2.0: Agronomic Practices
2.7 Weed Management
Perennial and most annual weeds are a problem in soybean especially during the early growth stages. A properly timed weed management program can minimize the effects of weeds on growth, development and yield of soybean. Weed control in soybean could be manual or chemical or both. Manual weed control: Carry out the first weeding at 2 weeks after planting and subsequent weeding operations can be done when deemed necessary upon looking at the nature and their infestation in the field. Avoid weeding immediately after rains as this would lead to transplanting the weeds. Poor hoe weeding or delay in weeding could cause significant reductions in soybean yields.
Chemical Weed Control: Herbicides, if used properly, are safe and effective in controlling weeds in soybean. The choice of herbicide, however, depends on the pre dominant weed species and the availability of the herbicide. Herbicides are available for pre-emergence (dual magnum) and post-emergence weed control in soybean. If herbicide is applied at planting, one weeding may be required at 5–6 weeks after planting. Knowledge of the weed problems in a field and proper weed identification are essential when making herbicide decisions. Most herbicides selectively control certain weeds when applied as directed on the herbicide label. Weeds not listed on the label probably will not be controlled. Always read the label carefully and follow directions concerning application rates, timing, spray additives, application technique, personal protective equipment, and any restrictions when using chemicals.
Cultural Practices: Cultural practices can have a tremendous influence on the type and severity of weed problems. Crop rotation, tillage system, livestock wintering, and other field-management practices affect weed populations and competition in soybeans. Production practices that encourage quick soybean emergence and canopy development can give the crop a competitive advantage over many weeds. Proper seed placement, fertility management, planting date, and seeding rates can help establish a healthy, competitive soybean crop. Removing weeds in the field before they have reached physiological maturity (formed seeds) is an important weed control strategy for the next crop in the following cropping season even if you have already harvested your crop. This prevents the weeds from producing seeds that would have germinated in the next cropping season.
2.8 Soybean Insect Pests and Their Control
Several insects occur in soybean fields but few are of economic importance, and the species that cause damage are usually not abundant enough to warrant control measures. However, during vegetative stage, the crop can be attacked by caterpillars commonly soybean lopper, leaf miners and leaf rollers insects that feed on the foliage make up the majority of the insects that attack soybeans and if not controlled may affect final yield because they reduce the plant photosynthetic area.
Termite may also attack soybean plants and cause significant yield losses. Termites can attack soybean plants at any stage of development from the seed to the mature soybean plant particularly when there is prolonged dry spells. The first sign of termite attacking roots on seedlings or older plants is wilting. Eventually some plants die or fall over. Pulling out the affected plants and examining the roots and lower stem for live termites and tunnelling will confirm the presence of termites. Plant roots and stems may be completely hollowed out and soil-filled. In general, damage by termites is greater in rain-fed than irrigated crops, during dry periods than periods of regular rainfall, in lowland rather than highland areas, and in plants under stress (lack of moisture, disease or physical damage), rather than in healthy and vigorous plants. Weeds competing with crops for nutrients, light and water may lead to stress and hence increased susceptibility to termite attack. Crop rotation may be useful in reducing the build-up of termites since intensive monoculture for long periods makes plants more susceptible to termite attack. The removal of residues and other debris from the field may reduce potential termite food supplies and hence lead to a reduction in termite numbers and subsequent attack. However, the extent to which termites are a problem to agricultural crops, the nature of loss they cause and the plant species they infest are very much related to the geographic region concerned.
From flowering onwards, soybean becomes attractive to pod-sucking bugs that can seriously reduce seed quality. Insect pests can be controlled with a single spray of Cypermethrin + Dimethoate 10 EC; read the chemical label for application rates and conditions.
2.9 Soybean Diseases and Their Control
For Malawi, just like many other countries within the region where soybean is grown, Soybean rust, is the only disease of economic importance. Other diseases such as frogeye, bacterial pastule, soybean rust and red leaf blotch also occur but are relative less important economically. Except for soybean rust and soybean cyst nematode, soybean diseases normally do not result in major yield losses under Malawian growing conditions. From time to time however, weather and growing conditions can combine to produce significant losses from diseases such as Phytophthora root rot, seed-ling blights, and Phomopsis pod and stem blight. These diseases can be caused by fungi, bacteria and viruses.
Soybean Rust: Asian soybean rust, caused by Phakopsora pachyrhizi, is one of the most important foliar diseases for soybean in Malawi. The infected leaves have small tan to dark brown or reddish brown lesions on which small raised pustules (or bumps) occur on the lower surface of the leaves (Fig. 10). Pustules produce a large number of spores. Brown or rust-coloured powder falls when severely infected leaves are tapped over a white paper or cloth. Severe infection leads to premature defoliation and yield losses up to 80%. The disease is of great economic importance in the areas where rainfall and humidity are high. Late planted soybean is prone to soybean rust infection. It is therefore recommended that planting should be timely at least before end December.
Figure 10, Rust infested soybean field
Soybean Cyst Nematode: The soybean cyst nematode (SCN), Heterodera glycines, is important in soybean . It can be present in the field without causing obvious above-ground symptoms and yield losses caused by SCN are often under-estimated. In heavily infested fields, SCN can cause yield losses of more than 30%. “Yellow dwarf” is an appropriate description for symptoms that are commonly caused by SCN. Root galls (swellings) are diagnostic and quite different from nodules produced by the N-fixing bacteria. When soybean plants are severely infested, they become stunted, canopy closure doesn’t occur, and leaves may become chlorotic. Unfortunately, these symptoms are not unique to the disease caused by SCN and may be confused with symptoms caused by other crop stresses such as nutrient deficiencies, injury from agricultural chemicals, feeding of the soybean aphid, and infection by other plant pathogens. SCN infection may limit nodulation by nitrogen-fixing bacteria. Severely infected plants may die before flowering, especially during dry periods where soils have poor water holding capacity.
Good soil fertility and adequate moisture increase tolerance of soybean plants to SCN and reduce the severity of above ground symptoms in fields. Good crop production practices can also help reduce severity of nematode infestation. For example, a 1 to 2 year rotation with maize or cotton (non-hosts) has proven effective for many soybean growers. In addition, soybean resistant varieties such Ocepara 4 are being successfully used in SCN management.
Virus Diseases: They are commonly transmitted by insects such as whitefly. Depending on genotype and age of infection symptoms range from mosaic and mottling, leaf curling, green vein banding, and stunting (Fig 8) . Most severe symptoms are observed in plants infected at early stages of growth (pre-flowering) and significant reduction in pods.
Soybean is also susceptible to several viruses transmitted by aphids and beetles prevailing in Malawi. Most of the virus infection results in foliar symptoms such as mosaic and mottling, thickening or brittling of older leaves, puckering, leaf distortion, severe reduction in leaf size, and stunting of plants (Fig11). Infected plants should be rouged and burnt.
In general, soybean diseases can be controlled by:
Use of certified seed to avoid seed-borne infection or use seed that are produced away from the infection sources.
Use of resistant varieties to prevailing diseases in the area.
Early planting may also escape diseases that can late in the season but also encourage good establishment of soybean plants to enable them withstand disease attack
Avoid planting seeds obtained from mosaic-affected plants
Roguing (uprooting and destruction) symptomatic plants. This can reduce the incidence of insect-transmitted viruses.
Do not use seed that is cracked or broken as it is easily invaded by seedling disease organisms. Care should be taken in harvesting and handling to reduce mechanical damage.
Since many soybean diseases such as pod and stem blight, brown spot, and bacterial blight can infest crop debris, rotating soybeans with non host crops such as maize or sorghum is good control practice.
Eradicate the weeds and voluntary plants in the vicinity of the soybeans farms as these act as host agents of soybean diseases.