| The blast, caused by the fungal pathogen (Magnaporthe grisea (Hebert)Barr., anamorph: Pyricularia grisea Sacc.), is the most devastating disease of rice worldwide. Heavy losses occur frequently in many rice-producing areas of China. For years, development of resistant cultivars has been considered the most effective strategy to control the disease. Although a number of cultivars with various degrees of blast resistance have been released in China in the past several decades, disease prevalence remains high. This approach needs to be more efficiently explored in Chinese rice breeding programs through better resistance evaluating method. However the complexity of the pathogen population for the targeted areas will make it difficult to circumvent the low efficiency problem in blast resistance breeding. To better understand the population dynamics of the pathogen at disease nursery and field level, and their correlation, we studied the pathogen population at two disease nurseries located in Shanghang and Jiangle county and their nearby fields with virulence analysis based on CO39 near isogenic lines (CO39 NILs) and DNA fingerprint patterns defined by a repetitive element (Pot-2) based polymerase chain reaction (rep-PCR).A total of 207 single spore isolates were tested for pathogenicity using CO39NILs,C101LAC (Pi-l(t))> C10A51 (Pi-2(t)X C104PKT (Pi-3(t)X C101PKT(Pi-4aX C105TTP-4L-23 (Pi-4b) and CO39. In 1999, 9 pathotypes from thnursery and 11 from the field were detected in Shanghang county. The sameprevalent pathotype in two populations was FJ20.1. Whereas in 2000, 8 pathotypes in the nursery and 12 in the field were identified, as FJ24.1 and FJ34.1 were the most and the second-most frequent pathotypes respectively. Seven pathotypes in another nursery and 4 in its nearby field were detected in Jiangle county in the early cropping season of 2001. In the nursery, pathotype FJ34.1 and FJ14.1 were present at fairly high frequencies of 31.6% and 15.8% respectively in the nursery. In the field population, pathotype FJ14.1 and FJ30.1 consisted of 38.5% and 30.8% respectively. However, 9 pathotypes in the nursery and 8 in the field were detected in the late cropping season. In the nursery, the most and second-most frequent pathotypes were FJ20.1(33.3%) and FJO. 1(20.7%) respectively, whereas in the field, the most and second-most frequent pathotypes were FJ30.1(29.4%) and FJ20.1(23.5%), respectively.A total of 202 isolates were analyzed for DNA fingerprint patterns by rep-PCR. Ten genetic lineages were found by clustering at 80% of similarity in DNA fingerprinting. Lineage I was the most common lineage in the nursery and in the field in Shanghang county in 1999, but shifted to lineage VII in 2000. Lineage VI was the most prevalent among the nursery and field isolates in Jiangle county throughout 2001 despite sampling at different cropping seasons. No correlation was found between virulence analysis and DNA fingerprinting.These results showed that the general composition of M grisea pathotypes and genetic lineages in the nursery represents the population in the commercial field. However, the spatial and temporal differences of the fungal population between the nursery and the field indicate the necessity of resistance evaluation in different disease nurseries continually.Use of traditional and molecular techniques will improve the efficiency ofscreening new varieties for resistance. It will also be beneficial to deepen the understanding of the relationship between the pathotype and genetic lineage, and breed for durable resistance in rice varieties.
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