My tomato plant leaves, for example, are yellowing (originally started at bottom of plant but now are slowly affecting rest of leaves), some with brown "target" spots, some are curling, but mostly all three with the yellowing being most prevalent. How do I keep the plant, since the tomatoes look so healthy, but prevent the disease from reaching or discoloring the rest of the plants? My beans are also turning yellow at the bottom, yet at some of the top leaves, they are browning slightly at veins. One cucumber plant that gets morning sun is a lime green color, yet it is producing nice cucumbers. Not sure what is going on here. Did notice that the base stem where it was planted looks like some insect, maybe a borer? Has had a picnic cutting into it.
How Do I Prevent Early Blight From Destroying My Vegetable Plants?
This is kind of a difficult question to answer, because your vegetable difficulties are not caused by one problem. In general, the best way to prevent diseases in your garden is with correct cultural practices such as starting with good soil, disease resistant cultivars, good watering, light, and fertilizing, watching for pests while plants are growing, removing and destroying infected leaves both during growing and after when you clean up the garden. Here are some articles with more information on the types of problems you're seeing: