Heirloom varieties do not have much foliage disease resistance so suspect those lower and interior leaves have septoria, or late or early blight disease. Lower and interior leaves also can be naturally lost as the plant develops and there is more productive foliage in full sun. Those leaves would be a clear yellow without spots as the plant moves resources out of the leaves. if you are not growing these plants organically, there are fungicides available to stop the spread within a plant. Culturally, carefully removing the impacted leaves as soon as any spots show up, water the soil, not the plants and do not splash soil up onto lower leaves. Mulching can prevent splashing during a rain event. staking or caging to keep foliage off the ground helps too. If you have lots of space, rotation helps, but most often it is not possible in a home garden setting.