Diagnosis: It's generally recommended that seedlings are transplanted when they get to be about four inches (find tomato transplanting tips here.) But if you seeded at the recommended time and they're still small or spindly, it's probably a question of heat and light, both of which are crucial for tomatoes.

Treatment: Since most tomatoes are started indoors, it can be difficult to make sure they get enough heat and light. Heating pads and grow lights are a must if you don't have a greenhouse. At the start, tomato seeds need a consistent temperature of around 80 degrees for at least a week. After germination, they'll still need that 80 degrees during the day, and around 65-70 degrees at night for 3-4 weeks. They'll also need about 12 hours of light daily. If you haven't transplanted them yet and it's well past your last frost date, simply getting them in the ground may be exactly what they need.

Diagnosis: There are a variety of reasons why tomato leaves might be pale or yellow. It generally means they're stressed due to an imbalance of heat, light, water, or fertilizer, or that they need more space.

Treatment: We've already emphasized the importance of adequate light and heat--but it's important to keep in mind that an excess of those things can be just as damaging. Not watering enough, for instance, can cause stress that will manifest as yellow leaves, but over-watering can just as easily cause problems. The same goes for fertilizer. Although it's important to apply an organic fertilizer to your tomatoes as they grow, it's not always a good idea to fertilize seedlings. The minerals can build up in their tiny cells in levels too high for them to handle. Additionally, make sure your seedlings have enough space. If they've been in their cells for a while, they may be outgrowing them, and if you planted more than one seed per cell, be sure to cut all but one of them at the base to avoid overcrowding. All things told, leaf color is a good marker of whether or not you're striking the proper balance of resources for your seedlings.

Diagnosis: If young seedlings' stems wither at the soil level and the seedling topples over and dies, the cause is almost certainly damping off. This is a fungal disease caused by a combination of too much moisture, poor air circulation, and moderate temperatures.

Treatment: We've already addressed over-watering, and here's another reason why to avoid it. Although seeds need a constant supply of moisture to germinate, you can decrease the amount of water after they sprout, just making sure they don't completely dry out. Additionally, maintain air flow by providing them with a fan or moving them out of closed, stuffy rooms. For many, however, it may be long past that early vulnerable period, and once they are transplanted into well-draining soil, the threat of damping off is over.

Diagnosis: Sometimes as seedlings grow, the leaves will develop a purple hue in the leaves. This is generally due to phosphorous deficiency.

Treatment: Seedlings can run out of phosphorous as they grow and exhaust the resources in their cells. Yet again, the best thing to do is go ahead and transplant them. As long as they are planted into well-composted soil, they're almost certain to recover right away.

Diagnosis: Nothing gets a tomato grower's heart pounding like blotches of brown on leaves, since this can be an indicator of disease, which tomatoes are particularly susceptible to. But before you fear for the worst, remember that leaves can turn brown for any number of reasons, and in fact to some extent a bit of browning is unavoidable. It could be a symptom of stress, as discussed before, or simply sun scorching.

Treatment: If your seedlings are still in trays and you think they might be stressed, it's time to transplant them into fresh soil where they'll have more nutrients, sunlight, and space. Sun scorching can happen after transplanting while tender seedlings are acclimating to outdoor conditions, especially if they weren't hardened off properly. This can be minimized by transplanting on cool and cloudy afternoons, but with time, tomatoes will adjust on their own, growing newer, stronger leaves that can withstand direct sunlight. If browning continues to spread, however, or leaves and stems begin to wilt, you may be looking at a tomato disease such as Early Blight or Fusarium Wilt. There isn't much to do in this case except prune off the affected leaves or branches and discard them away from the plant. Much of the battle against disease is preventative care. Always be sure trays, pots, and trellising materials are sterilized if they have been used before. If possible, don't plant tomatoes where you planted them the year before. Additionally, water can help the spread of disease, so only water at the base of the plant, avoiding leaves, and don't prune or harvest if the plant is wet. If you think your tomatoes might be contracting a disease, you can turn to Cornell's Tomato Disease Identification Key, and if you need more advice for avoiding blight, they've got a guide for that too.

Diagnosis: It happens without warning--suddenly your seedling has been chewed through at the bottom. You've been struck by the dreaded cutworm! Cutworms are moth larvae that nibble a circle around the full circumference of the stem and then chomp down. It mainly affects tomatoes, peppers, and eggplants, but occasionally visits the bottoms of brassicas too.

Treatment: Prevention is the name of the game here. An easy way to protect your seedlings against cutworms is to put two toothpicks alongside and touching the stems of your transplants, one on each side of the stem. Do this at transplant time. The cutworm won't be able to fully encircle the seedling and will accept defeat.As you've probably gathered, if you haven't already transplanted your tomatoes, most of their ailments can be solved by biting the bullet and planting them outside. Chances are with the proper bed prep, they'll thrive.