Plant Problem Identifier

When a plant looks wrong, the hardest part is knowing where to start. Yellow leaves, wilting, holes and spots can each have half a dozen causes, from simple watering mistakes to insects, diseases and nutrient problems. This plant problem identifier narrows it down. Tell it what you see, where on the plant it appears, how moist the soil is and what kind of plant it is, and it ranks the most likely causes with what to check and what to do. It covers the problems home vegetable gardeners run into most often across the United States.

Use it as a starting point, then confirm with a closer look. The guidance below explains how experienced gardeners and plant diagnosticians work through a problem, so you can get to the right answer faster.

How to use the plant problem identifier

1. Choose the main symptom. Pick the thing you noticed first, such as yellow leaves, wilting or holes.
2. Say where it appears. Whether a problem starts on old leaves, new growth or the whole plant is one of the most useful clues.
3. Check the soil. Push a finger 2 to 3 inches into the soil near the plant and choose wet, moist or dry.
4. Choose the type of plant if it is listed. Some problems only affect particular crops.
5. Read the ranked causes. Start with the most likely, and use the “what to check” advice to confirm or rule it out.

For specific problems, the dedicated tools go deeper: the yellow leaf identifier, the overwatering checker and the pest identifier.

Diagnose like a plant clinic

Plant diagnosticians at university extension clinics follow a consistent process. You can use the same steps in your own garden.

1. Know what is normal

Some “problems” are just how a plant grows. Lower leaves of tomatoes and squash naturally yellow and drop as plants mature. Many large-leaved plants droop in afternoon heat and recover by evening. Purple tints on some varieties are genetic. Before treating anything, make sure it is actually abnormal for that plant at that stage.

2. Look at the pattern

Patterns are the best clues you have. Ask yourself:

• Which leaves are affected? Problems that start on old, lower leaves often point to mobile nutrients such as nitrogen, to watering, or to soil-splash diseases. Problems on new growth point to sap-sucking insects, immobile nutrients such as iron, or chemical damage.
• Is it on one plant or many? A problem on many different kinds of plants at once usually has a non-living cause, such as weather, water or soil. A problem on one crop, or spreading from plant to plant, suggests insects or disease.
• Is it spreading? Living causes such as diseases and insects tend to spread and get worse over time. Damage from a single event, such as a late frost or a hot day, usually stops where it is.
• Is there a pattern in the bed? Damage along one edge can point to drift from a neighbor’s spray. Damage in a low spot can mean waterlogging.

3. Check the soil and roots

A surprising number of problems that show in the leaves start below ground. Check soil moisture a few inches down, not just at the surface. If a plant is seriously ill, gently dig beside it and look at the roots. Healthy roots are usually white or light tan and firm. Brown, soft roots that smell sour point to root rot from waterlogging. Knotted swellings on roots can indicate root-knot nematodes.

4. Look closely at the plant

Turn leaves over. Many insects hide on leaf undersides, and many diseases show their spores there. A cheap hand lens reveals mites, aphids and eggs that are invisible to the naked eye. Look at stems near the soil for holes, sawdust-like material, or dark, sunken areas.

5. Think about recent events

What has changed in the last week or two? A heat wave, a cold snap, heavy rain, transplanting, fertilizing, spraying nearby, or a change in watering can each explain sudden symptoms.

Common symptoms and their usual causes

Yellow leaves

Yellowing is the most common complaint, and its location is the key clue. Even yellowing on older leaves often points to a shortage of nitrogen or to normal aging. Yellow between green veins on new leaves suggests iron shortage, often caused by alkaline soil. Yellowing with soggy soil points to overwatering. The yellow leaf identifier works through these patterns in detail.

Wilting

Check the soil first. Wilting in dry soil is thirst. Wilting in wet soil can mean the roots are rotting and cannot take up water. Wilting in moist soil that does not recover overnight may mean a wilt disease or, in squash, a vine borer. Afternoon wilting that recovers by evening is normal in hot weather.

Holes in leaves

Holes almost always mean something is eating the plant. Tiny round shot holes suggest flea beetles. Large ragged holes with slime trails suggest slugs. Large holes with green caterpillars and dark droppings suggest cabbage worms or tomato hornworms. Lacy leaves with only the veins left point to beetles. The pest identifier narrows it down by crop.

Spots and blotches

Brown spots, sometimes with yellow halos or concentric rings, often mean fungal or bacterial leaf diseases. Many start on lower leaves, where soil splashes up during rain or watering. Fine pale speckling across a leaf usually means spider mites or other sap-feeding insects.

White powder

A white, flour-like coating on leaves is almost always powdery mildew, very common on squash, cucumbers and melons in late summer.

Curled or twisted leaves

Curled new growth with sticky residue usually means aphids. Twisted, cupped new growth after nearby spraying suggests herbicide drift. Tomato leaves that roll upward in hot weather while the plant otherwise looks healthy are often showing a harmless physiological response to heat.

Sticky leaves and black mold

Sticky honeydew comes from sap-feeding insects such as aphids, whiteflies and scale. A black, sooty mold often grows on it. Control the insects and the mold fades.

Slow, stunted growth

Plants that sit and do nothing are often struggling with cold soil, poor or compacted soil, too little light, or crowding. A soil test is the quickest way to rule nutrient problems in or out.

Problems with fruit

A dark, sunken patch on the blossom end of tomatoes, peppers or squash is blossom end rot, usually linked to uneven watering that disrupts calcium uptake. Cracks appear when heavy rain or watering follows a dry spell. Flowers that drop without setting fruit often reflect very hot or cold weather, or too few pollinators.

Seedlings falling over

Seedlings that pinch at the soil line and topple are usually suffering from damping off, a fungal problem favored by overwatering and stagnant air. Seedlings cut cleanly at the base overnight point to cutworms.

Living versus non-living causes

Plant problems fall into two broad groups. Living causes include insects, mites, slugs, fungi, bacteria, viruses and animals. Non-living causes include watering, temperature, light, nutrients, soil conditions and chemicals.

Non-living problems are actually more common than many gardeners expect. Watering issues alone account for a large share of the problems people bring to plant clinics.

When to get expert help

Many state Cooperative Extension services run plant diagnostic clinics or Master Gardener help desks that will examine samples or photos, often for free or a small fee. They are especially helpful for diseases, which can look alike, and for problems affecting valuable trees or shrubs. When you contact them, have this information ready:

• The plant name and variety, if you know it.
• When you first noticed the problem and how it has changed.
• Photos of the whole plant, the affected parts, and leaf undersides.
• Recent weather, watering, fertilizing and any sprays used nearby.

Preventing problems in the first place

• Grow in good soil. Compost-rich, well-drained soil prevents many problems. See the compost calculator.
• Water deeply and evenly at the base of plants. The garden watering calculator helps set amounts.
• Space plants properly. Air flow between plants reduces many leaf diseases. The plant spacing calculator shows how many fit.
• Rotate crop families each year to break pest and disease cycles.
• Choose resistant varieties where available. Seed catalogs list disease resistance codes for many tomatoes, cucumbers and other crops.
• Encourage beneficial insects with small-flowered plants. The companion plant finder suggests good pairings.
• Walk the garden often. Problems caught early are far easier to fix.

Problems to watch for through the season

Many garden problems follow the calendar. Knowing what tends to appear when helps you catch it early.

Spring

Seedlings are most vulnerable now. Watch for damping off indoors, cutworms cutting transplants at the soil line, and flea beetles riddling young eggplant, radishes and brassicas with tiny holes. Cold nights can turn warm-season transplants pale or purplish. Slugs are active in damp spring weather, especially in mulched beds.

Early summer

Aphids multiply quickly on tender new growth. Cucumber beetles arrive on squash and cucumbers, and cabbage worms appear on brassicas. Colorado potato beetles emerge on potatoes and eggplant. This is the time to have row cover in place and to start checking leaf undersides every few days.

Midsummer

Heat and uneven watering bring blossom end rot and fruit cracking on tomatoes. Spider mites thrive in hot, dry weather. Tomato hornworms grow large, squash vine borers cause sudden wilting, and Japanese beetles skeletonize leaves in many regions. Afternoon wilting becomes common and is usually just heat.

Late summer and fall

Powdery mildew coats squash and cucumber leaves, and leaf blights work their way up tomato plants. Many of these late-season problems are a sign that plants are tiring rather than a reason for alarm. Clean up diseased plant material at the end of the season so problems do not carry over to next year.

Keep a simple garden log

A notebook or a note on your phone makes diagnosing problems much easier, this year and in future seasons. Record planting dates, varieties, when you fertilized, unusual weather, and when problems first appeared, ideally with a photo. Patterns emerge quickly. You may notice that powdery mildew always arrives in the same week of August, that one bed always stays wet, or that a particular tomato variety resists the blight that troubles others. Those notes are exactly what a plant clinic will ask for, and over time they become the most useful diagnostic tool you own.

Frequently asked questions