CO-Horts Blog

Wednesday, February 10, 2021

Jeffco Clinic’s Top 10 Plant Problems of 2020

Usually, a top ten list highlights the 10 best of something.  Unfortunately, when looking at plant problems, we are looking at the ten problems most frequently seen...the 10 worst!  

Here is a summary of the top 2020 plant problems that were seen on samples submitted to the Jefferson County Extension Plant Diagnostic Clinic in Golden.  Some plant problems are consistently seen in the Jeffco Plant Clinic every year, while others are influenced by factors like weather conditions, and clientele.  For example, freeze damage to plants was everywhere this year; highly managed golf courses will experience different turf diseases than home lawns.  Since we only report on what has been submitted to the clinic, you may have experienced slightly different problems.  

#1 – Winter Damage (22 samples, 9.8%)

Freeze damage on Bosnian pine

In the Jeffco Clinic, the most frequently diagnosed problem in 2020 was plant damage caused by severe winter weather.  With three sudden freezes within a 12 month span across the Front Range, the majority of the plant problems that came into the Clinic were diagnosed as “Winter Desication” or “Freeze Damage”.  With the freezes of October 2019 and September 2020, plants had not yet hardened off for winter, resulting in freeze damage to existing growth.  In the spring freeze of April 2020, many plants had started to come out of dormancy:  tender young buds and leaves were frozen.  The cumulative effect of these 3 freeze events continued to show up all season long as needle browning and branch death. 

#2 – Drought Injury (22 samples, 9.4%)

Drought injury is an environmental disorder that occurs when more water is lost through leaves or needles than can be taken up by the roots, causing plants to become dehydrated.  The cause of drought injury can be due to either cultural (insufficient irrigation) or environmental factors such as prolonged heat, low humidity, drought, wind, and root damage.  

Deciduous trees typically develop leaf scorch.  

Symptoms on conifers include needle discoloration (yellow, red, brown or purple, depending on variety) as well as tip dieback of needles progressing from the top of the tree down.

#3 – Necrotic Ring Spot on Kentucky Bluegrass, 13 samples, 5.5%

Necrotic Ring Spot (NRS), is a perennial disease caused by a fungal pathogen of Kentucky Bluegrass that causes circular (sometimes donut-shaped) patches of dead grass.   It thrives in grasses that have poor drainage, and that are overwatered.  Good turf management practices are the best means of prevention and control.

Necrotic Ring SpotHoward F. Schwartz, 

#4 - Herbicide Injury, 10 samples, 4.3%

 Herbicide injury was diagnosed on vegetables as well as herbaceous and woody plants.  Plant symptoms include deformed leaves and stems, scorch, stunting and dieback, depending on the active ingredient. 

Paul Bachi, University of Kentucky Research and Education Center,

Herbicides can reach non-target plants in several ways:  via vapor movement from nearby properties, drift of sprays, and through manure or straw mulch.  With good cultural care, plants can outgrow sub-lethal doses over one to several seasons.  

For more information see  Vegetable Herbicide Damage and Herbicide Damage to Trees.

#5 - Eriophyid mite damage, 7 samples, 3.0%

Eriophyid mite scabbing on Oak

Eriophyid mites are tiny (microscopic) mites that feed on plant sap, causing plant damage such as leaf galls, scabbing and distortion.  Most years we see finger galls on Linden leaves, but in 2020 we received multiple samples (Maple and Oak) that had these scabbing symptoms due to the mite feeding.

The damage is primarily aesthetic and typically doesn’t warrant control.  


#6 - Planting Issues, 6 samples, 2.6%

The most common planting issue that we see is deep planting of both herbaceous and woody plants, evidenced in trees by lack of trunk flare. When roots are planted too deep in the soil profile, then they don’t get enough oxygen.  Poor growth, or even death, is a result.  Other planting issues can include choosing the wrong plant for your hardiness zone or soil, or planting in the wrong spot.  See Planting Trees and Shrubs.

    Deep planting (above)

Deep planting (no trunk flare) above
 Overwatering (below)

 # 7 – Overwatering, 5 samples, 2.1%

When plants are overwatered, water replaces oxygen in the soil, causing oxygen starvation (aka suffocation).  Plant symptoms can include leaf discoloration and leaf drop from the bottom up, stunting, root rot, and death.  See Environmental Disorders of Woody Plants.

# 8 – Cultural problems, 4 samples, 1.7%

Sprinkler coverage or malfunction

“Cultural problems” is a category that we use to include plant symptoms and diagnoses resulting from sub-optimal cultural care (water, fertilizer, core aeration of turf, etc.).  

The samples with this diagnosis in 2020 were typically lawns that had become thin & weedy due to improper sprinkler coverage, failure to apply sufficient water to the roots, and lack of aeration. See Lawn Care and Waterwise Landscape Design for recommendations.

 #9 - Spider Mites, 4 samples, 1.7%

Spider mites feed on plant sap, causing leaf damage that ranges from stippling/flecking to scorch, leaf drop and plant death.  Providing appropriate water to the plant is the first line of defense.

Two-spotted spider mites, Frank Peairs, Colorado State University,

# 10 Tomato Spotted Wilt Virus, 3 samples, 1.3%

Tomato Spotted Wilt Virus (TSWV) was reported in both tomato and pepper plant samples.  Symptoms of TSWV include distorted leaves with purplish veins, brown to purple spots on leaves, wilting, and yellow to orange/brown rings on fruit.  

With no treatment available, infected plants should be pulled and discarded.  See also  Recognizing Tomato Problems 


Regardless of what plant disease 2021 sends our way, prevention is the best management tool.  Awareness of what diseases can affect a plant host enables the gardener to monitor and intervene before disease develops.  

Want to know more? To see additional summary data, CLICK HERE.  If you have plant problems, or need help identifying insects or plants, please contact the Jeffco Plant Clinic at 303-271-6627,  We look forward to hearing from you!


  1. Very nice visual summary. Thanks Mari

  2. Though I'm familiar with a lot of the Extension publications, many of these were new to me. Thanks so much for the great post!