Seed Rot and Seedling Diseases

Seed rot and seedling diseases are most often caused by the oomycetes Achlya klebsiana and Pythium spp. Oomycetes are a group of organisms often referred to as water molds. Once classified as fungi, advances in our understanding of genetic relationships have revealed that these organisms are unrelated to true fungi and are more closely related to algae. Seed rot and seedling diseases often appear within a few days of planting. Whitish outgrowths of oomycete mycelium emerge from cracks in the seed glumes or from the collar of the infected seedling's plumule.

After a few days, the mycelium resembles a halo that radiates from the infection point on the seed or seedling. Various algae colonize the mycelium, turning it green. In some cases, the infected seed appears within a dark circular spot on the soil surface. This, too, may be the result of algal growth but is most likely caused by secondary invasion of the seed and mycelium by various aquatic bacteria. Early infection of germinating seeds, especially when temperatures are cool, will often result in seed rot or seedling mortality.

If seedlings produce primary leaves and roots before infection occurs, they usually survive but are typically stunted. Leaves and sheaths become yellow or chlorotic and further development is hindered. The typical halo of mycelium is usually evident. If infection occurs after seedlings are well established, there is generally little apparent effect.

The diseases, which are prevalent throughout the rice-producing areas of California, are generally more severe when cool temperatures, which are unfavorable for rice growth, occur at, or shortly after, planting. Pathogens that cause seed rot and seedling diseases persist between crops of rice in the field soil. If conducive conditions for development of seed rot and seedling diseases occur, water-sown rice is often severely affected within a few days after seeding, resulting in sparse stands with fewer seedlings or reduced seedling vigor.

Appropriate water management to promote rapid, uniform stand establishment is critical in reducing the impact of these diseases when conditions favor their development. Once seedlings are well established, these pathogens generally have minimal impact on the rice plants during the remainder of the season. Uneven stands caused by seed rot and seedling diseases can be partly compensated for by increasing seeding rates.

Plant certified seed with at least an 85% germination rate when temperatures are favorable for rice seed germination and growth of rice seedlings (preferably above 70°F). Water-seeded rice sown into a deep continuous flood will favor disease development as deeper water slows seeding development. Laser level fields and maintain a uniform water depth of 4 inches to promote rapid germination and stand establishment without the loss of weed control often associated with draining for stand establishment. If the field has had severe problems in the past with seedling diseases or temperatures are expected to be cool at, or shortly after, planting, an increased seeding rate may be advisable. In extreme cases where there is a high incidence of disease, fields may need to be drained to promote seedling establishment.

All cultural controls discussed above are acceptable in a certified organic crop.