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Plant Problems

A page about a plant problem almost always does two things: lists the causes, then names something to spray. On every problem below, one of those two is broken — and which one is the most useful thing we can tell you. Sometimes the list cannot work, because the three caterpillars people call cabbage worms are all green and no color chart separates them. Sometimes the spray cannot work, because the grub is sealed inside the leaf, or because calcium cannot cross into the fruit however you apply it. Each guide starts from what you are actually looking at, says plainly where two causes cannot be told apart by eye, and where the honest answer is nothing now, something earlier in the season, it says that instead of selling you a bottle. Timing question rather than a health one? The frost date lookup is the place to start.

A blue-green cabbage leaf close up, chewed through with ragged holes and one patch eaten down to a pale translucent film. Three green caterpillars rest on it at very different sizes — a large one humped up into a loop, a medium one stretched along a leaf rib, and a small slender one. A banded sun sits low on the horizon behind.
Plant problems

Cabbage Worms: Three Green Caterpillars, and You Tell Them Apart by Poking Them

Imported cabbageworm, cabbage looper and diamondback moth are all green and all called cabbage worms. Color will not separate them. What they do when you touch them will.

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Two beetles on a green squash leaf with one chewed hole above them, seen from above against cream paper. The left beetle is yellow-orange with three black stripes — one down the center seam and one on each wing case. The right is yellow-green with about twelve small black spots and a solid black head.
Plant problems

Cucumber Beetles: Stripes Mean the Grubs Are in Your Roots, Spots Mean They Are Not

Two quarter-inch beetles share the name and look alike. Only the striped one's larvae feed on cucurbit roots — UC IPM says exclusively. That decides what a wilting plant means.

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A cucumber vine climbing a wooden trellis, its big leaves patched with angular yellow blotches that stop dead at the veins, with dark green cucumbers hanging from it.
Plant problems

Cucumber Leaves Turning Yellow: The Straight-Edged Patches Are the Tell

Straight-edged yellowing that stops at the veins narrows it to two — downy mildew or angular leaf spot — and the underside of the leaf decides which. The cucumber differential, extension-sourced.

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A ripe red tomato hanging on the vine against aged paper. Its top is still glossy and healthy, while the entire lower half is a wide sunken patch of dry, wrinkled dark brown. A band of scattered colored pixels cuts horizontally across the fruit where the red meets the brown.
Plant problems

Blossom End Rot: It Is a Calcium Problem, and the Calcium Spray Cannot Fix It

The fruit really is short of calcium. Your soil usually is not. Clemson names the spray as a myth for two mechanical reasons — and the actual fix is how you water.

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A seed tray of dark compost. Four small seedlings stand upright at the back while seven others lie toppled flat on the soil, each with a thin pale stem and bright green leaves that still look perfectly healthy. A banded sun disc glows above the tray.
Plant problems

Damping Off: There Is No Cure, So the Whole Job Is Getting Seedlings Past It Fast

Several unrelated organisms share the name, and unusually it does not matter which one you have. Wisconsin's advice is to germinate warm so seedlings 'rapidly grow out of the stage'.

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A large cucumber leaf held by its stalk and turned over to show the pale underside with its veins standing out. Dark purple patches of fuzzy growth spread across it, each one stopping in a straight line where it meets a vein. A cucumber vine and trellis post stand behind.
Plant problems

Downy Mildew: The Straight-Edged Patches Are a Cucumber Thing

The angular, vein-bounded lesion everyone teaches is cucumber's tell. NC State says it is less reliable on watermelon and cantaloupe — and basil's downy mildew is a different genus entirely.

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A thin green tomato stem cut straight across and held upright between two fingers, its cut face toward the viewer. Just inside the green skin runs a ring of dark brown, wrapped around a pale ivory core. Tomato foliage and an evening sky behind it.
Plant problems

Fusarium Wilt: The One in Your Tomato Bed Cannot Touch Your Beans

It is not one disease. Each fusarium wilt is locked to its own host — UC IPM says the tomato one 'infects tomatoes only.' That changes what you can plant there next year.

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A single large chard leaf with deep green blades and red stems, lit from behind against a violet-to-gold sky. One pale tunnel snakes across the middle of the leaf and widens as it goes, and its far end breaks apart into drifting squares of light.
Plant problems

Leaf Miners: By the Time You See the Tunnels, Spraying Is Too Late

The grub is sealed inside the leaf where sprays cannot reach it. Minnesota Extension also says the damage barely matters — unless the leaf is the part you eat.

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The underside of a large squash leaf, with a tight cluster of coppery bronze eggs tucked into the V where two leaf veins meet.
Plant problems

Squash Bugs: Why Spraying the Ones You Can See Does Not Work

By the time you notice adult squash bugs, spray is the weakest tool you have. The whole fight is eggs and freshly hatched nymphs — here's the timing, extension-sourced.

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Two compound tomato leaves lying on dark soil. The leaflets of the left leaf are covered in small brown spots, each with concentric rings inside it and a yellow halo around it. On the right leaf, one leaflet is almost entirely taken over by a single large dark-brown blotch.
Plant problems

Tomato Blight: One Name, Two Diseases, and Opposite Instructions

Early blight is yours to manage over a season. Late blight is a water mold that blows in, kills in days, and puts your neighbors' plants at risk. Telling them apart decides what you do today.

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Two tomato leaves on dark soil, both curled and curled in opposite directions. The left leaf's leaflets are folded upward along their midribs into shallow troughs, the leaf still a healthy deep green. The right leaf's leaflets bend downward and droop, narrowed and strappy. A banded sun sits low between them.
Plant problems

Tomato Leaves Curling: Which Way They Roll Tells You Whether Anything Is Wrong

Rolling upward from the lower leaves is physiological leaf roll — UC IPM says it is not caused by a pathogen and will not harm plants. Rolling downward with twisted growth is something else.

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A large green caterpillar gripping the stem of a tomato plant, its back and sides covered with dozens of small white cocoons lying flat against its skin.
Plant problems

Tomato Hornworm: How to Find It, and the One You Must Not Kill

Find hornworms by their droppings, not by hunting green on green. And if yours is covered in white cocoons, leave it — it's already dead and working for you.

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A tomato plant tied to a wooden stake in bare soil. Its lowest leaves have gone yellow and are drooping, while the top of the plant stays green and carries small ripening tomatoes.
Plant problems

Tomato Leaves Turning Yellow: How to Tell Which Cause It Is

Yellow tomato leaves have five common causes and the yellow alone can't tell them apart. Use the spots, the pattern, and which leaves are hit — extension-sourced.

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A young cabbage-family seedling in a raised bed of dark soil, its leaves peppered with dozens of small round holes chewed clean through.
Plant problems

Flea Beetles: When the Shot-Holes Matter and When They Don't

Tiny holes peppered through your leaves, and beetles that jump when you lean in. On a two-week-old seedling that is urgent. On a grown plant it is not.

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A speckled green leaf bronzing at the tip, with two smears on the paper below it, one green and one orange.
Plant problems

Spider Mites: Why the Spray Often Makes It Worse

Too small to see, so you diagnose the leaf. Three extension services name the sprays people reach for as a cause of spider mite outbreaks. Here is what to do instead.

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A stream of green and copper beetles flies past a bulging full trap bag and on to roses eaten down to lace.
Plant problems

Japanese Beetles: Why the Trap Makes It Worse

Metallic green head, coppery wings, leaves eaten to lace. Two extension services find the traps pull in more beetles than they catch. Here is what to do instead.

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A green stem packed with aphids, and a spiny orange-and-gray ladybird larva working through them, leaving bare stem behind.
Plant problems

Aphids: How to Be Sure, and When to Leave Them

Every aphid has two tiny tailpipes at the back end, and nothing else on your plants does. Minnesota says most aphid populations can simply be ignored. Here is when that holds.

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A tomato leaf covered in small dark-rimmed spots, with one larger spot showing tiny black dots inside its pale center.
Plant problems

Septoria Leaf Spot: The Tell Is Inside the Spot

Many small spots, pale centers, and tiny black specks in the middle of each one. That last detail is what separates Septoria from early blight, and it decides what you do next.

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