Your Tomato description is classic for foliage blights. They overwinter in the soil, then get splashed up onto lower leaves in a rain event or while watering. Treatments would need to begin before you see the problem to prevent it. In your case where you have had it before suggests you will get it every year. Early blight, Septoria, Late blight are the three common ones. Plants that are overcrowded, where air movement to dry the leaves is impeded are prime targets. Tomatoes that are trained on trellises or in a cage where suckers are removed fair better. Using a straw mulch or layers of newspapers to seperate leaves from splashing soil is also quite helpful. Once you see it if is often too late to manage it very well. You end up with a few leaves at the top of a stem and poor fruit production. Yellowing leaves on your cucumbers could be caused by the plant moving resources into the fruits, an underdeveloped root system, soils too wet, foliar diseases, etc. Plant disease resistant vegetable varieties if you can to lessen the impact. With all the weekly rainfall we have had until recently, foliar diseases on our vegetables and woody plants has been above normal.
