Phymatotrichopsis root rot (abbreviated as PRR) is commonly referred to as Cotton root rot, Phymatotrichum root rot, Texas root rot, or Ozonium root rot. This disease is caused by a soilborne fungus Phymatotrichopsis omnivora (abbreviated as PO) that attacks the roots of susceptible plants, causing sudden wilt and death. This pathogen has a wide host range and can attack more than 2,300 dicotyledonous (broadleaf) plant species including many ornamental and important agronomic crops (Lyda, 1978). In contrast, the fungus colonizes but does not kill monocotyledonous plants (grasses), which are highly tolerant or resistant to this pathogen. Phymatotrichopsis can be found in a wide range of soils, but is more prevalent in calcareous clay soils with a high pH range of 7.0 – 8.5. It is one of the most destructive fungal pathogens of pecan, pistachio, cotton, alfalfa, grape, fruit trees, shade trees, and ornamental plants like conifers. The economic losses attributable to this important disease are over hundreds of millions of dollars annually in the southwestern and south central United States.
RR is prevalent in all of major pecan production areas in Arizona. Average tree mortality due to this disease is thought to be greater than 10% annually. Consequently, the negative impact of PRR on AZ pecan industry is significant. For instance, in pecan growing areas where valuable resources (i.e., water, fertilizer, and labor) have been used to raise a young seedling transplant for 3 - 4 years, and sometimes even in trees of 12 years of age and older, the sudden loss due to root rot infection makes a waste of these high cost resources. The value of loss in new transplants indicated by farm managers are upwards to $2,000 per tree. In 2016, for Cochise County, it was reported that 600 immature trees in two separate locations died from this disease. This is an estimated $1.2 million dollar combined loss. Therefore, the need for researching economically viable methods in mitigating PRR’s impact on tree mortality is critical and of high interest by a large number of perennial crop stakeholders