Dusting the Soil for Fungus-Prints: Spinach Seed Production and One of its Greatest Threats

Spinach, one of the most popular dark-green leafy vegetables in the American diet, is grown across the United States and the world. Yet spinach seed only can be produced in a few regions that share specific climatic conditions: summers characterized by long days, mild temperatures and low humidity with little rainfall. Consequently, the ideal climate in western Oregon and western Washington makes this the only spinach seed production region in the United States and the home of up to one-fifth of the global spinach seed supply.

Although the climate in western Oregon and Washington is optimal for spinach seed production, the low pH of the soils in this region is conducive to a devastating disease called Fusarium wilt. This disease is caused by a soil-dwelling fungus that infects the roots of spinach plants and enters the water-conducting tissues of the plant known as the xylem. The plant defends itself by clogging up the xylem to mitigate spread of the fungus. Unfortunately, this also blocks the flow of water from the roots to the leaves, causing the plant to wilt and die. Spinach seed growers can lose entire crops because of Fusarium wilt. Even worse, the fungus can survive in acid soils in western Oregon and Washington for greater than 10 years, rendering fields risky for spinach seed production for extended periods. My research at Washington State University focuses on helping spinach seed growers protect their plants from the fungus that causes Fusarium wilt.

Currently, spinach seed growers have no affordable management practices that can eliminate the fungus from soil. However, growers can assess the level of risk in their soil for Fusarium wilt. My advisor, Dr. Lindsey du Toit and a previous graduate student in her program, Dr. Emily Gatch, developed a method of testing soil, called a bioassay, for determining the risk of spinach Fusarium wilt in a grower’s field before planting a seed crop. In short, spinach seed growers deliver a representative sample of soil from their fields in which they intend to plant spinach seed crops. Our research program processes the soil, dispenses the soil into pots, plants spinach seed in the soil and then monitors the developing plants for wilt. The severity of wilt is used to determine if the field is suitable for planting a spinach seed crop that spring.

Although the soil bioassay is an effective tool, it is time- and labor-intensive, requiring six to eight weeks to yield a risk estimate. Since the soil bioassay requires greenhouse space, we are limited by the number of soils we can test each year. For my Ph.D. dissertation, I am developing a tool that could, potentially, be used to estimate the risk of Fusarium wilt faster than the soil bioassay, with fewer resources and higher throughput.

Typically, soils with more of the spinach Fusarium wilt pathogen are riskier than those with less of the pathogen. Therefore, if I can count how much of the pathogen is in soil, it should be possible to estimate the Fusarium wilt risk in a grower’s field. To do this, we have identified several fungal genes that are unique to the spinach Fusarium wilt pathogen. The presence or absence of these genes is like the characteristic whorls and folds of skin that compose our fingerprints, a trait that makes us unique. Just as a forensic analyst dusts for fingerprints at a crime scene in search of a suspect, I can search for genetic fingerprints in a grower’s field using the tool I am developing. After counting these genetic fingerprints, I can infer how much of the spinach Fusarium wilt fungus is in the soil. Finding more of the fungus indicates a greater risk of wilt, which can be relayed to growers within days rather than weeks.

The outcomes of this research will help inform growers about the Fusarium wilt risk in their fields faster than when using the soil bioassay. If successful, the tool will also complement the annual soil bioassay, which provides an invaluable service to growers as a direct test of risk. This research protects spinach seed producers in one of the few locations in the world where this crop can be grown, securing the supply of this healthy leafy green vegetable and economic sustainability for spinach seed growers in the Pacific Northwest.

My research is supported by the FFAR Fellows program in conjunction with Pop Vriend Seeds BV, Rijk Zwaan, Sakata Seed America and the Washington State University Robert MacDonald Fellowship. The FFAR Fellowship has provided me with many opportunities to advance my career beyond my technical skills, for which I am thankful.