Seeds from storage may carry seed rotting microbes. These microbes may have been carried into the storage from fields in the previous season. Moreover, secondary infection can also be caused from soil or water. Certain environmental conditions such as puddled spots also favor seed rotting.
Rotting seeds become mushy and may be surrounded by gooey substances or white moldy growth due to microbial activities.
Seedling diseases can be caused by multiple factors:
Microbes: Multiple bacterial and fungal microbes may be responsible to cause complex symptoms in rice seedlings. Problematic seedlings may show brownish discoloration below or above the soil line causing seedling blight.
When diseases are severe the seedlings are stunted, turn yellow, and eventually could die, resulting in a poor stand.
Seedlings may also have darker rot at the base of the plant. Pathogen structures such as mycelia may easily be detected on collar of infected seedlings or may be seen radiating from rotted seeds.
Environmental factors: Low temperatures and wet conditions can make soil conditions unfavorable for normal growth of rice seedlings. Cool temperatures at or shortly after planting intensify seedling problems. Likewise, warmer temperatures under wet conditions also favor some fungi to cause seedling diseases.
Water seeding: Seeds for water seeding are not often treated with chemicals and hence, seedling diseases are usually severe. Even if treated, the chemicals can quickly wash off, lowering the benefit to seedling protection.
As a result, chemical seed treatment for water seeding is not generally recommended. Therefore, water molds in water-seeded rice can cause more severe seedling diseases than in drill-planted fields.
Freezing nights: Rice seedlings in early spring often show white bands (rings) at the leaf that started at soil line.
Freeze leaf spots are formed when dew or water drops on young and tender leaves freeze at night. Seedling stands are not usually affected by the white bands or freeze spots as long as the seedlings continue growing.
Soil types: Seedlings emerge faster and easier in sandier soil than in clay soil provided water and seed quality are not limiting for seeds to germinate.
Rice cultivar genotypes: Rice varieties may not emerge equally and uniformly even on the same soil type. Different rice cultivars clearly show differences in emergence and seedling vigor.
Knowledge about your field in relation to cultivar emergence is useful to match the right cultivar with the right soil type. The faster the seeds germinate and emerge, the higher the chance of escaping the early-season disease complex.
Herbicides: Chemical compounds applied pre-emergence or postemergence can substantially affect young and tender seedling tissues making seedling disease diagnosis difficult. The white bands (rings) formed due to freezing can be confused with symptoms caused by herbicide damage as with the herbicide Command.
Sometimes damage caused by herbicides can be more substantial than actual seedling diseases.
Herbicide damage on seedlings may also be caused by herbicide carryover or drift from other fields.
Insects: Grape colaspis is known to cause substantial loss in seedling stands. It may also give rice seedlings a sick appearance, making seedling diseases more complex and difficult for diagnosis.
Salt damage: Different soils have different salt levels. Higher levels often kill rice seedlings.
Zinc deficiency in soil can show a distinct leaf bronzing symptom in rice seedlings. Zinc deficiency affects crop growth.
Symptoms are more prominent at seedling stage. However, the problem can persist throughout the growth cycle of the crop, ultimately affecting the grain yield unless corrected in a timely manner.
To read more on zinc deficiency, go to chapter 9 Pages 92-93 in http://www.uaex.edu/publications/pdf/mp192/chapter-9.pdf.
Consider the following to ensure adequate seed germination, good seedling stand by reducing seed rots, and seedling disease complex at early stage of your rice crop.
• Plant healthy-looking, plump seeds treated with appropriate fungicides and insecticides.
• Higher rates of seed treatment containing mefenoxam, fludioxonil, metalaxyl, trifloxystrobin, either individually or in combinations of two fungicides need to be used for early planting or even for late planting if the season stays cold and wet. A field with a known history of disease such as rice blast requires higher rates of the appropriate fungicides.
• Gibberellic acid seed treatment may be considered on cultivars with weak seedling vigor to increase emergence and seedling vigor.
• Make sure seeds are treated uniformly.
• Avoid using seeds stored inadequately for a lengthy period. For instance, seeds stored under high moisture and temperature can lose their viability within a few months.
• Before planting, test for germination. Seeding rates may need to be adjusted based on your germination results.
• Check for adequacy of soil moisture to promote seed germination.
• Correct low areas that puddle in your field so you can have more uniform emergence and less seed rotting.
• To maximize crop tolerance to diseases, correct nutrient deficiencies in a timely fashion.
Yeshi Wamishe is the Extension Rice Plant Pathologist with the University of Arkansas. Contact him at [email protected].