Soybean Diseases PDF

Course Title: Diseases of Field and Horticulture Crops and their Management-I Course Code: (PP-353) Topic: Diseases of Soybean Dr. L. Sanajaoba Singh Assistant Professor (Plant Pathology) College of Agriculture, Kyrdemkulai,...

Course Title: Diseases of Field and Horticulture Crops and their Management-I Course Code: (PP-353) Topic: Diseases of Soybean Dr. L. Sanajaoba Singh Assistant Professor (Plant Pathology) College of Agriculture, Kyrdemkulai, CAU-I, Meghalaya  Diseases of Soybean Disease Casual organism 1. Rhizoctonia Blight/ Aerial blight/ web blight: Rhizoctonia solani 2. Bacterial tan spot Corynbactrium flaccumfaciens 3. Seed and Seedling rot Phytophthora spp, Pythium spp, Rhizoctonia spp and Fusarium spp 4. Mosaic Soybean mosaic virus (SMV) 1. Rhizoctonia Blight/ Aerial blight/ web blight: Symptoms: It is one of the destructive foliar disease which causes rapid defoliation of the plants in warm, humid regions. It cause the symptoms like leaf and pod spots, leaf blight, defoliation, stem and petiole lesions, cob web like mycelium and sclerotia developed over infected leaves. In Foliar, symptoms appears on the leaves, stem and pods. Infection begins from the lower or middle parts of the plant and gradually moves up. Initially, water soaked spots appear on the leaves which soon take on a greenish brown to reddish brown appearance and later become tan, brown or black. Spots are initially small which later coalesce to cover large areas of the leaf. In the aerial blight phase, infected leaves droop and adhere to the pods and stem beneath them, thus becoming sources of infection for the pods and seeds. On the petioles and stem brownish lesions forms. On the pods, lesions may be small, brownish spots or may blight the whole pod.  Etiology: Pathogen: Rhizoctonia solani Kuhn; Sexual stage: Thanatephorus cucumeris. The fungus survives in the soil or previous crop debris as sclerotia. As disease progresses, sclerotia (overwintering structures) may form inside of the web-like hyphae. It has a very wide host range, and rotational crops like peanut, cotton, tobacco, and other common vegetable rotations are susceptible. Under the field condition, a web like mycelium with micro and macro sclerotia may form on the host surface. Hyphae growing on host tissue are hyaline or light brown, branched, septate and 5-6 µm wide. Micro and macro sclerotia are irregular or oval, white when young, changing from light to dark brown at maturity and ranging from 2- 7 mm in diameter and 1-3 mm thick. Fig: Mycelium Fig: sclerotia Favourable Conditions: : High relative humidity (96-97 %), high temperature (30-32°C). Closer planting. Heavy doses of nitrogenous fertilizers. Disease Cycle: Rhizoctonia solani overwinters as sclerotia in soil or plant debris from the preceding crop. With the onset of soybean growing season and presence of favourable environmental conditions (high RH and 77 to32 °C), the sclerotia germinates and the mycelium spreads inside the host. The infected plants show typical disease symptoms viz., development of web like mycelium. The fungus spreads to adjacent plants via rainwater runoff, flood irrigation and direct contact. When the crop is harvested, the fungus survive as sclerotia and dormant mycelium in soil and plant debris respectively  Management: Avoid dense planting. Completely cover plant residue by clean ploughing the field soon after harvest. Destroy infected stubble. Seed treatment with Thiram + Carbendazium (2:1) @ 3g/kg seed. Use Mancozeb or copper fungicide at 2.5gm/l or carbendazim 1 g/lit  2. Seed and Seedling rot: Symptoms: Phytophthora can attack and rot seeds prior to emergence, and can cause pre- and post- emergence damping off. It produces tan-brown, soft, rotted tissue. At the primary leaf stage, infected stems appear bruised and soft, secondary roots are rotted, the leaves turn yellow, and plants frequently wilt and die. Pythium can attack and rot seeds and seedlings prior to emergence and can cause post- emergence damping-off under wet conditions. The characteristic symptom of most Pythium infections is soft, brownish-colored, rotting tissue. Thus, Pythium causes symptoms similar to Phytophthora in seedlings, and can only be distinguished by laboratory examination. Although Pythium causes most damage to seeds and seedlings, roots of established plants can be rotted and plants may be stunted. Rhizoctonia can damage seeds and plants prior to or after emergence. In seedlings and older plants, a firm, rusty-brown decay or sunken lesion on the root or on the lower stem is a characteristic symptom. The infections can be superficial and cause no noticeable damage, or they can girdle the stem and stunt or kill plants. Fusarium is also a common pathogen that can damage seeds and seedlings. It causes light to dark brown lesions on roots that may spread over much of the root system and may appear shrunken. Fusarium may attack the tap root and promote adventitious root growth near the soil surface and may degrade lateral roots.  Etiology/ Pathogen: The most important pathogens involved in seed and seedling diseases of soybeans are fungi or fungal- like organisms: Fusarium spp, Rhizoctonia solani, Phytophthora sojae, and Pythium spp. In some situations and locations, other soilborne or seedborne pathogens that may be important include Macrophomina, Colletotrichum, and Phomopsis. Phytophthora sojae is a fungal-like pathogen that survives in soil for up to five to 10 years in association with decomposed soybean tissues. Soybean is the only known crop host for this pathogen. It is favored by saturated, warm soil. Pythium is a soilborne, fungal-like pathogen. Several different species damage soybeans. The various species of Pythium that infect soybean have a wide host range that can include corn and many other crops. Pythium tend to be favored by cool and soil, but some species may do more damage in warm soils. Rhizoctonia solani is a common pathogen with a wide host range. The most common strains of this pathogen (anastamosis groups, AG) that infect soybean are AG-2-2 and AG-4. AG groups can have different optimal conditions for infection. Fusarium seed and seedling blight of soybean is caused by a complex of different species that may prefer different conditions. For example, some species may prefer warm and dry soils and others may prefer cool and wet soils. Some Fusarium species may also have a broad host range that includes corn and wheat. Favourable condition: General conditions that promote seed and seedling disease include wet, poorly-drained and compacted soils. However, the different pathogens have different optimal conditions. For example, Phytophthora and Rhizoctonia are favored by wet and warm soils, whereas Pythium is typically favored by wet and cool soils. The diseases may be enhanced by slow germination and growth of soybeans, poor quality seed, and plant stress. Management: In general, these diseases can be reduced by planting good-quality seed in well- drained, non-compacted fields. Delaying planting until soils are warmer than 55 ° F and relatively dry to allow for rapid emergence and growth can be beneficial. Crop rotation and tillage may be of some benefit. Genetic resistance in soybean varieties is only clearly available for managing Phytophthora infections. Seed treatments with two or more active ingredients should typically be used. For example, products containing mefenoxam (ApronXL®) or metalayl (Allegiance®) can be effective against Pythium and Phytophthora, and products containing fludioxonil (Maxim®) or a strobilurin product (azoxystrobin, trifloxystrobin, or pyraclostrobin) may help to reduce damage from true fungi such as Fusarium and Rhizoctonia.  3. Mosaic: Soybean mosaic virus (SMV) Symptoms of SMV vary with variety, virus strain, environment, and age of plant at infection. Most varieties will be stunted and have fewer pods. Trifoliate leaves will have a mosaic of light and dark green areas that may become blistered or raised with time. Leaves may appear distorted, generally with the leaf margins curling downward. Symptoms are most severe during cool weather and infection is rarely evident during summers. Seed from infected plants can be mottled black or brown depending on hilum colour. SMV and BPMV (Bean pod mottle virus) act synergistically. Symptoms on plants with both viruses are very severe and terminal death may occur. Yield losses associated with combined infection can be as high as 66 - 86%. The combined infection also increases the level of seed transmission of SMV. SMV infection will reduce oil content, seed germination, and seed quality due to seed coat mottling. Yield losses due to SMV generally range from 8% to 35%, however losses as high as 94% have been reported. Terminal death  Etiology/ Pathogen: It is caused by Soybean mosaic virus - a Potyvirus (genus). Flexuous particles 750 - 900nm long, ss RNA genome, linear and unipartite. The viron are composed of 5.3 per cent nucleic acid and 94.7 per cent protein. SMV is transmitted by aphid species and seed. In most varieties seed transmission Fig: Properties of the Soybean mosaic virus (SMV)–soybean is less than 5%, but much greater pathosystem. (A) Typical symptoms induced by SMV in a susceptible soybean. (B) Negatively staiProperties of than seed transmission for BPMV. the Soybean mosaic virus (SMV)–soybean pathosystem. (A) Typical symptoms induced by SMV in a susceptible soybean. This disease also can be spread (B) Negatively stained filamentous SMV particles of about 750 nm in length. (C) Alate Rhopalosiphum padi which through soybean aphids which can transmits SMV non-persistently. Rhopalosiphum padi is one of the most important aphid vectors of SMV in the Midwest of vector this virus the USA. (D) Seed coat mottling of soybean seeds induced by SMV. ned filamentous SMV particles of about 750 nm in length. (C) Alate Rhopalosiphum padi which transmits SMV non-persistently. Rhopalosiphum padi is one of the most important aphid vectors of SMV in the Midwest of the USA. Favourable condition: Symptoms most pronounced during cool weather and disappear during hot weather. Planting bin run seed from infected fields can increase this problem. The virus is transmitted by aphids and also seed-borne at a rate less than 5% in most varieties. Environmental conditions favouring aphid reproduction can favour this disease if the virus is present to be spread. Disease cycle: Soybean mosaic virus is seed borne. The SMV can be transmitted through sap, 32 aphid species are involved in transmission. Management: Deep summer ploughing. Use resistant or tolerant varieties. Use healthy/certified seeds. Keep the field free from weeds. Rogue out infected plants and burn them Pre-sowing soil application of Phorate@ 10 kg/ha. Two foliar sprays of Thiamethoxam 25 WG @ 100 g/ha or Methyl demeton 800 ml/ha at 30 and 45 days after sowing.