Read the spot before you reach for the spray.

Spots on leaves are among the most common things gardeners worry about and among the least often correctly identified. Fungi, bacteria, sunburn, cold, fertilizer, herbicide drift, and insects can all leave marks that look alike at a glance.

The good news is that most leaf diseases are managed the same way — by changing the conditions that let them spread — and that many of them, especially late in the season, do not threaten the plant at all. This guide helps you tell what you are looking at, decide whether it matters, and act in the right order.

The short answer

1. Rule out the non-disease causes first. Damage that appeared all at once, on several unrelated plants, and is not spreading, is probably weather, chemicals, or care — not a pathogen.
2. Look for the organism. White powder, fuzzy growth, colored pustules, and tiny black specks inside spots are signs of a fungus. Angular, water-soaked spots bounded by veins suggest bacteria.
3. Decide whether it matters. Late-season mildew on a squash near the end of its harvest, or spots on a shade tree after a wet spring, rarely justify treatment.
4. Change the conditions. Water the soil, not the leaves; water in the morning; space and prune for airflow; mulch to stop soil splashing onto leaves; remove infected leaves and fallen debris.
5. Use a fungicide only to protect healthy growth, after you know the disease. Fungicides do not cure existing spots.

Rule out the non-disease causes

Plant pathologists divide problems into biotic ones, caused by living organisms such as fungi, bacteria, and insects, and abiotic ones, caused by weather, chemicals, soil, and care. A surprising share of the samples that reach plant clinics are abiotic. The pattern of damage separates the two better than any single spot.

The common look-alikes:

• Sun scorch. Bleached or brown patches on the most exposed side of a plant, often after a move into stronger sun or a sudden heat wave.
• Cold injury. Leaves that turn limp, water-soaked, and then dark after a frost or cold snap, on the most exposed growth.
• Drought and heat scorch. Browning that starts at leaf margins and tips, especially on the sunny side.
• Fertilizer or salt burn. Brown margins after feeding, sometimes with a white crust on the soil.
• Herbicide injury. Cupped, twisted, or strap-like new growth, often on tomatoes, beans, and peppers. Drift from a neighbor's spray and contaminated compost or manure are the usual sources; the compost guide explains how to test for the second.
• Pest damage. Pale stippling from spider mites and silvery scars from thrips are often mistaken for disease. See the pest guide.
• Edema. Small corky or blistered bumps on leaf undersides, from roots taking up water faster than leaves can lose it. It follows cool, cloudy, wet spells and overwatering, and is common on geraniums and some vegetables indoors.

Look for the organism, not just the damage

Pathologists separate symptoms — what the plant does in response, such as yellowing, spots, wilting, and dieback — from signs, which are the pathogen itself. A symptom narrows the list. A sign usually settles it.

With a 10× hand lens, look for:

• Powdery white or gray growth you can rub off with a finger: powdery mildew.
• Fuzzy gray, purple, or white growth, usually on leaf undersides or on flowers and fruit: downy mildew, late blight, or gray mold.
• Orange, yellow, or brown pustules that leave colored powder on a tissue: rust.
• Tiny black dots inside a spot, often in rings: fungal fruiting bodies that release spores.
• Sticky or slimy ooze, or spots that look greasy and water-soaked: often bacteria.

Remember the disease triangle: an infection needs a susceptible plant, a pathogen, and the right environment at the same time. Spores are almost always present. What decides whether you get disease is usually the weather and the conditions around the plant — which is why the prevention section below matters more than any spray.

Powdery mildew

What it looks like: white to gray, powdery patches on leaves, starting as small spots that spread to cover much of the leaf surface, usually on upper surfaces first. Heavily infected leaves yellow, brown, and die. It is common on squash, pumpkins, cucumbers, melons, peas, roses, lilacs, phlox, bee balm, zinnias, and many others. Most powdery mildew species infect only a narrow range of plants, so mildew on your lilac is not a threat to your squash.

Confirm it: rub the powder with a finger; it smears off. Check with a hand lens that you are not looking at waxy insects such as mealybugs or whiteflies, or at a residue from spray or hard water.

What favors it: unlike most leaf diseases, powdery mildew does not need wet leaves to infect. It thrives on dense, shaded, crowded plants in moderate temperatures with high humidity around the leaves, and on soft, fast growth pushed by heavy nitrogen feeding. That explains why it appears in dry late summers when other diseases do not.

What to do:

• Decide whether it matters. On squash and cucumbers it usually arrives late in the season, and if the harvest is mostly in, the right response may be none. On lilacs, phlox, and bee balm in late summer, it is largely cosmetic.
• Choose resistant varieties. This is the most effective control for crops and flowers that get it every year. Seed catalogs mark mildew-resistant cucumbers, squash, melons, peas, phlox, and bee balm.
• Give plants sun and air. Space plants at the recommended distance, thin crowded perennials, and avoid excess nitrogen fertilizer.
• Remove early infections by picking off the first spotted leaves, if there are only a few.
• If a spray is warranted, use a labeled product early. Sulfur works only as a protectant, before infection is established. Horticultural oils, including neem oil, and labeled potassium bicarbonate products can reduce light to moderate infections. Never apply oil within two weeks of a sulfur spray; the combination injures leaves.

Powdery mildew is also the exception to the rule about wet leaves: water on leaf surfaces can kill its spores. That does not make overhead watering a good idea, because it favors nearly every other leaf disease in this guide.

Downy mildew and rust: the look-alikes

Downy mildew is not closely related to powdery mildew, and it behaves in the opposite way. It needs cool, wet, humid weather. The upper leaf surface shows yellow patches, often angular and bounded by veins, that turn brown; the underside, directly below, carries a gray, purple, or white fuzzy growth, most visible early in the morning. Cucumbers, melons, basil, lettuce, grapes, and impatiens are common hosts, and it can spread fast. If you see fuzz underneath rather than powder on top, pull a leaf and check a local Extension guide for the crop: downy mildews call for different management than powdery mildew.

Rust shows as small orange, yellow, reddish-brown, or black pustules, usually on leaf undersides, with pale or yellow spots on the upper surface above them. Wipe one with a white tissue: rust leaves a colored powder. Some rusts need two unrelated hosts to complete their life cycle. Cedar-apple rust, for example, alternates between junipers and apples or crabapples, so the orange gelatinous galls on a juniper in a wet spring are connected to the spotted apple leaves later on. Removing infected leaves, improving airflow, and choosing resistant varieties are the main controls.

Leaf spots

Leaf spot is a description, not a single disease. Dozens of fungi and bacteria cause spots, and most of them spread in the same way: splashing water carries spores and bacteria from infected debris and soil onto leaves, and wet leaves let them infect. That is why leaf spots appear first on low leaves and build after rainy spells.

Fungal or bacterial?

The difference matters because fungicides do nothing for bacteria. You cannot always tell by eye, but these clues help:

• Fungal spots are usually roundish, often with distinct borders or concentric rings, and may cross leaf veins. With a hand lens you may see tiny black specks in the center — spore-producing bodies. Many have a yellow halo.
• Bacterial spots are often angular, bounded by the small leaf veins, and look water-soaked or greasy when young, turning brown or black. Some have a yellow halo too. In wet weather, bacterial spots may ooze.

If you cannot tell and the plant matters to you, a plant clinic can (see lab diagnosis).

Common leaf spots in home gardens

Septoria leaf spot of tomato. Many small spots, about an eighth of an inch across, with dark borders and tan or gray centers containing tiny black specks. It starts on the lowest leaves, usually after the first fruit sets, and works upward, with heavily spotted leaves yellowing and dropping.

Early blight of tomato and potato. Larger brown spots, up to about half an inch, with concentric rings like a target, often surrounded by yellow tissue. It also starts on older, lower leaves and can girdle stems and rot fruit near the stem end.

Black spot of rose. Black spots with fringed, feathery margins on upper leaf surfaces, followed by yellowing and heavy leaf drop. Resistant rose varieties make the biggest difference.

Anthracnose of shade trees. Brown blotches, often along the veins, on sycamore, maple, oak, ash, and others after a cool, wet spring, sometimes with heavy early leaf drop. It looks alarming, but it is generally cosmetic: affected trees usually put out a new flush of leaves, and fungicides are rarely warranted on large trees. Rake up fallen leaves and keep the tree well watered in dry spells.

Shot hole. On cherries, plums, and other stone fruits, and some ornamentals, spots whose dead centers fall out, leaving round holes with a brown rim. These are easily mistaken for insect feeding; the brown, dry edge is the giveaway.

Blights: when spots turn into collapse

Blight describes rapid browning and death of leaves, shoots, flowers, or whole plants. Several are worth knowing.

Late blight of tomato and potato. The most destructive disease in the vegetable garden, and one that moves between gardens. Look for large, greasy, gray-green to brown patches on leaves, at least the size of a nickel, often at leaf tips and edges, with white, fuzzy growth on the underside in humid weather. Stems develop dark brown lesions, and tomato fruit develops firm, greasy brown patches. In cool, wet weather it can kill plants within a week or two.

It cannot be cured once plants are infected. If you suspect it:

1. Confirm quickly through your Extension office or plant clinic; many Extension services track late blight and want reports, because spores travel on the wind to nearby gardens and farms.
2. Pull infected plants, bag them in plastic, and put them in the trash. Do not compost them, and do not leave them in the garden.
3. Next year, start with clean plants. Late blight often arrives on infected transplants and seed potatoes, and it can survive the winter in potato tubers left in the ground or in cull piles. Buy certified seed potatoes and inspect transplants before you buy.

Gray mold (Botrytis). A fuzzy gray growth on flowers, fruit, and dead or damaged tissue in cool, humid weather, common on strawberries, peonies, and tomatoes. It usually starts on spent flowers and injured tissue. Remove fading flowers and damaged fruit, space plants for airflow, and avoid wetting foliage.

Fire blight. A bacterial disease of apples, pears, and relatives that turns shoot tips black and hooks them into a "shepherd's crook," as if scorched by fire. Management is mostly a matter of pruning well below the infection at the right time, which the pruning guide covers in detail.

Prevent the spread with conditions, not sprays

Most leaf diseases are managed by the same handful of practices, and they work for all of them at once.

• Water the soil, not the leaves. Use drip lines or soaker hoses, or water at the base of plants with a hose. If you must water overhead, do it in the morning so leaves dry quickly. The water-wise gardening guide covers drip timing.
• Space and prune for air. Crowded plants stay wet longer. Follow spacing on seed packets, stake or cage tomatoes to lift them off the ground, and remove the lowest tomato leaves that touch the soil.
• Mulch to stop splash. A layer of straw or shredded leaves under tomatoes and other crops keeps soil — and the spores in it — from splashing onto lower leaves. The mulch guide covers what to use in vegetable beds.
• Rotate crop families. Many leaf-spot and blight fungi overwinter on debris in the soil. Avoid planting tomatoes, potatoes, peppers, and eggplant — all one family — in the same spot more often than once every three or four years where space allows.
• Choose resistant varieties. Seed catalogs list resistance codes and descriptions for many diseases. Resistance is not immunity, but it shifts the odds more than any spray.
• Stay out of wet plants. Working among wet foliage carries spores and bacteria from plant to plant on hands, tools, and clothes. Harvest and prune when leaves are dry.
• Remove infected leaves early, while there are only a few, and take them out of the garden.
• Feed for steady growth, not a flush. Excess nitrogen produces soft, crowded growth that several diseases, including powdery mildew, favor.

Where fungicides fit

Fungicides have a narrow, real role in home gardens. To use them well:

Identify first. A fungicide does nothing for bacterial spots, sun scorch, herbicide injury, or mites. Spraying without a diagnosis wastes money and can harm the plant.

Understand that they protect; they do not cure. Most garden fungicides form a barrier that stops spores from infecting healthy tissue. They do not reverse existing spots. That means they are useful only when applied before or at the very start of an infection, and repeated at the interval on the label to cover new growth and replace what rain washes off.

Match the product to the disease and the plant. The label lists which crops and diseases a product is registered for, how long to wait between application and harvest, and the maximum number of applications. The label is the law, for organic products as much as synthetic ones. Common home-garden options include sulfur and potassium bicarbonate for powdery mildew, copper products as protectants against some bacterial and fungal diseases, and chlorothalonil for tomato leaf spots and blights.

Know when not to spray. Late-season mildew on a crop that is nearly finished, cosmetic spots on ornamentals, and anthracnose on large shade trees rarely justify a fungicide. The better investment is resistant varieties and better spacing next year.

Clean up without spreading it

Many leaf-spot fungi, rusts, and blights survive the winter on fallen leaves and plant debris, and splash back up onto new growth in spring. Removing that debris reduces next year's starting population.

• Rake up and remove fallen leaves from plants that had leaf spot, black spot, or anthracnose.
• Compost diseased material only in a pile that heats up properly. A cool backyard pile may not kill pathogens; the compost guide explains what "hot" means. If in doubt, put it in the trash or yard-waste collection.
• Never compost late blight. Bag it and send it to the trash.
• Disinfect pruning tools after cutting diseased tissue. The pruning guide covers alcohol and bleach.

There is no need to strip the whole garden bare. Healthy stems and seed heads shelter overwintering insects, including predators of pests. The fall tasks guide explains how to clean up for disease without clearing out everything else.

Get a lab diagnosis

When the plant is valuable, the problem is spreading fast, or you have tried the obvious fixes without success, a plant diagnostic clinic can identify the cause. Most land-grant universities run one, usually for a modest fee, and county Extension offices and Master Gardener volunteers can often help first.

A good sample makes a good diagnosis:

• Send a range of symptoms, from early to advanced, plus some healthy tissue for comparison. For small plants, send the whole plant, dug up with the roots intact.
• Do not add water, and do not wash the sample. Wrap it in dry paper towels or newspaper, then place it in a plastic bag. Washing removes the evidence; water encourages secondary rot organisms that obscure it.
• Collect it fresh and deliver or ship it promptly. Refrigerate it if you cannot send it the same day.
• Include the story: the plant and variety, when symptoms started, how they spread, recent weather, and any fertilizer, pesticide, or herbicide used nearby. Add photos of the whole plant and its surroundings.

Call or check the clinic's website first; some have forms, shipping instructions, or specific days for accepting samples.

Sources and further reading

• UC Statewide IPM Program: Powdery mildew on vegetables — infection without free water, resistant varieties, and sulfur, oil, and bicarbonate options.
• UC Statewide IPM Program: Powdery mildew on ornamentals — host range, conditions, and the two-week interval between sulfur and oil.
• Utah State University Extension: Powdery mildew on vegetables.
• Iowa State University Extension: Biotic vs. abiotic — distinguishing disease problems.
• Penn State Extension: The role of patterns in diagnosing crop disorders.
• Colorado State University Extension: CMG GardenNotes #331, Plant pathology — signs, symptoms, and the disease triangle.
• University of Minnesota Extension: Early blight in tomato and potato.
• University of Minnesota Extension: Quick guide to insects and diseases of tomatoes.
• University of Maryland Extension: Septoria leaf spot of tomatoes.
• Iowa State University Extension: Septoria leaf spot.
• Rutgers NJAES: Diagnosing and controlling fungal diseases of tomato in the home garden (FS547).
• UMass Extension: Late blight on tomatoes and potatoes.
• University of Maryland Extension: Late blight of tomato and potato.
• Cornell University: Late blight on tomatoes — photos of symptoms and common look-alikes.
• UMass Extension: Bacterial diseases of vegetable crops — leaf spots and blights.
• University of Maryland Extension: Bacterial leaf spot disease on flowers.
• University of Maryland Extension: Fungal leaf spot disease on flowers.
• Iowa State University Extension: Anthracnose on shade trees.
• University of Maryland Extension: Anthracnose diseases of shade trees.
• University of Minnesota Extension: Managing plant diseases in the home garden.
• UNH Extension: Guidelines for collecting and submitting plant material to the plant diagnostic lab.
• New Mexico State University: Collecting and submitting plant specimens for disease diagnosis (A-154).