Several of the tangerine trees in my inherited home orchard (30 trees, mostly citrus limetta and dancy tangerine) are experiencing dieback.

Basically I would like to know how to stop the dieback and then on the trees that have been damaged whether they can be brought back and how.

Below are the tree details and a link to photos.
http://s22.photobucket.com/albums/b336/yoggity/Trees/?albumview=grid

Environemnt:
-Central Mexico, temperate climate, rainy season july-october, no rain after that.
-Irrigated by well water manually with giant hose filling basins around tree trunk
-Dense clay lakebed soil, very poor drainage.

Symptoms:
-Branch dieback, severe in some trees, mild in others, none in others
-upward leaf curl
-yellow spotted leaves on some trees, yellowing between veins on others (this happens actually on seedless orange and citrus limetta trees I have, not on tangerines)
-black sooty mold and white fuzzy substance under leaves of some trees (could not determine what pest this was)
-minor problem with leaf cutter ants, but not on tangerines
-could not find any spider mites or other pests

Measures taken:
-Reduced watering schedule
-Fertilizing with Urea per recommended dosages
-Foliar fertilizer 3% iron 4% zinc plus some other minerals
-Fumigated with Malathion and now applying insecticidal soap

I am very eager to learn and appreciate any feedback. I have many questions so I have tried to keep it as simple as possible.

Thanks,
JNR

How long have you had your orchard? And how familiar are you with growing citrus?

Do you live in an area where canker or greening disease is present?

Yellowing between veins is usually a sign of deficeincy. What is the pH of the soil?

Black sooty mold is usually a sign of scale. Ideally this should be washed off because it is blocking sunlight from reaching the leaf surface and also not allowing oxygen to enter the leaf.

White fuzzy substance could be mealy bugs or a fuzzy scale called cushion(?) scale.

At this point stop all the spraying. The leaf pores may be getting clogged and not allowing transpiration to occur.

Cornmeal. This is something that I once read about and now use. Although its success will depend on what the real problem is, but it will not hurt anything and may help. Get a few kilos of cornmeal (ground corn), sprinkle very liberally around the tree, rake it in and water. In 3 to 4 weeks you may see healthier tree.

If you have questions about anything I've written, please feel free to ask.

Lisa

This worked for me with my Key lime, which started to decline. Just repotting it did not stop the decline, but it turned around after the cornmeal was applied.

People say that there are other places to buy it like feed stores, but the only place that I found it was the grocery store.

Those green leaves that have brown spots that are surrounded by a yellow coloring around them reminds me of Alternaria.

I would like to write more, and will wait for you to reply--maybe not till this evening.

plant54 this is a link to an album with the photos in case you still cannot see them http://s22.photobucket.com/albums/b336/yoggity/Trees/?albumview=grid

To start off, I would agree with everyone that the general problem is poor drainage. This is former lakebed and very dense sticky clay (I have broken trowels right off the handle in the stuff). Recently I dug a hole for a new tree and filled it with water to see how it drained and it took almost 48 hours for it to drain.

This said, I have some questions about making the best of the situation:

Watering:
1) We are cutting back watering to every 14 days and generally just giving a pretty good soaking. Should we be more measured in the actual amounts of water?
2) Being a tropical climate with heavy sun, even though 6" below the surface it is still moist or even pure water, the top bakes and cracks within a day or 2 after watering. Would plastic or mulch covering around the tree help
3) Is it advisable to not water for an extended period of time (say 3-4 weeks) to make sure the soil really dries out once and for all, then aerating and going back on the appropriate regimen? Would this help? Would it hurt?

Aerating:
1) What is the best method to aerate the soil? Can it be done with the special aerating drill bits I have seen?
2) Once aerates, should we mix in some organic mulch, liquid humus, or something?
3) Is this best done during certain time of year? We are entering a brief spell of cold (never freezing) and then a long, super sunny dry season until July.

As for you other questions regarding soil pH and the pests, let me get outside and do some legwork and get back to you with more details. This is a home orchard, we are not professionals, do not sell but rather give away the fruit to neighbors, and consume much of it. Again, I appreciate the responses and hope to continue hearing from you.

Thanks,
JNR

No sign of spider mites or scale.

Wooly Whitefly:
I believe this is the main culprit. Explains sooty mold, shrivelled leaves, dieback, fuzz under leaves.

Citrus leaf-miner:
I believe this is at work on some trees, although I have yet to find a live bug. I did find a folded over edge with a cocoon in it. As I understand that this pest isn't totally devastating.

Greening Disease:
This was very disturbing as I read it because so many of the symptoms coincide with my situation. However, upon a more detailed reading, I do not think it is at work because the yellowing does not (yet) affect whole sections of the tree and is symmetrical across the vein. Also, I still have exceptional fruit quality.

Canker:
Fruit are good, even on sick trees. Leaf-miner mottled leaves do not have canker. I recall last year finding a cankerlike fruit, but it is very rare. Definitely no infestation.

Here is what I am doing:
1) Ant control... not sure exactly what I will do yet (I heard cornmeal works here too), tanglefoot is not available in Mexico that I know of, so probably baits
2) Powerwash leaves, if I can find a powerwasher
3) Insectidal soap with horticultural oil spray
4) Look for natural predators to control biologically, again not sure if available down here
5) Rethink chemical fumigation so as not to kill natural predators

I would appreciate any commentary and especially opinions/tips on my control strategy as far as what works, how often, etc.

Thanks,
JNR

However there is an occasional small blob of gummosis resin, usually on branches and upper trunk, never at graft or lower trunk. This would support the lack of drainage and excess soil humidity right?

As for your tangerines -- water damage to the roots would produce symptoms like that. You don't want to let the trees dry to the point of severe wilting, but it's probably good that you're watering only every 2 weeks. Do you know the soil pH? It may be high. If you can test it, try to get the pH below 7, and preferably below 6.5 with sulfur or iron sulfate. But I would not just throw some of those materials on, unless/until I'd done an actual test of the pH.

Malcolm M. Manners
John and Ruth Tyndall Professor of Citrus Science
Florida Southern College
Lakeland, FL