Bay Laurel Leaf Problem

Recently, I've come across a problem with my Bay Laurel planted last summer. The trees have been indoors since September and have been doing beautifully. This week I noticed a problem, searched your Q&A section and referred to several of my herb books for a solution. I've had no success and am hoping you can help.
I have two groupings of the trees. One grouping is healthy, the other is failing. The undersides of the leaves show small medium brown colored circles. The lower leaves are affected the most and are dried and dead. As I view the leaves going up the plant, nearly all have the small brown circles. The leaves at the very top of the plant are completely healthy. I see no signs of mites or other "bugs" and am baffled. The stems are healthy. Recently two of my smaller trees leaves dried, turned brown and died. Now, my tallest and best Bay is showing signs of this affliction. This plant is one of my absolute favorites. Please help!! What is this? How do I treat it? Is it species specific? Can it harm my very numerous other houseplants and indoor herb garden? Please note, the only thing I've done out of the norm is add 1/2 inch layer of worm castings to the soil. Also, I never wet the leaves of the Bay.

I presume you are familiar with scale insects since they look like small slightly raised circles and absolutely love bay laurels. In case you do not, they come in all colours, but the most common on bays is the soft brown scale- which forms brown circles. These circles are tiny shields that the insects hide under. This makes them impervious to all but systemic sprays when in this stage. Once mature the part under the shield will turn completely into masses of eggs that hatch slowly over the span of several months. The crawlers that hatch are not protected by a shield and can be killed by something like pyrethrum soap spray. For obvious reasons the spraying has to be repeated once to twice per week for at least a couple of months to get the crawlers. To speed up the cleaning up process remove as many of the scales as possible with a rag soaked in insecticidal soap.
Since you don't see any mites it might be some species of false spider mite. They are about a tenth the size of ordinary spider mites and are not visible to the unaided eye. Under a dissecting scope they can be seen as transparent reddish or palest straw coloured uglies! Something like short, but multi-legged fat aphid miniatures. There will be only one or two in each huge NEW lesion. They inject toxins into the plant to suck the juices and cause disproportionately large dead areas. Washing throughly once a day with a rag soaked in pyrethrum soap spray for a week should get rid of them.
It could also be a fungus or bacterial infection. To control it remove all infected leaves and burn them, then spray with a sulphur spray. Repeat spray twice a week for 3 weeks.
Both scales and false spider mites would spread to yourother plants. The fungus or bacterial infection might- it all depends on what it is. Since there are not likely to be any reservoirs of infected bay laurels in your area, it is more likely that your problem organism has a large host range.