Great Blue Lobelia trouble
Two Great Blue Lobelias have developed yellowing leaves. The yellowing pattern is quite different on each plant. The largest plant started slowly developing yellow speckles on its lower leaves a month ago. I thought it might be a nutrient deficiency so I fertilized it and that did not help. The speckles spread to the entire leaves and the entire plant and now the leaves are turning brown. For the large plant, please see the two speckled leaves and the brown leaf in the photo. The second, smaller plant has now started to turn yellow, but the yellowing pattern is very different- there is yellowing between the veins and not speckles. For this plant, please see (same photo) but the one yellow-between-the- veins leaf beside the brown leaf-- this is from the small plant. These plants are a few feet apart in a border bed with other lobelias which are unaffected, so far. I planted the lobelias last year as small transplants from a plant exchange and this is the first year they have bloomed.
Montgomery County Maryland
While we aren't positive as to the cause of the leaf flecking, it does look suspiciously like virus symptoms. (There are multiple plant viruses and we cannot determine which exact one this may be.) If that is the case, the plants are incurable and while some plants can live with virus for some time, they eventually succumb and may spread it to other plants in the meantime. Because of that, it would be safest to remove them, as sap-feeding insects can transmit the virus as they move from plant to plant. (It can also be spread by sap contaminating pruning tools, so clean them after use if you have trimmed recently.)
If you are uncertain and wish to keep them, see if they return and if they leaf-out normally next year. While normal-looking leaves might appear at first due to weather conditions in spring, they will eventually become symptomatic again if this is a systemic disease.