Race nonspecific resistance for potato late blight

The late blight fungus (Phytophthora infestans) rots susceptible species of potato plants. None of the major varieties of potato (Solanum tuberosum) grown in the USA is resistant to US-8, the most prevalent genotype of the fungus. Now, Junqi Song, James Bradeen and colleagues have cloned the RB gene from the wild diploid potato species, Solanum bulbocastanum, using a map-based approach in combination with long-range PCR. Transgenic plants containing the gene, normally fully susceptible, displayed broad-spectrum late blight resistance.     

Prosystemin-antimicrobial-peptide fusion reduces tomato late blight lesion expansion

Antimicrobial peptides offer a new method for controlling pathogens, however, many promising peptides are too small for direct production in plants. A protein delivery system was developed based on a proteolytic mechanism used by Solanaceous plants to produce the very small (18 amino acid) signaling peptide systemin from the polypeptide prosystemin. Fusion of the gene encoding the 23 kDa protein prosystemin with the antimicrobial peptide (pep11) sequence, replacing the systemin sequence, allowed for expression in transgenic tomato plants. Six days after inoculation with the late blight pathogen Phytophthora infestans, detached leaflets of transgenic tomato (Rutgers) exhibited a reduction in lesion size of at least 50 percent.     

Biological control of post-harvest late blight of potatoes

Introduction of US-8 genotypes of Phytophthora infestans has coincided with an increase in severity of potato late blight in North America. As alternatives to chemical fungicides, 18 bacterial strains patented as biological control agents (BCA) of both sprouting and Fusarium dry rot were cultivated in three liquid media and screened in wounded potato bioassays for their ability to suppress late blight incited by P. infestans (US-8, mating type A2). Washed or unwashed stationary-phase bacteria were mixed with fungal zoospores to inoculate potato wounds with 5µL containing ～10⁸ bacterial CFU/mL and 2×10⁴ zoospore count/mL. Disease suppressiveness was evaluated after tubers were stored a week at 15°C, 90% relative humidity. One-fifth of the 108 BCA treatments screened, reduced late blight by 25–60%, including among other strains Pseudomonas fluorescens S22:T:04 (showing most consistency), P22:Y:05, S11:P:12 and Enterobacter cloacae S11:T:07. Small-scale pilot testing of these four strains, alone and in combination, was conducted under conditions simulating a commercial application. Suspensions of 4×10⁴ P. infestans sporangia/mL were sprayed at a rate of 1.6 mL followed by 0.8 mL of bacteria treatment at ～5×10⁹ CFU/mL per each of 90 unwounded potatoes. Three replicate boxes per treatment (30 tubers per box) were randomized in storage and maintained 4 weeks at 7.2°C, 95% relative humidity. All BCA treatments significantly reduced disease; and unwashed bacteria outperformed those washed free of culture broth. Disease suppression ranged from 35% up to 86% the first test year and from 35 to 91% the second year. Highest overall performance rankings significantly above the control were achieved by the following strains in culture broth: four-strain mix > P. fluorescens S22:T:04> P. fluorescens S11:P:12. Combined with previous demonstrations of dry rot and sprout suppression, the consistent late blight control by these strains and strain mixtures suggests the commercial feasibility of a single treatment for broad spectrum suppression of post-harvest potato diseases and sprouting.     

Assessment of horizontal resistance to late blight of potatoes

Ten potato clones were assessed for horizontal resistance to late blight in the foliage, in replicated plots for 4 years. Plots were scored on several occasions each year and this paper explores the most reliable ways of using the data to detect differences between clones. Using the genetic component of variance (heritability in the broad sense, h²) as criterion, mean resistance over all scoring dates was found to be the best measure out of several tried. Though good discrimination was achieved within years (mean h² > 90%), there were large clones × years interactions and changes of rank order such that h² fell to about 65% over seasons, and many years of testing would be required to discriminate between similar clones. There was no reason to think that the interactions were caused by the erratic expression of R genes, though one case of vertical resistance was detected. Some results from parallel testing of diverse potato breeding materials are described, and implications for breeding methods and objectives are briefly discussed.     

Genetic diversity among late blight resistant and susceptible potato genotypes

RAPD polymerase chain reaction analysis was used to study the genetic diversity among a wild potato variety Solanum demissum (very resistant to late blight) and six potato cultivars (Hanna, Lady-Olympia, Lady-Rosetta, Spunta, Diamant and Cara) varied in their resistance to Phytophthora infestans. Cluster analysis of six potato genotypes showed that, all tested genotypes were separated into two clusters (1 and 2). Cluster 1, included only the wild potato variety (S. demissum), whereas cluster 2 divided into two groups (G1 and G2). Late blight high resistant cultivars Hanna and Cara were grouped in G1. Group 2 included the moderate resistant cultivar Spunta and the susceptible cultivars Diamant, Lady-Rosetta and Lady-Olympia. The potato cultivars that showed highest genetic similarity to the wild potato variety were the resistant cultivars Hanna and Cara. Lowest genetic similarity was obtained with the susceptible cultivars Lady-Rosetta, Diamant and Lady-Olympia. RAPD primer K17 yielded a band with molecular weight of 936 bp found in all susceptible potato cultivars (Lady-Rosetta, Lady-Olympia and Diamant). On the other hand, band with molecular weight of 765 bp were detected in the wild potato and the resistant cultivars Hanna and Cara. Results of this study suggested that, the RAPD marker technique could be beneficial for revealing the genetic variability of different genotypes of potato varied in their resistibility to late blight.     

The late blight resistance locus Rpi-bib3 from Solanum bulbocastanum belongs to a major late blight R gene cluster on chromosome 4 of potato

Late blight, caused by Phytophthora infestans, is one of the most devastating diseases in cultivated potato. Breeding of new potato cultivars with high levels of resistance to P. infestans is considered the most durable strategy for future potato cultivation. In this study, we report the identification of a new late-blight resistance (R) locus from the wild potato species Solanum bulbocastanum. Using several different approaches, a high-resolution genetic map of the new locus was generated, delimiting Rpi-blb3 to a 0.93 cM interval on chromosome 4. One amplification fragment length polymorphism marker was identified that cosegregated in 1,396 progeny plants of an intraspecific mapping population with Rpi-blb3. For comparative genomics purposes, markers linked to Rpi-blb3 were tested in mapping populations used to map the three other late-blight R loci Rpi-abpt, R2, and R2-like also to chromosome 4. Marker order and allelic conservation suggest that Rpi-blb3, Rpi-abpt, R2, and R2-like reside in the same R gene cluster on chromosome 4 and likely belong to the same gene family. Our findings provide novel insights in the evolution of R gene clusters conferring late-blight resistance in Solanum spp.     

Resistance of tomatoes to late blight (Phytophthora infestans)

Race T₁ of Phytophthora infestans was first isolated in Central Europe. Tomato varieties carrying the dominant gene Ph were easily infected by T₁ but showed a different degree of field resistance. In September 1967 more than 80% of all isolations from the experimental plots at Greifswald were determined as the aggressive race T₁. This race was also isolated in a large screening test over several provinces of the GDR from potato and tomato fields where no plants carrying dominant resistance genes were present in the neighbourhood. The fungus was able to pass from potato leaves or tubers to detached tomato leaves or intact tomato plants (cv. Fanal) without any reduction in sporulation. The field-resistant tomato variety Ru?ový Ker and the highly field-resistant variety Atom were crossed. In F₂ and subsequent generations, young plants were selected following inoculation of 14 mm leaf-discs. Weak sporulations on the leaf-discs were almost invariably correlated with a reduced level of infection of fruit in the field. Data from crosses of Atom with more or less susceptible varieties suggested the presence of incompletely dominant genes. It is concluded that at least two genes in the variety Atom control field resistance and the gene-symbols Phf and Phf-2 are proposed for them.     

The R1 gene for late blight resistance in early and late maturing potato cultivars

The method of polymerase chain reaction was used to amplify a fragment of the LZ-NBS-LRR receptor kinase gene R1; the gene was transferred into potato (Solanum tuberosum) from its wild-growing relative S. demissum and confers the race-specific recognition of the pathogen Phytophthora infestans. To verify this method as a test for the presence of the late blight resistance gene R1, the amplified genome fragment was cloned from the potato hybrid comprising the germplasm of S. demissum. The primary structure of this fragment, which corresponded to the receptor domain of kinase, did not practically differ from the matching sequence in S. demissum. In addition, the method was verified by scoring the set of plant differentials, wherein the presence of R1 was established with race-specific Phytophthora isolates. By screening 70 potato cultivars, we established a significant relationship between the presence of the gene R1 fragment and the phenotypic characters of late blight resistance and late maturity. This evidence supports the idea that R1 was introgressed from short-day S. demissum into potato plants together with some gene(s) conferring late transition to flowering.     

马铃薯致病疫霉研究进展

马铃薯致病疫霉(Phytophthora infestans)属卵菌纲(Oomycetes)霜霉目(Peronosporales)腐霉科(Pythiaceae)疫霉属(Phytophthora),是马铃薯和番茄晚疫病病原菌。由于晚疫病对马铃薯生产的毁灭性和严重性,对致病疫霉的研究一直是关注的重点。本文首先对病害引起的症状、发生特点及流行规律进行阐述,对有性生殖发生的遗传规律和多种交配型共存的大环境下病原菌群体结构变异特点进行归纳总结。随着2009年致病疫霉基因组测序的完成,本文比对了疫霉属目前已完成测序各个种的基因组学特点,介绍了致病疫霉在效应子克隆方面的研究进展及线粒体基因组研究现状,阐述了功能基因组学的两个重要技术:高密度遗传连锁图谱(high density linkage mapping)和全基因组关联分析(genome-wide association study,GWAS),及其在挖掘致病疫霉重要功能基因上的应用。本文有助于了解致病疫霉研究热点及后续突破方向,可为深入解析致病疫霉的功能基因及致病机制提供参考,对开发马铃薯晚疫病菌药物靶标及预测病害的大规模流行趋势也具有重要意...     

Silencing of six susceptibility genes results in potato late blight resistance

Phytophthora infestans, the causal agent of late blight, is a major threat to commercial potato production worldwide. Significant costs are required for crop protection to secure yield. Many dominant genes for resistance (R-genes) to potato late blight have been identified, and some of these R-genes have been applied in potato breeding. However, the P. infestans population rapidly accumulates new virulent strains that render R-genes ineffective. Here we introduce a new class of resistance which is based on the loss-of-function of a susceptibility gene (S-gene) encoding a product exploited by pathogens during infection and colonization. Impaired S-genes primarily result in recessive resistance traits in contrast to recognition-based resistance that is governed by dominant R-genes. In Arabidopsis thaliana, many S-genes have been detected in screens of mutant populations. In the present study, we selected 11 A. thaliana S-genes and silenced orthologous genes in the potato cultivar Desiree, which is highly susceptible to late blight. The silencing of five genes resulted in complete resistance to the P. infestans isolate Pic99189, and the silencing of a sixth S-gene resulted in reduced susceptibility. The application of S-genes to potato breeding for resistance to late blight is further discussed.     

Resistance of new Polish cultivar Awizo F1 to late blight

Two different types of resistance introduced into cultivar Awizo F₁ resulted in good quality high marketable yield with very limited chemical protection against late blight. Cultivars Granit and Lima gave good yield only in condition of full chemical protection. Awizo F₁ did not differ much from Granit and Lima, typical processing varieties in the content of dry matters, sugar and vitamin C. High chemical quality of fruits tested of all cultivar indicated their suitability both for processing and fresh market.     

马铃薯晚疫病生防木霉菌的筛选及鉴定

采用马铃薯活体筛选法从268株木霉菌中筛选获得两株对致病疫霉有较强抑菌活性的木霉菌株R-5和T-15。这两株木霉菌代谢液可抑制病原菌生长及孢子囊萌发。温室防病试验发现,接种两株木霉菌可以减轻晚疫病的发生。田间试验进一步证明,两株木霉菌对晚疫病具有良好的田间防治效果,防效分别达到了72.4%和70.0%。经分子生物学方法鉴定,两株木霉菌分别为拟康氏木霉和棘孢木霉。实验构建的以活体筛选为基础的生防木霉菌筛选方法是一种可行高效的生防木霉菌筛选方式。     

Chitin and chitosan derivatives as elicitors of potato resistance to late blight

Water-soluble low-molecular-weight (3–10 kDa) chitosan obtained by enzymatic degradation of high-molecular-weight chitosan, as well as its deaminated derivatives, can be used as elicitors of resistance to late blight in potato.     

Conditional QTL underlying resistance to late blight in a diploid potato population

A large number of quantitative trait loci (QTL) for resistance to late blight of potato have been reported with a "conventional" method in which each phenotypic trait reflects the cumulative genetic effects for the duration of the disease process. However, as genes controlling response to disease may have unique contributions with specific temporal features, it is important to consider the phenotype as dynamic. Here, using the net genetic effects evidenced at consecutive time points during disease development, we report the first conditional mapping of QTL underlying late blight resistance in potato under five environments in Peru. Six conditional QTL were mapped, one each on chromosome 2, 7 and 12 and three on chromosome 9. These QTL represent distinct contributions to the phenotypic variation at different stages of disease development. By comparison, when conventional mapping was conducted, only one QTL was detected on chromosome 9. This QTL was the same as one of the conditional QTL. The results imply that conditional QTL reflect genes that function at particular stages during the host-pathogen interaction. The dynamics revealed by conditional QTL mapping could contribute to the understanding of the molecular mechanism of late blight resistance and these QTL could be used to target genes for marker development or manipulation to improve resistance.     

Tuber surface microorganisms influence the susceptibility of potato tubers to late blight

Potato tubers (cvs Cara and Bintje) were grown in compost in a glasshouse and immature tubers harvested 57, 68 and 78 days after planting. Two moisture levels were imposed after the first harvest by disconnecting the water supply to one of the treatments and allowing the soil in that treatment to dry naturally. Tubers from wetter compost (59.4% moisture holding capacity) were more resistant to Phytophthora infestans than those from drier compost (14.7% moisture holding capacity) 78 days after planting. The potential causes of this difference were investigated. Aqueous extracts of wet compost did not inhibit the growth of P. infestans. The susceptibility of the internal tuber tissue, from which the periderm had been removed, was different to whole tuber susceptibility. The internal tissue of tubers from wet compost was more susceptible (cv. Cara), or as susceptible (cv. Bintje) as that of tubers from dry compost 78 days after planting. Fungi were isolated from the surface of whole tubers and there were no differences between the populations of potentially antagonistic fungal genera on tubers from wet and dry compost. As the experiment progressed, the number of bacteria per gram fresh weight on tubers grown in wet compost increased, whereas that on tubers from drier compost decreased (cv. Bintje) or remained similar (cv. Cara). There were significantly (P= 0.008) more bacteria on the surface of tubers from wet compost 78 days after planting. When P. infestans was co-cultured in Petri dishes with randomly selected tuber surface bacteria, some isolates (≤ 16.7%) inhibited the growth of the fungus. The percentage of the total bacterial population that was antagonistic to P. infestans was not significantly affected by soil moisture level (P= 0.368). The greater numbers of bacteria, of which a proportion are antagonistic to P. infestans, on the surface of tubers grown in wet compost may account for the greater resistance to tuber blight in that instance.     

Argentinian wild diploid Solanum species as sources of quantitative late blight resistance

Greenhouse and field evaluations of quantitative resistance to late blight in genotypes of the wild Argentine diploid species Solanum chacoense, Solanum commersonnii, Solanum microdontum and Solanum maglia were performed with a complex race containing the virulence factors 1, 3, 4, 5, 7,10, 11 of Phytophora infestans and using the percentage of leaf area affected by late blight estimated at weekly intervals. From these readings the area under the disease progress curve (AUDPC) was calculated. Highly resistant and susceptible genotypes were identified. Resistance and variability were found in S. chacoense, which together with S. commersonnii showed a significantly higher level of resistance compared to the susceptible checks. Field evaluations of an F₁ progeny of 165 individuals, obtained from non-inbred diploid parents segregating for foliage quantitative resistance in S. chacoense, were also done by means of AUDPC. The polygenic nature of the resistance and some indication of the presence of both additive and interaction effects were evident. Received: 30 August 1999 / Accepted: 30 December 1999     

Resistance to late blight in Solanum bulbocastanum is mapped to chromosome 8

Somatic hybrids between potato and Solanum bulbocastanum, a wild diploid (2n=2x=24) Mexican species, are highly resistant to late blight, caused by Phytophthora infestans. Both randomly amplified polymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP) markers that are closely linked to the resistance have been noted by analysis of three different backcross-2 populations derived from two different somatic hybrids. With reference to previously published potato and tomato maps, resistance appears to be on the long arm of chromosome 8 and is flanked by RFLP markers CP53 and CT64. In a population of BC₂ plants derived from a cross between the BC₁ line J10lK6 [(S. tuberosum PI 203900+S. bulbocastanum PI 243510) ×Katahdin)]×Atlantic, late blight resistance cosegregated with RFLP marker CT88 and RAPD marker OPG02–625. Received: 26 November 1999 / Accepted: 22 December 1999