Sheath Rot of Rice

This PPT describes about the Sheath Rot disease of Rice,it's Symptoms, pathogenic organisms, disease cycle, Management strategies(preventive, cultural, botanical and chemical methods) and a Self evaluation questions.

1. DISEASE NAME: SHEATH ROT CAUSAL ORGANISM : SAROCLADIUM ORYZAE Presented by kaviraj.s faculty of agriculture annamalai university

2. Sheath rot reduces the grain quality by causing the panicles to rot and grain to become discoloured. It`s incidence increases with the increasing plant density and in plants that provide entry points for the fungus, in the form of injuries or wounds caused by the insects such as stem borers/leaf hoppers/mite at the panicle initiation stage. Sheath rot lesions are sometimes confused with the sheath blight lesions. However, sheath rot occurs higher on the plants than sheath blight normally does and interferes with the panicle exertion. Infected seeds and mycelium carried by the rice crop residue play an important role as source of inoculum for primary infection.

3. Symptoms : Inflorescence – Discolouration in panicle. Inflorescence – L esions on glumes. Inflorescence – Twisting and Distortion . Leaves – Irregular lesions/spots. Leaves – Whitish powdery fungal growth. Seeds - Discolourations . Seeds - Rot .

4. Discoloration in the flag leaf sheath. Rotting occurs on the leaf sheath that encloses the young panicles. Irregular spots or lesions with dark reddish brown margins and grey center . Lesion enlarges and often enclosed and may cover the entire sheath. Severe infection causes the entire or parts of the young panicles to remain within the sheath.

5. Unemerged panicles rot and florets turn red brown to dark brown. Whitish powdery growth inside the affected sheaths and young panicles. Infected panicles and grains are sterile, shrivelled, partially or unfilled and discoloured. The disease is important during the Heading towards the maturity stage of the rice crop .

6. CONFIRMATION OF SHEATH ROT: Lesions develop on the uppermost leaf sheaths that encloses the panicles. Some panicles do not emerge or emerge partially. Rotting of the sheath and the development of the whitish powdery fungal growth is usually observed.

7. Identification of the pathogen: The fungus produces the white mycelium, sparsely branched, septate , and measures 1.5-2mm in diameter. Conidiophores arising from the mycelium are slightly thicker than the vegetative hyphae, branched once or twice each time with 3-4 branches in a whorl.

8. In some cases, the fungus infects the sheath in combination with the bacterial pathogen attacking the sheath and causing the grain discoloration ( eg ., Pseudomonas fuscovaginae ). The fungus invades the rice through the wounds and stomatas and grow intercellularly in the vascular bundles and mesophyll tissues. The sheath rot fungus survives as the mycelium in infected residues and on seeds.

9. DISEASE CYCLE: The disease spreads mainly through air borne conidia and also seed borne. Primary source of inoculum is by means of infected plant debris. Secondary spread is by means of air borne conidia produced on the leaf sheath.

10. Ro Sheath rot pathogens are seed transmitted,resulting in infected seedlings. Infected seedlings can die. Resulting in infected plant debris. Or survive. P.fuscovaginae can colonize the whole plant as an endophyte or survive epiphytically and infect the inflorescences at booting stage. The seedling transmission of the fungi pathogens is less well understood. Secondary infections result from conidia or bacterial cells released from infected plants. Conidia or bacterial cells are spread by wind or rain to healthy plants. Plants at booting stage are especially susceptible to infection. In the case of S.oryzae , insects and mites can also spread conidia and facilitate infection by creating wounds. Rot occurs on the sheath enclosing the young panicles; grains on affected tillers become chaffy and discoloured. Grains infected with Fusarium sp. can become contaminated with mycotoxins . Pathogens can spread to new field via contaminated grains. After harvest, infected plant debris will remain in the field. Serving as inoculum for the next growth cycle.

11. Favourable conditions: High amount of nitrogen. High R elative Humidity: 65-85%. Dense crop growth. Optimum temperature: 20-28˚C.

12. Management strategies: Preventive methods : Since it is a seed borne disease, use healthy seeds (Certified seeds). Avoid monoculture in the same field. Limiting insect population in rice fields as they are involved in disease transmission. Adopting different cultural practices for limiting the diseases attack impact: Field sanitation, Crop residue management, Control of weeds etc ,. Bacterization with Pseudomonas fluorescens reduced sheath rot severity by 20-42%, enhanced crop growth, and increased grain yield .

13. Seed treatment with Pseudomonas fluorescens @ of 10g/kg of seeds followed by seedling dip @ of 2.5kg/ha (2.5cm depth; 25sq.m area of Nursery) dissolved in 100 litres and dipping for 30 minutes. Soil application of Pseudomonas fluorescens @ of 2.5 kg/ha after 30 days of transplanting (this product should be mixed with 50kg of FYM/sand and then applied). Foliar spray at 0.2% concentration Pseudomonas fluorescens commencing from 45 days after transplanting at 10 days interval for 3 times depending upon the intensity of disease/even with the micronutrients reduces the disease incidence and increases the grain yield.

14. botanical METHODS: Application of NSKE ( Neem Seed Kernel Extract) 5% or Neem oil 3% or Ipomea or Prosopis leaf powder extract 25kg/ha. First spray at the boot leaf stage and second 15 days later. The identification od antifungal activity of extracts from leaves and flowers of Tagetes erecta . These extracts exhibited the inhibition of S.oryzae by 100%.

15. CULTURAL METHODS: Removal of infected stubbles after harvest (because the fungi survives on the rice crop residues even after harvest and cause infection). One of the best method of reducing the carryover inoculum is field burning of rice residues after harvest. Other effective method is that harvesting the rice at ground level and removing the straw from the field.

16. Minimize insect infestation in the rice field. Insects causes injuries to the plants that allow fungi to enter the plant and cause infection. Optimum plant spacing can reduce the disease. Application of potash at tillering stage is also recommended./ Gypsum 500Kg/ha at two equal splits one basally and another at active tillering stage. Control weeds and keep field sanitation.

17. CHEMICAL METHODS: For control of sheath rot, spray the fungicides at the time of panicle emergence. Ediphenphos may be applied by high volume sprayers only. Application of a systemic pesticide, T ridemorph (a fungicide) and P hosphamidon (an insecticide) in combination, protect the plants from sheath rot.

18. Seed treatment with fungicides such as Mancozeb and Benomyl effectively eliminate seedborne inoculum. At booting stage, foliar spraying with Carbendazim , Edifenphos or Mancozeb was found to reduce sheath rot. Spray Carbendazim 250g or Chlorothalonil 1kg or Edifenphos 1lit/ha. Foliar spraying with Benomyl and Copper- oxychloride were also found to be effective.

19. TEST OUT: (In case of any doubts regarding these questions, Please comment below) Where the disease was first isolated? What are the synonyms/ other names used for various pathogenic organisms that cause Sheath rot? What is the actual rate of growth of the pathogenic organism in PDA media? Spot/lesions approximate size? Optimum temperature and Relative Humidity? W hy excess Nitrogen application causes Sheath rot? Why dense crop practise is avoided? Why Potassium and Gypsum is applied to control Sheath rot? What are the Resistant Varieties against Sheath Rot? Methods of Crop Intensifictaion practices followed to control? Why Neem coated Urea is preferred? When to apply Potassium? What are the other pathogenic microorganisms that are associated with sheath rot disease? What are their main characteristics of other pathogenic organisms that cause Sheath rot? What are the Alternate hosts?

20. Name the pathogenic metabolites/ toxins produced by the pathogenic organisms that cause sheath rot? What are their role(pathogenic metabolites/ toxins)? How do they affect? How pest attack causes sheath rot? Sterile panicle is the result of which pest attack? Why breeding of rice against sheath rot was limited? What are the types of F usarium sp. (Broadly) that causes sheath rot? What are the symptoms of sheath rot caused by F usarium sp.? What are the symptoms of sheath rot caused by pseudomonas sp.? What are the pathogenicity determinants/toxins of F usarium sp.? List out the main toxins involved in sheath rot disease caused by various pathogenic organisms? General cause and method of control of sheath rot disease?