Abstract: Crude aqueous and methanol extracts of 121 plant species from 41 families, collected from the west of Iran, were screened for antifungal activity against mycelial growth of Phytophthora drechsleri. The bioassay used was based on the paper disc diffusion method with four replicates. Extracts of 38 of 121 (about 31%) plant species had inhibitory activity against this phytopathogenic fungus, among which 23 species measurably inhibited the growth of Phytophthora drechsleri. A methanol extract of Xanthium strumarium had the strongest inhibitory activity (17.79±1.35 mm) against P. drechsleri followed by extracts of Glycyrrhiza glabra, Verbascum sp., Hypericum perforatum, Centaura depressa, Centaura sp., Lamium amplexicaule, Haplophyllum perforatum. An investigation of the efficacy of extracts of different plant parts on mycelial inhibition of P. drechsleri, using the paper disc method, indicated that the inflorescence and fruits of cocklebur (Xanthium strumarium) showed significantly more inhibitory effect than the other plant parts against the fungus. Two common species of cocklebur, X. strumarium and X. spinosa, grown around the city of Kermanshah, inhibited mycelial growth of the tested fungus, but extracts of X. strumarium had significantly more inhibitory effect against P. dreschleri than X. spinosa. The results of fractionation of leaf, fruit and inflorescence by thin layer chromatography (TLC) indicated that yellow and blue fractions (under UV) with relative fronts (Rf) equal to 0.93 and 0.98 of leaf, fruit and inflorescence fractions exhibited the highest inhibitory effect against P. dreschleri. These results suggest that cocklebur has potential for control of P. dreschleri and further green house and farm studies are recommended.
Abstract: The bacterial wilt in tomato caused by Ralstonia solanacearum infection is common and widespread, especially in hot and humid environments. Combating this disease is difficult due to unstable host resistance and the variation and diversity of the bacterial strains. Thus, the molecular mechanisms underlying tomato resistance against Ralstonia solanacearum remain unknown. Here, we isolated a homolog of tomato Solanum lycopersicum Jasmonate-Resistant 1 (SlJAR1), named SIJAR1-like 1 (SlJRL1), and generated transgenic tomato lines harboring an inducible promoter-driven SlJRL1 construct. SlJRL1 shares 99% amino acid identity with SlJAR1. Intriguingly, SlJRL1 showed preferential expression in aerial parts and SlJAR1 in roots. DNA gel blot analysis revealed multiple copies of SlJRL1 in the tomato genome. Transgenic tomato containing a single copy of the transgene SlJRL1 exhibited high levels of SlJRL1 expression two days after dexamethasone (DEX) induction. Moreover, DEX-induced SlJRL1 expression could delay the symptoms of tomato bacterial wilt, and efficiently reduce the amount of Ralstonia in stems. The phytohormone jasmonic acid may play a role in this resistance response. This study of inducible SlJRL1 expression in transgenic tomato contributes to the molecular understanding of tomato resistance against bacterial wilt.
Abstract: Late blight disease caused by Phytophthora infestans (Mont.) de Bary is one of the major diseases of economic importance to potato production in Uganda causing yield losses of 40 to 60%. Use of host resistance is most economical and environmentally feasible solution to control this disease. This study was carried out to identify potato clones with high levels of field resistance to late blight pathogen (P. infestans) and high yields so that well adapted and tolerant clones could be developed and released as varieties. Sixteen potato clones of population B3C2 were accessed from International Potato Centre (CIP) in Lima Peru and evaluated for field resistance to late blight pathogen population in Uganda. Six potato clones (396027.205, 396034.103, 396026.103, 393280.82, 396038.107, and 395015.6) out of 16 clones were identified to posses high field resistance to late blight and high yields. The mean late blight disease severity measured as relative area under disease progress curve (rAUDPC) ranged between 15 to 38% for no sprayed while for sprayed it ranged from 5 to 22%. Yield performance under no spray treatment ranged from 7 to 20 T/ha while under spray treatment, yield ranged from 13 to 25 T/ha. Four of these clones (396026.103, 396034.103, 393280.82, and 396038.107) have been recommended for release as potential varieties for farmers.
Abstract: A replicated field trial containing Rhododendron cultivars, species, and experimental hybrids was repeatedly flooded during one growing season to test for resistance to Phytophthora cinnamomi under stress conditions. At the end of the season root rot disease scores were assigned based on visual assessment of root, crown, and shoot necrosis using a numerical rating scale of 1 (healthy fine roots) to 5 (dead plant). Under flooding conditions, the average disease score of three resistant cultivars (controls used as benchmarks) was 4.1, which was a 90 percent increase above their previously determined average of 2.2 under non-flooded conditions. In contrast, disease scores of the resistant species R. hyperythrum were 35 percent higher under flooded (2.7) than non-flooded (2.0) treatments. Eight F1 hybrids derived from R. hyperythrum had an average disease score of 3.3 and were significantly less diseased than the resistant benchmark cultivars under flooded field conditions. Loss of root rot resistance in flooded soils could result from conditions that favor pathogen development and infection and from physiological changes in host plants that predispose them to disease. Under flooding conditions, R. hyperythrum appears to be less predisposed to root rot than resistant genotypes with different genetic backgrounds. While the basis for this difference in stress response is not currently known, it appears to be heritable in the F1 generation and represents a valuable trait for root rot resistance breeding.
Abstract: Phytophthora cinnamomi is a soil-borne plant pathogen that has caused widespread damage to vulnerable native ecosystems and agriculture systems across the world and shows no sign of abating. Management of the pathogen in the natural environment is difficult and the options are limited. In order to discover more about how resistant plants are able to defend themselves against this generalist pathogen, a microarray study of plant gene expression following root inoculation with P. cinnamomi was undertaken. Zea mays was used as a resistant model plant, and microarray analysis was conducted using the Affymetrix GeneChip Maize Genome Array on root samples collected at 6- and 24-h post-inoculation. Over 300 genes were differentially expressed in inoculated roots compared with controls across the two time points. Following Gene Ontology enrichment analysis and REVIGO visualisation of the up-regulated genes, many were implicated in plant defence responses to biotic stress. Genes that were up-regulated included those involved in phytoalexin biosynthesis and jasmonic acid/ethylene biosynthesis and other defence-related genes including those encoding glutathione S-transferases and serine-protease inhibitors. Of particular interest was the identification of the two most highly up-regulated genes, terpene synthase11 (Tps11) and kaurene synthase2 (An2), which are both involved in production of terpenoid phytoalexins. This is the first study that has investigated gene expression at a global level in roots in response to P. cinnamomi in a model plant species and provides valuable insights into the mechanisms involved in defence.
Abstract: Phytophthora root rot (PRR), caused by Phytophthora sojae Kaufmann & Gerdemann, is one of the most destructive diseases of soybean [Glycine max (L.) Merr.]. Deployment of resistance genes is the most economical and effective way of controlling the disease. The soybean cultivar 'Yudou 29' is resistant to many P. sojae isolates in China. The genetic basis of the resistance in 'Yudou 29' was elucidated through an inheritance study and molecular mapping. In response to 25 P. sojae isolates, 'Yudou 29' displayed a new resistance reaction pattern distinct from those of differentials carrying known Rps genes. A population of 214 F2:3 families from a cross between 'Jikedou 2' (PRR susceptible) and 'Yudou 29' was used for Rps gene mapping. The segregation fit a ratio of 1:2:1 for resistance:segregation:susceptibility within this population, indicating that resistance in 'Yudou 29' is controlled by a single dominant gene. This gene was temporarily named RpsYD29 and mapped on soybean chromosome 03 (molecular linkage group N; MLG N) flanked by SSR markers SattWM82-50 and Satt1k4b at a genetic distance of 0.5 and 0.2 cM, respectively. Two nucleotide binding site-leucine rich repeat (NBS-LRR) type genes were detected in the 204.8 kb region between SattWM82-50 and Satt1k4b. These two genes showed high similarity to Rps1k in amino acid sequence and could be candidate genes for PRR resistance. Based on the phenotype reactions and the physical position on soybean chromosome 03, RpsYD29 might be a novel allele at, or a novel gene tightly linked to, the Rps1 locus.
Abstract: Random amplified polymorphism DNA (RAPD) and bulk segregant analysis (BSA) approaches were used to characterize the molecular marker linked to the Phytophthora infestans resistance gene Ph-3 in tomato. A total of 800 RAPD primers were screened. One RAPD marker UBC#602 was identified to be tightly linked to the Ph-3 gene. The marker was successfully converted into a co-dominant sequence characterized amplified region (SCAR) marker. The SCAR marker SCU602 was used to analyze 96 F2 progenies and fitted the expected 1:2:1 Mendelian segregation ratio. Forty one tomato inbred lines were screened using the SCAR marker in comparison with a reference marker linked to the Ph-3 gene and both markers gave the same results. SCU602 was further validated for association to resistance and its potential in MAS in 72 tomato lines and cultivars. The marker identified three genotypes harbouring the resistance allele. This SCAR marker can be used in breeding programs for the selection of the Ph-3 gene for Phytophthora infestans resistance.
Abstract: A novel disease resistance inhibitor gene (inhibitor of P. capsici resistance [Ipcr]), found in the chile pepper (Capsicum annuum) variety 'New Mexico Capsicum Accession 10399' (NMCA10399), inhibits resistance to Phytophthora capsici but not to other species of Phytophthora. When a highly P. capsici-resistant variety was hybridized with NMCA10399, the resultant F1 populations, when screened, were completely susceptible to P. capsici for root rot and foliar blight disease syndromes, despite the dominance inheritance of P. capsici resistance in chile pepper. The F2 population displayed a 3:13 resistant-to-susceptible (R:S) ratio. The testcross population displayed a 1:1 R:S ratio, and a backcross population to NMCA10399 displayed complete susceptibility. These results demonstrate the presence of a single dominant inhibitor gene affecting P. capsici resistance in chile pepper. Moreover, when lines carrying the Ipcr gene were challenged against six Phytophthora spp., the nonhost resistance was not overcome. Therefore, the Ipcr gene is interfering with host-specific resistance but not the pathogen- or microbe-associated molecular pattern nonhost responses.
Abstract: Phytophthora root rot (PRR) is a disease of chickpea (Cicer arietinum L.) caused by the Phytophthora megasperma Drechs. f. sp. medicaginis (Pmm). It is the most serious disease for this crop in Australia because there is no strong resistance exists in current cultivars. Therefore, it is vitally important to identify the new source of resistance for the improvement of chickpea cultivars. A range of chickpea genotypes were evaluated in this study to identify sources of resistance to this disease. Three Australian chickpea varieties (Jimbour, Flipper, and Yorker) were used to assess the pathogenicity of 4 Pmm isolates (4019, 4021, 4027, and 4046). The most aggressive pathogenic isolate 4027 and a mixture of two other isolates were selected to assess the resistance of 16 international and Australian chickpea genotypes to PRR. The general score (GS) (0.5 × plant infection rate+0.5 × plant death rate) was used in this study to indicate the severity of the disease. According to the GS value derived from the most aggressive isolate 4027, 4 genotypes (FLIP97-114C, ICCV 05111, ICCV 98818, and ICCV 96852) were considered resistant with a GS of 3.9-4.4, 3 genotypes (Bumper, ICCV 98801, and Yorker) were classified as moderately to resistant with a GS of 4.7-4.9, and the rest of them were susceptible with GS scores above 5.8. The large variation in pathogenicity observed for these isolates suggested that the 4 Pmm isolates represented different pathogen strains. Significant differences in plant infection rate, plant death rate, and disease development were observed among the chickpea genotypes. These findings indicated that the basis of resistance differed among the chickpea genotypes evaluated. The PRR resistance in chickpea is a multi-gene controlled trait. These resistant genotypes provided different sources of resistance and could be combined in breeding program to produce durable and high level of resistant cultivars.
Abstract: For the program towards the development of resistant cacao (Theobroma cacao L.) to the black pod rots disease caused by P. palmivora, the diallel crossing analysis is expected can be used to provide genetic parameters on quantitative traits. The objective of the present study was to determine genetic parameters of cacao resistance to the disease caused by P. palmivora, using half diallel crosses. The crosses used five cocoa clones as parental clones (ICCRI 3, TSH 858, DR 1, ICS 13, and Sca 6) having resistance levels from susceptible to resistant. The experiment was arranged in a Randomized Block Design with three replications. The treatments were 10 hybrids (F1) and 5 parental clones. Observation was conducted 3 days after inoculation to the spot area caused by P. palmivora infection. The results showed that there was no interaction between genes in determining the resistance to black pod rots disease caused by P. palmivora. The resistance was more affected by the additive gene action. Characters of resistant to the disease were controlled by the recessive genes. The results in the present study also showed that the dominant genes were more in the parents. It is expected that the opportunity to produce cocoa hybrid owned by ICCRI 3 and Sca 6. Furthermore, the estimated value of broad sense heritability (h2BS) and narrow sense heritability (h2NS) was high for the spot area, while, based on the disease intensity it was moderate to high.
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