Early season soybean diseases
This is another interesting year for soybean production and early season soybean diseases in Missouri. According to the Missouri Agricultural Statistics Service as of June 23, 2013, “soybeans were 84 percent planted which is 24 days behind last year but the same as normal. Emergence was 67 percent, 20 days behind last year and 5 days behind normal. Emergence was reported as uneven across the state.” Certainly weather has been a key factor in this. Unusually cold, wet conditions across most of the state delayed planting and have resulted in some uneven or poor stands.
Although weather is a key factor this season, soil-borne pathogens could still be contributing to some of the uneven stands and poor vigor in seedlings. A heavy rain event and slow emergence due to compaction could have given Pythium species an opportunity to attack developing seedlings. Plants which are struggling to send out roots and to survive could be targets for Rhizoctonia or Fusarium species. Plants with comprised root systems were more prone to desiccation from warm, drying winds during the recent spell of higher than normal temperatures. Some marginal browning of leaflets, wilting of plants and even premature death of plants may occur in drier areas of fields or across large areas of fields. Thus far this season Rhizoctonia seems to be the most prevalent problem.
Soybean seedling blights have the potential to cause losses in Missouri soybean fields every year. The specific seedling blights that occur and their severity vary with the environmental conditions each season. When checking stands this season, it is important to take into account soil conditions and environmental stress as well as checking for seedling diseases.
Pythium and Phytophthora are favored by wet conditions and are more likely to be serious problems when wet conditions exist at or just after planting. Rhizoctonia and Fusarium are not as restricted by soil moistures and soil temperatures but still need some moisture to initiate infection. Macrophomina phaseolina grows best at temperatures between 82-95°F. Infection of seedlings with Macrophomina is most likely to occur if conditions of high soil temperatures and low soil moisture exist during the first two to three weeks after planting.
Symptoms of Pythium damping-off range from seed rot or preemergence damping-off to early postemergence damping-off. Affected tissue develops a soft, watery brown rot. Pythium damping-off is most likely to occur in cool (50-550F), wet soils.