Mechanisms causing root death

[K-Sci]

If orchid roots rotted because infections set in when roots remain wet too long, neither growing in sphagnum nor semi-hydroponics should be viable growing options. Obviously the mechanism causing death from over watering requires something more than prolonged exposure to moisture. Some of the candidate modes of action sometimes theorized include:

1. Suffocation when sodden potting media blocks oxygen availability.
2. Suffocation when aerobic organisms consume the available oxygen when breaking down media.
3. Extremely low local pH caused by carbonic and organic acids produced resulting from decay.
4. Hydrogen sulfide produced by decay occurring in the absence of oxygen.

The growing techniques orchid growers use to prevent root death from over watering address these in two ways.

• They eliminate or reduce the organisms of decay that could deplete oxygen, produce acids, or release hydrogen sulfide. Examples include potting in scoria, Stalite slate, charcoal, Styrofoam, sphagnum, or LECA.
• They assure high oxygen levels throughout the root zone. Common techniques include growing on mounts, in baskets, and the use of sphagnum and LECA.

Semi hydroponics and growing in sphagnum are special cases because they appear in both the preceding lists. Sphagnum resists decay and contains many tiny air spaces that assure oxygen availability to roots. This is true so long as the moss isn’t so saturated that the spaces are filled with water. The LECA used in semi hydroponics does not decay and is porous to oxygen.

K-Sci

[Shadeflower]

not sure if this is an extract from a future book in the making or if we are meant to discuss it but I'd have issues with some of these statements if I read them in a book.
I don't think the substrate rotting displaces oxygen or even if it does I don't think that is the big issue. The issue is that rotting material literally infects other material.
You place a rotting banana next to a fresh banana and the fresh banana will go off much faster because it is next to a rotting banana.
It's the same principle with roots, if they sit next to some rotting bark that starts developing fungus it will start to spread onto live roots and affect them too.
The ph dropping will not affect the oxygen but it will affect the plants ability to absorb nutrients. If the ph in the pot drops too low then the plant can't absorb certain nutrients, becomes deficient and is more susceptible to get sick as a result.

If you look at seedlings in a flask, they sit in a wet substrate in a sealed jar with no air exchange. This should be according to the assumptions be a sure fire way to disaster but it isn't. It only start to become a problem if the flasking media becomes contaminated and that in turn contaminates the roots. If the flasking media becomes contaminated then the seedlings suffer from dampening off very often. Dampening off is a fungal infection that can kill the whole plant so maybe it's not quite the same as root death but it is all related to the plant health and arguably far worse than just losing a root to suffocation.

I'd also have the question why lecca or moss are special cases compared to mounts or baskets but maybe I am skipping some chapters that adresses my points seperately. Like I grow using perlite, pumice and bark. These haven't even been mentioned

I don't want to place any less importance on roots growing airily via the methods used, I think it is vitally important but because I have discovered that orchids suffer before they lose roots. You can grow orchids "too wet" - like full water culture for example but in my experiments, reduced air exchange around the roots causes reduced growth. Long term it leads to roots dying but ideally you want to avoid that stage by providing good growing conditions that not only will encourage good growth but as a result also discourage roots dying.

So air around the roots is important. More important than just preventing roots from dying and if they do then the orchid will have already been suffering before that happens.

[SouthPark]

ksci ------- certainly - as shade mentions ------ various considerations can be made, which include particular portions of root possibly getting negatively impacted due to stagnating regions within the pot, as in not much or no water movement. Temperature will be a consideration too, and anything else that's in the media at various time ----- including pH. And also predicting what's happening at various different layers down in the depths of the pot or media.

I sometimes associate the growing of orchids with aquarium fish keeping. In general, the experienced aquarium keepers have some knack of keeping their fish alive for really long periods of time ------ avoiding disease and dying etc ------- by doing what they can (and know) to keep the conditions in the tank satisfactory - temperature, pH, clean enough, water adequately oxygenated, and anything that stops fish from developing diseases or getting attacked by parasites.

I guess it's like us looking after ourselves and other pets. Providing suitable conditions, and warding off disease, and providing suitable treatments to get out of bad situations when needed (and if possible).

[Ray]

First of all, healthy tissue does not rot. The roots need to be weakened sufficiently for a pathogen to attack them, or literally die before microbes decompose them.

In my opinion, by far, the most significant cause of root death is suffocation. That can happen if the potting medium is too fine and dense, allowing surface tension to hold water between the particles, stifling gas exchange. With coarser mixes, or when using a potting medium that absorbs more fully over time, that is less of an issue, until of course, the medium starts decomposing (bark) and/or is simply compressed by overhead watering (sphagnum), both of which converts large voids to small ones.

The presence of microbes is apparently not much of an issue, because the plant can fend them off to some degree, and those we add as probiotics are perfectly capable of decomposing necrotic tissue, yet we see a reduction in ailments through their use, not acceleration.

In the case of Quantum probiotics, they are packed under H2S, as that apparently puts the microbes into “hibernation” in the bottle, giving it an extended shelf life.

Another significant factor in root death, in hot-growing plants especially, is letting them get too cold when wet. Cold is bad enough, but also being wet seems to be much worse. I haven’t given the mechanism of that much thought, but just try to avoid it.

[isurus79]

Originally Posted by Ray

Another significant factor in root death, in hot-growing plants especially, is letting them get too cold when wet. Cold is bad enough, but also being wet seems to be much worse. I haven’t given the mechanism of that much thought, but just try to avoid it.

I've always wondered if hot growing species with strong dormancy (e.g. Catasetums) actually shut down their disease fighting mechanisms along with everything else while they're "asleep." The cold and wet could just facilitate microbial blooms and the lack of disease fighting mechanisms inside the plant permit infection. And yes, I'm totally making this up with no scientific evidence for this hypothesis whatsoever.

[estación seca]

Staying dry for too long, and low humidity, also kills some roots.

Based on posts here to Orchid Board, suffocation is the commonest cause. Improper temperatures are probably the next commonest cause. Pathogens are everywhere in the environment. They rarely attack healthy plants except in hot and very humid climates. Ensuring proper growing conditions - temperature, humidity, light - is the most important part of keeping plants free of disease.

[Ray]

Originally Posted by isurus79

The cold and wet could just facilitate microbial blooms and the lack of disease fighting mechanisms inside the plant permit infection. And yes, I'm totally making this up with no scientific evidence for this hypothesis whatsoever.

Equally unscientifically, I suspect microbes multiply faster with warmth, but that may be extremely microbe-specific.

[isurus79]

Originally Posted by Ray

Equally unscientifically, I suspect microbes multiply faster with warmth, but that may be extremely microbe-specific.

Seems like we've got a thesis topic here for any aspiring graduate students out there who might be reading this!!

[Leafmite]

Plants definitely keep the mind working and wondering. It is just astounding at the variety and how they have all managed to adapt to different environments.

[K-Sci]

Originally Posted by SouthPark

ksci ------- certainly - as shade mentions ------ various considerations can be made, which include particular portions of root possibly getting negatively impacted due to stagnating regions within the pot, as in not much or no water movement. Temperature will be a consideration too, and anything else that's in the media at various time ----- including pH. And also predicting what's happening at various different layers down in the depths of the pot or media.

I completely agree.

I sometimes associate the growing of orchids with aquarium fish keeping.

Similar chemistry challenges. Note, also, that different fish require different conditions - what's ideal for some will kill others. I think it is likely that we've not completely understood the needs of some orchids, such as C. dowiana, which are notoriously prone to abrupt death due to rhizome infections.

I guess it's like us looking after ourselves and other pets.

Ironic, I think, how so many people fail to adequately understand themselves and adequately look after their own or their family's care.


k-Sci

---------- Post added at 08:33 PM ---------- Previous post was at 08:11 PM ----------

Originally Posted by Ray

First of all, healthy tissue does not rot. The roots need to be weakened sufficiently for a pathogen to attack them, or literally die before microbes decompose them.

A healthy flourishing phalaenpisis will develop crown rot if water sits in the crown for more than a few hours. Don't you think it is possible for infections to occur in healthy tissue.

In my opinion, by far, the most significant cause of root death is suffocation. That can happen if the potting medium is too fine and dense, allowing surface tension to hold water between the particles, stifling gas exchange. With coarser mixes, or when using a potting medium that absorbs more fully over time, that is less of an issue, until of course, the medium starts decomposing (bark) and/or is simply compressed by overhead watering (sphagnum), both of which converts large voids to small ones.

I think you're right. I use slotted pots, baskets and large Orchiata and LECA for most of my orchids for just this reason. However, there are a number of secondary factors that contribute to the poor gas exchange. My post was intended to spark discussion of these.

Consider that roots can grow fully submerged, so more than just the presence of water is required, isn't it.

The presence of microbes is apparently not much of an issue, because the plant can fend them off to some degree, and those we add as probiotics are perfectly capable of decomposing necrotic tissue, yet we see a reduction in ailments through their use, not acceleration.

Can't the presence of microbes include both beneficial and extremely pernicious ones? Microbes come in lots of different flavors. What do you think will happen to roots when the lack of oxygen favors anaerobic sulfur dioxide producing bacteria?

In the case of Quantum probiotics, they are packed under H2S, as that apparently puts the microbes into “hibernation” in the bottle, giving it an extended shelf life.

I must confess that I know nothing about orchids and probiotics, but it stands to reason that aerobic microbes would stop functioning when H2S replaces O2.

Another significant factor in root death, in hot-growing plants especially, is letting them get too cold when wet. Cold is bad enough, but also being wet seems to be much worse. I haven’t given the mechanism of that much thought, but just try to avoid it.

I think you've gone full circle with this thought. Growing orchids at the wrong temperature weakens them (sentence 1 of your post).

---------- Post added at 08:38 PM ---------- Previous post was at 08:33 PM ----------

Originally Posted by isurus79

I've always wondered if hot growing species with strong dormancy (e.g. Catasetums) actually shut down their disease fighting mechanisms along with everything else while they're "asleep." The cold and wet could just facilitate microbial blooms and the lack of disease fighting mechanisms inside the plant permit infection. And yes, I'm totally making this up with no scientific evidence for this hypothesis whatsoever.

It seems likely that something like this must occur. Another effect I suspect is involved is that a dormant plant doesn't initiate the growth required to heal from microbial, mechanical, or chemical injuries.

---------- Post added at 08:56 PM ---------- Previous post was at 08:38 PM ----------

Originally Posted by estación seca

Staying dry for too long, and low humidity, also kills some roots.

There is no question that this is true. It seems likely that this results, at least in part, from dehydration, but I also suspect that orchids will use moisture in their roots to sustain the rest of the plant when put under drought stress. I can't say this with any certainty, however.

Based on posts here to Orchid Board, suffocation is the commonest cause.

I'm not disagreeing, but keep in mind that conventional wisdom is often completely wrong. Chills don't cause colds, bock beer doesn't come from the bottom of the barrel, binding broken ribs was a common medical practice until they discovered that it causes pneumonia, and the sun doesn't rise, the horizon moves up.

Improper temperatures are probably the next commonest cause. Pathogens are everywhere in the environment. They rarely attack healthy plants except in hot and very humid climates. Ensuring proper growing conditions - temperature, humidity, light - is the most important part of keeping plants free of disease.

Agree.
K-Sci

---------- Post added at 09:03 PM ---------- Previous post was at 08:56 PM ----------

Originally Posted by Ray

Equally unscientifically, I suspect microbes multiply faster with warmth, but that may be extremely microbe-specific.

Then doesn't it also stand to reason that global warming could be net beneficial - it accelerates biological processes increasing the food supply.

K-Sci