Need help with root problem

[RosieC]

Originally Posted by Ray

As beginners, we all focus on watering - and the problems we have in doing so properly - but nobody ever switches the focus to air, which is actually more important.

It's sort-of an iterative process, but if you start by figuring out how to keep the root system airy, then you have to figure out how to get the plant sufficient water. If what you've done requires too much labor, you then have to make the root zone retain more water without sacrificing airiness.

Particle packing also plays a role (I have a piece about that on my Free Info page, too). I don't know about the rest of you, but I learned that adding small perlite particles was supposed to make the mix more open. In reality, mixing particle sizes actually reduces that.

I had started to realise by reading your posts here over the last few months, that the key to not overwatering was nothing to do with water, but all to do with air flow. Your link helps me get a better understanding of that.

I agree with you about the perlite. I'm using it at the momment but have begun to feel unhappy about the air spaces those small grains are filling. Glad I'm not the only one.

[Orchid126]

Thanks Ray. Your idea about Air Management answers a lot of questions and makes a lot of sense.

[Ray]

A few thoughts concerning water culture and S/H reservoirs...

1) Root cells are tailored to the environment they are in as they grow. Once grown, they do not change. That is why it is important to have repotting (all, not just moving into S/H culture) coincide with the initiation of new roots - the new ones will be "perfect" for the new environment, while the old ones might fail. The greater the dissimilarity between the old and new environments, the more critical that is.

2) Roots that grow into liquid will be tailored to live in liquid, so will be fine. (There is plenty of dissolved oxygen in water, by the way). Submerge roots that grew in another environment, and it's an entirely different story!

3) "Rotting" does not spontaneously occur on healthy tissue, but only on dead tissue.

It is interesting to note that some rots take a really complex series of events to occur. Consider. for example, this scenario, which we would normally hear as "over watering caused root rot":

• Your potting medium is decomposing, so becomes compact.
• You water it "normally", not knowing any better, which really saturated the compact medium.
• Water droplets bridge the gaps between particles, cutting off air flow.
• Gas exchange at the roots is compromised.
• As a response to the suffocation, the plant emits phenolic compounds to "defend" itself from the stressing agent (those are what turns the roots dark brown and black, by the way).
• The phenols however (as the stressing agent is not removed), build up sufficiently to kill the roots.
• THEN the bacteria and fungi can attack, and rot the roots.

[You know, I get paid to discuss this stuff with societies. I think I'll send each of you an invoice....]

[Undergrounder]

I agree with you about the perlite. I'm using it at the momment but have begun to feel unhappy about the air spaces those small grains are filling. Glad I'm not the only one.

Rosie if it makes you feel any better, i have many (many) plants growing in 100% coarse perlite and even rubbishy dusty small grained perlite and they love it. Obviously though its a very wet medium and so it would kill off 'dry' adapted roots similar to the way S/H does, but probably more dramatically. Deflasked, sphagnum moss and S/H roots, though, love it.

Undergrounder the situation in s/h is no different than bark you get very good air circulation since the medium never packs down.

Water culture is a totally different situation. Since rot can not grow without air the total lack of air in water prevents root rot. But you must keep the roots under water. If you let the water levels drop and refill after a while the rot will develop.

Thanks Jerry i'm interested in the theory about fungus and rot in water. My own experience agrees with yours, and often when i let reservoir roots dry off (and expose them to air), they do get attacked by fungus. But is this cause or effect? Also, there are lots of rots that love water too, ie: Pythium, pseudomonas, etc.. Do such rots require access to air at some point in their life cycle? And surely water doesn't stop rot from occurring, or we would be preserving dead animals in water and not alcohol.

So i wonder whether "fungus/rots can't grow underwater" is really the reason. I think its probably the special adaptations of 'water roots' that leads to their survival. And the protection from traditional fungus attack that requires air is probably a smaller, secondary benefit.

Roots that grow into liquid will be tailored to live in liquid

Thanks Ray, before i say the next bit, i just want to say that you seem to be one of the few people who come across to me as really understanding orchid physiology, and i respect everything you say, so keep in mind I’m not trying to be argumentative. I just want to understand things better.

I agree with you and I hate people using the word 'over-watering' as if watering is somehow harmful, or as if its possible to give a plant too much water. Its a generalisation that confuses a lot of people into making mistakes in their culture.

But i wonder whether the air explanation is falling into a similar trap. Water culture shows that orchids don't need air, they need oxygen. And however they transpire gases in standard culture, or however it may be blocked by water, transpiration is clearly not affected in roots that are submerged from the beginning. You could extend this to say that roots that grow into a fine mix that is constantly kept wet will grow and transpire just fine as well.

Talking about lack of air killing roots sounds like the same kind of talk that says too much water kills roots. I know its a small distinction between air and oxygen, but it means people out there will continue to be viciously skeptical of water culture, hydroponics, and other new methods of culture, as if the roots won't grow in water because there's no air.

This has a practical result, because in S/H culture, why do we care about the particle size at all? if its only going to be the new roots that adapt best to S/H in the first place, why not go go the whole hog and pot up orchids in small size LECA (or perlite, horticultural sand) The small particle size would wick the water better and the roots would do just as well.

It seems to me like the amount of air in the pot is virtually inconsequential in S/H for the reason that the roots will adapt to the conditions (from 100% water to 100% air) regardless. And the benefits to wicking ability and water-holding capacity (which reduces re-watering time) would suggest that small particle sizes are actually better.

for example, this scenario, which we would normally hear as "over watering caused root rot":

Would you say that example is the most common cause of root rot in 'over-watered' plants? I know you said that "rotting" only occurs on dead tissue, and i think i agree concerning most "rots". But fast root death in over-watered sphagnum seems to be caused by direct fungal attack. I've seen pin molds and white/orange spot fungus rip through roots within a day. It starts with a few spots that germinate on the wet sphagnum, which then sporulate and spread to roots. Just one point of infection and the roots go mushy within a matter of hours and by then you can't stop the death of that root.

The wet sphagnum still contains plenty of air pockets, and the death occurs far faster than any buildup of phenols could possibly account for. In all the cases of death by over-watered sphagnum it's been this that caused root death, and not lack of air exchange.

To prove this, i started pressure-cooking my poor quality moss before i used it. I've never had the same problem since.

I grow tonnes of leafless orchids as well, and they get plenty of air. I have seen many examples in them of a single point of fungus hyphae infecting and killing the whole root. I watch them daily, and the fungus didn't infect the root -after- it was dying, but it definitely seemed to initiate the death. I might be wrong, it's impossible really to tell whether the fungus took advantage of a sick root, or whether the fungus itself made the root sick, but i can say there was nothing visually wrong with that root before the white hyphae germinated.

This is why i am thinking that at least in many cases of 'over-watering', death is actually a result of you provide conditions that allow pathogen spores to germinate and spread, killing the roots. I'm sure blocked transpiration and phenolic buildup is a long-term cause of root loss in roots that aren't used to those conditions, but is it the main cause? Is it the most common cause? I don't know.

[Ray]

Great discussion, Undergrounder!

Maybe my use of "air" is oversimplified (my term "air management" is meant to encompass overall gas exchange within the medium), but I'm not convinced it's solely oxygen, either. I think the cause of root death in a compact medium is likely a combination of suffocation and poisoning from waste products originating with nutrient uptake and respiration.

Rod Venger (Venger's Orchid in Colorodo, long-since closed) originated the concept of water culture for orchids 20 years ago. He found that he needed to change the water periodically, or it became a smelly mess that was toxic to the plants. Letting a good bloom of algae remain in the container mitigated the issue to a certain extent, probably by consuming some of the waste products.

I speculate that his water culture experience was simply a more extreme case than we see in a pot, due to the even lower gas exchange and the affinity of the ions for water.

As to the root rot scenario, I think it is the most likely cause for soppy media situations. I did not mean to imply that it was the only way a rot would occur - you're right, there are lots of pathogens that can "get" us, and their vectors vary all over the map.

[Undergrounder]

Originally Posted by Undergrounder

This is why i am thinking that at least in many cases of 'over-watering', death is actually a result of you provide conditions that allow pathogen spores to germinate and spread, killing the roots. I'm sure blocked transpiration and phenolic buildup is a long-term cause of root loss in roots that aren't used to those conditions, but is it the main cause? Is it the most common cause? I don't know.

I had a bit of time to think about this, and another thread about using H202 got me thinking and made me do some research.. I'm starting to think that root-rot caused by over-watering in the vast majority of cases might be slightly different.

Firstly, phenols are a natural defense against pathogen infection, - that is their main adaptive function. Phenolic compounds create quinones and free radicals that attack fungus directly (through oxidation) and indirectly (by inactivating pathogenic enzymes created by the fungus). Essentially the production of oxidative phenols by the root around the infection site protect the roots from the infection.

This back and forth process between fungal attack and phenolic plant defense is well documented (Hammerschmidt, 2005). On the other hand i don't know how 'stress' or inhibited respiration is supposed to lead to phenol production directly. Phenols are released in the process of general tissue degeneration, but this seems to be a process occurring after an initial infection. This point is stressed in Hew & Yong (1997) in the chapter on oxidases and respiration, p.121

Now importantly, in order to produce these phenols, the root needs lots of oxygen. It has been shown that the rate of respiration increases four-fold in orchid tissues around the site of fungal infection, as the roots gather the oxygen they need to produce the phenols (ascorbic acid oxidase, peroxidase and polyphenol oxidase) (Hew & Yong, 1997). So immediately following infection, the roots take in a massive amount of oxygen from the surrounding environment.

Now when the root-zone is water-logged and deprived of oxygen, the orchid's roots are unable to produce the oxidative phenols that it normally uses to resist the fungus, because it can't get the oxygen it needs from the root-zone. As a result, the fungus is able to infect the root, killing it.

Fungal Attack + Lack of O2 = Lack of phenol defense = Infection = Root Death

Several studies support this link between microorganism pathogenicity and oxygen levels in soil:

Anoxia and hypoxia aggravate many soil-borne diseases. For example, when oxygen was artificially raised in hydroponics; that is, from a control 5.8–7% to 11–14% (Cherif et al., 1997), colonization of tomato roots by Pythium spp. was significantly reduced... Beans growing in soil under controlled conditions and infested with Fusarium solani were more severely injured by the pathogen when subjected to short periods with near zero O2 in the soil air than plants growing in well-aerated soil (Miller and Burke, 1975). More injury also occurred in the field when the soil was excessively wetted during flood irrigation. It was concluded that aggravation of root rot by low oxygen diffusion rate is the principal cause of plant stunting and yield reduction that results from temporarily excessive wetting of the soil in infested fields (Miller and Burke, 1975)... Disease development may arise due to a decrease in plant resistance to infestation or an increase in the population or virulence of the pathogen...

(Surya, Bhattarai & Midmore, 2005)

So i think that the what people call over-watering might be something more like this:

• Your potting medium is decomposing, so becomes compact.
• You water it "normally", not knowing any better, which really saturates the compact medium.
• Water droplets bridge the gaps between particles, cutting off oxygen from replenishing the root-zone.
• Lack of oxygen severely hampers the roots' natural ability to defend against fungal attacks.
• Fungi infect and metabolise the roots, killing them.

-------------

Hammerschmidt, R. (2005). Phenols and plant–pathogen interactions: The saga continues, Physiological and Molecular Plant Pathology, 66, 77 - 78.

Hew, C. S. & Yong, J. W. H. (1997). The Physiology of Tropical Orchids in Relation to the Industry, Singapore: World Scientific.

Surya P., Bhattarai N. S. & Midmore, D. J. (2005). Oxygation Unlocks Yield Potentials of Crops in Oxygen-Limited Soil Environments, Advances in Agronomy, 88, 313 - 377.

[Connie Star]

Hi Undergrounder, I'm curious about the references you cite- can you give more information about how to access them? I am interested in the physiologic details- have a master's in plant physiology and an MD.

[Undergrounder]

Hi Connie, pm'd