Development of specific PCR primers for identification and detection of <Emphasis Type="Italic">Phytophthora capsici</Emphasis> Leon

A PCR-based method was developed for the identification and detection of Phytophthora capsici in pepper plants. Three PCR primers (CAPFW, CAPRV1 and CAPRV2) specific for P. capsiciwere designed based on the sequence of its internal transcribed spacer regions. CAPFW/CAPRV1 amplify a 452 bp product from P. capsici DNA whereas CAPFW/CAPRV2 a 595 bp fragment; neither set amplifies DNA from pepper or several fungi pathogenic to pepper. In conventional (single-round) PCR, the limit of detection was 5 pg DNA for both primer sets, whereas in nested PCR the detection limit for both was of 0.5 fg. However, when the dilution series of target DNA were spiked with plant DNA, amplification declined two-fold in both conventional and nested PCR. The CAPFW/CAPRV2 set in conventional PCR was used to detect P. capsici DNA in inoculated plants. Detection occurred as soon as 8h post-inoculation in stem samples from infected but still symptomless plants. The method was also tested to detect fungal DNA in infected soils.     

Markers, old and new, for examining Phytophthora infestans diversity

Late blight, caused by Phytophthora infestans , is an ongoing threat to potato and tomato crop production worldwide and considerable fundamental and applied research is conducted with the long-term aim of improved disease control. Understanding the mechanisms, processes and rates of P. infestans evolution is an important factor in predicting the effectiveness and durability of new management practices. A range of phenotypic and genotypic tests has been applied to achieve this goal, but each has limitations and new methods are sought. Recent progress in P. infestans genomics is providing the raw data for such methods and new high-throughput codominant biomolecular markers are currently being developed that have tremendous potential in the study of P. infestans population biology, epidemiology, ecology, genetics and evolution. This paper reviews some key applications, recommends some changes in approach and reports on the status and potential of new and existing methods for probing P. infestans genetic diversity.     

Effect of foliar-applied potassium chloride on septoria leaf blotch of winter wheat

The effect of foliar-applied potassium chloride on Septoria tritici , the anamorph of Mycosphaerella graminicola , was quantified and possible modes of action investigated during controlled-environment and field experiments. A field experiment in harvest year 1997 showed c . 50% reduction in the area of leaf 2 of winter wheat plants affected by septoria leaf blotch after foliar application of potassium chloride, compared with untreated controls. Similarly, in harvest year 1998 potassium chloride reduced, by about one-third, the area of the flag and penultimate leaf affected by S. tritici . However, a significant yield increase was not observed, although grains m⁻² did show an increase of borderline significance. Applications of epoxiconazole reduced the area of leaf 4 affected by S. tritici compared with untreated controls, whereas applications of chlorothalonil, potassium chloride or polyethylene glycol proved ineffective against disease development. This may suggest that potassium chloride is relatively immobile and possesses contact activity similar to that of chlorothalonil. In 1998, similar reductions in leaf area affected were observed with the inert osmoticum polyethylene glycol in the field, suggesting that the control provided by potassium chloride may be achieved by adverse osmotic effects on the pathogen. Scanning electron microscopy of germinating conidia on wheat plants showed inhibition of conidial germination by both potassium chloride and polyethylene glycol at the same calculated osmotic potential on the leaf surface.     

Molecular detection of Fusarium solani f. sp. glycines in soybean roots and soil

A polymerase chain reaction (PCR)-based method was developed to detect DNA of Fusarium solani f. sp. glycines , the cause of soybean sudden death syndrome. Two pairs of primers, Fsg1/Fsg2 designed from the mitochondrial small subunit ribosomal RNA gene, and FsgEF1/FsgEF2 designed from the translation elongation factor 1-α gene, produced PCR products of 438 and 237 bp, respectively. Primer specificity was tested with DNA from 82 F. solani f. sp. glycines , 55 F. solani non-SDS isolates, 43 isolates of 17 soybean fungal pathogens and the oomycete Phytophthora sojae , and soybean. The sensitivity of primer Fsg1/Fsg2 was 10 pg while that of FsgEF1/FsgEF2 was 1 ng when using F. solani f. sp. glycines total genomic DNA or down to 10³ macroconidia g⁻¹ soil. Nested PCR increased the sensitivity of the PCR assay 1000-fold to 10 fg using primers Fsg1/Fsg2, and 1 pg using primers FsgEF1/FsgEF2. F. solani f. sp. glycines DNA was detected in field-grown soybean roots and soil by PCR using either single pairs of primers or the combination of two pairs of primers. The occurrence of F. solani f. sp. glycines was determined using nested PCR for 47 soil samples collected from soybean fields in 20 counties of Illinois in 1999. F. solani f. sp. glycines was detected in soil samples from all five Illinois Agricultural Statistic Districts including 100, 89, 50, 92 and 50% of the samples from East, Central, North-east and West Districts, respectively.     

Pathogenesis-related gene expression in rice is correlated with developmentally controlled Xa21-mediated resistance against Xanthomonas oryzae pv. oryzae

Disease resistance mediated by the resistance gene Xa21 is developmentally controlled in rice. We examined the relationship between Pathogenesis Related (PR) defense gene expression and Xa21-mediated developmental disease resistance induced by Xanthomonas oryzae pv. oryzae (Xoo). OsPR1a, OsPR1b, and OsPR1c genes were cloned and their induction was analyzed, in addition to the OsPR10a gene, at the juvenile and adult stages in response to a wildtype Xoo strain that induces a resistance response (incompatible interaction) and an isogenic mutant Xoo strain that does not (compatible interaction). We found that the adult stage leaves are more competent to express these OsPR1 genes and that the Xa21 locus is required for the highest levels of induction.     

娄彻氏链霉菌XL-6的抑菌活性及对茄子幼苗的防病促生效应

本文研究了娄彻氏链霉菌Streptomyces rochei XL-6无菌发酵液在自然状态下的抑菌活性及对茄子幼苗的防病促生效果。结果表明,菌株XL-6发酵液的抑菌活性物质可耐70℃高温,在pH 5~8其抑菌活性保持稳定;该抑菌活性物质不受紫外线、蛋白酶K和胰蛋白酶的影响,在4℃条件下贮藏28 d仍可保持良好的抑菌活性。采用不同浓度的发酵液处理茄子幼苗均可提高茄苗对青枯病的抗性,其中,施用40倍发酵液显著降低茄子青枯病发病率和病情指数并显著提高PAL和PPO酶活性,对盆栽茄子的防治效果达64.78%。同时,40倍发酵液显著提高茄子种子发芽率、鲜重、叶绿素含量及根系活力,较病原菌菌体处理和无菌水处理,茄苗干重增幅分别达15.81%和27.95%,对茄苗的促生效果显著。说明适宜浓度的拮抗菌XL-6发酵液兼具防治茄子青枯病和促进植株生长的效果。