Screwbean Mesquite Research

Sponsors and Partners:

Federal and State Agencies

Project Duration: 2021 to Current
Pure cultures; black is Neoscytalidium dimidiatum

Project Overview

Screwbean Mesquite Research

The Mystery of the Great Screwbean Mesquite Die-off

In the middle part of the 20th century, Screwbean Mesquite bosques (Spanish for forest) were one of the dominant forest types in riparian systems of the southwest, comprising over a third of riparian vegetation along the Lower Colorado River and adjacent tributaries. These trees produce a distinctive seed pod that is an abundant and nutritious food for animals as well as people. Native peoples of the southwest used the screwbean as a source of food, building material and firewood.

After half a century of slow decline due to land conversion and damming of rivers, screwbean mesquite suddenly began dying rapidly and in large numbers. The die-off was first noticed in the early 2000s by tribes in the Yuma area of southwestern Arizona. Early reports describe a “mysterious malady” which was sweeping through populations, leaving only a few unhealthy survivors in its wake. Within years the die-off had moved up the Colorado, Gila and Salt Rivers. 
Despite the ecosystem-changing implications of losing this foundational species, no concerted attempt was made to figure out what was causing so many trees to die so quickly. Restoration proved unsuccessful. The trees would succumb to the mysterious killer a few years after being planted in the wild. Many land managers gave up trying to use the tree in restoration plantings and soon a community-defining tree species had become a rarity over most of its southern range. Fortunately, large populations were still thriving in the north, along the Virgin River and in more scattered populations of southern Nevada.  
After a quiet period of half a decade, though, the second wave of the die-off began. An avian biologist working in Shoshone, California watched as the large, healthy stand of screwbeans in which he’d been monitoring bird nesting for over a decade was decimated within two years. By 2014 only a few stragglers remained. He reached out to biologists and land managers and the community formed a working group to figure out what was causing the trees to die and whether something could be done about it. That is when EcoCulture got involved.

A Fungal Culprit and the Importance of Disturbance

In 2021, with financial support from state and federal agencies, we began investigating the loss of this ecosystem-defining species. We mapped the current distribution of the species and identified areas of active die-off across its range. By mid-2022 we had identified the fungal pathogen causing the destruction, a regional endemic fungus known by the common name sooty canker. Though the fungus had been known to afflict cultivated trees and crops since the mid-20th century, it was undocumented on screwbean mesquite. We also identified a flat-headed boring beetle as a prominent insect pest on the wild trees, likely attacking and killing trees already weakened by the fungus. 
The next phase sees us working with agencies and tribes to develop remediation methods for combating this threat to a highly-valuable tree species. We are identifying potentially resistant genotypes from surviving remnants of decimated populations. We have also found that regular natural disturbance cycles such as periodic flooding can help suppress the disease and build more resilient populations. We are exploring the use of techniques such as coppice cutting and controlled burns as a means of mimicking natural processes and protecting this unique and irreplaceable tree species. Stay tuned!