MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Rust

So far, soybean rust hasn’t spread very quickly this year in part due to unfavorable weather conditions down south. Soybean rust has been confirmed on soybean in several southern states, including Texas, Alabama and Florida. Since the first of this year, soybean rust has been reported in two counties in Alabama; one county in Georgia, 16 counties in Florida; three counties in Louisiana; one county in Mississippi, and four counties in Texas, mainly on kudzu. Rust was also reported in three states in Mexico on jicama and soybean, but these have been destroyed or are no longer active. However, the effect of recent hurricane activity on the spread of soybean rust may soon be detected as scouting activities continue. Rainfall traps monitored by the USDA Cereal Rust Lab have recently shown an increase in frequency and detection of rust spores, although the three rust spore trap locations in Michigan have shown no detection of spores as of August 1.

White mold

Conditions in parts of Michigan are favorable for white mold (Sclerotinia sclerotiorum), especially in irrigated fields, or fields with lush growth and dense canopies, and it is beginning to show up. We’ve had cool morning and evening temperatures and high humidity, which provide a favorable environment for the disease to develop particularly as plants are flowering. Even if it has been hot during other hours of the day, remember that temperatures in the crop canopy may be considerably cooler than air temperatures. Hopefully, you won’t be seeing white mold if you planted partially resistant varieties, which should be the first line of defense in managing this disease. An in-state source with ratings for white mold resistance, including partially resistant varieties can be found at the MSU Soybean Variety Trials website: http://www.css.msu.edu/varietytrials/soybean/whitemold.htm

White mold symptoms first appear as wilting and dying upper leaves, with dead leaves staying attached to the stems. Infections take place through blossoms and develop at the stem nodes as water-soaked lesions. Cottony tufts of white mycelium appear on stems and other infected parts. Later on, stems look bleached, papery or shredded. Black pieces of dormant mycelium (sclerotia) that look like rat droppings appear loosely attached to stems, in stem pith and sometimes in pods.

Cropping practices that promote a dense, lush canopy including narrow rows, high planting rates (more than 200,000 plants/acre), irrigation and high soil fertility, contribute to an environment that favors white mold. Once the disease is present, there isn’t much that can be done to treat it in soybeans. There aren’t many fungicides registered for controlling white mold in soybeans, canopy penetration and coverage are major issues and currently registered products have limited efficacy. Soybeans’ extended bloom period and tendency to rebloom on the lower stem, creating new infection sites for the fungus, makes control with a single fungicide application difficult. Although fungicides are rarely used to control white mold in soybeans, they are used in dry beans. Achieving good control of white mold in dry beans is still a challenge, but there are more fungicides with greater efficacy to choose from, and reblooming low on the stem isn’t an issue to confound the effectiveness of treatments. Page 95 of the extension bulletin E-1582 Insect, Nematode and Disease Control in Michigan Field Crops lists products registered for use on dry beans.

Most of the strategy for managing white mold is based on using good management practices. These include rotation to non-hosts such as corn, sugar beets, alfalfa or small grains for two to three years to reduce the number of sclerotia in the soil, and controlling broadleaf weeds during the non-host crop rotation (many broadleaf weeds are hosts for this disease).

During years when soybeans are planted, in addition to selecting varieties with partial resistance, having lower planting rates and wider row widths that promote air circulation and rapid drying of plants and soil surface create a less favorable environment for the white mold fungus. Seed used should be clean and free of sclerotia and can be treated with one of several seed treatment fungicides labeled for white mold if it is present on the seed. Irrigation should be limited to what is needed for optimum plant growth.

Root rots

Some soybean fields are just not growing well this year, with stunted plants, yellow foliage, and bare patches evident in fields. Some of this can be attributed to earlier weather conditions, but Rhizoctonia root rot, phytophthora root rot and fusarium root rots are appearing in some fields.

Symptoms of Rhizoctonia usually appear as the weather becomes warm and plants become drought stressed, although infection occurs under wet conditions. Soil temperatures between 77°F - 84°F, wet soils, and high organic matter provide favorable conditions for development of the pathogen. Seedling stem discoloration by Rhizoctonia is usually limited to the outer (cortex) layer of the main root and hypocotyls (portion of the stem below the first leaves). Infected stems have a reddish brown color and do not become mushy; they remain firm and dry. On older plants, the reddish brown discoloration of the main stem tend to stay below the soil line and may develop into sunken lesions and girdle the stem. Management options for this disease are limited.

Soybean varieties differ in their susceptibility to Rhizoctonia seedling and root rot, but none are resistant. Crop rotation is of limited value because so many crop species are hosts for Rhizoctonia. At planting time, using high quality seed will help, and several seed treatments are labeled to control damping-off caused by Rhizoctonia.

Phytophthora can develop any time from plant emergence to early maturity, but is most often seen shortly after emergence and during early flowering. Young plants turn yellow, collapse, and die, leaving gaps in row plantings. Foliage of older plants wilts and turns yellow starting at the bottom of the plant and moving up. Dead leaves may remain attached. A chocolate-brown discoloration of the lower stem starts near the soil line, and spreads upward. Root systems are rotted. Conditions favoring the disease include poorly-drained or saturated soils, and soil compaction, phytophthora prefers soil temperatures that are above 80°F.

A primary strategy for managing phytophthora is to plant soybean varieties that are tolerant. Look for soybean varieties with specific resistance genes Rps 1c or Rps 1k and good partial resistance/field resistance to all races of phytophthora. Rotate with a non-legume crop for at least two years. Improve soil drainage, avoid working or driving on wet soil, and tillage practices that contribute to soil compaction. At planting time, seed treatments labeled for phytopthtora can be used for fields with a history of the disease.

Several species of fusarium cause seed and seedling rots and root rots, including F. solani, complex, F. graminearum, F. oxysporum complex and others. Isolates within some species of fusarium may vary widely in their level of pathogenicity, ranging from saprophytes, to those that cause increasing amounts of root rot. Seedlings may not be killed outright, but may be severely stunted. Although infection can take place early, symptoms may not be noticeable until early flowering. Fusarium root rot often is found in wet, poorly drained areas of the field such as low spots. Infected plants may have poor root systems and poor nodulation with reddish brown or dark brown lesions on lateral roots and taproots. Plants may appear stunted or unthrifty, and there can be a yellowing of the leaves with the veins remaining green for a short time. The leaves eventually turn completely yellow, die from the edges inward and fall from the petioles. The foliar symptoms are different from those caused by sudden death syndrome (fusarium virguliforme), where veins remain green.

Fusarium root rot may be observed in association with stressed plants, such as in drought conditions or with herbicide damage. The root rot predisposes plants to damage from other stresses. High populations of the pathogen in the soil, however, may result in disease development under good growing conditions. The pathogen may interact with other pathogens such as rhizoctonia or the soybean cyst nematode to cause disease. Disease severity may be greater in plants showing iron chlorosis

There is no known resistance to fusarium. Resting spores can remain in soils for extended periods of time. Conditions that delay germination and emergence such as cold, wet or compacted soils favor fusarium seedling blight and root rot. To reduce losses from fusarium seedling blight and root rot, plant good-quality seed with a good germination rate under good seedbed conditions, and minimize or avoid stresses that delay germination and emergence. Several fungicidal seed treatments are labeled to control fusarium damping-off. Seed treatments have not consistently reduced SDS.