April, 2000 um 21:23:02

Antwort auf: Re: Question regarding pot culture von Alastair Robinson am 30 Maerz, 2000 um 12:04:58:

: “While I am not certain about the link between root rot and the pH of your water, I can tell you that damping off (Phytophthora spp.) isn't greatly affected by pH, though some claim that it is limited under more acid conditions. A variety of factors do affect this fungus, including the type of soil used and consequently its constitutive microflora.”

I have also experienced a lot of trouble with a “damping off” type of organism in seedlings grown under lights. I suspected Phytophthora (is that how to spell it?) and sent samples of plants newly keeled over, plus compost samples and water samples from the water butt, to the Government Plant Pathology Lab – they found nothing but requested more samples. The second time they isolated Bortrytis (no surprise! but I think a secondary infection of dying leaf material)and a Verticillium sp which they told me had not been isolated from orchids before. Verticillium also cause a variety of wilts in other species. I suspected the organism came from composted bark I had mixed into the compost (some Verticillium affect trees) I was advised to treat with a fungicide called “Fongarid” (can’t remember the name of the active ingredient but I have it at work). This is used as a compost drench and seems to prevent further spread but will not cure seedlings already affected – it doesn’t harm the seedlings.

I have changed the organic compost fraction as a result of this trouble and now use beech humus instead. Beech leaves contain chemicals which inhibit fungal decay and I hope this will make life difficult for the pathogens. I am also running an experiment treating some seedlings with a routine monthly drench of Fongarid during the growing season and so far have no trouble in those batches which are now 12 months in compost. I have toyed with the idea of sterilizing the seedling compost before planting and was interested to learn recently that in South Africa Disa seedlings are only weaned into sterilized compost (a very damp Sphagnum based mixture).

There is no doubt that the damping off is worse early in the season, in damp poorly ventilated conditions and encouraged by watering too much. I think everyone who grows these plants is going to encounter this or similar organisms sooner or later. Carl’s tip of watering from below early in the season will I am sure prove to be very helpful and also the pine duff in his compost is very free draining so shouldn’t get too wet and soggy.

I think the real question is why do some seedlings in a community pot die of fungus infection while others getting identical treatment grow into healthy plants? My guess is that the ones that live just chance upon a symbiotic fungus first which invade the tissues and protects from further invasion by pathogens.


Peter



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