Why are my tomato plants leaves turning brown?

From looking at your photo, I see several culprits: the tomato cage is keeping too many of the plants branches together and they are not getting enough sun and the trapped humidity is making them rot. Use tall wooden stakes for tying up your plants and remove the lowest and fruitless branches as the plants grow. You also have too much other vegetatation, i.e. grasses, weeds, too close that is encouraging leaves to die. Trim off all of the lower leaves and dying leaves immediately and dispose of them--not in your compost pile but in your trash pick-up. You may have the onset of a virus that you don't want on your soil. Also, next year NEVER plant vegetale species in the same spot as the soil can harbor last year's problems. Your tomatoes need much more room to grow and more sunlight. Your ground looks damp---do not overwater! Tomatoes like hot, sunny areas in loam soil. Good Luck!

Not Enough Water

Blight......use Daconil......it helps very much.

check for aphids. very small (pin head)and same color as plant stem.

it could be any of the things mentioned by the other home talkers but definitely would recommend a sunny spot and let the soil dry before watering and leaving plenty of space around them. Another thing I want to mention is that when you plant your tomatoes next year toss some crushed egg shell and an aspirin in the hole when you plant. It seems crazy but it has worked very well for me. The calcium and aspirin helps the plant get the fertilizer and nutrition from the soil and will keep from them having blossom rot. Another wise tale about tomatoes is if you whack them with a broom handle and break braches( don't go nuts) just like a hail storm they will produce a ton of tomatoes. We had baseball sized hail last year and I thought it killed them. Like just stems sticking out of the ground. It didn't I got 150lbs of tomatoes off 6 plants. It took me like a month to can them all. Hope this helps!

Too much water. We can't do anything about rain really, but never water your tomatoes, so that the leaves get wet. Prune off all the dead and spotted leaves and get rid of them. If you look at a healthy tomato leaf, you will notice that it is very rough and water doesn't run off them well, like it would on a smoother leaf like bell peppers or green or lima beans. This spring i watered my tomatoes twice and it rained twice and with a little pruning of the bottom 10 inches or so and I have the healthiest and best looking tomatoes I have ever had. Other years when we have gotten several rains, my tomatoes look like yours.

I agree with the previous posts, but there are a couple of issues that most people do not know. First, Tomato plants do need a lot of room, but it is better to use a welded wire fence about four to five feet tall, placed in a ring for support. His helps them to grow vertically without choking them. Pulling the suckers is very important as well. The biggest No-No that people do is water the leaves! Tomatoes HATE getting their leaves wet, it sends the tomato into shock every time they get wet. Yes, this means protect them from rain as well. Water them from the ground, and keep the leaves dry. It looks like the lower branches might have been scalded, a possibility. They look like they were frozen as well, but we can eliminate that since it is obviously growing season, and not the fall of the year. My average tomato is about the size of a softball, and all I use for fertilizer is worm castings. Think about the last post, the fuzzy leaves trap water causing issues. If you prevent the leaves from ever getting wet, and keep them watered well from the ground you will have more and larger tomatoes, earlier than any of your neighbors... I know I consistently beat my neighbors, they ask why, but refuse to do this one simple little thing. Think about it, when a plant goes into shock, they stop growing and producing until they recover. If your plants do not go into shock, they never stop growing, putting you ahead of everyone else. I have been growing tomatoes for over 25 years, and have never had a failed crop.

All of the above are good ideas, but my first thought was cats! Males like to scent & where the brown is showing is about the right height. Good Luck.