Abstract: The central highlands of Mexico are considered to be a center of genetic
diversity for both the potato late blight pathogen and for tuber-bearing Solanum spp. Recent work conducted in Mexico and South America sheds new light on the biology and evolution of Phytophthora infestans and other related Phytophthora pathogens. It now appears that Mexican Solanum species, which coevolved with P. infestans and were previously known for providing a source of R-genes, also provide a source of quantitative, rate-reducing resistance that is highly effective, stable, and durable. It is now apparent that Mexico is the center of origin not only of the potato late blight pathogen P. infestans, but also of several related Phytophthora species including P. mirabilis, P. ipomoeae, and possibly P. phaseoli. We close with the hypothesis that these Phytophthora species evolved sympatrically from one ancestral host through adaptive radiation onto their respective four host families.
Abstract: Eighty-five isolates of Phytophthora infestans from 33 tomato and 8 potato fields in North Carolina, South Carolina, and Tennessee were collected from 1993 to 1995 and tested for mating type, sensitivity to metalaxyl, and allozyme genotype at glucose-6-phosphate isomerase (Gpi) and peptidase (Pep) loci. One isolate collected from potato in eastern North Carolina in 1994 was the A1 mating type, whereas all other isolates from potato and tomato were the A2 mating type. Six isolates were sensitive to metalaxyl (<40% growth at 1.0 μg of metalaxyl per ml), nineteen isolates were intermediate in sensitivity to metalaxyl (>40% growth at 1.0 μg of metalaxyl per ml and <40% growth at 100 μg of metalaxyl per ml), and sixty isolates were resistant to metalaxyl (<40% growth at 1.0 and 100 μg of metalaxyl per ml). Four different allozyme genotypes at the Gpi and Pep loci were identified. The single A1 isolate found on potato in eastern North Carolina had the dilocus allozyme genotype Gpi 86/100, Pep 92/100 and was identified as the US-1 genotype. Fifty-five isolates had the dilocus allozyme genotype Gpi 100/111, Pep 100/100 and were classified as the US-7 genotype, whereas twenty-four isolates were Gpi 100/111/122, Pep 100/100 and were classified as the US-8 genotype. Two isolates that were sensitive to meta-laxyl and seventeen isolates that were intermediate in sensitivity to metalaxyl were found among the US-7 and US-8 genotypes. In addition, five isolates had the allozyme genotype Gpi 100/100, Pep 92/100 (similar to the previously reported US-6 genotype), but they were the A2 mating type and either sensitive or intermediate in response to metalaxyl. These isolates composed a new genotype not previously reported in the United States and were designated as US-18. The US-7 genotype was more frequent on tomato in western North Carolina and the US-8 genotype was present on potato in eastern North Carolina, indicating that different inoculum sources are responsible for epidemics on the two crops in different regions of the state.
Abstract: Secondary plant compounds are important signals in several symbiotic and
pathogenic plant-microbe interactions. The present review is limited to two groups of secondary plant compounds, flavonoids and strigolactones, which have been reported in root exudates. Data on flavonoids as signaling compounds are available from several symbiotic and pathogenic plant-microbe interactions, whereas only recently initial data on the role of strigolactones as plant signals in the arbuscular mycorrhizal symbiosis have been reported. Data from other plant-microbe interactions and strigolactones are not available yet. In the present article we are focusing on flavonoids in plant-fungal interactions such as the arbuscular mycorrhizal (AM) association and the signaling between different Fusarium species and plants. Moreover the role of strigolactones in the AM association is discussed and new data on the effect of strigolactones on fungi, apart from arbuscular mycorrhizal fungi (AMF), are provided.
Abstract: To test the hypothesis that isolates of Phytophthora infestans attacking wild Solanaceae exhibit specialization for particular host species, 115 isolates of P. infestans were collected from cultivated potatoes, nontuber-bearing Solanum spp. of the Basarthrum section and wild tomatoes from five departments in the northern and central highlands of Peru, and
characterized using several neutral markers. All isolates belonged to one of four clonal lineages described previously in Peru: EC-1, US-1, PE-3 and PE-7. There was a strong association of three lineages with host species: PE-3 was only isolated from cultivated potato, while PE-7 and US-1 were only isolated from nontuber-bearing Solanum spp. (Basarthrum section and wild tomatoes). EC-1 was isolated from all host groups sampled. A subset (n= 74) of the isolates was evaluated for metalaxyl resistance. High levels of resistance were found almost exclusively in EC-1 and PE-3, while US-1 and PE-7 isolates were generally sensitive. In a detached-leaf assay for lesion diameter using five EC-1 isolates from S. caripense and seven EC-1 isolates from cultivated potato, there was a significant interaction between isolate origin and inoculated host, caused by higher aggressiveness of EC-1 from cultivated potato on its host of origin. In a comparison of EC-1 (seven isolates
from cultivated potato) and US-1 (three isolates from S. caripense), each pathogen lineage was more aggressive on its original host species, causing a highly significant interaction between isolate origin and inoculated host. Wild tomatoes and nontuber-bearing Solanum spp. harbour several pathogen lineages in Peru and could serve as reservoirs of inoculum that might contribute to epidemics on potato or tomato. Potential risks associated with the use of wild Solanum hosts as sources of resistance to P. infestans are discussed.
Abstract:Phytophthora palmivora causes bud rot, fruit and immature nut fall in India, and causes significant coconut yield losses. Genetic variability was estimated in 70 isolates representing seven populations from the Kerala and Karnataka regions using random amplified polymorphic DNA (RAPD) markers. RAPDs generated a total of 163 markers with 15 decamer primers, of which 92% were found to be polymorphic. The number of polymorphic loci within a single population varied from 45 to 92 with estimated hetrozygosities ranging from 0.08 to 0.23. The majority of the genetic diversity was distributed within populations (68.25%) and only 31.75% were among populations. Genetic relationships estimated by the unweighted pair-group method with arithmatic averaging (UPGMA) revealed clear separation of Kerala and Karnataka populations.
Abstract: Mefenoxam is one of the most commonly used fungicides for managing diseases caused by Phytophthora spp. on ornamentals. The objectives of this study were to determine whether Phytophthora nicotianae, a destructive pathogen
of numerous herbaceous annual and perennial plant species in nurseries, has developed resistance to mefenoxam, and to evaluate the fitness of mefenoxam-resistant isolates. Ninety-five isolates of P. nicotianae were screened for sensitivity to mefenoxam on 20% clarified V8 agar at 100 a.i. ug mL–1. Twenty-five isolates were highly resistant to this compound
with EC50 values ranging from 235·2 to 466·3 ug mL–1 and four were intermediately resistant with EC50 values ranging from 1·6 to 2·9 ug mL–1. Sixty-six isolates were sensitive with EC50 values less than 0·04 ug mL–1. Nine resistant and seven sensitive isolates were tested for mefenoxam sensitivity on Pelargonium × hortorum cv. White Orbit. Mefenoxam provided good protection of pelargonium seedlings from colonization by sensitive isolates, but not by any resistant isolates. Four resistant and four sensitive isolates were compared for fitness components and their relative competitive ability on Lupinus Russell Hybrids in the absence of mefenoxam. Resistant isolates outcompeted sensitive ones within 3 to 6 sporulation cycles on lupin seedlings, regardless of their initial proportions in mixed zoospore inoculum. Resistant
isolates exhibited greater infection rate and higher sporulation ability than sensitive ones when they were applied separately onto lupins. These results suggest that fungicide resistance may pose a serious challenge to the continued
effectiveness of mefenoxam as a control option for nursery growers.
Abstract: Analysis of 12 polymorphic simple sequence repeats identified in the genome sequence of Phytophthora ramorum, causal agent of sudden oak death, revealed genotypic diversity to be significantly higher in nurseries (91% of total) than in forests (18% of total). Our analysis identified only two closely related genotypes in US forests, while the genetic structure of populations from European nurseries was of intermediate complexity, including multiple,
closely related genotypes. Multilocus analysis determined populations in US forests reproduce clonally and are likely descendants of a single introduced individual. The 151 isolates analysed clustered in three clades. US forest and European nursery isolates clustered into two distinct clades, while one isolate from a US nursery belonged to a third novel clade. The combined microsatellite, sequencing and morphological analyses suggest the three clades represent distinct evolutionary lineages. All three clades were identified in some US nurseries, emphasizing the role of commercial plant trade in the movement of this pathogen.
Abstract: A total of 743 single-lesion isolates of Phytophthora infestans were collected in summer 2003 from Denmark, Finland, Norway and Sweden. Most of the isolates were tested for mating type, and subsets were tested for sensitivity to fungicides and virulence (host specific pathogenicity). Approximately 60% of the isolates were A1 mating type in each country. Both mating types were present in 40% of the fields where more than one isolate was tested, indicating strong potential for sexual reproduction. The proportion of metalaxyl-resistant isolates dropped to under 15% from the 60% observed in the early 1990s in Norway and Finland, possibly due to lower selection pressure because of decreased use of metalaxyl. Propamocarb-HCl sensitivity remained unchanged in the Nordic countries compared to the situation in 1997–2000 in Finland. Four isolates collected from Finland and Sweden were able to sporulate in the presence of this fungicide at a concentration of 1000 mg L–1. In Norway and Finland the frequencies of virulence factors and pathotypes remained nearly unchanged since the 1990s, but the mean number of virulence factors per isolate increased from 5·6 to 6·3. In Denmark and Sweden virulence factors 2 and especially 6 were more common than in Norway and Finland. In addition, in the Swedish population the frequencies of pathotypes were quite even while in other countries pathotype 1,3,4,7,10,11 was most prevalent.
Abstract: Incidence of Phytophthora blight in bell pepper fields that were sprayed for the first time with Ridomil Gold (mefenoxam) according to labeled recommendations was higher in North Carolina in 1997 than in previous years. Mefenoxam is the more active enantiomer contained in the racemic fungicide metalaxyl. A total of 150 isolates were obtained from 17 fields at eight
grower locations. Among isolates from all locations, 30% were classified as sensitive, 10% as intermediate, and 59% were resistant to mefenoxam. Mefenoxam-resistant isolates were found in 82% of the fields sampled (14 of 17 fields). The proportion of resistant isolates in individual fields ranged from 28 to 100%. The mean effective concentration (EC50) values for mefenoxam-sensitive isolates was 0.568 µg ml–1 (ranging from 0.12 to 1.1 µg ml–1), whereas the mean EC50 value for mefenoxam-resistant isolates was 366.5 µg ml–1 (ranging from 3 to 863 µg ml–1). The mean EC50 value for metalaxyl-sensitive isolates was 0.27 µg ml–1 (ranging from 0.00002 to 1.3 µg ml–1) and for metalaxyl-resistant isolates was 470.34 µg ml–1 (ranging from 10 to 966 µg ml–1). The greatest proportion of resistant isolates came from fields where mefenoxam was used alone rather than in combination with other fungicides. Both mating types were found among resistant isolates, suggesting that these isolates may persist in soil in subsequent years. Field isolates of Phytophthora capsici resistant to mefenoxam on pepper have not been reported previously and now pose new challenges for management of this important disease.
Abstract: The sensitivity of 127 Phytophthora infestans isolates to flumorph was determined in 2003 and 2004. The isolates originated from two geographical regions and showed similar levels of sensitivity in both years. Baseline sensitivities were distributed as a unimodal curve with EC50 values for growth of mycelia ranging from 0·1016 to 0·3228 µg mL-1, with
a mean of 0·1813 (± 0·0405) µg mL-1. There was no cross-resistance between flumorph and metalaxyl. Laboratory studies were conducted to evaluate the risk of P. infestans developing resistance to flumorph. Mutants resistant to metalaxyl or flumorph were obtained by treating mycelium of wild-type isolates with ultraviolet radiation. Metalaxyl-resistant mutants were obtained with a high frequency and exhibited resistance factor values (EC50 resistant/EC50 sensitive phenotypes) of more than 100, while flumorph-resistant mutants were obtained at much lower frequencies and had very small resistance factors (1·5–3·2). There was cross-resistance between flumorph and dimethomorph, but not with azoxystrobin or cymoxanil. Most flumorph-resistant mutants showed decreases in hyphal growth in vitro and in sporulation both in vitro and on detached leaf tissues. These studies suggested that the risk of resistance developing was much lower for flumorph than metalaxyl. However, as P. infestans is a high-risk pathogen, appropriate precautions against resistance development should be taken.
Abstract