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Abgeschickt von Alastair Robinson am 18 April, 2000 um 23:37:49
Antwort auf: Re: "damping off" von Peter Corkhill am 14 April, 2000 um 21:23:02:
:I suspected Phytophthora (is that how to spell it?) :
Yup. It is interesting to hear that they were able to isolate Verticillium from your material. As you rightly point out, it is not one of the fungi readily associated with orchids. Species such as Verticillium dahliae demonstrate rather species specific toxins, though others, such as V. albo-atrum, have wider host ranges. Did they detail the species isolated, or was it not identified at that level?
Verticillium is one of a number of so called 'vascular wilt' fungi (the group includes the commonly observed Fusarium and also Ophiostoma) - species of this genus often bring about the rapid death of any plant that they successfully infect, with treatment being difficult, as you observe, because they are able to invade the vascular tissue (mainly the xylem) and reach all parts of the plant.
This genus has a particularly fascinating mode of spread within the host. Entry is through the apical region of the root, where the endodermis is not fully differentiated. The fungi are able to grow through this region and reach the developing protoxylem, infecting the rest of the plant from here by growing through the vascular vessels and tracheids, passing from cell to cell via pits. Rather uniquely, further long-distance movement is achieved by the production of microconidia (small asexual spores) which are carried in the transpiration stream to all parts of the infected organism where they may germinate.
This mode of spread is much more rapid than would be possible by mycelial growth, and though the Verticillium are fairly tissue specific, the pathogens can become virtually systemic.
: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:
While this is established, would it not also be possible that the advent of new roots early in the season might present a temporarily increased risk of infection? Prior to their maturation, they are comparatively soft and vulnerable to damage, and so one would think that organisms that characteristically infect plants by jumping into the rapidly dividing apices of roots would have a party during this period.
: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:
I can't give you any examples for orchids, but would back you up on this as it is extremely common in other families of monocots and eudicots alike; Trichoderma harzianum and Paecilomyces lilacinus are both fungi that are used commercially to protect specific plants from certain pathogens. Do note that such biocontrol is not always the works of *symbionts*, per se, but often of beneficial commensals; as you will know, many a time, orchids are degraded by the fungi that they are supposed to depend upon.