When yield is low, plants are not performing well or simply aren’t growing, there’s a chance a plant pathogen has infected the crop. Plant Pathologist Lindsey du Toit, associate professor at Washington State University, is one of several scientists in the world searching for ways to prevent fungi, viruses and bacteria from infecting seed, soil and plants so growers can expect healthy crops year after year.

There are hundreds of thousands of plant pathogens, which fit into two main categories: foliar or soilborne. Foliar pathogens mainly infect plant tissues aboveground. Although some foliar pathogens can infect seeds, these are usually easier to manage over time because they cannot survive in soil without host plant tissue. When infected plant tissue dies and falls to the ground, it biodegrades in the soil; when decomposition is complete, the pathogen dies.

Soilborne pathogens can live in the soil for decades, even in the absence of a host plant or host tissue. These pathogens can affect plant growth, reduce yields and reduce the quality of seed.

Some foliar and soil-borne plant pathogens can become seed-borne. Seed transmission occurs when infected seed planted in healthy soil transmits pathogens into seedlings developing on another plant.

Pathogens attack seeds in a variety of ways:

  • Systemically—pathogens infect a plant’s roots and move up the plant into the developing seed.
  • Via pathogenic spores—when wind or rain disperses pathogen spores onto a plant’s maturing seeds, the pathogen infects those seeds.
  • Via insects—insects that feed on plants sometimes carry pathogenic viruses, bacteria and fungi.
  • Via contaminated seed lot—healthy seeds planted from an infested seed lot can carry the pathogen into the soil. The pathogen infects the plant from the infested soil and can be transmitted to the seeds developing on the plants.
  • Via pollination—pollen can carry some viruses that infect developing seeds.

Researchers throughout the world are exploring ways of preventing seed crop infestation and infection by studying how pathogens reproduce and spread, where pathogens are present, where they originate and how to prevent them from entering other geographic regions. Scientists also want to learn how to prevent seed infections once a pathogen has already entered a region.

When a pathogen has entered the soil, crop rotation is an effective method for eradicating pathogens from the ground given the proper amount of time. Foliar pathogens can die in soil within two to six years. However, soilborne pathogens can live in the ground for decades. The longer a pathogen persists in the soil, the less effective crop rotation is for eradicating it. Factors that determine whether crop rotation will be effective include the number of crops susceptible to the pathogen that might grow in the area and the ability of the pathogen to feed off other host plants in the absence of the main host plant.

Among the many pathogens du Toit is currently focusing on are two soilborne fungi that affect spinach: Fusarium oxysporum f. sp. spinaciae (fusarium wilt) and Verticillium dahliae (verticillium wilt).

Fusarium wilt is caused by a soilborne fungus that’s also seed-borne. Western Washington’s acidic soils tend to be highly conducive to this long-lived soilborne fungus. As a result, spinach seed growers in this region must utilize a 10 to 15-year crop rotation cycle to minimize risk of fusarium wilt infestation. Unfortunately, even lengthy crop rotation cycles don’t always guarantee a safe seed crop. Du Toit recently visited a farmer who had planted a spinach seed crop in a field after a 16-year rotation. The soil was still infested with a devastating case of fusarium wilt, and the farmer lost his crop.

Although soil testing can identify several Fusarium oxysporum types, the microorganism that causes a spinach crop to fail is indistinguishable in a petri dish from other Fusarium oxysporum species. Some species of the fungus are pathogenic, but others are beneficial. Du Toit and her team are working with a USDA plant pathologist to develop a DNA-based means of identifying the spinach pathogen. With a clear genetic fingerprint, scientists should be able to develop a DNA-based probe to test fields. Du Toit’s team is also in the process of developing a risk assessment for fields to help growers identify fields where it’s safe to raise spinach-seed crops.

Due to the global nature of the seed industry and the current practice of shipping seed around the world, plant pathologists worldwide are exploring the relationship between the centers of origin and genetic diversity of crop species. They want to learn about resistance from plants that have survived in the presence of pathogens. Resistant plants may also help reduce infection of seed and reduce growers’ losses to seed-borne pathogens.

Next month, this series will continue with information about seed health testing and the establishment of thresholds to protect seed supplies from plant pathogens.

Infection vs. Infestation
A seed infection results when a pathogen establishes itself inside the seed. A seed infestation results when a pathogen contaminates a seed lot or is present on the surface of the seed, but does not enter the seed.

The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.