Hi there! I've had this rosemary in a large pot (20 gallon) on the side of my house for a few years. It's grown but it hasn't exactly thrived. As you can see, it's fairly yellowed, and the leaves are a little mottled. I thought it was a white fly issue so i treated for that, but the flies appear to be gone but the issue persists. Thanks in advance!
Multnomah County Oregon
Thank you for your question. Your plant appears to be yellowing, a symptom of a nutrient deficiency, typically nitrogen. Do you fertilize it regularly? Here is an article on their care: https://web.extension.illinois.edu/herbs/rosemary.cfm Give a balanced (10-10-10) time release fertilizer and it should perk up. Good luck!
In addition to nutrient problems as described above, we have seen several cases of the leaf stippling on rosemary, catmint, and lavender in our clinic this month. We finally pinpointed the culprit. In every case it was the Sage Leafhopper, Eupteryx melissae, (http://www.tsusinvasives.org/home/database/eupteryx-melissae#:~:text=Description,to%20that%20of%20a%20leopard.) which are found on many herb species and potted plants such as: lavender, mint, oregano, rosemary, thyme, sage, lemon balm, and Phlomis species. They are very tiny, and unless you're looking for them, you'll never even see them hopping. We were able to catch some by waving a sticky trap within the foliage. https://pnwhandbooks.org/insect/hort/landscape/common/landscape-leafhopper
If you are able to confirm the presence of leafhoppers, you have some options. You can use the sticky traps to catch the adults, but unfortunately, you'll also catch beneficial insect that are present, too. The same goes for any insecticides you might choose to use. Attached you'll find an excerpt from PNW Handbook's Chemical Control for Landscape Pests. If you decide to use chemicals, always choose the least toxic options. I've highlighted them. Be sure to read and follow all label instructions to protect yourself, the environment, and pollinators. Avoid using any chemicals when your plant is flowering. https://pnwhandbooks.org/sites/pnwhandbooks/files/insect/horticultural-landscape-ornamental/content/pdf/pdfs/landscape-chem-control.pdf