EARLY SEASON AND SEED-BORNE DISEASES
Phytophthora root and stem rot is the most serious soybean disease in Ohio and is present everywhere soybeans are grown. Damage to the crop by Phytophthora is most prevalent in fields with poor drainage, high number of years with soybeans, and reduced tillage systems. Varieties are susceptible at all stages of growth. Saturated soil with a temperature above 60 degrees Fahrenheit provides the ideal conditions for infection. Susceptible varieties should not be grown in poorly drained soils or on soils known to have a history of the disease. Seed of varieties with good partial resistance should be treated with a fungicide that aids in the control of Phytophthora damping off. Varieties with Rps genes should also be treated to control Phytophthora damping off because these Rps genes are not effective in every field nor across the whole field. Planting early, well before soil temperatures reach 60 degrees Fahrenheit, often allows varieties with high levels of partial resistance to escape early infection if the soil does not become saturated.
Pythium and Rhizoctonia root rots are also common in Ohio and most varieties are susceptible. Damage to plant stands is greatest on poorly drained soils and during sea- sons of high rainfall.
Phomopsis seed rot can be severe when rainfall occurs intermittently during grain dry down and harvest. The longer soybeans are in the field after ripening, the greater the incidence of seed rot. Harvesting soon after the soybeans mature (15 to 20 percent moisture) decreases the amount of seed damage. Using varieties with a range of maturities allows for a more timely harvest of each field. Many varieties are resistant to Phomopsis seed rot; if Phomopsis develops in a variety, look for a different variety for future years. Crop rotation and tillage are excellent management tools for this seed rot pathogen as it survives on old crop residue.
Phomopsis seed rot can reduce overall germination in certain seed lots and Phytophthora, Pythium and Rhizoctonia can kill seeds and seedlings after they are planted. One of the management tools for these seed and soil-borne plant pathogens is to use fungicide seed treatments. No one fungicide is highly effective for all pathogens. Choosing a mix of several compounds will provide broad spectrum control. Seed treatments are best used on fields with poor drainage, a history of stand establishment problems, and reduced tillage systems.
MID-SEASON TO LATE-SEASON SOYBEAN DISEASES
Soybean cyst nematode (SCN) exists in production fields throughout Ohio. In some fields, the population of SCN is currently quite high (>10,000 eggs per cup of soil). Populations of SCN may take eight to 10 years from introduction to reach damaging levels throughout a field. In a variety test in west central Ohio on a fertile, dark-colored soil, varieties resistant to SCN yielded over 50 bushels per acre, whereas those susceptible to SCN yielded from 24 to 39 bushels per acre. Although these studies were conducted in problem fields, the estimated yield loss from SCN in other Midwestern states is 8 to 12 percent.
In the vast majority of fields in Ohio, SCN causes no above-ground symptoms. The only difference that growers will see is that yields may be 5 to 10 bushels less than fields with similar yield potential. In more severe situations, where SCN populations are high, injury is easily confused with other crop production problems, such as nutrient deficiencies, injury from herbicides, soil compaction or other diseases. The first field symptoms are usually detected in circular to oval patches of stunted, yellowed plants. Symptoms are most evident in late July or August when plants are under drought stress or in fields with low fertility. When populations of nematodes are high, the symptoms may even occur under normal to optimal growing conditions. Affected areas of a field may increase in size each year in the direction of tillage. In these affected areas, SCN females can often be found feeding on the roots.
Soybean cyst nematode is best managed with crop rotation, rotating non-host crops such as wheat, corn, alfalfa or red clover and rotating sources of SCN resistance. Never plant a SCN resistant variety without checking your SCN population levels first. When a non-host crop is planted, SCN populations will decline by as much as 50 percent annually. Soybean cyst nematode resistance is measured by a reduction in the number of females that feed on roots, but a few females will reproduce. Thus, over time, populations will adapt to these sources of resistance and reproduce in increasing numbers.
To determine what your SCN levels are, soil samples should be collected. Each field should be divided into sections not exceeding 10 acres and each section sampled by taking 15 to 20 subsamples in a zigzag pattern. This level of sampling is necessary to obtain relatively accurate counts of the nematode population and to make meaningful recommendations for control. The soil samples should be moist, but not wet, packaged in double plastic bags, and protected from becoming too warm. Mail samples to: C. Wayne Ellett Plant and Pest Diagnostic Clinic, The Ohio State University, 8995 E. Main Street, Bldg. 23, Reynoldsburg, Ohio 43068. The telephone number for the clinic is: 614-292-5006. A short video showing how to properly sample for SCN can be found here: youtube.com/watch?v=FQgg-UPQdcs&feature=youtu.be.
Phytophthora stem rot will continue to infect plants throughout the growing season. This late season phase of the disease can only be found in fields where heavy rains or saturated soils have occurred, varieties with ineffective Rps genes and low levels of partial resistance. If the Rps genes are effective against the P. sojae population, then no disease will develop; however if they are no longer effective, the stem rot will develop. We have found from a number of years and locations that varieties with high levels of partial resistance rarely develop stem rot. One reminder that not all seed companies use the same scoring system.
Sclerotinia stem rot is present throughout most of Ohio and may be severe (50 percent of plants in a field infected) when wet weather occurs prior to and during flowering. Varieties with resistance to Sclerotinia have fewer numbers of plants infected but all are susceptible to some degree. Stem symptoms first appear as water-soaked lesions followed by cottony growth and eventually, black irregular-shaped sclerotia which resemble mouse droppings. Wide rows (30 inches) aid in control by permitting air to move through the canopy to dry plant leaves and the soil surface but also reduce yield due to less sunlight fixation. Reduction in plant populations (160,000 to 180,000) and planting in 15-inch rows can reduce overall incidence of Sclerotinia stem rot without negatively impacting yields.
Brown stem rot can severely reduce yield. This fungus enters the plants through the roots and slowly colonizes the stem and the xylem, where it interferes with water transport. The disease symptoms develop after flowering and are identified by an internal browning of the stem in August. Foliar symptoms are rarely seen in Ohio, but the leaves of infected plants may suddenly wilt and dry 20 to 30 days before maturity and drop from the plant. Crop rotation is an excellent control for this disease.
Sudden Death Syndrome (SDS) is another late season disease that always appears to be associated with soybean cyst nematode and areas of the field with very poor drainage. Symptoms are very similar to brown stem rot in that brown spots develop in the leaves between the veins, surrounded by a bright yellow chlorosis. In SDS, the roots are very degraded along with the crown. One of the key diagnostic tools is the color of the pith, which remains white and healthy with SDS and is brown and decayed with brown stem rot. This fungus survives in soil for long periods of time, so to prevent rapid build-up of the pathogen, crop rotation and improving soil drainage are key.