(Adapted from Ohio State University: “Twenty Questions on Plant Diagnosis”)
Kirsten Ann Conrad, Extension Agent
Diagnosis of plant problems are complex questions to resolve.  Many times that is no one simple answer about why a plant is not doing well and in almost every case the arrival at a decision about how and whether to treat a plant problem involves a discovery process. While it helps to have some experience with recognizing plant problems, even beginners can attempt the discovery process to be able to explain what is going on to a Plant Health Professional.
It is helpful to remember that while many plant problems will not be diagnosed with your first effort and sometimes you might never get to the real answer, but some problems are very obvious and while there is often a desire to choose a plan of action quickly, there are times when it will be better to take a sit back and watch approach to avoid assumptions and a tendency to make the symptoms fit the diagnosis.
This check list will provide good starting points to get to the facts of a plant disease or insect infestation.

  1. What kind of plant is it? Many plant problems are unique to a particular type of plant or family of plants.  It is also true that most plants have known problems that we can expect to see on a particular plant. Knowing the plant name can help you to identify the insect or plant disease problem.
  2. What is normal for this plant? Some variations in plant coloring, form, or structure can be mistaken for a plant problem.  For instance, plants infested with viruses sometimes look like those with variegated leaves. Decreasing leaf size can be a sign of stress or illness- what size of leaves is it supposed to have?
  3. What are the common problems of this plant? Know your plants.  I always suggest that people know the names of plants in their gardens, their growing habits and needs, and the common diseases and pests that that plant is subject to.   The annual Pest Management Guide for Home Grounds and Animals is one publication put out by VT.edu that helps us know the common problems of hundreds of plants.
  4. What are you really seeing when you look at a struggling plant? What is abnormal about it?   Do you see the SYMPTOMS of a leaf that has been infested with a fungal mildew disease or to you see the SIGNS of the white fungal mycelium on the leaf?  It is good to separate the effects that the plant is experiencing from the causal agents because similar kinds of symptoms can be caused by different causes and will be impossible to treat without knowing exactly what is causing them.
  5. What is the overall health of the plant? How much of it is infected?  Is one leaf of the plant affected or are half of the leaves affected?  Is the plant vigorously growing but has holes in the leaves or does the damage appear to be stunting the growth of the plant?  It is good when making health assessments to look at the color and size of the leaves.  Look at the internode lengths to determine if the plant’s vigor has declined.
  6. Where exactly is the damage and what can you really see? Are the veins in the leaves a different color from the other parts of the leaf?   Are damaged leaves, chewed or shredded?  Is there a yellow halo around a spot or is it a different color or not there at all? Many times it is is helpful to look for patterns of injury and determine if it is one side, or on the top vs bottom, or older or newer growth.   Sometimes we can look at a sick plant and see that leaves are dropping off and then go over all the many reasons that plants can drop leaves-  damage, drought, heat, flooding, season or weather damage, disease, and chemical damage-  and begin to rule out what is not probable.
  7. What’s going on with other plants around our sick one? Are others of the same kind affected?   Are many different kinds of plants affected?  What is growing over, under, or around our sick plant?    Because biotic factors like bacteria, viruses, and fungi and even insects are attracted generally to specific plants, whenever multiple different plants are affected at the same time in the same landscape, it is usually an abiotic problem like heat, cold, drought, or flood that is affecting them.
  8. What are the growing conditions like? Every plant has preferred conditions in which it will thrive.  When we deprive that plant of the correct pH or the preferred light conditions or amount of water, it will become stressed. Stressed plants attract and are more subject to damage by plant pests and pathogens. Right Plant. Right Place is the very most basic tool to maintaining healthy plants.
  9. What do we know about the problem, the history of the planting, the history of this problem plant’s maintenance? How much mulch has been used? Much more than 3 inches can be damaging to a tree. Was it transplanted recently?  Knowing when or if it was fertilized, how much water it gets, how old it is, and how long the problem has existed is valuable information that will help determine how and or if we can help a plant become healthier.  Problems have a logical progression and it can be very telling to discover when symptoms of damage or disease first occurred.
  10. Have site conditions changed over time? Many established plants begin to have problems when an overhanging tree is lost and the light levels suddenly increase or construction over a site changes soil compaction or water drainage patterns. Sometimes plants can leaf out in spring and a sudden freeze can result in damage to new growth and flowers.  Summer droughts can be long and very hot and without adequate replacement of rainwater, plants can become stressed and invite disease and insect problems.

Plant problem diagnosis can be challenging but there are some basic skills and tools that will help a home gardener to do this. Accurate Identification of the plant and the pest are an absolute must.  Get help on this from your local Extension Master Gardener plant clinic or help desk. Local botanical gardens and local nurseries can also be helpful. Keep accurate records of lime, fertilizer, mulch, and pesticide treatments and record observations about weather conditions, rainfall and onset of plant problem symptoms.