Don’t Jump to Conclusions when Diagnosing Tree Problems

February 2, 2019

Question:

The Texas red oak, live oak, and pecan trees in my yard were looking bad going into the fall, as were my neighbor’s honey locust and maple. When I searched for problems online, I found different possible pests and diseases for each tree. Can you help me narrow down the possibilities?

- Richard V., Hobbs, NM

Answer:

This is Part II of the column on diagnosing tree problems. Last week, we learned that water stress and weed whacker injury are the most common tree problems in our landscapes, and that the rooting area necessary for large trees to survive and grow is much bigger than most folks realize.

We also touched on the reasons why symptoms are rarely sufficient for conclusive diagnosis of a tree disorder. This is partly because symptoms may point to secondary or tertiary problems. Many—but not all—insect pests and pathogens are more likely to attack trees that are already stressed.

Plant stressors can be broadly divided into two categories: biotic and abiotic. Biotic stressors are caused by living or once-living organisms, like insects, bacterial and fungal pathogens, and animal pests.

Nature's Notebook is a national, online program with the USA National Phenology Network that uses amateur and professional naturalists to record plant and animal observations in a given location over time. The steps for becoming a volunteer are straightforward. And you can set your backyard as a location or pick a public space and get a group to sign up together.

While visiting the City of Albuquerque’s Botanical Garden last February with a Nature’s Notebook group, we saw severe trunk damage way up in the canopy of some of the huge bosque cottonwoods. It was hard to see, but big strips of bark had been peeled off. The tree wasn’t very happy about it, but I was thrilled when someone pointed out the culprit hanging out up there: a porcupine! This type of trunk injury can cause branch dieback and adversely affect overall tree health, but controlling the porcupine population sounds too sad to me.

Red arrow is pointing to a porcupine munching on tree bark way up in the canopy of a cottonwood at the Albuquerque Botanical Garden in February 2018. Photo credit M. Thompson

Abiotic stressors include environmental conditions, like soil quality and type, cold and hot temperature extremes, drought, storms, and wind. Cultural practices that are harmful to plants can also be considered as abiotic stressors. This includes selecting species that won’t perform well in our environment, poor planting techniques (e.g., planting too deep!), irrigation issues, fertilizer stress, or root damage when caused by things like water stress or trenching. When they are human-related, these stressors should also count as biotic factors.

When you’re trying to narrow down the potential causes of any symptom, here are some questions to consider: What’s the history of the symptoms? How did the tree perform last year? Did it leaf out normally? What’s the landscape history? Have changes been made to hardscapes (walls, sidewalks, etc.)? What are the irrigation methods and frequencies? Which structures are showing signs of a problem, older or newer growth? Have the symptoms come on suddenly or were they slow to develop? In a very general sense, plant pathology guidelines suggest that symptoms caused by biotic diseases tend to develop more slowly, and when a report comes in of tree symptoms that occurred overnight, it’s more likely an abiotic factor to blame.

Answers to these questions may be hard to come by, but you’ll be amazed what kind of telling details come to the surface with a little extra attention.

I had the opportunity to visit Hobbs last week, where Wayne Cox, the Lea County Extension Ag Agent, and I checked out Richard V.’s trees firsthand. We didn’t find an obvious answer to why the huge landscape trees are suffering. No evidence of borers, crown gall, or trunk damage, and the irrigation history seemed adequate, although watering deeper might be helpful.

While deciduous tree branches become more exposed during dormancy and bark problems are easier to identify, winter is not the best time to diagnose disorders affecting leaves. Tender leaves are often the first to scream for help. We’ll check back in with the Hobbs trees this spring to see what they can tell us.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!