Regular scouting can catch problems such as this leaf-spot disease early, before the whole tree loses its leaves (defoliates). © George Weigel

So much can go wrong in a garden. The tough part for many gardeners is figuring out what’s causing, or nearly causing, a plant to fail. Because plants can’t point to where it hurts, you’ll need to diagnose differently: Instead of playing doctor, play detective.

Know your plants

In order to know something’s out of the ordinary, gardeners should first know what’s normal for their “victim” plants. Bark peeling off a birch tree is normal. And white pines are prone to dropping many yellowed needles each fall, while larches are genetically programmed to drop all of their needles every fall.

Look for clues

When you’re sure there’s a problem, start digging. Many times, the answer to what ails plants only becomes apparent as a process of elimination. Start by collecting evidence. Small brown pellets on the ground next to a nibbled pansy could indicate an animal problem. Deformed or discolored leaves might conceal dot-sized black fungal spores on their undersides. A glossy film on those holly leaves might be the aftermath of a scale insect attack.

Sweat the small stuff

What seems like nuance could be the solution to the puzzle, so attention to detail is key. Symptoms could point to different, or even opposite, causes. A plant slowly yellowing from bottom up could mean it’s suffering from disease; a plant that’s browned just on one side may have been hit by herbicide drift or salt spray. Wilting can happen because a new plant hasn’t been watered for three weeks, or because the roots have rotted from excess water.

Know your plants so you can tell what’s normal and what’s not. These yellowing pine needles in fall are normal. © George Weigel

Play P.I. often

Routine scouting and observation is the best way to catch a problem early. Take regular walks to enjoy your plants, but while you’re gawking and sniffing, watch for early signs and symptoms that something’s amiss – leaves yellowing or losing their green, little white flecks on evergreen needles, or noticeable thinning of a tree’s canopy. You might not know what’s causing the plant trouble yet, but realizing there’s trouble is where a plant P.I. starts the investigation. County Extension offices, garden centers, and a host of university and botanical garden Web sites make reliable “witnesses” when trying to catch a garden crook red-handed.

A few common plant problems with some possible causes:

• Plant is yellowing all over: Poor soil fertility; extreme heat; light is too intense or lacking; plant is pot bound.
• Young leaves are yellow: Not enough light; iron or manganese deficiency in the soil; excessive fertilizer.
• Old leaves are yellow: Nitrogen, magnesium, or potassium deficiency in the soil; overwatering; natural aging of leaves; plant is pot bound; roots are rotting.
• Leaves, needles yellowing/browning sporadically: Mite damage; herbicide spray drift; root or stem injury; stem galls.
• Dead or yellow spots on leaves: Fungal, bacterial, or viral infection; excessive fluoride in the soil; pesticide damage.
• Holes in leaves: Caterpillar, slug, or other bug damage; fungal leaf spot disease; hail or wind damage.
• Leaves brown around the edges: Wind damage; excessive salt in the soil; lack of water; excessive fertilizer; pesticide damage; air pollution.
• Leaves falling off: Excessive fertilizer; lack of water; reaction to move or transplanting; cold damage; pesticide damage; lack of light; rotting roots; natural life cycle of plant.
• Wilting leaves: Under- or over-watering; excessive fertilizer; roots or stems rotting; rodent damage to roots; pesticide damage; frost damage; excessive heat.
• Weak growth and/or gradual dieback of branches: Lack of water; root injury or girdling roots; compacted soil; plant was planted too deeply; excessive mulch; poor soil nutrition; lack of light.