Also (is it bad if I'm double-posting? I can't find an edit button!), as long as I'm asking for advice, any idea what would cause a marigold plant to do so badly at the beginning? It's only about a month in the pot but there are grey spots all over the leaves and it is not doing well at all. The flowers seem to get rotted and 'spoil' quickly. I am not giving it too much or too little water, either. ;)

hi riverminty, it's blossom end rot on your tomato which is usually caused by irregular watering which causes problems with the plants taking up calcium. It's not infectious and wont infect the other tomatoes, but if the watering is too uneven, your other tomatoes may develop it too.

Huh. That stinks. My mom warned me I was being a bit neglectful of the garden. :( Is this a reversable condition??

Thanks very much!

What you have is most likely Blossom End Rot, but the cause is more complcated than simply irregular watering, although that could be one of the causes.
Blossom End Rot is caused by a Calcium deficiency, most often at fruit set. That Ca deficiency could be the result of low soil CaCo3 levels (soil pH problems), lack of sufficient soil moisture so the plant cannot uptake needed nutrients (the irregular watering thing), a nutrient imbalance in the soil that causes a plant to uptake things it does not need instead of what it needs, a sudden growth spurt that gets the fruit started before the heavy calcium can reach that fruit, just not quite enough moisture available to the plant to move this heavy nutrient up the plant to the developing fruit.
Preventing BER next year starts now with a good, reliable soil test so you know what your soils pH is and what may need to be done to correct that, if necessary, as well as what nutrients may need help. Along with that good, reliable soil test there are these simple soil tests that can help you know your soil better,
1) Structure. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. A good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer you soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.
and know what you need to do to make that soil into the good, healthy soil needed to grow strong and healthy plants.

Charlie Nardozzi, staff horticulturist at National Gardening, wrote an article on why good tomatoes go bad some time back and I have a copy of it that I could post but it is 3 pages long, but he says in that article essentially what I have written above.

Wow, I didn't know growing tomatoes was so compilcated.

I had blossom end rot one year and I was watering everyday. I heard about the calcium and didn't have any bone meal, so I just crushed up some calcium tablets I was taking and sprinkled some around the tomato plants and bingo, tomatoes started to mature normally.

But I guess I should have been sniffing and squeezing the soil and routing around for earthworms. 8-)

In my experience, how to avoid blossom end rot is:

1. Mulch (so soil moisture levels stay consistent)
2. Don't grow Roma tomatoes.

Not too complicated :-).

Growing tomatoes free of things such as Blossom End Rot is not at all complicted, just be sure you have good, healthy soil that is evenly moist. Putting things such as a high Calcium antacid, egg shells, etc., spraying the plant with Epsom Salts, all of those old mythical remedies to prevent BER really do not work because the Calcium available in those sources will not be available to plants for at least a year after they are put in the soil, and Epsom Salts (Magnesium Sufate) cna make BER worse since excess Mg can displace necessary Ca.

It is difficult to maintain watering at an even level. Especially if you live in a drought zone! Even with this I have never had blossom end rot. At planting time and every 4-6 weeks, I feed my plants with the cheapest calcium tablets that I can find. The dollar stores are good bets for these. On the watering, dig down adjacent to the plant. How moist is the ground. It should be moist not soaking wet at 4-8 inches.

My heavens, I'm new to gardening, but with all these issues, I think I may be better off just buying my veggies at the supermarket.

Water every other day & mulch the plants to keep in water & keep down weeds. Epsom salt, just a pinch when you water next should help & will not hurt.
Issues are a part of life,we all have to learn how & why we do things. I have been planting tomatoes for 42 years,started young on a farm, & still learning things.
If it was really easy it would be boring & you would have less fun.

The wee bit of Epsom Salt (Magnesium Sulfate) added to water and dumped on or near the plant really does very little to help with this Calcium deficiency, even though it may be caused by too little Magnesium in the soil (or because there is too much Magnesium) since plants need some Magnesium to properly utilize the Calcium.
The best way to prevent Blosson End Rot is to start the growing season with a good, healthy soil well endowed with organic matter so the soil has balanced nutrients and will be evenly moist, but well drained. Mulching the soil will also aid in soil moisture retention, so you may not need to water daily which can create more problems because a too wet soil will also keep the plants from uptaking needed nutrients.

In my experience the BER is usually confined to the first tomato or 2 as the first fruits ripen. In fact the first fruit usually

IS bad and the rest ok all season.

KIMMSR I think your post is very informative, I'll have to bookmark it! Thanks!

We are still doing the 5 earth worm thing.
A whole thread on the myth was not enough.

Kimmsr hit on the true solution and that is the addition of magnesium... He suggests spraying epsom salts... but magnesium is the key... it helps prevent root born fungus such as blossom rot... I have an age old proven trick that is so easy... I save my banana peel in a separate area near my compost bin... Serious key tomato nutrients are found in banana peel... magnesium, potassium and a large concentration of nitrogen... I cut them into chunks let em rot and add a handfull to the soil for each plant I set out in the spring!

KimmSr did not say that Magnesium is the solution for Blossom End Rot and spraying Epsom Salts is not the answer, Blossom End Rot is caused by a calcium deficiency and that has many causes ranging from too little soil moisture to plants growing too fast for that mineral to get where it is needed when it is needed.
Spraying some Epsom Salts, the usually recommended dosage is 1 cup Epsom Salts per gallon of water, is not enough to do much of anything except waste your money. The people most interested in convincing you to use
Epsom Salts are those that are selling that product.

Here is a link that might be useful: Epsom Salts for BER