One of my tomato plants (Better Boys if that matters) has a few leaves that have started yellowing on the lower branches. Not sure if they aren't getting enough water, or if they are diseased or what.
What is causing this? If it's a disease how can I prevent it from spreading to the rest of my plants?
You probably can pinch the lower branches off, and actually help the growth of the plant. Your plant can grow pretty big (4 feet) so to encourage tall growth (and keep the leaves away from the dirt, which probably caused the yellowing) pull them out.
If it goes on, pull the plant out (don't compost it) and be thankful it is early in the year.
It's probably not the water, unless you're watering directly on top of the plant and causing them to get muddy. Try watering at the base and making a little dirt moat around the plant so that the water goes into the ground and the roots have to grow down to get it.
This could also simply be a Nitrogen deficiency. The plant will transfer Nitrogen from the lower, older leaves up to the upper, newer ones if it doesn't have enough available Nitrogen in the soil. Feeding with fish emulsion (There are a few brands, and what your local store has will probably depend on what region you're in: My local Home Depot carries it). would give it an immediate boost, or you could use something else with a medium to high N number on the fertilizer bag.
Yellowing of the lower leaves can be caused by a few different things (and these are different things than if it starts with the upper leaves). How the leaves are yellowing can give you clues as to what is wrong.
If the whole leaf is yellowing, nitrogen deficiency is often the cause. If just the edges of the lower leaves are yellowing, potassium deficiency is often the cause. Too much potassium may cause lower leaf yellowing because it can inhibit magnesium uptake. Magnesium deficiency can cause yellowing of older leaves, but the veins may remain green.
Too much boron may also make the edges of the leaves yellow.
Large amounts of calcium may inhibit potassium, and too much magnesium will do the same in at least water plants. I guess this explains why giving my plants basalt rockdust seemed to give them potassium deficiency, with the edges of the leaves yellowing (rockdust is high in calcium).
I've heard a rumor that too much nitrogen inhibits potassium uptake, and perhaps vice versa (so, if true, too much of either can potentially cause some form of yellowing). I have not, however, found a reliable source to verify either of these claims. My personal experience makes the rumor seem as if it might be true, but not enough to be certain. I used to propagate this rumor, thinking it was something a lot of people knew (because I heard it more than once), but now that I've actually looked into it more, it may just be a rumor, after all. Too much nitrogen may inhibit roots, however (and potassium is supposed to help the roots), and, again, too much potassium can cause yellowing due to inhibiting magnesium.
In my experience, if you have lots of plants close together in a warm, humid environment, this may also cause yellowing of lower leaves, particularly the edges (probably due to some fungus in my area). It looks like potassium deficiency, but I've had it happen on plants that I had already given plenty of potassium previously. I'm not sure why this happens, but yellow discolored circles or patches, or bumps on the leaves can result, too (and usually do instead of it mimicking potassium deficiency). Giving the plants a cooler, dryer environment with more space has easily fixed this problem, for me. Lots of light is also important, but not as important as temperature, dryness and space. This happens much less on plants with enough potassium, even though it still may happen.
