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

Table 3. Pathogen profiles closely associated with rice production situations (PS) and potential yield losses causedby rice diseases (adapted and modified from Savary et al 1998, Savary and Willocquet 1999).PS1 PS2 PS3 PS4 PS5 PS6 Yield loss(%)Actual yield (t ha –1 ) 4.8 4.6 3.5 6.7 3.8 3.9DiseaseBlast a L L M M 1–3Bacterial blight L L L L L 0.2Bakanae VL 0.0Brown spot L L VH H H 6.6Sheath blight VH VH M VH H H 6.4Sheath rot complex M M H M 0.5Grain discoloration M M H M 0.1Characteristics of environmentsMineral fertilizer m l l h m hFallow period l l m s m sDrought stress l l h l h mWater stress l l l h h hCrop establishment tr tr tr ds ds dsHerbicide use m l l m l lInsecticide use m m m m m mFungicide use l l l h h hPrevious crop rice rice w/b w/b rice riceaIn the surveys, rice varieties possessing resistance to blast and bacterial blight diseases. For characteristics of environments, m = moderate, h =high, l = low, tr = transplanted rice, ds = direct-seeded rice, s = short, w/b = wheat or barley. For diseases and grain discoloration, L = low, M = medium,H = high, VH = very high.Disease and infection cyclesFigure 2 shows how seedborne inoculum reinfectsthe seed during the development of a disease epidemic:seedborne inoculum → disease establishment→ disease development in the field (infection cycle)→ crop damage or yield loss (effect of seedborneinoculum) → reinfection of infestation of seed (potentialdissemination to other fields, regions, or countries).There is voluminous information on seedbornepathogens of various crops derived from routine seedhealth testing for either certification or issuance ofphytosanitary certificates. Information on transmissionof the pathogen from the infected or infestedseed to disease development in the field is scarce.Various factors that affect the infection cycle areweather conditions, cropping practices, resistance orsusceptibility of the variety, virulence of the pathogen,and amount of incoculum produced for secondaryspread and efficiency of the inoculum.It is often assumed that, for a pathogen to beseedborne, it must be seed-transmitted. McGee(1995) indicated that in only very few seedbornepathogens is the transmission clearly established.When conditions in the nursery bed and theecosystem where rice is grown re taken into account,there is inadequate documentation on plantquarantine to guide decision making. It is notknown under what specific conditions seedbornepathogens are transmitted to the crop at the seedlingstage. Blast caused by P. oryzae and bakanaecaused by F. moniliforme, are two of the betterknown diseases (Ou 1985). Once a disease is establishedin a crop, its intensity will depend on factorsthat influence the infection cycle. Climaticconditions and crop management practices arecrucial to disease development.In rice, the infection frequency of P. oryzae isvery low, yet the disease potential under a conduciveenvironment (e.g., upland, subtropical, andtemperate) is very high. Once seedlings are infectedfrom seedborne inoculum, even at a lowinfection rate, millions of conidia are produced forsecondary infection. On the other hand, seedborneF. moniliforme often induces bakanae with onlyone cycle of infection. Therefore, the initial inocu-7