A Handbook of Rice Seedborne Fungi TW Mew and P ... - IRRI books

Functions of seed health testingSeed health testing is done to determine microbialinfection or contamination for quarantine purposes(e.g., international seed exchange or movement). Itidentifies the cause of seed infection that affects theplanting value of seed lots for seed certification byseed growers to supply seed to farmers. Seed testingaffects policies on seed improvement, seed trade,and plant protection. Neergard (1979) brought out theimportance of pathogens carried by seeds and thedisease potential assigned to pathogens.Several routine activities are undertaken duringseed health testing. These include dry seed inspection,the standard blotter test for seed infection andcontamination, postentry planting for field inspectionof undetected plant diseases of seedborne and seedcontaminatedpathogens, and certification. In seedmultiplication for export, crop inspection prior to seedharvest offers an additional means to link seedbornepathogens and diseases of mother plants. All theseactivities provide preventive measures to eliminatethe introduction of undesirable pathogens into a regionor country. Seed health testing offers a powerfultool for documenting microorganisms associated withseeds. Information on microorganisms, however,needs to be associated with a database on yield lossand information on pathogens that cause diseases.Catalouging pathogens of cropsFor rice, seed health testing has been done on morethan 500,000 seed lots following International SeedTesting Association (ISTA) rules (1985). A total ofmore than 80 fungi were detected on rice seeds(Table 1). The detection frequency varied. About 20species of fungal pathogens were detected from riceseed at any one time. Not all of them cause notablediseases in the field and it was not ascertainedwhether diseases were all seed-transmitted and, if so,what their transmission efficiency was. The role of arice seed in a fungus life cycle is not clear.Pyricularia oryzae, the rice blast pathogen, althoughconsidered a very important rice pathogen,has the lowest detection frequency. The level variedaccording to seed source. Except Fusariummoniliforme, the seedborne inoculum of the otherpathogens may not serve as an important source ofsecondary inoculum in the field. The infection levelof P. oryzae is likely to be higher in temperate orsubtropical environments than in tropical environments.The data set provides insights into the occurrenceof rice fungal pathogens. The detection frequencyand infection level are very high for Alternariapadwickii (80–90%) (Fig. 1). In tropical Asia,stackburn, the disease it causes, is hardly observed inthe field.Detection methodsMany detection methods have been developed overthe years for various seedborne pathogens. We foundthe blotter test to be a common but efficient methodof detecting seedborne fungal pathogens in rice seed.Following ISTA rules, the method involves plating400 seeds on some layers of moistened filter paper.Below is a list of the different detection methods usedin routine seed health testing. Descriptions of thesemethods can be found in the references listed (seepage 82).Seed health testing procedures involve techniquessuch as• Direct examination of dry seeds• Examination of germinated seeds• Examination of organisms removed by washing• Examination after incubation (both blotter andagar plates)• Examination of growing plants (for example,the seedling symptom test)• Embryo count methods• Molecular and serological techniquesOther methods include a selective medium forspecific pathogens. With advances in moleculartechniques, emphasis in fungal identification andtaxonomy has changed from a morphological approach(for example, spore size and spore shape) toa more functional approach based on aspects of thelife cycle, mechanisms of spore production and release,DNA relationships, and physiological attributes.DNA analysis techniques such as the polymerasechain reaction (PCR), and random amplifiedpolymorphic DNA (RAPD) analysis are the mostcommonly used tools.These are powerful techniques for detecting andfor establishing the relationship between the inocu-3