How to Manage Pests
UC Pest Management Guidelines
Seed Rot and Seedling Diseases
Pathogens: Achlya klebsiana and Pythium spp.
(Reviewed 4/04, updated 4/04, pesticides updated 10/15)
In this Guideline:
Seed rot and seedling diseases often appear within a few days of planting. Whitish outgrowths of fungal mycelium emerge from cracks in the seed glumes or from the collar of the infected seedling's plumule. After a few days, the fungal mycelium resembles a halo that radiates from the infection point on the seed or seedling. Various algae colonize the fungal growth 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 algae growth, but is most probably caused by secondary invasion of the seed and fungus 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 retarded. The typical fungal halo is usually evident. If infection occurs after seedlings are well established, there is generally little apparent effect.
COMMENTS ON THE DISEASE
The disease, although prevalent throughout the rice-producing areas of California, is generally more severe when cool temperatures, which are unfavorable for rice growth, occur at or shortly after planting time. If ideal conditions for development of seed rot and seedling diseases occur, water-sown rice often is severely infected within a few days after seeding, resulting in reduced plant stands or reduced seedling vigor.
Seed treatments with fungicides provide protection against seed rot and seedling diseases, especially if seed is planted early or if environmental conditions are unfavorable to germination and seedling growth. Uneven stands caused by seed rot and seedling diseases can be partly compensated for by increasing seeding rates.
Plant high quality rice seed (preferably certified seed) with 85% germination or more and a bushel weight of 44 pounds or more. Plant when water temperatures are favorable for rice seed germination and growth of rice seedlings (preferably above 70°F or 21°C). Maintain a uniform water depth of about 4 inches or 10 cm; this will also improve germination, rice stand establishment, and tillering. 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.
Organically Acceptable Control Methods
Recommended cultural controls are organically acceptable.
The benefits from fungicide seed treatment in water-sown rice are most obvious during the early planting season when environmental conditions are often unfavorable for germination and seedling growth. Chemical seed treatment increases the percent stand establishment, thus reducing the need to replant and helping to establish uniform rice stands.
UC IPM Pest Management Guidelines: Rice
C. A. Greer, UC Cooperative Extension, Colusa County
Acknowledgments for contributions to the disease section:R. K. Webster, Plant Pathology, UC Davis