[align=left]I have a 10y/o Rosemary in a container that has done beautifully until this summer, which was hot and humid. The plant is about 2'x3'. During a very humid spell the leaves, mainly on the top branches, turned first brown (top half of leaf) and then yellow below that, but did not drop. Thinking that the plant was root-bound or over-watered, which it was, I repotted it, The problem seems to be worse now. Leaves (not all)*on the lower branches turned completely yellow, but did not fall off.* A local nurseryman diagnoses it as Phytophthora (which is root rot) but*I'd like a second opinon.[/align]
[align=left]I sprayed it with Neem and BayerAdvanced (to no effect). Cut back on water (to no effect). Any suggestions would be appreciated. Thanks.[/align]
So sorry to hear this.* Generally once a plant gets any form of root rot it's life is over.* To my knowledge there is no treatment to control or cure it.* Scroll about half way down the page here to 'Problems'.
You can try letting it dry out and then drenching the soil with a fungicide that is used as a soil drench and approved for Rosemary.* Not all fungicides can be used on rosemary and I would have some concern about using the rosemary in my food after using the recommended phosphioric acid on the plant.* From this site from half way down the page under 'Chemical Control'.
"Based on a 2004 UF survey, some Florida herb growers applied fungicides on up to 100 percent of their acreage, while others used none at all. There was reported use of azadirachtin and sulfur. Other fungicides registered for all herbs in Florida in 2004 were Bacillus subtilis, Coniothyrium minitans, fludioxonil (seed treatment only except mint), harpin, mefenoxam (foliar for all herbs and seed treatment for dill), potassium bicarbonate, and Trichoderma harzianum. A blend of peroxyacetic acid and hydrogen peroxide is labeled for post-harvest processing water. As with insecticides, certain fungicides are just labeled for one or several herbs. Azoxystrobin is labeled for use on cilantro; copper hydroxide is labeled for dill, and phosphoric acid is labeled for rosemary. For mint, azoxystrobin, phosphoric acid, and trifloxystrobin were registered for use as of 2004. For parsley, azoxystrobin, fosteyl-Al, phosphoric acid, and pyraclostrobin were actively registered for use as of 2004."
Here's info on phosphoric acid.* This first site are products that contain it and what percentage.* At the very bottom of the page is a Nematode and root rot cure.
When I click on the product it brings me to here:
Click again on the phosphoric acid and you get here.* You can read about the health hazards and more if you like.
So, unless this plant is so precious that you will be devistated to lose it or you can find a fungicide that you feel comfortable with, I would suggest you try and save it by changing the soil or replace it.* :?
[align=left]I agree with your conclusion. If it is root rot or Phytophthora then I will get rid of the plant. However, I'm still not 100% sure of the diagnosis. Except for the brown/yellow leaves and now some pure yellow ones, the plant keeps on sending out new shoots and leaves. When I re-potted it last month, though I didn't check closely, the root mass look typically like a pot-bound plant. I didn't notice any mushiness or rot.[/align]
[align=left]Is it possible that over-watering and/or over-fertilizing would show the same symptoms? I could give the plant a good soaking to flush it out somewhat, but it would probably stay wet for at least a week or more. I don't want to get into exotic/toxic fungicides: too much trouble, expense, and it defeats the use of the plant.[/align]
[align=left]I could also just get a sulphur based fungicide which I believe is non-toxic and try that...Lastly I think I'll take a sample over to the extension service.[/align]
[align=left]What's your opinion on the above?[/align]
You said, "When I re-potted it last month, though I didn't check closely, the root mass look typically like a pot-bound plant. I didn't notice any mushiness or rot."
It's always best to check the rootball and loosen the outer roots if the roots are circling tightly.* There are plants with delicate roots that can't tolerate the root disturbance.* If a plant is severely rootbound and you overwater, the water can stay in the tight mass longer then is good for the plant.* The other side to that is the tightly wound mass can't take up enough water.
You said, "Is it possible that over-watering and/or over-fertilizing would show the same symptoms?"
With root rot you will usually see browning of the stems.* With fertilizer burn the leaves closest to the base of the plant and the older leaves will usually burn first.
I think I would unpot the plant and take a close look at the roots.* If it's severely rootbound I'd try and do as they did in the first link above or even wash off all the soil.* Cut away any brown or broken roots and any brown stems.* You might lose the plant, but at this point you may be able to save it.* I would wash the pot in solution of one part bleach to nine parts water (wear old clothes as bleach will splash), rinse with a 50/50 mix of vinegar and water, rinse again with plain water and let it dry.* Repot with fresh soil so you are certain it's not infected with any fungal spores.
I'm not a fan of sulfur based fungicides with edibles as some people are allergic to sulphur.