Seedling diseases can prevail in any rice field. However, their likelihood can be reduced with proper management. Seed rots and seedling diseases are generally “complex” meaning there can be more than one causal agent in a field. In fields with seedling problems, pathogen structures such as mycelia may easily be detected on the collar of infected seedlings or may be seen radiating from rotted seeds. Rotting seeds become mushy and may be surrounded by gooey substances or white moldy growth as a result of primary microbial activities. Moreover, secondary infection from soil or water microbes may also enhance rotting. Problematic seedlings may also show brownish discoloration below or above the soil line and this is referred as “seedling blight”. When the disease is severe seedlings are stunted, turn yellow, and eventually could die. Seedlings may also have darker rot at the base of the plant. For all these symptoms multiple factors may be responsible.
In addition to seedborne and soilborne microbes, temperature and wet conditions can make soil condition worse. Cool temperatures at or shortly after planting aggravate seedling problems. It is important to note that some fungi can also be favored by warmer temperatures under wet conditions. Therefore, we need to be diligent with our management at all times. Water-seeded rice fields may have reduced plant stands with less seedling vigor due to seedling disease complex. Seedling diseases, particularly water molds, are worse in water-seeded rice than drill-seeded rice. Emerging seedlings which encounter freezing nights in early spring often show white bands (rings) (Arkansas Rice Update 4-12-14). The white bands generally are formed at the leaf portions near the soil line. The whitened portion of the leaf remains white in the growing young leaf blade without affecting the seedling stand.
Moreover, all cultivars do not emerge equally well in different soil types. Our preliminary greenhouse experiment in 2013 showed quite a difference in emergence among eleven cultivars tested on heavy clay soil. Seeds from each cultivar in this test were healthy but none were chemical treated. To read more on this go to Plan ahead to minimize rice seedling diseases. Therefore, knowing your field history 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 to escape the early-season disease complex.
Furthermore, young and tender seedling tissues could suffer considerably from herbicide and insect damage. The white band (ring) formed due to freezing is sometimes confused with symptoms caused by herbicide damage such as Command. In past years, the damage we observed due to herbicide (drift or direct) effect in ricepossiblywas much higher than damage due to seedling diseases (Cold, wet weather optimal for herbicide injury; Herbicide drift from GE crops). Wet and cool conditions in 2013 were tougher on seedlings than the previous hot and dry year.
Zinc deficiency could also show early on seedlings with a distinct leaf bronzing symptom. Zinc deficiency affects plant growth. Although the symptoms may develop early in the season, the problem will continue throughout the growth cycle of the crop – ultimately affecting the grain yield unless corrected. To see symptom pictures and read more on zinc deficiency go to chapter 9 Pages 91-93 in MP192.
Management tips to reduce rice seedling disease complex
- To ensure field germination, avoid using seeds stored inadequately for lengthy period. Seeds stored under high moisture and temperature can lose their viability. Germination testing before planting could help.
- To tackle the seedling disease complex, 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 severe disease situations. To read more go to (Plan ahead to minimize rice seedling diseases).
- To minimize seed rotting and encourage emergence, plant high-quality seed treated with appropriate fungicides and insecticides.
- To maximize seed treatment value, make sure seeds are treated uniformly.
- To increase emergence and seedling vigor, particularly with cultivars with weak seedling vigor, Gibberellic acid seed treatment may be considered.
- To speed up emergence, avoid using poor quality seed.
- To have uniform emergence, correct low areas that puddle in your field.
- To maximize crop tolerance to diseases, correct nutrient deficiencies timely.
- To maintain active crop growth, temperatures need to be adequate for rice (leave this part for nature). This year, as rain continues to delay planting, cool temperatures also can reduces seedlings vigor which means germination will be retarded and crop growth will be slowed.