Soil borne and Seed Borne Diseases

1. Soil borne and Seed Borne Diseases End Next

2. Soil borne disease The diseases that are caused by fungal pathogens which persist (survive) in the soil matrix and in residues on the soil surface are defined as soil borne diseases. Thus the soil is a reservoir of inoculums of these pathogens, the majority of which are widely distributed in agricultural soils. End Previous Next

3. Key features of soil borne plant pathogens • Disease Symptoms • Many soil borne fungal plant pathogens cause disease of the roots or stem disrupting the uptake and translocation of water and nutrients from the soil. Therefore they commonly cause similar symptoms to drought and nutrient deficiencies; these include wilting, yellowing, stunting and plant death. End Previous Next

4. Host Range The majority of these pathogens has a wide host range and may cause different types of diseases on different hosts. A particular species may cause severe root rot on one host but only cause superficial and symptomless infection on the roots of another host. Epidemiology and Survival In general these fungi can persist for long periods in soil in the absence of susceptible crops. Some species persist mainly as resistant hyphae in plant residues, in large pieces of stubble or small fragments. The inoculum of such pathogens declines gradually as the residues decompose. Many species also persist as propagules which are adapted for long-term survival in soil. These include thick walled spores such as oospores and chlamydospores, together with sclerotia and microsclerotia. End Previous Next

5. Nature of diseases caused by soil borne plant pathogens Root rot These diseases are caused by a diverse group of fungi and related organisms. The most important genera include Pythium and Phytophthora, Rhizoctonia, Sclerotium, Sclerotinia, Fusarium, Cylindrocladium and Armillaria. These diseases are characterised by a decay of the true root system. Wilt The main species of fungi that cause these diseases are Fusarium oxysporum and Verticillium spp. T he symptoms of these diseases include wilting of the foliage and internal necrosis of the vascular tissue in the stem of the plant. Some species of bacteria can also cause similar types of diseases. End Previous Next

6. Seedling blights and damping-off Various common names are used for diseases of seedlings such as seedling blight and damping-off. The fungi that commonly cause seedling death include Pythium, Phytophthora, Rhizoctonia, Sclerotium and less commonly Fusarium spp. These fungi can infect the seedling during the germination, pre-emergence or post-emergence phases of seedling establishment. End Previous Next

7. Seed borne diseases • Pathology is the science deals with micro-organism infecting seeds. • Seeds are attached by various fungi, bacteria and virus. • Seeds are attached by various stages, • The mother plant get infected by the pathogen, it attack seed also. • During processing. • At the time of transportation. • During storage Introduction End Previous Next

8. There are three types of infections 1) Internally seed borne 2) Externally Seed borne 3) Fruity bodies/spores End Previous Next

9. Internally Seed borne: Pathogen attacks seed sod, endosperm & embryo Externally seed borne: Pathogens externally carryover on the seeds. End Previous Next

10. Effects of seed infection: • Germination % get reduced. • Due to changes is morphology, the market level get reduced. • Due to infection it induces the changes in the content get reduced. • Due to infection it induced the secretion of toxic chemicals ex: Aflatoxn, Rubra toxin, ochre toxin, chitrinin, patulin etc. End Previous Next

11. Methods of testing for the presence are of four main types. The seeds may be • (1) directly examined , • (2) examined after incubation, • (3) plated on agar so that the pathogens grow out into colonies and • (4) subjected to techniques which allow the use of modern serological developments viz., ELISA, SSEM and IF. End Previous Next

12. Methods for Detecting Seed Borne Fungi: Examination of dry seeds: • It is applied for detection of seed borne fungal pathogens which cause discoloration of the seed or change the shape and size of the seed. • Also applicable for detecting fungal structures present in, on or with seed. • Examples: • Karnal bunt of wheat Neovossiaindic • Ergot of bajraclavicepsfusiformis End Previous Next

13. Washing test: • This method is used particularly for smut and bunt fungi in gramineous hosts except loose smut of wheat and barley. • It can also be used for downy mildew (peronospora Manchuria) of soybean and tumor disease (protomyces macrospores) of coriander. NAOH seed soak method: Applied for Karnal bunt of wheat and bunt of rice. End Previous Next

14. Blotter method: • This method is widely used. • All kinds of cereals, vegetables, crucifiers, legumes, omamentals and forests seeds are tested by this method. Seedling symptom test: • This test is applicable for those fungi which are capable of producing symptoms on the root and shoot of the young seedlings. • This test for certain pathogens, provide information perta • ining to field performance of the seed lot. End Previous Next

15. Important Seed- Borne Fungal Diseases of Major Crops End Previous Next

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17. Detection Method For Seed Borne Bacteria: • Several methods have been developed worldwide to detect various phytopathogenic bacteria in seed and planting materials. • The detection techniques generally employed are: Examination of Dry Seeds: As in Fungi ‘Growing on ’test: • The ‘growing out’ bioassay of a working seed sample involves the sowing of test seeds into seedlings under conditions optimal for the disease development in glass house or closed environmental chambers. • ‘Growing –on’ test has been successfully used for a large number of Xanthomonads and pseudomonad’s. End Previous Next

18. Isolation on agar medium: As in fungi Indicator test: • Working seed sample is sterilized with (2.6%) sodium-hypochlorite for 15 min. and rinsed with sterile water. • The seed sample is incubated for 18-24 h in sterile water. • The water suspension is inoculated by infiltration into the primary leaf node of 10 day old bean seedlings. • The appearance of lesions followed systemic necrosis is positive reaction. End Previous Next

19. Serological Technique: • Serological tests are based on ‘In vitro’ reactions between antigens and antibodies. • This specific recognition of antigens by antibody has offered the basic principle for the development of various serological methods for detection and identification of phytobacteria. • The washing of the working seed samples are cultured for 36 h using sterile distilled water. • The supernatant is tested with antiserum of the suspected pathogen. End Previous Next

20. Detection of Seed- Borne Viruses: Viruses are obligate parasi tes and they differ in their genetic make up and require special technique for detection. 1.Dry seed examination Visual observation of seeds may reveal abnormalities like discoloration, shriveling, reduced seed rice size, staining and seed coat necrosis. End Previous Next

21. 2.Growing on test: Growing on test is used to determine virus seed transmission. Seeds are planted in blotter paper, sand, soil, vermiculite or in other growth medium. Seedlings are examined at regular intervals for the viruses End Previous Next

22. 3.Indicator test: Viruses can be detected in seeds by assaying the extracts of different parts of seeds and seedlings raised from infected seeds on suitable indicator plants. Susceptible hosts which produce local lesions or systemic symptoms are used as indicator plants. This test has been used to detect BCMV in bean and urd bean, LMV in lettuce, TMV in tomato and tobacco ring spot virus in soybean. End Previous Next

23. 4. Serological methods: Serological methods are used for characterization and determining relationship between viruses. The tests are based on the reaction between an antiserum, a blood serum containing specific antibodies produced by injecting laboratory animals with a pure virus preparation, and an antigen-virus protein. The tests are specific since an antibody combines only with the antigen which contain which contains similar grouping of amino- acid sequences. The union of antigen and antibody can be detected in the form of precipitation and agglutination End Previous Next

24. To sum up • The diseases that are caused by fungal pathogens survives in the soil matrix and in residues on the soil surface are defined as soil borne diseases. • Many soil borne fungal plant pathogens cause disease of the roots or stem disrupting the uptake and translocation of water and nutrients from the soil. Therefore they commonly cause similar symptoms to drought and nutrient deficiencies; these include wilting, yellowing, stunting and plant death. • Seed borne diseases are caused by micro-organisms infecting seeds. Seeds are attacked by various fungi, bacteria and viruses at various stages viz., in the field ,during processing, at the time of transportation, and during storage. End Previous