Sudden death syndrome (SDS), a disease in soybean plants, quickly spread across the southern United States in the 1970s, eventually reaching most agricultural areas of the US. SDS is caused by multiple Fusarium fungi in the Fusarium solani complex. Fusarium virguliforme is the sole causal agent in North America. In South America, Fusarium brasiliense, F. cuneirostrum, F. tucumaniae, and F. virguliforme are all causal agents. Losses could exceed hundreds of millions of dollars in US soybean markets alone making it one of the most important diseases found in Soybeans across the US.
Importance Sudden death syndrome (SDS) has become one of the most impactful yield-reducing diseases in North American soybeans. After making its first appearance in Arkansas in 1971 SDS soon spread to the surrounding states of Tennessee and Mississippi, and then traveled up the Mississippi River to Midwestern states. Currently, the disease affects an area stretching from South Dakota to North Carolina, putting the majority of American soybean producers at risk. The disease has the potential to cause up to 80% yield loss, making it a devastating disease for producers if not detected early and managed correctly. Its presence is currently increasing in northern Midwest states and western Corn Belt states including Minnesota, Nebraska, and Wisconsin, and may coincide with the spread of the Soybean Cyst Nematode (SCN).
Symptoms and signs Most of the SDS symptoms can be confused with other factors like nutrient deficiencies and some other diseases like brown stem rot and stem canker. Usually the first symptom seen is interveinal chlorosis, which is the yellowing of the plant material between the leaf veins. When leaves begin to die, puckering and mottling can also be observed along with the chlorosis. As severity increases, necrosis (death of cells) occurs and eventually these leaves will fall off, leaving only petioles left on the stem. If the conditions are right (cool and wet), these symptoms can appear suddenly, causing large yield reductions. Normally, this is seen in mid or late July around the time of flowering and pod production. In addition to foliar symptoms, the stem of the soybean plant can show symptoms as well. If a soybean stem with SDS is split, the pith will be visibly white while the cortical tissue around the pith will be tan to light brown in color. If the pith is brown in color (or if the whole stem looks brown on the inside), it is likely that the plant has brown stem rot, rather than SDS. Along with the above-ground foliar and stem symptoms, the roots usually show some kind of rotting and decrease in vigor compared to other healthy soybean roots. If soil conditions are moist, roots are also likely to show blue masses of spores (macroconidia) around the taproot just below the soil surface. Blue fungal masses, found along with the foliar and stem symptoms, are strong diagnostic indicators for SDS.
Disease cycle F. virguliforme overwinters as asexual macroconidium and chlamydospores and currently, no research has found a successful sexual stage with this pathogen. When conditions are favorable, these spores germinate on seedling roots and infect the plant. From the V1 to R1 stages (seedling to first flower) of soybean growth, the fungus colonizes within the plant cortex and only goes up the stem a few inches above the soil surface. Toxins are produced when the pathogen colonizes the lower parts of the soybean cortex. These toxins travel up the xylem to the leaves, causing leaf chlorosis and necrosis, eventually leading to leaf and pod drop. Blue Fungal spore masses are produced on the roots of the plant where macroconidia are formed. Macroconidia are one of the overwintering phases of the pathogen and can persist in the soil and plant residue for many years. Between growing seasons, F. virguliforme is also found in the form of chlamydospores in the crop residue and freely in the soil. These thick-walled overwintering structures can withstand large temperature fluctuations within the soil and even resist desiccation. There is no known sexual reproduction in certain SDS causal agents such as F. virguliforme. However, F. tucumaniae has recently been shown to produce perithecia, a sexual fruiting body, which provides evidence for the existence of a sexual reproduction cycle in that species.
Environmental factors Cool and wet soil is the most ideal condition for F. virguliforme, the pathogen that causes SDS. The presence of the disease is often able to be tracked with storm fronts moving across the country. Symptoms of SDS are usually common 10–14 days after heavy rains. While the disease prefers cool soil, symptoms often do not manifest themselves until July or August after a soaking rain saturates the soil. Early planting can leave seeds susceptible to SDS, as can planting vulnerable varieties. SCN infection of a population of plants also creates a favorable environment for SDS, as the roots of the plants are already under stress from the nematode, leaving them vulnerable to infection from F. virguliforme. The presence of SCN is also one of the most important environmental factors for SDS. Limiting soybean exposure to SCN is much easier in comparison to SDS, so limiting a field's vulnerability to SCN is vital to preventing the introduction of SDS. Additionally, SDS has been shown to be more severe in highly fertile soils, specifically those with high phosphorus, magnesium, and organic matter. Environmental factors are some of the largest variables that will determine both the presence and severity of an SDS infection. The ability to control and react to as many of these factors as possible is key to protecting a soybean crop from SDS.
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