All that heat plus reflected heat damaged the leaves. And yes, it can and does happen that fast.

Part of the problem is that the roots are cramped into a small space which heats rather rapidly. Another part is the sudden increase in temperature.

i'm not sure its sunburn. seems more like over fertilization.

new growth is generally a bit on the light green side. if your older leaves are a medium/dark green then your tree has enough N.

also, what time of the day do you water? do you water the whole plant or just the soil?

Wouldn't over fertilization start showing at the leaf edge?

This tree is getting the same treatment as my others - it has Osmocote Plus (CRF) in the pot and it gets 1 tsp/gallon of Foliage Pro (9-3-6) once a week. I understand that Osmocote releases fertilizer based on temperature - so if the rootzone temperature got up above 90 degrees (not unlikely) then maybe it IS fertilizer burn since the Osmocote might have released a lot? But it doesn't look like fertilizer burn?

I water in the morning. I almost never wet the foliage unless there is a problem with mites or some other insect. I thought that was best practice with pretty much every kind of plant to prevent fungus and etc.

None of the leaves on the tree are dark. It used to have dark leaves but over the winter during the branch dieback it started going pale. All of the leaves are the same pale green (new and old). Seems like N deficiency to me but I've been sticking with the same fertilizer regime as every other plant - I assumed there was an overwatering problem while I was out of town that would be corrected eventually.

The only difference is that this tree came in a somewhat sandy soil mix which I haven't yet replaced. Maybe the sandy soil leaches nutrients more readily and N deficiency explains the paleness? But then how to explain the brown spots?

Did you ever figure this out? It doesn't look like primary-caused sunburn to me either, more like the heat burned off the dead/dying portions of the leaf (from a different cause). But jean is right it can happen very quickly. I don't know the cause without doing more research myself.

Goodmorning everybody!

Actually Jean is most likely on target since most of the time she is. I tend to side with her on this.

That very same thing happend to mine a few years back when I put a couple of mine on my hot flat roof, especially after a cool period. Within one day the leaves turned like that.

As for over fertilization? The damage to the plant would always start on the leaf tip and then onto the margins. They would either turn brown or yellow and recede back further. Also you can get sudden light green compacted growth in the full sun which then declines rapidly.
I would of actually thought it was fertilizer spray damage, but you don't fertilize this way.

You can grow a plant in pure sand, if you fertilize almost everyday, so I am going to guess if anything, that plant is not fertilized often enough even with the slow release osmocote in the mix. If the weather gets too hot, it doesn't work, and if it gets too cold, it does nothing for your trees which is exactly what you described.

Your fertilizing regimend is very good as long as the temps cooperate.

You may still have root issues going on with that one if root damage was a problem as recently as the last few weeks. It can take a long time for recovery.

Ps. Nothing wrong with spraying your plants down as often as you want with water any time of the day and in fact it is much welcomed for everyone one of mine:-)
For some reason I just don't make it a practice to spray mine after dark.

Mike

No. I never did get a definitive answer.

I treated this citrus just like every other I have with the exception of its location. This means that I kept an eye on its moisture with a meter/dowel and had it on a CRF and 1/4tsp per watering regime of fertilizer.

The speed of the decline still makes me think it has something to do with reflected heat from the patio. I don't know what else could take the leaves from glossy (if light) green to collapsed in a day like that.

Maybe an insect but I didn't see any. Nor do I have any damage that looks like that on the other citrus which are nearby but not on the patio. I'd imagine that I would see similar damage if it was either fungal or insect related.

That branch has since died back. After it did I repotted it into the gritty mix and I'm keeping it in a cooler location. It has a single new branch very similar to the old one (ie: new, weak, light green, large leaves). I'm keeping it in a filtered sun spot for at least another week or so.

There is also the unexplained winter branch dieback. Sadly wasn't there to see it so I don't know if it happened in exactly the same way...

Sorry I was thinking about its *current* fertilizer regime which is 1/4tsp at every watering (since it's in the gritty mix now).

The old regime was 1 tsp FP per week.

OK so I did more research. I walked out to my backyard and looked at some trees ;-)

Sure enough, the trees that were exposed to the most sun, in our 108F weather last week, have almost identical spots shown in your pics. These particular trees were also recently exposed to the extremes (were in semi-shaded but then were in full afternoon sun). The part that threw me in your picture was the waxy outer leaf surface burned though, then it browned/burned up. Whereas my trees that were mostly in full afternoon sun that have sunburned leaves are tan with what appears to have the waxy cuticle intact; in time they turn very pale, almost bleached white if left in full sun and then they lose the waxy surface but in large sections.

So I guess the assumptions about the fast extreme swings in temps can cause small circular sunburned sections on the leaf whereas more gradual transitions into burns affect the larger leaf surface (what I'm use to seeing).

I suspect none of this is related to your branch die-back. That could be related to the low N scenario.

Thanks cebury!

I'm not able to exactly envision what you are describing. If you have time I'd love to see a photo for future reference.

I'm pretty sure the rootzone temperatures were well above 95 degrees in that pot as well the day it turned. It was in a black nursery pot (ick) and sitting on the brick patio in ~98-100 degree weather. I gather that citrus roots stop functioning above 95 degrees which then causes transpiration to cease and increases the leaf surface temperature.

There was a thread somewhere about how citrus love high temperatures but I assume they were only talking about in-ground citrus. I can imagine (though I haven't read about it anywhere) that they can burn just as easily as any other plant if the roots stop pushing moisture?

I tend to agree with cebury and meyermike. I've had this also happen on some of the newer, more tender leaves, especially if it had been dewy in the morning. The little beads of water act like little magnifying glasses, and can make these odd burn marks on the leaves.

Patty S.

Redshirt: Let me go snap some photos for you and I'll post em tomorrow.

That sucks about the black plastic pot accident. I have that love/hate relationship with them. Their UV resistance is so great they far outlast other plastic containers and are very durable (esp. at the rolled rim top), but they just get so dang hot.

RE: the rootzone. Yeah, I recall the figures for ideal root zone temps is ~93F (root hair/tip growth) or ~81F (elongation). However, it's interesting because add a few more degrees above 93 and the growth stops completely.

However, I believe the leaf temperature (which is what we're discussing in the photos above) has a max of ~115F where photosynthesis stops, but in reality I think it's much lower. Most people don't realize that citrus really only need the morning light until noon, because the tree is efficient at photosynthesis production when light is ~1/3 full sunlight and temps are cool. Once ambient temps get warm, it produces less and then nearly stops when ambient air temps reach ~98F. This is purely photosynthesis production and not discussing how heat affects fruit.

Patty: The little beads of water act like little magnifying glasses. Pretty sure that is a myth. Unless the water contains salts or other minerals that would do the same thing, absent the solution of water.

Hi Patty!!!

How have you been anyways? Good to see you again!

Cebury. I always enjoy reading your offerings:-)

Mike

Good to know that, cebury. Maybe for me, then, it could be due to a higher amount of salt in our water? One of our issues here so close to the ocean. I've only seen that kind of leaf damage pattern on one of my citrus, on the very newest leaves, and the tree gets full (blazing) south sun. And Mike, I've been well. Extremely busy, but good :-) I'm trying to pick up 3 more citrus trees: An Ortanique Tangor, an Ugli Tangor (highly unlikely I'll find this one in Calif.), a Nova Mandarin and a Tango Mandarin. I'll squeeze them into my front slope. Trying to figure out if my Oro Blanco grapefruit is just starved for nutrients, or if I might actually be looking at (horrors) Citrus Greening. Verdict is still out. I've sprayed with a micronutrient spray (Grow More Citrus Grower Blend), and will spray, again, on Friday evening. It did green in up a wee bit, but it looks suspicious to me. We have had some very isolated reports in N. San Diego county of Citrus Greening, so it technically is possible, but in reality, would be unlikely. Just that all my other citrus are nice and green except this poor Oro Blanco. Roots are not waterlogged, but I'm going to try to dig down tomorrow morning just to be sure that's not the issue, too.

So, good all in all :-)

Patty S.