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Plant Diseases/Wilting agastache


Black stalk
Black stalk  

Hi, I have an Agastache Blackadder that suddenly started wilting - not just flopping at the tip as they can but all the leaves absolutely wilted.  This happened on one stem, and then on other stems.  I had watered the plant lightly and the wilting was only on particular stems, which I ended up removing  as they did not recover.  The affected stems were black at the bottom.  Other leaves are yellowed with brown patches, but this could be unrelated.  The weather has been warm (I live in the UK so not too hot) and there has been a little rain.  Other stems so far seem unaffected.

I hope you can help.

Dear Sophie, First I must sincerely apologize for the lateness of this answer.  I have been away from functional internet lately, and I do believe that you have a serious problem.  I am not entirely sure exactly what disease your agastache has, although the likely culprit is Phytophthora.  This disease causes fast wilting, usually starts with one stem and then other stems quickly succumb.  Unfortunately, there isn't a cure for this disease, which is a type of water mold.  However, we aren't sure yet that this is the case.  Now, if by the time you get this response, the plant has continued to wilt, then Phytophthora is probably the culprit, and you should probably pull the plant up and keep that area clear for a while.  But if the plant is still surviving, then perhaps you have a fungal infection which can be cured.  You could try cutting off any part of the plant that has even started to wilt or yellow.  Be sure to disinfect your shears with bleach afterward.  Then spray the agastache with the strongest fungicide you have available.  Spray thoroughly until the plant is dripping and then allow to dry thoroughly as best you can.  If you do have a fungus, then the plant should start to show some change after you spray it once or twice, a few days apart.  I wish I had better news, but if you do have Phytophthora, and if you do notice other nearby plants becoming affected, and if the fungicide doesn't help, then remove any affected plant immediately.  I wish you the best of luck, and if you have more questions, or other symptoms to report, please write back.  I'm back again to reliable internet so I can respond much more quickly.  Good luck, Melissa

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Melissa Johnston


Plant diseases, landscaping, tropical plants, roses, herbs, plant care, grafting, horticulture, plant identification, anything about plants I can likely answer.


35 years experience in various plant businesses, 1984 Certified Texas Master Gardener.

Master Gardener Association of Texas charter member

none as of yet, but I have a plant q&a book I am in process of submitting.

Magna cum laude graduate of Texas A&M, 1978

Awards and Honors
Plant growing awards, highest grade for Texas Master Gardener graduates.

Past/Present Clients
Past member of and was very highly rated. Owned landscape company in the past, Almost Paradise, and was very successful despite little equipment, no help, and no advertising. Lived well for two years until 9/11.

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