Many things make plants unwell. Diseases are only one cause of poor plant growth. In diagnosing problems of plants from the production area, the retail area, the landscape, or home all the possible causes of poor or abnormal growth must be considered.

The following key lists some but certainly not all of the possible causes of plant ill health. Your interrogation of the person growing the plant or a review of the history of the plant should eliminate some of the causes listed while close examination of the plant should eliminate others. Use this key as a guide and add to it as your experience increases.

Seeds And Seedlings

Seed Emergence Poor

Temperature and moisture good

• Old seed
• Planted too deep
• Planted too shallow and dried
• Soil too compact
• Too much fertilizer close to seed
• Fungus rotted seed

Temperature or moisture poor

• Too dry
• Too wet
• Too hot
• Too cold
• Fungus rotted seed

Seed Emergence Good but Growth Poor

Seedlings normal but not growing

• Inadequate light
• Insufficient fertilizer
• Too cool

Seedlings appear swollen, twisted, or distorted

• Mite feeding injury
• Herbicide in soil

Seedlings with leaf tip burn

• Excessive fertilizer
• Pesticide spray damage

Seedlings dying at soil line

• Damping-off due to fungi

Established Plants

Leaves

General yellowing of plant

• Low fertility
• High temperature
• Light conditions too intense or insufficient
• Plant pot bound

Young leaves yellow

• Low light intensity
• Iron or manganese deficiency
• Overfertilization

Old leaves yellow

• Nitrogen, magnesium or potassium (major elements) deficiency
• Overwatering
• Natural aging of leaves
• Plant pot bound
• Root rot

Dead or yellow round spots on leaves (or elongated spots on linear leaved plants)

• Fungal, bacterial, or virus infections
• Fluoride toxicity
• Pesticide damage

Dead or yellow irregular spots or flecks

• Thrips feeding
• Air pollution
• Pesticide damage
• Cold water damage

Mosaic pattern of light and dark green

• Virus infection
• High temperature injury
• Pesticide damage
• Major element deficiency

Leaves with abnormal color pattern

• Virus infection
• Mutation in small area of tissue
• Nutrient deficiency (reddening or purpling of leaves)

Leaves very dark green with water soaked, limp appearance

• Cold injury
• Mechanical damage (crushing)
• Bacterial infection

Leaf tips and margins dead

• Overfertilization
• Underwatering
• Pesticide damage
• Vascular wilt disease

Leaves too small

• Low light conditions (note long or upright petioles or long internodes)
• Too much or too little fertilizer
• Low humidity conditions
• Root rot

Leaves very brittle

• Minor nutrient deficiency
• Mite feeding

Leaves falling off

• Overfertilization
• Overwatering
• Plant not acclimated to low light intensity
• Cold damage
• Pesticide damage
• Prolonged low light intensity
• Ethylene exposure
• Root rot
• Natural cycle of plants (Ficus benjamina drops leaves its dry season)

Leaves wilted

• Overwatered (soggy soil; algae on soil surface)
• Underwatered (soil pulling away from edge of pot)
• Overfertilization
• Root rot
• Stem rot
• Plant not acclimated to low humidity
• Cold damage (leaves blacken and die)
• Plant pot bound

Leaves with white powdery growth on surface

• Powdery mildew

Leaves with black growth in patches in general over surface

• Sooty mold (look for aphid and scale insect activity)

Leaves with sticky substance on surface

• Material excreted by insects
• Natural secretions of plant (note if material is located in the same general area on each leaf)

Leaves crinkled

• Natural habit of plant
• Virus infection
• Insect feeding

Stems

Stem tips too small

• Nutrient deficiency
• Mite feeding
• Herbicide injury

Too many stem tips

• Virus or mycoplasma infection
• Herbicide injury

Too few stems

• Low light intensity

Stems rotted at soil line

• Fungal or bacterial disease
• Overwatering

Stems rotted above soil line

• Fungal or bacterial rot
• Sunburn

Stem tips dead

• Fungal infection of tips
• Overfertilization
• Underwatering
• Root rot
• Insect feeding
• Pesticide damage

Stem near soil line abnormally swollen and galled

• Crown gall bacterium infection

Stem near soil line with abnormal 'cauliflower' growth

• Bacterial fasciation infection

Entire plant

Excessively slow growth

• Poor light conditions
• Potting soil too compact
• Poor fertilization or watering schedule
• Root rot
• Non-vigorous - Some plants in the population just do not thrive

Roots

No roots

• Cuttings not rooted (No hormones used or plant difficult to root)

Roots very close to soil surface

• Plant grown on too hot a surface (TV, radiator)
• Soil kept too wet
• Soil too compact
• Only enough water added to wet top layer
• Soil washed away during watering

Roots dark colored, limp

• Overwatering
• Overfertilization
• Root rots due to fungi or bacteria
• Nematode feeding

Roots swollen or galled

• Natural growth of many plants
• Root-knot nematode infection

Roots sticking up in air and foliage buried

• You've got a wise guy on your hands. Beware.

Similar keys are available from the following sources:

Vegetables:

• Averre, C. W., A. A. Banadyga, and K. A. Sorensen. Diagnosing common disorders of vegetable crops. North Carolina Agricultural Extension Service Publication AG 07. 13 pp.

Ornamentals:

• Joiner, J. N. 1981. Foliage plant production. Prentice-Hall, Inc. Englewood Cliffs, NJ. Chapter 13. Pages 330-339.