I have a salad table that I planted with different varieties of leaf lettuce this year (Lolla Rosa, Red Salad Bowl, Tangy, and 4 Seasons). I started them all from seed on 5/11, except the Red Salad Bowl, which I transplanted from seeds started earlier. It was fine for a while, although the seedlings started in May seemed slow to develop to harvest size, but now all the older leaves have tan spots on them. By the time the leaf is large enough to harvest, the spots cover the leaf. They seem to be through the entire thickness of the leaf, but I can't tell for sure (maybe start on surface, then eventually go through to underside). I can't scrape or rub them them off. It doesn't look like the pictures I've seen of Leaf Rust.or any other lettuce disease I come up with online. The area gets some morning sun and afternoon shade, since it's been hot (I can move it around, so I do depending on weather). The soil depth is 4-5", a mix of topsoil/leafpro I bought in bulk at a garden center, compost I make and vermiculite. I water when it seems really dry, but that's only been the past few weeks and this started in mid-late June.when it was still raining TOO much. At first I thought it was dirt splashed up on the leaves, but since it can't wash off, I realized that wasn't the problem. I have not fertilized. Water is either from hose or dehumidifier. The photos attached show entire plant with leaves in all stages. Center new growth unaffected, outer leaves more infected. Coin is a dime for size comparison. I want to replant lettuce for fall harvest but am not sure if I should do in this salad table or if I need to find another place. Thank you!
The leaf damage looks like spider mites or a small rasping insect. You can spray with horticultural oil or horticultural soap. Don't spray when temperatures are high. Read the label. Heavily damaged leaves will not be able to tolerate the sprays. Here is the info on lettuce: http://extension.umd.edu/growit/vegetable-profiles-lettuce and mites: http://extension.umd.edu/growit/insects/spider-mite
Lettuce needs moisture. Don't wait to water until the soil is thoroughly dry, but keep moisture in the soil (not soggy though). Water when the surface soil dries down about 1/2". (See the cultural info above.)
However, these plants are old. Since these plants have been producing for some time, we'd suggest pulling them and planting your fall crop rather than fool with trying to kill an insect pest.
We were surprised to see the extent of gravel in your planting mix. You may want to add more organic matter or good top soil at that time.