By Felix Cawich, Agronomist, M.Sc.
Worldwide, soybean is considered an important crop for the production of oil and as a source of protein for people and animals. In addition, it´s now being used for biodiesel production. According to the Worldwide Web Foundation (WWF), soybean production has grown 10 times in the last 50 years, covering over one million square kilometres. The increasing production area, like the increase in yield of monocultivation, has also led to higher presence of disease in soy plantations. Furthermore, the varying climate may favor rapid development of diseases.
Diseases originating from different microorganisms can cause blights, cankers, rots, rusts, wilts and others, causing damage to different parts of the plant at any given growth stage (Fig 1.). All growth stages are vulnerable to a disease infection, which may be triggered by existing soil growing conditions, weather, pest occurrence, etc. For example, from VE to V2, plants are susceptible to blights, leaf spots and stem and root rots, all caused by different microorganisms such as Pythium spp., Phytophthora spp., Fusarium spp., Rhizoctonia spp., Phomopsis spp. As the plant develops, the stages from bloom (R1) to pod fill (R5), are critical times where the appearance of other diseases may include bacterial blight, fusarium wilt, mildews, sudden death syndrome, antrachnose, among others (Paderson, 2009). Some of these are foliar diseases and are obvious to detect, while others damage below ground and go undetected, thus both causing yield reductions.
Diseases affecting the roots are common every year, causing primarily root rots. The most frequently occurring are root rots caused by different fungi agents like Phytophthora sojae, Rhizoctonia solani, Pythium spp. and Fusarium spp.; also the so named charcoal rot caused by Macrophomina phaseolina (Diaz, 2012). All are important and require treatment. For purpose of this article, Fusarium spp. will be the principal disease discussed.
One of the major disease attacking soybeans is Fusarium spp., which is a widespread, soil-borne pathogen causing diseases such as sudden death syndrome, root rot, crown rot, stalk rot, head blight and scab on cereal and grains (Nelson, 1993; Pedelson, 2010). Different environments may host this fungus, such as soil, roots and aerial plant tissues, plant debris and other organic substrates (Aoki, 2003), where they are capable of surviving for long periods of time, because they have resistant reproductive structures (chlamydospores) that can survive in inadequate conditions for years, remaining dormant until suitable conditions are present. Due to its widespread distribution, the disease can occur in all stages of plant development, from germinating seeds to plant maturity, depending on the host, climatic factors and composition of fusarium species (Mueller, 2010). Fusarium may cause disease individually or in complex with other causal disease agents, particularly in response to temperature and humidity (Doohan, 2000). Wet soils and cool temperatures often favor its infection in the early growing season; meanwhile, limiting soil moisture can cause plant stress making it susceptible to infection later in the growing season (Zhang, 2010).
Fusarium species, primarily root rot, is favored by cool temperatures and wet soils during early vegetative growth stages, thus affecting younger plants. One of the most important species, causing vast damages in young plants is called Fusarium oxysporum, which symptoms may appear as damping off, stunting, decreased vigor, chlorosis and decreased root masses (brown discoloration and/or lesions on the roots), eventually leading to the death of infected plants (Diaz, 2012). The presence of Fusarium oxysporum has been confirmed in Belize and it is difficult to differentiate from other diseases or stresses. The absence of a phytopathologic laboratory in the country makes difficult the accurate identification of these phytopathogens.
The presence of fusarium species infection has been linked to the soil, seeds and pest infestation. Monocultivation of legumes and persistence of infected plants, increase the damage under optimal environmental conditions. The use of infected seeds is another source of contamination where ten different fusarium species have been isolated, causing a reduction in germination (Pant, 2000). Also, soil containing nematodes that attack legumes may cause root damage, thus allowing them and other fungi to infect plants. Other conditions influencing the development of this disease include soil compaction, crop rotation history, soil pH, herbicide induced stress, iron chlorosis and soil type (Zhang, 2010).
Management of Fusarium
Managing fusarium includes different practices. The use of non-infected seed or resistant varieties must be considered. Avoid or minimize spread of disease from infested to non-infested fields through the use machinery, irrigation equipment, water and contaminated seed. Cultural practices such as planting in well-drained soils, reducing soil compaction, delaying planting and crop rotation (does not include legumes) may help to make conditions less favorable for this pathogen’s development (Davis, 2008).The use of fungicidal seed treatment is recommended when there is a history of fungal root problems. The use of biological products, such as Trichoderma spp. has proven to reduce fusarium infections (Milanesi, et al 2013). Good management practices are important for minimizing losses from this disease.
For good management of plant diseases, it is most critical to obtain a correct diagnosis!
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