@_Maq_
Pythium, Phytophthora, and
Aphanomyces -
oomycetes (water moulds) - they are more closely related to algae, many cause root damage.
Thanatephorus - true fungi, they can damage all parts of plants, usually those in contact with soil.
Botryotinia, Fusarium - true fungi, associated with damaging leaves and green stems.
As you mention, many adapt to reproduce under water (hydrophytes), damp conditions (hygrophytes), and many are fully aquatic. For instance, most Botryotinia probably prefer to sporulate in the air, whereas we know that most Oomycetes prefer water.
Baths can sterilise in-vitro plants, but rarely will you find a chemical fungicide that will persist once they have been planted or submerged. There have been a few persistent and systemic chemical fungicides developed over the years, but they tend to have associated toxicology. If you did start using synthetic chemical like etridiazole or tetrazole, then we might not be able to help you in terms of how safe this is for your aquarium. Certainly these chemicals are effective against oomycetes and may not harm your plants at the right concentrations, but it is hard to know how persistent they will be, or whether they will harm your microbial balance and aquarium animals. A wide variety of non-persistent chemicals are actually quite suitable for sterilising baths, but they probably do not achieve future protection. You can certainly try sprays like dilute potassium sorbate if you are doing a dry start, and want some level of interim protection against mildew and fungi that attack leaves and stems while they are exposed to the air. But when you submerge them into your tank this will be washed away. I would not suggest dosing your tank with potassium sorbate - I have to confess to
@seedoubleyou that this could probably harm other microbes, like your aerobic filter bacteria (at certain concentrations), so really this is only suitable for dry starts and emergent growing.
Your best method for protecting in-vitro plants is going to be to inoculate them with a broad-spectrum formulation of Biological control agents (BCAs) before planting. The fastest and most effective way to do this is to shake your BCA powder into a suspension, and then spray it over the plants at least 72 hours before planting. If your in-vitro plants are coming in tubs as opposed to packets, then you can leave them to inoculate for a lot longer, and in some circumstances you might be able to simply sprinkle your dry BCA powder over the top of them and wait (days or even weeks). The longer the better, because you get higher rates of inoculation. However, remember that many BCAs are susceptible to chemical fungicides/biocides, and this is significant if the producer has treated them with these beforehand. There are thousands of BCAs to choose from, and those in the
Trichoderma genus tend to consistently get the best results, and can offer the best all-round protection against all of the plant pathogens that you mentioned. It does get very species/variety-specific, both in terms of which pathogen is at play, and which variety of BCA defeats it. Greenhouse producers will often find that they need to try numerous strains before they find the right one to counter a resistant disease outbreak. For that reason, many commercial BCAs are mixtures of different strains and species.
There are a few BCAs that have been developed specifically for hydroponics that may be of interest. There is a lot of anecdotal support that hydroponic applications are effective, but I have seen very little research if any that has demonstrated that this is the case: One researcher feels that a <
lower water pH is beneficial>, but I would take that opinion with a big pinch of salt until it has been corroborated, because it seems like a
post-hoc ergo propter-hoc assumption. Other research is really still in it's infancy, so it's up to you. Potentially you could add BCAs to your water column, or mix them in with the aquarium soil, but sadly I have no idea whether this might work. The best "terrestrial" results usually come from spraying, irrigation, or mixing BCAs into the soil (usually in that order of precedence). Nobody seems to have developed any BCA products suitable for treating submerged aquarium plants. Even if they did, there isn't enough research to substantiate it would work, and it would be like trying to hit the bullseye on a dart board blind-folded. You best bet for now is to simply find a way of inoculating them before you plant them, perhaps in emergent growing tubs or with a dry start.
It would be reassuring if in-vitro aquarium plant producers considered inoculating their cultures with BCAs beforehand, and let us know what they included.
Other results I have seen indicate that some sort of UV light is effective, but this has personal health risks; and to be honest, you could just add a chemical fungicide to your water column and ride it out, aiming to establish your microbial balance and biological filtration at a later date, while the plants adapt to the stresses of growing under water. A holding tank would be rather useful for this purpose.
The point about ABA as a phytohormone is that it could potentially be added as a preservative. This would keep the in-vitro plant in a dormant state and avoid wilting until opened, but conversely could cause existing plants to shut their stomata and suffer stress if it is released locally into the water column in high concentrations, because in-vitro plants could be positively laced with ABA for all we know. This does sound like a crazy conspiracy, so I apologise, because it is exactly that. It comes from the theory that ABA can lead to odd events in aquariums after plants are cut and this hormone is released from existing natural plant tissue. On a related note, auxin seems to do <
odd things> to aquarium plants. The truth is that nobody can really prove that phytohormones (PGRs) are a big issue in aquariums, but to me they seem to be the missing link to resolving a whole host of problems. Organic molecules are like that... always a bit side-lined ...but to some of us very significant. I try not to mention them too much because it gets a massive debate going about allelopathy, bioremediation, and algal blooms; and hopefully the theories will become more commonplace if they appear in future aquarium plant books.
@ElleDee - PGRs are quite easy to add to hydroponics at known concentrations. I guess that it would be easy to add them to aquarium water. To me, Purigen seems to be the antithesis: I would imagine it works by removing them from the water, which I have always felt is why people feel it works well against algal blooms. But I do think that some of the synthetic PGRs have huge potential for delivering very interesting results in terms of plant growth, and possibly no impact at all upon algal growth.