Disease on a healthy tomato plant
Hard to know from this vantage point. There is actually quite a lot that could be
causing what you are seeing. Read more here;
Isn't always easy to determine whether disease or insect damage, but often it is both and looks as if it could be here too. You could send a plant sample to Diagnostic Services https://pestid.msu.edu/ for a small diagnosis fee but maybe not worth it. Since it is in a container, I might simply dispose of the plant and buy another. Plants can sometimes be infected at purchase so it is important to choose healthy looking plants to begin with at purchase, clean your container with a mild bleach solution, and use fresh quality potting soil. Tomato diseases are less prevalent in container plants than in plants in ground.
Further, some of the most common tomato plant problems are fungal. Fungicides
are readily available but keep in mind that fungicides do not cure. They stop progression but do not cure, meaning to be most effective should be used before
plants exhibit symptoms. Spraying every couple of weeks while plants are healthy could be cheap insurance.
Thank- you for your Early morning response! Am I understanding that I should be spraying my plants Before anything happens to prevent diseases? I’ve read or heard about this and was not 100% sure. I bought the Copper fungicide a few weeks ago, but still haven’t used it. I also have bought some mulch to spread around the bottom of my plants, but not to cover the stems to prevent splashing and to keep the moisture. Every year it’s something different -so please tell me how I’m doing in understanding this?
Correct, a fungicide will stop progression but is not a cure. So, spraying periodically while plants are healthy could block fungal activity. Be sure to read the label. Several pesky funguses such as early blight or septoria leaf spot are common to in ground tomatoes. Worse, they remain in the soil for years. In these cases mulching does help to avoid the splash, as does removing lower leaves. But container grown tomatoes are generally not affected, especially when using fresh sterilized potting soil each year.