Seedborne Disease and Control

Seedborne diseases refers to the particular plant diseases that are transmitted by seed. In some cases the transmission by seed is insignificant compared to the population of disease organisms that exist in soil or on weed species. In other cases, the transmission by seed is the primary means by which the disease spreads. While we are cautious about any type of disease on seed, it is this latter set of diseases that we must be most vigilant in controlling.
Planting infected seed may result in a widespread distribution of disease within the crop, and an increased number of initial infection sites from which the disease can spread.  As an example, consider the development of ascochyta blight in a chickpea crop.  Since there is a high rate of seed-to-seedling transmission of this disease, even a small percentage of infected seed can result in significant seedling infection in the field.  For a seed lot with 0.1 per cent ascochyta infection (one infected seed in 1,000 seeds) and a planting density of three to four plants/ft.2, 175 infected seedlings per acre could potentially result. This is a substantial amount of early infection for such an aggressive disease.
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. 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, 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 require a signed waiver with the purchase of watermelon seed even though it is not a problem in most growing regions.
Seed should be tested for germination to determine its suitability for planting.  Germination can decrease in the bin over the winter, especially if the seed was immature or damaged at harvest.  It is a good investment to re-test seed for germination in the spring, if quality was questionable in the fall.  Increasing the seeding rate will compensate for low germination, but only to a certain extent.  If the reduced germination was a result of disease, an increased seeding rate can introduce more disease into the field. In general, seed treatments may have either systemic or contact modes of action.  Controlling fungi that are carried within the seed requires a systemic product (i.e. smut in barley), whereas contact or protectant products are adequate for surface-borne or soil-borne fungi.  Systemic seed treatments are diluted quite quickly within the plant once the seed germinates and is actively growing.  Some treatments will protect a young seedling against early leaf disease or root rot infection, but in most cases, seed treatments are no longer effective after seedling emergence.