Mottling and yellow tips on oncidium. Disease, nutrition or...?

[sunfire]

Hello all.

I have this Oncidium Golden Mirth "Filini's Gold Dust" that I bought as a plug one year ago. It is growing in semi-hydro and has been in a grow tent for the last 6 months or so. It seems very vigorous, putting out new growths and roots with firm pseudobulbs (started as a single small p-bulb).

I have noticed over the last few months that the leaves seem to be mottling and the tips slowly turning yellow and I would like to know your thoughts on the matter. The mottling is most apparently on the older leaves but also subtly visible on the new ones.

I use rainwater and MSU rainwater formula. I probably had been under fertilizing it over the winter (fed at 50ppm N every week). I have increased the dose recently to 100 ppm every week. It otherwise has been in my grow tent at about 80+% humidity, 3 fans circulating air and 65-78F with grow lights on 9 hours a day. The roots seem healthy although they are covered with algae and probably cyanobacteria (I have tried some Physan20 rinses but it does not seem to go away).





New growth


Roots


The whole plant.

[estación seca]

It might not be getting enough magnesium. I think these do better with more fertilizer than 100ppm once a week. Consider treating with Epsom salts on a different day than you fertilize. A tablespoon per gallon of water should work. It can take weeks to resolve magnesium deficiency.

I don't worry about algae nor cyanobacteria.

9 hours of light daily probably isn't enough, unless they also get natural light. That isn't causing the yellowing but you might have trouble getting them to flower. Many people use 12 hours spring/summer/fall and 10-12 hours in winter.

[sunfire]

Thanks. I will increase the magnesium.

Originally Posted by estación seca

It might not be getting enough magnesium. I think these do better with more fertilizer than 100ppm once a week. Consider treating with Epsom salts on a different day than you fertilize. A tablespoon per gallon of water should work. It can take weeks to resolve magnesium deficiency.

I don't worry about algae nor cyanobacteria.

9 hours of light daily probably isn't enough, unless they also get natural light. That isn't causing the yellowing but you might have trouble getting them to flower. Many people use 12 hours spring/summer/fall and 10-12 hours in winter.

[Ray]

Originally Posted by sunfire

I use rainwater and MSU rainwater formula. I probably had been under fertilizing it over the winter (fed at 50ppm N every week). I have increased the dose recently to 100 ppm every week.

Let me throw out a couple of things that might help.

The MSU RO formula has plenty of magnesium, and supplementation is probably unnecessary, but...

A decent weekly feeding level is about 100 ppm NITROGEN, which for that formula, has a true TDS contribution of 740 ppm (i.e., 0.74 g powder per liter of pure water).

I don't know how you determine your concentration, but if you've been feeding at a true TDS of 50 or 100 ppm, you've obviously been drastically underfeeding, but...

TDS meters are notoriously inaccurate, so don't trust the reading to be solid indicators of reality. You can use it as a control device though, making your own "calibration curve".

I had two TDS meters (now both trashed), If I remember correctly, for a known 100 ppm N solution of MSU RO, one said the TDS was 375 ppm, while the other said about 450.

[sunfire]

Hi! Yes, I meant ppm N in my post. I calculated based on your site (made a conc soln of MSU and dose cc per vol of water I need at that moment).

Originally Posted by Ray

Let me throw out a couple of things that might help.

The MSU RO formula has plenty of magnesium, and supplementation is probably unnecessary, but...

A decent weekly feeding level is about 100 ppm NITROGEN, which for that formula, has a true TDS contribution of 740 ppm (i.e., 0.74 g powder per liter of pure water).

I don't know how you determine your concentration, but if you've been feeding at a true TDS of 50 or 100 ppm, you've obviously been drastically underfeeding, but...

TDS meters are notoriously inaccurate, so don't trust the reading to be solid indicators of reality. You can use it as a control device though, making your own "calibration curve".

I had two TDS meters (now both trashed), If I remember correctly, for a known 100 ppm N solution of MSU RO, one said the TDS was 375 ppm, while the other said about 450.