The population structure of Phytophthora infestans from the Toluca Valley of central Mexico suggests genetic differentiation between populations from cultivated potato and wild Solanum spp

Flier W G, Grünwald N J, Kroon L P N, Sturbaum A K, van den Bosch T B M , Garay-Serrano E, Lozoya- Saldaña H, Fry W E and Turkensteen L J. 2003. Phytopathology 93:382–390.

The population structure of Phytophthora infestans in the Toluca Valley of central Mexico was assessed using 170 isolates collected from cultivated potatoes and the native wild Solanum spp., S. demissum and S. ×edinense. All isolates were analyzed for mitochondrial DNA (mtDNA) haplotype and amplified fragment length polymorphism (AFLP) multilocus fingerprint genotype. Isolate samples were monomorphic for mtDNA haplotype because all isolates tested were of the 1a haplotype. A total of 158 multilocus AFLP genotypes were identified among the 170 P. infestans isolates included in this study. P. infestans populations sampled in the Toluca Valley in 1997 were highly variable and almost every single isolate represented a unique genotype based on the analysis of 165 AFLP marker loci. Populations of P. infestans collected from the commercial potato-growing region in the valley, the subsistence potato production area along the slopes of the Nevado de Toluca, and the native Solanum spp. on the forested slopes of the volcano showed a high degree of genetic diversity. The number of polymorphic loci varied from 20.0 to 62.4% for isolates collected from the field station and wild Solanum spp. On average, 81.8% (135) of the AFLP loci were polymorphic. Heterozygosity varied between 7.7 and 19.4%. Significant differentiation was found at the population level between strains originating from cultivated potatoes and wild Solanum spp. (P = 0.001 to 0.022). Private alleles were observed in individual isolates collected from all three populations, with numbers of unique dominant alleles varying from 9 to 16 for isolates collected from commercial potato crops and native Solanum spp., respectively. Four AFLP markers were exclusively found present in isolates collected from S. demissum. Indirect estimation of gene flow between populations indicated restricted gene flow between both P. infestans populations from cultivated potatoes and wild Solanum hosts. There was no evidence found for the presence of substructuring at the subpopulation (field) level. We hypothesize that population differentiation and genetic isolation of P. infestans in the Toluca Valley is driven by host-specific factors (i.e., R-genes) widely distributed in wild Solanum spp. and random genetic drift.

Corresponding author: W G Flier

email: Wilbert.Flier(at)wur.nl

Variations in the Phytophthora infestans population in Nepal as revealed by nuclear and mitochondrial DNA polymorphisms.

Ghimire S R, Hyde K D, Hodgkiss I J, Shaw D S and Liew E C Y. 2003. Phytopathology 93:236–243.

Phytophthora infestans isolates collected from potato and tomato crops from various parts of Nepal during the 1999 and 2000 crop seasons were characterized for nuclear and mitochondrial DNA polymorphisms using restriction fragment length polymorphism markers. The nuclear DNA probe RG57 detected 11 multilocus genotypes among 280 isolates. Three genotypes were detected 21 times or more, constituting 94% of the total population, whereas frequencies of other genotypes ranged from 0.004 to 0.014. The overall genotypic diversity as estimated by the Gleason index was 1.78. Most of the overall diversity was present at the highest level (i.e., interregional, 46%), indicating limited gene flow among regions. Cluster analysis of multilocus genotypes derived from RG57 and mating type data for Nepalese isolates and representative isolates worldwide showed Nepalese isolates grouping into four clusters. Characterization of 67 isolates for mitochondrial DNA polymorphisms revealed the presence of two mt-haplotypes, 1a and 1b with the proportions of 0.88 and 0.12, respectively. Polymorphisms in nuclear and mitochondrial DNA revealed a moderate level of diversity in this population. Genotype NP3 had an identical RG57 fingerprint to US-1 and had mt-haplotype 1b, confirming the presence of an old population in Nepal. Most of the genotypes had a different RG57 fingerprint than that of US-1 and mt-haplotype 1a, the common characteristics of new populations. The presence of a new population at high proportions in Nepal was consistent with the global trend of mt-haplotype distribution, and suggests the displacement of old populations. This study indicates at least three possible introductions of P. infestans to Nepal.

Corresponding author: S. Ghimire

email: sghimire(at)graduate.hku.hk

Phytophthora infestans populations from tomato and potato in North Carolina differ in genetic diversity and structure.

Wangsomboondee T, Trout Groves C, Shoemaker P B, Cubeta M A and Ristaino J B. 2002. Phytopathology 92:1189–1195.

Phytophthora infestans causes a destructive disease on tomato and potato. In North Carolina (NC) potatoes are mostly grown in the east, whereas tomatoes are grown in the mountainous areas in the western part of the state. Five genotypes of P. infestans were identified from 93 and 157 isolates collected from tomato and potato over a 5 year period between 1993 and 1998. All isolates collected from potato in eastern NC were the US-8 genotype, whereas only a single isolate was the US-1 genotype. Tuber blight was found on immature daughter tubers in a single field in 1997, however infection on mature tubers was not observed. Within potato fields, a range of sensitivity to metalaxyl was observed among isolates but all were either intermediate or highly resistant to the fungicide. In contrast, isolates from tomatoes included previously reported US-7 and US-8 genotypes and two new genotypes called US-18 and US-19 (A2 mating type, allozyme genotype Gpi 100/100 and Pep 92/100). These genotypes had unique restriction fragment length polymorphism banding patterns, were sensitive to metalaxyl, and have not been reported elsewhere. All genotypes, with the exception of the US-1, were the Ia mitochondrial haplotype. Thus, isolates of P. infestans from tomato were more genetically diverse over time in NC than those from potato and include two new genotypes that are sensitive to metalaxyl.

Corresponding author: J B Ristaino

email: Jean_Ristaino(at)ncsu.edu

Half-sib progeny evaluation and selection of potatoes resistant to the US8 genotype of Phytophthora infestans from crosses between resistant and susceptible parents.

Bisognin D A, Douches D S, Jastrzebski K and Kirk W W. 2002. Euphytica 125:129–138. © Kluwer Academic Publishers 2002

The objectives of this study were to evaluate the use of potato (Solanum tuberosum L.) late blight (Phytophthora infestans (Mont.) de Bary) resistant parents in cultivar development and identify superior clones possessing moderate to high late blight resistance combined with acceptable maturity and tuber quality. Ninety-five crosses were made between eight unadapted parents with reported late blight resistance (B0718-3, Bertita, Bzura, Greta, Libertas, Stobrawa, Tollocan and Zarevo) and susceptible parents (cultivars or advanced breeding clones) adapted to North American growing conditions. A total of 408 field selected clones were assessed for late blight resistance in the greenhouse and in the field using a mixture of US-8 P. infestans isolates (A2 mating type, metalaxyl resistant) that overcame all known R-genes except R8 and R9. Clones with = 10% infected foliar area in the greenhouse test or = 0.30 RAUDPC (relative area under the disease progress curve) value in the field in 1998 were re-tested in 1999. A total of 118 (29% of 408) putati blight resistant clones were selected. The eight late blight resistant parents differed in both the ability to transmit late blight resistance and in the level of resistance transmitted to the progeny. The Tollocan and B0718-3 families (half-sib progeny) had the greatest degree of resistance and frequency of resistant clones. Scott-Knott cluster analysis ranked 79 clones (67% of 118) in the high and moderate late blight resistant groups. Among these 79 clones, 19 clones had vine maturity equal to or earlier than mid-season combined with acceptable tuber quality. Further selection in 2000 resulted in eight advanced selected clones (six from Tollocan and two from B0718-3 families) with the same level of resistance as the parent combined with vine maturity and tuber quality equivalent to Atlantic, a standard cultivar for chip processing in North America. The results indicate that this breeding approach can be used to select parents for late blight resistance breeding and to identify superior clones with high levels of late blight resistance and marketable vine maturity and tuber quality.

Corresponding author: D S Douches

email: douchesd(at)msu.edu

Potato late blight management in the Toluca Valley: Field validation of SimCast modified for cultivars with high field resistance

Grünwald N J, Romero Montes G, Lozoya Saldaña H, Rubio Covarrubias O A and Fry W E. 2002. Plant Disease 86:1163–1168.

Management of potato late blight in the highland tropics is very costly and remains difficult. Reducing the impact of late blight through the use of resistant cultivars in combination with a fungicide forecasting system could lower the number of costly fungicide applications. Previously, we evaluated the fungicide advisory SimCast for use with Mexican cultivars of high levels of field resistance for use in the central highlands of Mexico and found that it predicted too many fungicide applications for cultivars of moderate to high levels of resistance. We adapted SimCast for cultivars with higher levels of resistance, and field validations conducted in 1999 and 2000 show that SimCast resulted in good disease control on cultivars ranging from susceptible to highly resistant. The number of fungicide applications forecast for cultivars with moderate to high levels of resistance was reduced. Precipitation was the environmental variable responsible for most of the forecasts made. Our work demonstrates that a predictive system can be portable from the temperate climate to a tropical highland climate. A user-friendly decision support system consisting of just a rain-gauge and the exclusive use of SimCast’s fungicide units could be a valuable and affordable tool in managing potato late blight in the highland tropics.

Corresponding author: N J Grünwald

email: ngrunwald(at)pars.ars.usda.gov

Containment of existing potato late blight (Phytophthora infestans) foliar epidemics with fungicides.

Stein J M and Kirk W W. 2002. Crop Protection 21: 575–582. © Elsevier Science Ltd. 2002.

Critical timing of application of foliar fungicides to limit further infection of potato foliage by Phytophthora infestans and the critical threshold of foliar infection level at which individual or combinations of specific fungicides limited further spread of infection with P. infestans were determined. In most seasons, key application timings at which the foliar late blight epidemic was contained to <50% of that in untreated control plots, were programs initiated 72 h before and 72 h after inoculation with P. infestans, and when foliar area with late blight lesions was estimated to be 1% or less. Delaying initiation of application of any fungicide until 5% and 10% estimated average foliar area with late blight lesions resulted in late blight development similar to the untreated control. Most fungicides and fungicide mixtures examined resulted in 30–50% reduction of late blight development in comparison to the untreated control as long as they were applied not later than 1% estimated average foliar area with late blight lesions.

Corresponding author: W W Kirk

email: kirkw(at)msu.edu

Influence of timing of harvest in relation to haulm killing and planting date on potato tuber rot caused by Phytophthora infestans.

Miller J S, Cummings T F, Mikitzel L J and Johnson D A. 2002. Plant Disease 86:264–268.

Killing potato haulms 2 to 3 weeks prior to harvesting tubers is recommended for the management of potato late blight to eliminate the foliage as a source of tuber blight inoculum. Some potato growers in the Columbia Basin of Washington and Oregon, however, harvest tubers within days of killing potato haulms or harvest tubers without killing potato haulms. The susceptibility of potato tubers harvested from green haulms to late blight is unknown. From 1996 to 1998, Russet Burbank and Ranger Russet seed tuber pieces were planted on three different dates and then harvested simultaneously to obtain daughter tubers at different levels of maturity as determined by physiological degree day (P-day) accumulation. Tubers from two trials in 1996 were harvested within 1 to 4 days of haulm killing, tubers from a single trial in 1997 were harvested 16 days after haulm killing, and tubers from two trials in 1998 were harvested 1 and 21 days after haulm killing. Tubers from each plot were divided into two samples; one sample was inoculated the day after harvest and the other sample was inoculated 6 months after harvest on the buds and periderm tissue with a US-8 isolate of Phytophthora infestans. Severity of tuber blight on tuber surfaces and internal tuber tissue was highly correlated. When tubers were harvested within 1 to 4 days after haulm kill, the youngest tubers had the highest percentage of skinning and the lowest percentage of tuber blight. These tests, however, were not designed to test the hypothesis that increased tuber skinning resulted in increased tuber rot. Severity of tuber blight was greater for Ranger Russet tubers than for Russet Burbank tubers. The correlation between the severity of tuber blight at harvest and after 6 months in storage was significant. In some cases, differences in susceptibility to tuber blight due to planting date were observed after 6 months in storage. When tubers were harvested within 1 to 4 days after vine killing, tubers from plants that had been growing longer were more susceptible to late blight tuber rot than tubers from plants that had been growing for a shorter period of time.

Corresponding author: J S Miller

email: jsmiller(at)uidaho.edu

The R1 gene for potato resistance to late blight (Phytophthora infestans) belongs to the leucine zipper/NBS/LRR class of plant resistant genes

Ballvora A, Ercolano M R, Weiss J, Meksem K, Bormann C A, Oberhagemann P, Salamini F and Gebhardt C. 2002. The Plant Journal 30:361–371. © Blackwell Science Ltd. 2002.

Late blight caused by the oomycete Phytophthora infestans is the most destructive disease in potato cultivation worldwide. New, more virulent P. infestans strains have evolved which overcome the genetic resistance that has been introgressed by conventional breeding from wild potato species into commercial varieties. R genes (for single-gene resistance) and genes for quantitative resistance to late blight are present in the germplasm of wild and cultivated potato. The molecular basis of single-gene and quantitative resistance to late blight is unknown. We have cloned R1, the first gene for resistance to late blight, by combining positional cloning with a candidate gene approach. The R1 gene is member of a gene family. It encodes a protein of 1293 amino acids with a molecular mass of 149.4 kDa. The R1 gene belongs to the class of plant genes for pathogen resistance that have a leucine zipper motif, a putative nucleotide binding domain and a leucine-rich repeat domain. The most closely related plant resistance gene (36% identity) is the Prf gene for resistance to Pseudomonas syringae of tomato. R1 is located within a hot spot for pathogen resistance on potato chromosome V. In comparison to the susceptibility allele, the resistance allele at the R1 locus represents a large insertion of a functional R gene.

Corresponding author: C Gebhardt

email: gebhardt(at)mpiz-koeln.mpg.de

Can the QTL for late blight resistance on potato chromosome 5 be attributed to foliage maturity type?

Visker M H P W, Keizer L C P, Van Eck H J, Jacobsen E, Colon L T and Struik P. 2003. Theoretical and Applied Genetics 106:317–325. © Springer-Verlag 2002

We investigated the association between late blight resistance and foliage maturity type in potato by means of molecular markers. Two QTLs were detected for foliage resistance against Phytophthora infestans (on chromosomes 3 and 5) and one for foliage maturity type (on chromosome 5). The QTL for resistance to late blight and the QTL for foliage maturity type on chromosome 5 appeared to be mapped on indistinguishable positions. We were interested whether this genetic linkage was due to closely linked but different genes, or due to one (or more) gene(s) with pleiotropic effects. We therefore developed an approach to detect QTLs, in which resistance to late blight was adjusted for foliage maturity type. This analysis revealed the same two QTLs for resistance against P. infestans, but the effect of the locus on chromosome 5 was reduced to only half the original effect. This is a strong indication that the two indistinguishable QTLs for foliage maturity type and for late blight resistance on chromosome 5 may actually be one gene with a pleiotropic effect on both traits. However, there was still a significant effect on resistance against P. infestans on the locus on chromosome 5 after adjusting for foliage maturity type. Therefore we cannot rule out the presence of two closely linked QTLs on chromosome 5: one with a pleiotropic effect on both late blight resistance and foliage maturity type, and another with merely an effect on resistance. In addition, the two QTLs for resistance to late blight showed an important epistatic interaction, suggesting that QTLs for resistance affect each other’s expression.

Corresponding author: M H P W Visker

email: M.Visker(at)plant.wag-ur.nl

These abstracts were reprinted with the kind permission of the American Phytopathological Society (www.apsnet.org ), Kluwer Academic Publishers (www.wkap.nl), Elsevier Science (www.elsevier.com/locate/cropro), Blackwell Science Ltd. (www.blackwellpublishing.com) and Springer-Verlag GmbH & Co. (www.springer.de)