Local information for growing soybeans in Trumbull County.
White Mold of Soybean
Causal Agent:
Sclerotinia stem rot of soybean is caused by the fungus Sclerotinia sclerotiorum. This pathogen has a very wide host range with more than 400 plant species. Alfalfa, canola, edible beans, cole crops (cabbage, broccoli), peanuts, pulse crops (peas, chickpeas and lentils) and sunflowers are some other examples of host plants. This fungus produces a survival structure called a sclerotium (pl. sclerotia), which is black, hard, and irregular in shape with a pink to white center (see below). The sclerotium is able to survive for many years in the soil, but under moist and cool conditions, the fungus can resume growth as myceliogenic (mycelium growth through the soil) or carpogenic (germination through formation of an apothecium). An apothecium is a small mushroom-like structure ranging from 0.5 to 2 mm in diameter, light-tan to brown in color (see below). The primary means by which the fungus infects a soybean plant is through carpogenic germination, where ascospores formed on the top of the apothecia are released and land on aging flower blossoms.
S. sclerotiorum cultured under laboratory conditions (A). Note the white mycelia and the production of sclerotia (black structures); Apothecia development from sclerotia (B).
Symptoms and Signs:
Diseased plants first appear wilted and with grayish green leaves that later turn brown to tan (see below). The stems also have grayish to bleached white lesions that are covered by white fluffy mycelia under conditions of high moisture. In later stages, sclerotia can be observed inside or outside of the stems and pods (see below). If the seeds were infected during the early stages of development, they may have a flat and shriveled appearance, or may be replaced by sclerotia. The amount of yield loss due to Sclerotinia stem rot depends on the number of infected plants in the field and how early in the season the plants became infected, as well as the number of plants that died prematurely.
Soybean plants affected by Sclerotinia stem rot appear wilted and present leaves that turn brown to tan. (A. Dorrance, OSU)
Symptoms and signs of Sclerotinia stem rot on plants in a soybean field (A and B); Sclerotia produced inside stems (C) and inside pods (D); Representation of the effect of the disease on pod filling: a branch from an infected plant in the right and a healthy plant in the left (E).
Disease Cycle:
Under favorable conditions in the spring and summer, sclerotia that are present in the soil germinate forming either mycelia or apothecia. When apothecia development occurs, ascospores are produced and forcibly ejected into the air where they are spread by wind or splashing rainwater to stems, branches, flowers and pods. Blossoms serve as an initial source of nutrients and subsequent infections occur on the soybean stem near a node where the fungus has colonized dead flowers. Later, contact between healthy and diseased adjacent plants may cause infection as mycelia can grow through leaves, stems, and petioles. Sclerotia produced outside or inside the stem can be mixed with seed during harvest, becoming a source of inoculum for other fields.
Disease Management:
Management of Sclerotinia stem rot is very challenging in Ohio for two key reasons: the pathogen can survive in the soil for several years and disease development is greatly affected by environmental conditions. There is no single tactic that can be used to manage the disease effectively; therefore, multiple strategies including cultural, chemical and biological methods should be applied to reduce yield losses. Management should be targeted towards those fields with a history of this disease.
Host resistance: There is no complete resistance reported for Sclerotinia stem rot of soybean. However, cultivars with high levels of partial resistance are commercially available. Soybean growers should talk to dealers and evaluate which cultivars have the highest levels of resistance to the disease. Overall, the selection of a soybean cultivar should take into account an open canopy of upright leaves, which would minimize a favorable environment.
Use of long crop rotation: It is important to note that the pathogen has a very broad host range and infects crops such as alfalfa, canola, edible beans, cole crops, peanuts, pulse crops and sunflower. In addition, good weed management is recommended because several weeds are hosts for the pathogen. Corn and wheat can be used in crop rotation with soybean to minimize inoculum build-up over time. Some sclerotia germinate and produce apothecia even with non-host plants, therefore, several years of rotation will be required to reduce inoculum in a field.
Tillage is recommended for fields with first occurrence of Sclerotinia stem rot in order to bury the sclerotia, while reduced tillage is preferred for fields with a long history of the disease. It does appear that one year of moldboard plowing will bury sclerotia at least 10 cm in soil, delaying the production of apothecia. Sclerotia located within 5 cm from the soil surface may germinate and produce apothecia.
Reduction of seeding rate may improve air circulation, reducing the humidity and increasing the temperature within the soybean canopy. Overall, factors that accelerate canopy closure or practices that result in dense canopies will favor carpogenic germination. Others have found that increasing row spacing to 30 inch rows may avoid complete canopy closure. However, growers should be aware that this practice also reduces soybean yields.
Chemical control with fungicides: Fungicides are available for management of Sclerotinia stem rot, however, none of them provide complete control of the disease. They should be used when a moderately to highly susceptible cultivar is planted in a field with history of the disease in the past few years. There is a very narrow window for fungicide application targeting the disease, which must take into account development of apothecia, ascospore release, and crop flowering. In addition, since the fungus has medium risk of development of resistance, rotation of active ingredients should be considered. In recent field experiments in Ohio, thiophanate methyl has not provided good efficacy in reducing disease incidence and severity. Also, strobilurin fungicides, such as pyraclostrobin should not be used, as an increase in disease severity occurred following the fungicide application. Boscalid has provided good efficacy in reducing disease incidence and severity compared to the untreated control.
Chemical control with herbicides: Lactofen is an herbicide able to modify soybean canopy, delay flowering and induce systemic acquired resistance response with an increase in the production of phytoallexin production such as glyceollin. Some of these herbicides may reduce overall disease severity, but they can also cause substantial yield loss on their own.
Biological control with the fungus Coniothyrium minitans has been used in other crops systems to reduce inoculum levels in the field. To improve contact with sclerotia and degradation, it is necessary to incorporate this material to a depth of two inches (5 cm).
Avoid introduction of sclerotia in the field: This is very important because once the pathogen is introduced in the field, it is able to survive in the soil for many years and its management become more and more difficult. The fungus can be introduced in a field through sclerotia present in seed-lots, equipment, and flooding. Seed-lots can be cleaned by spiral separators and gravity tables. Avoid harvesting heavily infested fields until the end of the season as equipment, especially combines, can become contaminated with sclerotia. Seed treatments with fungicides could be used to treat infected seed.
Sudden Death Syndrome of Soybean
Causal Agent:
Sudden Death Syndrome (SDS) is caused by the fungus Fusarium virguliforme (formerly Fusarium solani f. sp. glycines). This pathogen can infect a variety of other hosts including alfalfa, pinto and navy beans, white and red clover, and pea. Asymptomatic hosts include corn, wheat, rye grass, pigweed, and lambsquarters. Though F. virguliforme can independently infect soybean, the presence of soybean cyst nematode (SCN) increases the severity of SDS, and evidence suggests that the fungus may be able to travel plant to plant attached to SCN.
Foliar symptoms become visible in the early reproductive stages of soybean development (R1-R6).
Early foliar symptoms. Pale yellow spots develop between veins.
Severe foliar symptoms of SDS. Interveinal areas become entirely necrotic and leaves will drop prematurely.
Symptoms and Signs:
Symptoms most often occur in the crown, roots, and foliage. Infection is usually first identified as small yellow spots between the veins of the upper leaves of the plant at the time of flowering. The spots will expand rapidly into brown lesions with chlorotic halos until the entire leaflet becomes necrotic. Leaves will drop, leaving the petiole attached to the stem. Symptoms are most commonly seen at pod-fill stages (R3-R4). Characteristic blue-green spores may be visible on the taproot. The pith will retain a healthy appearance as opposed to brown stem rot, which can produce similar foliar symptoms, but will have a tan to dark brown pith.
Blue-green sporulation is sometimes visible on the taproot.
Disease Cycle:
Various spore stages of F. virguliforme overwinter in the soil for multiple years, leaving the possibility for large areas of a field to contain the pathogen even if non-host rotation is implemented. The fungus will begin colonizing the roots soon after planting. Between R1 and R6, the pathogen will grow deeper into root tissues and begin to produce toxins. Foliar symptoms will become visible at this stage. Leaves and pods may drop, containing hundreds of spores for overwintering to the following growing seasons. Heavy spring rains will cause higher infection rates and higher yield losses where the pathogen is present.
Disease Management:
Seed treatments- There are fungicides available for seed treatment for both SDS and SCN. Consult with your local extension agent and the current fungicide list (Ohio Soybean Diseases) to identify which seed treatments may work well in your system.
Plant resistant varieties- Use high-quality seed with resistance to both SDS and SCN. Resistant varieties should always be used in fields with history of these diseases. Resistance to SDS is quantitative, meaning different varieties will show varying levels of resistance. To measure this resistance, each company will have their own rating scale to measure the level of resistance found in each variety. For example, DuPont Pioneer uses a scale from 1-9 where 1=poor resistance and 9=excellent resistance. Rating systems are particular to each company, so be sure to identify the meaning of each rating before selecting varieties for the season.
SCN management- In many instances, fields in Ohio with SDS also have measurable levels of SCN. Crop rotation with a non-host or less susceptible host is ideal to reduce populations of SCN. However, corn, ryegrass, and wheat may serve as asymptomatic hosts of F. virguliforme. Monitor SCN populations by sampling and submitting soil for SCN counts (See SCN Fact Sheet).
Cultural practices- Planting when soils are warm, improving soil drainage, and reducing soil compaction will all help prevent SDS infection from occurring. SDS is most severe when soybean is planted in cool (<60°F), wet soils and has delayed emergence. Planting later in the spring when soils are warmer can help slow or eliminate SDS infection. Planting earlier maturing varieties is also thought to reduce the impact of this disease.
Wet soils provide a conducive environment for F. virguliforme. In order to reduce the incidence and severity of SDS, improve drainage either through the use of drainage tile or tillage.
SDS is often found in the areas of the field that are most compacted such as field edges and lanes.