I've been growing marjoram for years and years and never had this problem. You can see in the attached closeup photo that there are white spots all over the leaves. Whenever I water or move the leaves, I've noticed a lot of tiny white insects. I suspect they may be the culprits. The other photo shows you what else I have planted around the marjoram (this was on 8 November). If it is the tiny white insects, is there anything I can do about them? If I spray, then I can't eat the herbs. I've been tempted to cut them all down to the roots and start again. What's your opinion?
I'm in Melbourne, Australia, by the way.
It appears that you have white fly. I would recommend a combination of the following:
You can vacuum around the plants to catch any flying insects.
Put a yellow sticky trap in the pot. The flies are attracted to the yellow.
Spray the leaves (especially the undersides where there may be more eggs and insects).
The spray you use will not prevent you from harvesting and using the leaves.
Insecticidal soap - from the store or homemade. Lemon dishsoap may be the best thing to use if you make your own insecticidal soap. Not antibacterial or anything like that.
a combination of the two - just mix some insecticidal soap with the horticultural oil spray.
With any of the sprays be sure to get the entire plant including underneath the leaves as much as possible.
Spray the other plants in the pot in case there is white fly on them also.
Inspect the plants on a regular basis.
You may need to spray once a week for awhile in case any eggs or white fly was missed.
Just rinse off the leaves before eating. None of these sprays will be harmful to you.
If you harvest some of the leaves for use you will have less leaves to spray with the insecticidal soap and/or oil sprays.
Be sure that when harvesting any of the herbs that you don't accidentally harvest and eat the non edibles such as the lobelia.