Seed rotting microbes can be associated with seeds from storage, fields from the previous season, or soil and water through secondary infection. Rotting seeds become mushy and may be surrounded by gooey substances or white moldy growth due to microbial activities (Figure 1). Certain environmental conditions such as puddled spots also favor seed rotting.
Causes of Rice Seedling Diseases
Multiple microbes may be responsible for causing 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 (Figure 2). 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 such as temperature and wet conditions can make soil conditions unfavorable for normal growth of rice seedlings. Cool temperatures at or shortly after planting intensify seedling problems. Warmer temperatures under wet conditions also favor some fungi that compromise seedling health.
Water seeding favors water molds and severe seedling diseases appear more in water seeded than drill-planted fields. Seedling diseases are usually severe since seeds for water seeding are not chemically treated. Even if treated, the chemicals can quickly wash off lowering the benefit of seedling protection.
Freezing nights in early spring often result in white bands (rings) on the leaf at the soil line (Figure 3) or freeze spots (Figure 4). Seedling stands are not usually affected by the white bands or spots because the seedling continues growing.
Soil types play a substantial role in seedling emergence. Seedlings emerge faster and easier in sandier soil than clay soil (Figure 5) as long as there is enough water to help the seed germinate.
Rice varieties do not all emerge equally well in different soil types. Rice cultivars clearly show differences in seedling vigor even on the same soil type. A recent greenhouse experiment showed quite a difference in emergence among 13 cultivars tested (Figure 5). Healthy looking seeds were selected and planted without chemical treatment. This test is in agreement with a preliminary test conducted in 2013 http://www.arkansas-crops.com/2013/02/19/plan-ahead-to-minimize-rice-seedling-diseases/.
In another recent preliminary test where different “soil-sand mixtures” were treated with the panicle blight bacteria (Burkholderia glumae), seedling emergence greatly varied (Figure 6). 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 applied pre- or post-emergence can substantially affect young and tender seedling tissues making seedling disease diagnosis difficult. The white band (ring) formed due to freezing can be confused with symptoms caused by herbicide damage– for instance, Command (Figure 7). Sometimes damage caused by herbicides can be more substantial than actual seedling diseases. Awareness of herbicide residues and drifts will help in understanding seedling problems in some cases.
Insects such as grape colaspis can cause loss of seedling stand or render a sick appearance with the crop making a seedling disease more complex to pinpoint a cause.
Salt damage to the level of killing seedlings is also shown in Figure 8.
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 will 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 91-93 in MP192.
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 known field with a 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 (Figure 9). 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 of rice seedlings.
- To maximize crop tolerance to diseases, correct nutrient deficiencies in a timely fashion.
To read more go to http://www.arkansas-crops.com/2014/04/17/seedling-disease-management/.