The very first time I grew tomato plants, I believe, it was a fairly successful season. What I mean is, I harvested a decent amount of fruits and it tasted okay. At the time I did not know anything about tomato plants having diseases, I never thought anything about the yellow leaves, and leaves with spots or even a fruit now and then having a dark spot on it.
It was not until I did a little research about tomatoes and planting them to have more success in future growing seasons, that I found out my first tomato plant had diseases. I also found that there are numerous problems you can encounter, but fear not because my first season was okay even though I did not know anything.
Leaf Spot
Through my research I found that one problem I had is called Septoria leaf spot; it is a fungus and can occur at different times in the plants life, but it normally happens when the plat is setting fruit. If you want to know the scientific name it is fungus Septoria lycopersici, just do not ask me to pronounce it.
You will recognize this fungus by the dark colored edges and light colored centers on the leaves, which usually appear as water spots on the older lower leaves of the plant. Under the right conditions though, the fungus develops spores that can be spread by rain or overhead watering.
It was good for me that the right conditions did not last very long during my first season because I did not know anything about this fungus. I have that prevention is the best thing to do, as far as overhead watering is concerned, water at ground level and water early to give plants a chance to dry. If you notice you have leaf spot do not work on the plants while they are wet as it may spread the disease.
While winter condition may help with some other problems the leaf spot fungus can survive on tomato debris as well as on weeds, so keeping a weed free garden and getting rid of tomato debris will help. If you are buying seedling double check if they have any tomato plant disease, or you may be setting yourself up for problems.
Bacterial spot
Bacterial spot is another problem, this one affects the leaves, stems and the fruit, but as the name suggest it is not a fungus but a bacteria, the Xanthomonas campestris pv. vesicatoria bacteria to be exact. If you grow more than tomatoes in your garden, be aware this bacteria can spread to other plants.
You will first notice the dark spots that appear first and eventually get surrounded by a yellow halo, even holes can develop in the leaves. In the fruit dark raised dots similar to pimples or black heads appear, when they get bigger they change to grey-brown and scab like appearance with sunken centers.
Much like leaf spot prevention is the key, double check seedlings if you are buying them. Bacterial spot can also survive through winter on plant debris and weeds, so get rid of them. Practice crop rotation as this disrupts many disease. You can check your garden shop for copper spray if this bacteria is caught early it may reduce the damage.
Bacterial Speck
Bacterial speck is caused by the bacterium Pseudomonas syringae pv. tomato, an appears as dark spots on leaves that are surrounded by yellowish colored halos. If it affects your fruits, dark specks develop but do not penetrate deep.
As with the previous two diseases prevention plays a big role in keeping this bacteria out of your garden, You can follow the same disease fighting principles as noted above, and the copper spray will help here as well if caught early.
Try using disease resistant varieties if possible, and I have found by growing your own seedlings from good seeds, giving them a healthy strong start will go a long way in avoiding tomato plant diseases.