Although some forms of environmental stress (like adapting to changing weather conditions) can mimic infectious disease symptoms, in this case our primary suspect is Septoria Leaf Spot, especially given our recent stretch of damp weather. (In greenhouses, overhead watering that gets leaves wet can increase the risk of infection, especially if spores are splashing off of soil under the bench or from other pots or plants around them.) Plants started from seed tend to be behind in growth compared to those getting a head start in greenhouses, so that might explain why only this plant is symptomatic so far. Plus, different tomato cultivars have varying resistance/susceptibility to the diseases they can contract. Cornell's list of disease-resistant tomatoes lists 'Yellow Brandywine' as resistant to abiotic disorders (conditions causing damage that are not due to a pest or disease), but says nothing about resistance to other common infections.

For now, you can snip off the two lowest leaves that have heavier symptoms, but don't remove any more until the plant is older and has more foliage to spare. Use of a fungicide spray might suppress future infection, but pesticide use should be a last resort and it cannot cure any existing infection (it only protects healthy growth), nor do such treatments always work perfectly well to inhibit further infection. Additionally, some fungicides might pose a risk to pollinators, a consideration for a plant benefitting from bee visits to pollinate flowers.

Miri