Soybean Diseases
From Bugwoodwiki
Contents |
Nematodes
Namatodes that may affect soybean are:
- root-knot (Meloiclogyne spp.)
- cyst (Heterodera glycines)
- Lance (Hoplolaimus columbus)
- sting (Belonolaitmus spp.)
The root-knot nematode is the most common nematode pest of soybeans in the southern states, and has more than 2,000 plant hosts. Four different species attack soybeans. All cause similar symptoms, including severe stunting and formation of galls or knots on the roots. Also, plants may appear to suffer nutrient deficiencies and may wilt during hot periods of the day. Nematode damage reduces yield and lowers quality.
Nematodes have a fairly simple life cycle. Infected soil contains eggs and larvae. The larvae are egg-like, non-segmented round worms about 1/65 inch (0.4 mm) long. Larvae enter and feed within the roots of a host plant. There they develop into adults which lay eggs outside the root. Each egg mass may contain several hundred eggs. The life cycle requires only 3 to 4 weeks, so there may be several generations in one season.
Soil fertility, moisture and temperature influence the severity of root-knot and most other nematodes. Therefore, damage may range from minor losses to complete crop failure with the same level of nematodes present.
The soybean cyst nematode is a relatively new soybean disease which restricts root development and causes an absence of nitrogen-fixing nodules. Plants are stunted in an irregular pattern that may resemble manganese toxicity, or moisture or potash deficiency. High nematode populations often result in death of plants.
There are two important stages in the life cycle of the cyst nematode—egg and larva. When the female's body becomes filled with eggs, it erupts from the root and resembles a lemon-shaped cyst. These cysts contain 300 to 500 white eggs which turn yellow and then brown. The brown cysts are the overwintering forms of the nematodes. Unlike the root-knot nematode, cyst nematode eggs must be fertilized before they will hatch. A single generation of cyst nematode may be completed in about 21 days.
Seed Decay and Seedling Diseases (fungi Pythium spp., Rhizoctonia spp.)
The fungi responsible for these diseases are primarily debris- and soil-borne. Poor quality seed is a major cause of seedling disease. Inadequate stands and death of young seedlings are characteristics of seedling disease. Damage may occur before the seed germinates or emerges, or young plants may die after seedling emergence. The presence of dark brown or reddish lesions on the stem and lower main root confirms seedling disease. Microscopic examination of the damaged tissue is sometimes necessary to identify specific disease pathogens involved. Other factors. such as improperly applied fertilizer or pesticides and poor soil environment, can cause similar damage.
Southern Stem Blight (fungus Sclerotium rolfsii)
Southern stem blight damages a wide range of host plants, including soybeans. The fungus is most active during summer, and survives in the soil and on plant debris. The typical symptom is a white cottony mold, often with brown "mustard seed-like" structures, on the main stem at the soil surface. Soybean losses to the disease vary considerably, but generally the damage occurs as localized areas of dead plants scattered across fields. Economic losses occur in some instances.
Pod and Stem Blight (fungus Diaporthe phaseolorum var. sojae, Plomopsis sp.)
Pod and stem blight occurs in every soybean-producing area of the United States and in many foreign countries. It is usually a late season disease, favored by warm, humid weather. It is a major cause of seed damage which results in poor seed quality. The fungus lives from season to season on seed or on crop residue. The most characteristic symptom of this disease is the arrangement of black fruiting structures in linear rows on the stems. Infected seed may crack and shrivel and are often covered with white mold. Such seed may fail to germinate, or may produce weak seedlings with brownish-red pinpoint lesions on the cotyledons. Another symptom is reddish-brown streaks on the seedling near or below the soil line.
Charcoal Rot (fungus Macrophomina phaseolina)
This disease is caused by a soil and plant debris-borne fungus common in soils of the southern United States. Charcoal rot is primarily a root and basal stem disease, but may be seen on above-ground parts of infected plants. Most of the overwintering inoculum is in the above-ground parts of diseased plants.
Infected soybean plants exhibit many tiny, black specks on the roots and lower stem just beneath the skin or bark. The affected area has a "charcoal" appearance. The disease often girdles the lower stem, causing wilting and death.
The disease is severest when plants are under stress from moisture or nutrient deficiencies, soil compaction, nematodes or other diseases. Yield losses are difficult to measure because there are no known resistant varieties or fungicidal controls, and it is difficult to find a test site where the fungus does not already exist.
