Our plants often give us clues as to why they're struggling. Before reaching for fertilizer or a spray for leaf disease, or throwing your hands up in frustration, it helps to take a moment and look closely at which leaves are affected and where on each leaf the damage starts. By observing the plant, it's often easy to figure out what is going wrong and how to fix it. And many of the fixes are quite easy.
If you see a few yellow leaves at the very bottom of a healthy, growing plant, it's usually nothing to worry about. When a plant grows, its oldest leaves can get shaded by new growth above them and turn yellow. The same thing often happens at the end of the season; annual plants will start yellowing from the bottom up as they die off.
What separates normal aging from a real problem is how fast the yellowing happens and how much of the plant it affects. If a leaf or two drops each week while the top keeps putting out healthy growth, the plant is fine. But if the yellowing affects several leaves in a few days, something else may be going on.
Yellowing
If the leaves at the bottom of the plant are pale or yellow all over, the plant usually needs more nitrogen. Or the soil might be too wet. These two problems can cause the same type of yellowing leaves, and the way to tell them apart is to check the soil. Push your finger an inch or two into the soil; if it isn't soggy but the plant looks washed out and is growing slowly, it's probably lacking nitrogen.
If the soil is soggy and the plant is wilting anyway, the roots are drowning; roots need air as much as water. Another sign of overwatered plants is that they will turn soft or dark at the base of the stem, right at the soil line. It's important to figure out which issue is the problem because watering won't help a hungry plant and extra fertilizer won't fix a drowning one.
When the leaves turn yellow, but the veins stay green, the plant is most likely lacking magnesium or iron. If it is a magnesium shortage, the older, lower leaves are affected first. On the other hand, iron shortages start in the newest growth at the top. Iron problems are rarely about the soil running out of iron, since most soils have plenty. The usual culprit is soil that's too alkaline, which locks up iron so the roots can't access it.
Browning
Brown, crispy leaf edges and tips mean the plant is losing water faster than it can replace it. The edges turn brown first because they are farthest from the veins, which carry water through the plant. Most of the time, the plant just isn't getting enough water. A thirsty plant usually wilts before it browns, and it will perk back up within a few hours of a good soaking.
Fertilizer salts can cause the same crispy edges because they make it harder for roots to take in water. This happens most with container plants, where salts have nowhere to go. If you see a white crust on the soil surface or around the rim of the pot, it's a good giveaway.
When browning starts at the edges of the older leaves and creeps inward, often with a yellow band between the brown edge and the green center, the plant is short on potassium. It looks a lot like drought damage, but it starts on the lower leaves and keeps spreading no matter how much water the plant gets.
If bleached white or papery tan patches appear in the middle of the leaves, the plant is sunburned. Young transplants often get sunburned because their leaves haven't toughened up yet. Older plants can burn too. This can happen during a sudden heat wave or after a heavy pruning puts previously shaded leaves in direct sun. The burned patches won't heal, but the plant will grow new leaves that handle the sun just fine.
Curling
Tomato leaves that are rolled upward are usually reacting to stress, like a hot spell or a heavy pruning. This is called leaf roll, and it mostly affects the older, lower leaves. It looks alarming, but it rarely hurts the harvest. The leaves may stay rolled for the rest of the season, and that's fine. Many plants cup or fold their leaves to hold on to water, then flatten back out once the evening cools off.
When there is curling on the newest leaves at the top of the plant, especially if they look twisted or puckered, it might be insects. Aphids like to cluster on the undersides of new leaves and suck the sap out, which curls and stunts them.
If new growth comes in twisted and oddly narrow, a weedkiller is likely the cause. Tomatoes are especially sensitive to this. Even if you don't use weedkiller, the spray can drift over from a neighbor's property. The leaves can twist from a surprising distance. Manure, or compost made with manure, can do the same thing. Some long-lasting weedkillers used on hay pass right through the animals that eat it, and they stay active in the finished compost for more than a year. The compost will look perfectly normal, which is frustrating.
Spotting
Spots that start on the lower leaves and work their way up the plant are a sign of a fungal infection. This is more common after a stretch of wet weather. The spots are often round with a darker border. Early blight, a common tomato disease, has rings inside each spot that look like a little target. Many of these fungi spend the winter in the soil and old plant debris, then when it rains, they splash onto the lower leaves of the plant. That's why the bottom of the plant gets hit first.
Dark, greasy spots are likely caused by bacteria. Bacterial spots often have angular edges because the infection stops at the veins, and some have a yellow ring around them.
If there are tiny pale dots all over the top of the leaf, it could mean spider mites. They're tiny and look like moving specks of dust. In a bad infestation, there will be fine webbing stretched between the leaves and stems, and the whole plant takes on a dull, dusty look.
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