Field Report: Seed-Borne Disease and Organics
by Micaela Colley, Bulk Seed Representative

Recent increases in the production and sale of organic seed has heightened the scrutiny of organic seed quality and in particular brought attention to concerns of seed-borne disease contamination. Seed-borne diseases are pathogens such as bacteria, fungus, or viruses, that live on the surface or interior of seed and have the potential to spread disease to the subsequent crop. Conventional seeds are often treated or coated with chemical fungicides to kill pathogens, a practice not allowed in organic production. Organic seed companies must instead practice careful monitoring and management of seed crops to prevent disease and utilize organically approved treatments to clean infected seed. It is always prudent to seek high-quality seed and work with a reputable company. However, seed-borne diseases are not ubiquitous in all crops, and high concern should be placed primarily on specific diseases known to be seed-borne that pose a risk in your growing region. The use of organic seed does not pose any higher risk than conventional seed if high-quality seed is used.

The risk of seed-borne disease infection varies widely by crop, disease, and location. Many diseases will only become a problem if grown in a region or environment conducive to the disease. Commonly diseases present on seed may also be soil-borne or air-borne and the ultimate fate of the crop may be as dependent on the variety resistance and crop management practices as on the presence of seed-borne innoculum. There are additionally many microorganisms present on seeds that have no known negative effects and some feel may hold potential positive effects, although there is no current research documentation. It is still important to start with high-quality, clean seed. Seeds of Change tests for a host of common seed-borne diseases on brassicas, tomatoes, and peppers and is working with a Ph.D. plant pathologist in our seed production and quality control program. In select instances the spread of specific pathogens from seed may introduce the disease to the system with devastating effects. Such is the case of Watermelon Fruit Blotch, a bacteria that can be seed- or soil-borne and difficult to manage once in the system; particularly in the warm, humid southern region of the United States. The disease is very difficult and expensive to test for and for this reason most companies, including Seeds of Change, require a signed waiver with the purchase of watermelon seed even though it is not a problem in most growing regions. Seeds of Change only grows watermelon seed in regions with no known Watermelon Fruit Blotch pressure.

Disease pressure is often regionally based and in some instances seed use is regulated by government authorities. In these regions intensified agricultural production and high regional economic dependence on a specific crop justifies this regional management approach. Such is the case with bean seed planted in Idaho. All seed must pass stringent disease testing before planting. Most of Seeds of Change's bean seed, including all varieties offered in the Bulk Seed Catalog, are grown in Idaho and produced from certified, disease-tested seed. Lettuce seed in the Salinas Valley of California is also monitored and must pass stringent tests for Lettuce Mosaic Virus prior to planting as this virus is quickly spread by leaf hoppers and the intensive lettuce production in the region poses a high economic risk. This disease is not usually a problem outside of this region. Potatoes in certain states are also certified by government agencies as meeting disease standards and clean-handling practices.

Disease Management in Seed Crops
In the Seeds of Change Quality Control Program we work with our seed growers to avoid disease in seed production, we carefully inspect and clean our seed at our own certified organic seed cleaning facility, and we test many of our seeds for specific diseases of concern. One of the most important factors in avoiding disease in seed crops is producing in regions with minimal disease pressure. Many of the Seeds of Change growers are located in western regions where dry summers aid in minimizing disease pressure and allow harvesting seed under dry, late-summer/fall conditions. Additionally we work with our growers and professional breeders to select several crops for disease resistance.

Careful field management in seed production can significantly mitigate seed-borne diseases in seed crops. Many of the same practices that prevent disease in field crops are used to prevent disease in seed crops. Disease promoting conditions are avoided by not using overhead irrigation under moist conditions, using drip irrigation, and cutting water to allow seed crops to dry before harvesting. Spacing plants adequately and orienting rows with the prevailing winds aids in increasing air flow—an important factor in minimizing disease. Rotating crops is important to avoid disease build-up in the soil. Of course, starting with clean, disease-free seed for planting stock is also crucial. Additionally, seed growers must use care in harvesting and cleaning seed to avoid post-harvest infection.

Seeds of Change works with our seed growers to select varieties for disease resistance. In this program disease is intentionally allowed to build up in an isolated field for breeding and selection purposes. Seed crops are selected under the disease conditions for resistance to the disease. In this case care is taken to ensure the stock seed is not harboring seed-borne diseases when used as planting stock for seed production. Many of Seeds of Change beet varieties have been selected in this manner for resistance to Rhizoctonia, a bacteria that causes scarring and lesions on beet roots. Many of the lettuce varieties offered have also been selected for resistance to Downy Mildew and Sclerotinia. Seeds of Change growers regularly practice rouging (selection) in seed production fields by removing diseased plants from the field.

Over 1,500 microorganisms have been shown to be related to seeds of all types (McGee, University of Iowa, pub IA-50011). However only a small percentage of these organisms pose a threat of disease to the following crop. The majority of common diseases are not seed-borne, but initiate from the local environment. As most diseases are regionally based it is valuable to check with local extension services and growers to identify which seed-borne diseases pose a threat in your ecosystem and then place extra care in sourcing varieties of these crops. If there are crops of particular concern, ask your seed company for recommendations and information about their disease management program. And always use good organic disease management practices in the field.

Micaela Colley
Seeds of Change Bulk Seed Field Representative

Photo caption: Author Micaela Colley with plant breeders John Navazio and Frank Morton in a lettuce disease nursery.

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