Yellow is an alarming shade to find in the greenhouse, especially when you’re not expecting it. From a limey pallor to the startling lemon of truly sick foliage, yellow leaves glare out like warning signs against the lush deep greens of healthy foliage.

The trouble is, yellow leaves are the plant health equivalent of a headache — a general symptom that could mean anything. Throbbing temples in humans can be caused by a brain tumor or just a good party the night before. To get to the bottom of exactly what’s wrong, a doctor must investigate further, and it’s the same with plants: while yellow leaves are a cause for concern, you’ll need to find other symptoms before you can decide on a treatment.

Essentially, when a leaf turns yellow, it is dying. We’re all familiar with the process: it happens every fall, as deciduous trees shed their foliage ready to slip into dormancy for the winter. Previously green leaves turn spectacular shades of red, orange, or yellow as they die, then detach from the tree and float to the ground.

The fall leaf drop is when trees reveal their ‘true’ colors. The green pigment in most leaves is a chemical, chlorophyll, which enables the plant to photosynthesize and turn sunlight into sugar. As the leaf dies, a layer of cells form along the base of the stalk attaching it to the plant, effectively sealing off the pathway of sugar from leaf to plant. The leaf stops photosynthesizing, levels of chlorophyll drop, and the leaf reveals the underlying pigment — usually yellow carotenoids (also responsible for orange carrots and yellow corn).

Eventually, this color also fades and becomes the brown of dead tissue (or the necrotic black of rot).

Read also: 10 Common Marijuana Leaf Problems and How to Fix Them

So, a yellowing leaf is a natural process. But in a healthy plant in midsummer, when leaves should not be dying, it’s also a signal something is up. There are lots of possible causes, so we’ve put together a symptom checker of the seven most common reasons for yellowing leaves to help you decide what’s wrong, and what to do about it.

1. Mineral Deficiency

Feeling pale, a little jaded, under the weather? All symptoms of anemia, or shortage of iron. It’s the same for plants: when plants are short of the minerals they need to thrive, they do the plant equivalent of going pale — its leaves turn yellow.

Most soils contain a good mix of minerals, but they aren’t always available to your plants. Acid-loving blueberries grown in alkaline soil are unable to absorb iron; forget to feed plants in pots and they quickly use up the nitrogen in the potting compost and start to starve. Waterlogging and drought can lock up minerals away from plants, too.

Other symptoms to look out for:

Green veins on older leaves: Yellowing leaves with green veins could mean magnesium deficiency, sometimes caused by over-feeding. Apply Epsom salts as a foliar spray every two weeks.

Brown, crispy leaf edges and green veins on young leaves: This is iron deficiency, common in acid-loving plants grown in alkaline or neutral soils. Grow in pots of ericaceous compost instead.

Spindly growth: Yellowing, weak, slow-growing plants are often short of nitrogen. Put it right with a good feed of nitrogen-rich liquid feed, followed up with a slow release feed and mulch.

Purple tints: Yellow leaves blotched with purple can mean potassium deficiency, especially if plants aren’t fruiting well. Remedy with a dose of potassium-rich liquid tomato feed.

Read also: Why did my little cucumber buds yellow and die?

2. Virus Infection

Luckily for gardeners, viruses aren’t as common in plants as they are in humans. But they work the same way: tiny microscopic parasitic organisms infiltrate a plant’s system cell by cell, distorting and stunting growth and turning leaves yellow, mottled, striped, and streaked.

There is no cure, so the only way you can deal with a virus is to stop it infecting your plants in the first place. Viruses can be carried by aphids and other insects, so keep on top of pest control; weeds also act as host plants. Dip pruners in disinfectant between plants as a precaution, too.

Other symptoms to look out for:

Mottling: Common in cucumber mosaic virus, which infects cucurbits (cucumbers, squashes, and pumpkins) and potatoes as well as many other plants.

Crinkly leaves accompanied by mottled yellowish patches could mean tobacco mosaic virus, which can affect tomatoes, eggplants, and peppers. The edges of the leaves can also dry out.

Stunted, twisted growth: Most viruses will cause plants to look ‘odd’ — twisted, curled leaves, sometimes streaked white, or stunted stems, plus brown patches on fruits are all virus symptoms.

3. Fungal Diseases

The world of fungi is a mysterious place. There are billions of them, all mostly beneath your feet, from microscopic mycorrhizal soil fungi to giant Armillaria ostoyae, one of which is now the largest living thing in the world, its underground mycelia covering almost four square miles of Oregon.

Read also: Is It Time to Say Goodbye to Your Cannabis Fan Leaves?

Sometimes, as in powdery and downy mildews, the fungal growth will be obvious — in this case, as a dusty coating on the leaf. Mostly, you won’t see the fungi infecting your plants; all you’ll notice is the symptoms, which often include yellow, sickly-looking leaves.

Other symptoms to look out for:

Rusty orange patches: Rusts first appear as orange spots; affected leaves then turn yellow and die prematurely. There’s no cure, but you can slow the spread by picking off affected leaves.

Wilting: If an otherwise healthy plant yellows and then wilts, suspect verticillium wilt, a fungal disease which infects water-carrying vessels so plants die of thirst. Once it’s in your greenhouse borders, you’ll have it for years: grow in containers or grow bags of clean compost instead.

Black spots: Yellow rose leaves with black or dark purple spots are a sure sign of blackspot. Prune out infected stems and pick up and dispose of fallen leaves carefully; some types, especially older ‘species’ type hip-bearing roses like Rosa rugosa, are less susceptible.

4. Pests

Plant leaves yellow when under attack from sap-sucking insects because they are literally having the life sucked out of them. Colonies can number thousands of microscopic creatures, every one of them plugged into your plants’ veins like so many leeches. It’s no wonder they turn pale.

Turn affected leaves over and look on the undersides, as this is where any pests will be hiding. Sometimes you’ll find them on the shoot tips, too, where the leaves are tender and tiny bug mouthparts don’t have to work so hard.

Other symptoms to look out for:

Cobwebs: You’ll need a magnifying glass to spot red spider mites, but their silk-like ‘cobwebs’ are a giveaway. Spray with insecticidal soap or release the biological control Phytoseiulus persimilis.

White moths: Whitefly are invisible while plants are undisturbed, but brush the leaves and they flutter up in clouds. Your best defence is a biological control like the parasitic wasp Encarsia formosa.

Wilting: This usually means damage at the root level. Suspect cabbage root fly maggots — easily prevented by laying a cardboard ‘collar’ around seedlings — or vine weevil grubs, especially in container-grown plants. Tip plants out and wash the roots clean of compost, then repot. Or prevent damage with the biological control nematode Steinernema kraussei.

Read also: 11 Common Pests That Can Destroy Your Cannabis Crop

5. Weather

We all get a little haggard in a long winter; it’s no coincidence that you’re more likely to fall sick in chilly, damp conditions when you’re uncomfortable and run down.

It’s much the same for plants, especially in these days of increasingly dramatic changes in our climate. Extreme weather conditions can happen any time of year and they’re really tough for growing plants. Heavy rain can flood the soil, drowning roots and washing out nitrogen completely. Long, hot, dry spells lock up nutrients so roots can’t get at them. And frosty days freeze and damage vulnerable plant cells.

All these conditions cause leaves to turn yellow, so sometimes when you’re faced with a sickly-looking plant, it’s a good idea to look up at the sky.

Other symptoms to look out for:

Standing water: Poorly drained soils waterlog easily, filling soil air pockets with water and drowning roots. Digging in organic matter helps open up heavy soils, or you can install drainage pipes.

Scorched leaves: Yellowing leaves with brown ‘scorched’ tips in summer often mean your plant is gasping for water. Water new plants particularly well in the year after planting as they haven’t yet sent roots into the soil.

Read also: Managing Your Landscape with Microclimates

Frost: Yellowing leaves in winter could be cold-induced chlorosis, caused by soil microbes slowing down in cold weather so they don’t deliver nutrients to plants. They usually recover but covering plants with horticultural fleece and cloches keeps them warmer longer.

6. Weedkiller

Sometimes, we are our plants’ own worst enemies. A slip of a spray gun, the wind blowing in the wrong direction, and a splash of herbicide lands in the wrong place. Result: yellow leaves. Protecting nearby plants with sheets of cardboard helps, but it’s better for them and the environment not to spray at all.

Herbicide damage can happen indirectly. Aminopyralid is a herbicide commonly used on grazing pasture and can end up contaminating manure, causing serious damage when used, unwittingly, by gardeners. Source manure carefully; if you can’t be sure, a ‘test sowing’ of susceptible plants like fava beans will tell you if your batch of compost is clean or not.

Other symptoms to look out for:

Stunted growth: Stalled growth and yellowing leaves are classic signs of general broadleaf herbicide damage. Prune off affected leaves as quickly as possible, and water thoroughly to dilute chemicals.

Curled leaves: When seedlings emerge with distorted, fernlike leaves that curl in on themselves, it’s likely you’ve been using manure contaminated with aminopyralid. Ditch affected plants, as they won’t survive. The chemical breaks down in soil, so after a year compost should be safe to use.

Brown patches: When leaves yellow in patches which die back leaving the rest of the leaf healthy, your plants may have been exposed to a contact herbicide such as diquat, which kills only where it touches. It’s sometimes used by farmers to clear fields prior to sowing. Rinse thoroughly with water and if the plant is only lightly damaged it should recover.

Read also: Advantages of Organic Weed Control

7. Normal Ageing

Yellowing leaves look alarming — and sometimes they are. But just occasionally, plants need to lose a leaf or two. We’re familiar with the fall leaf drop, but what’s less well known is that evergreen trees do this too, shedding a few older leaves gradually through the year. Brassicas (like cabbages, calabrese and sprouts) also do this naturally as they mature.

As long as you can’t see any other more sinister developments, such as wilting or the loss of young leaves, there’s nothing to worry about. This is a perfectly natural process, known as senescence, in which the leaf stops growing or photosynthesising as the plant no longer needs it. It loses its green coloring, dies and then drops away, leaving the rest of the plant to grow on as normal. It’s worth picking up the yellowing leaves, so they don’t become havens for fungal diseases like botrytis — but otherwise, you don’t need to do a thing.

When you spot that yellow warning light among your plants, it’s worth sitting up and taking notice. But you’ll need to look closer to find out what’s really ailing them. Once you’re confident of your diagnosis, act quickly, apply the right medicine, and in most cases, the patient will make a full recovery.