Search

With 2 figures and 2 tables The purpose of this research was to identify the genetic basis of foliar and tuber resistance to Phytophthora infestans in a potato breeding population developed from a cross between two tetraploid Solanum tuberosum lines, NY121 and NY115. The parent with high foliar resistance, NY121, was highly susceptible to tuber blight. Foliar resistance was assessed in field trials, while tuber blight was assessed both in the field and in the laboratory. A quantitative trait locus (QTL) explaining ca. 50% of the phenotypic variance for foliar resistance was located on the lower arm of linkage group V, and was loosely associated with tuber blight resistance (significantly in one of two trials). This QTL was confirmed in a separate sample from the same population. Tuber blight detected via laboratory assays was not correlated with tuber blight incidence in the field. Most markers associated with tuber blight resistance were not associated with foliar resistance and most markers associated with foliar resistance were not associated with tuber blight resistance. [ABSTRACT FROM AUTHOR]

Phytophthora infestans, the causal agent of late blight, threatens potato production worldwide. An important tool in the management of the disease is the use of resistant varieties. Eleven major resistance genes have been identified and introgressed from Solanum demissum. However, new sources of resistance are continually sought. Here, we report the characterization and refined genetic localization of a resistance gene previously identified as Rber in a backcross progeny of Solanum tuberosum and Solanum berthaultii. In order to further characterize Rber, we developed a set of P. infestans isolates capable of identifying each of the 11 R-genes known to confer resistance to late blight in potato. Our results indicate that Rber is a new resistance gene, different from those recognized in S. demissum, and therefore, it has been named R Pi-ber according to the current system of nomenclature. In order to add new molecular markers around R Pi-ber , we used a PCR-based mapping technique, named MASP-map, which located R Pi-ber in a 3.9 cM interval between markers CT240 and TG63 on potato chromosome X. The location of R Pi-ber coincides with an area involved in resistance to different pathogens of potato and tomato. [ABSTRACT FROM AUTHOR]

Ten institutions in nine countries joined together to test the stability of resistance of 14 potato genotypes to the oomycete pathogenPhytophthora infestansin three separate trials. Seven of the genotypes were tested in one trial involving seven locations, and all 14 were tested in two subsequent trials, each involving eight locations. Stability of resistance was tested with nonparametric tests and with an additive main effects and multiplicative interaction (AMMI) model. Overall, resistance toP. infestanswas robust; resistant genotypes were consistently resistant in all locations and trials. The nonparametric analysis indicated that specific genotypes were basically stable across sites for resistance. In trial 3, theZstatistic for overall stability was significant at 0·05%, indicating a significant level of interaction across the trial, but there were no significant interactions for specific genotypes in this trial. The genotype by environment (G × E) effect of the AMMI model was highly significant in both trials, but the mean square of G × E was less than 10% of the genotype effect in each trial. The first two principal components (PCA1 and PCA2) of the AMMI analyses together explained 75 and 80% of the interaction effects in trials 2 and 3, respectively. Based on both nonparametric and AMMI analyses, Ecuador and Argentina were locations of relatively high interaction effects for both trials 2 and 3, although in Ecuador this interaction was not associated with any particular potato genotype. Other locations also had high interaction effects, but these occurred in only one trial. The genotypes Chata Blanca and, to a lesser extent, Torridon were relatively unstable in trials 2 and 3, but in the case of Torridon, resistant, this did not represent a significant loss of resistance. [ABSTRACT FROM AUTHOR]

Host-pathogen interactions of a new species of Phytophthora , causal agent of late blight of tree tomato ( Solanum betaceum Cav.), identified as Phytophthora betacei , were investigated with four different cultivars. Thirty-six P. betacei isolates, collected from southern Colombia between 2008 and 2009, were used to inoculate common tree tomato cultivars, Común, Híbrido, Injerto, and Holandés. Data on incubation and latent periods as well as infection efficiency, lesion development, and total sporulation were collected via detached leaf assays. Significant differences in susceptibility, based on the parameters measured, were observed. Común was the most susceptible cultivar, followed by Injerto, Híbrido, and Holandés. The mean incubation period was lowest for Común at 125.6 h post-inoculation (hpi) and highest for Híbrido at 139.4 hpi. No significant differences in latent period were observed. All 36 isolates produced necrotic lesions on Común, and 33, 24, and 21 caused infection on Injerto, Híbrido, and Holandés, respectively. Two isolates were able to cause infection only on Común, and 13 isolates were able to infect all four cultivars. Infection efficiency was significantly higher for the cultivar Común, followed by Injerto, Híbrido, and Holandés. Average lesion size was larger on Común than on any other cultivar. An inverse relationship of lesion size and total sporulation was observed. Común had significantly lower total sporulation than Híbrido and Holandés, which had the smallest average lesion sizes. These data show variation in pathogenicity of P. betacei isolates, under controlled conditions, and differential susceptibility of four distinct S. betaceum cultivars.

The focus of this research was to evaluate genotypes for cold-tolerant germination from wild switchgrass (Panicum virgatum L.) populations collected in the Northeast USA. Switchgrass nurseries were established in 2008 and 2009 with seed collected from native stands of switchgrass in the Northeast USA between 1991 and 2008. Switchgrass seed harvested from individual genotypes was evaluated for cold-tolerant germination in a series of laboratory experiments. Germination assays of seed of 13 switchgrass genotypes harvested in the fall of 2016 are the primary focus of this reported research. The selected genotypes were evaluated for cold-tolerant seed germination in three experiments, during the spring of 2017, fall of 2017 and spring of 2018, (with and without stratification) using a 10/15 °C regime with a 12 h photoperiod. Germination tests showed that several genotypes had significantly higher percentage germination as well as faster germination rates expressed as T50 (number of days required to reach 50% maximum germination) when compared to Cave-in-Rock, a moderately sensitive cold-tolerant commercial cultivar established in the original switchgrass nursery as a control. A final germination test was conducted to compare seed from the original population (no selection cycle 0), with one of the top performing cold-tolerant germination genotypes, and a commercial cultivar, 'Espresso', developed for low seed dormancy and low temperature germination. In this test, the selected genotype had significantly higher percentage germination in the stratified treatment and was not significantly different than Espresso in the non-stratified test. These data indicate successful selection for cold-tolerant germination in switchgrass genotypes from native germplasm collected in the Northeast USA., Competing Interests: The authors declare no conflicts of interest.

Genetic resistance is a valuable tool in the fight against late blight of potatoes but little is known about the stability and specificity of quantitative resistance including the effect of defeated major resistance genes. In the present study we investigated the effect of different isolates of Phytophthora infestans on the mode of action of R(Pi-ber), an R-gene originating from Solanum berthaultii. The experiments were conducted on progenies derived from two reciprocal inter-specific backcrosses of Solanum tuberosum and S. berthaultii. The plant-pathogen interaction was tested in diverse environments including field, greenhouse and growth chamber conditions. The R(Pi-ber) gene provided complete resistance against a US8 isolate of P. infestans in all trials. When isolates compatible with R(Pi-ber) were used for inoculation, a smaller, but significant resistance effect was consistently detected in the same map position as the R-gene. This indicates that this R-gene provides a residual resistance effect, and/or that additional resistance loci are located in this genomic region of chromosome X. Additional quantitative resistance loci (QRL) were identified in the analyzed progenies. While some of the QRL (such as those near TG130 on chromosome III) were effective against several isolates of the pathogen, others were isolate specific. With a single exception, the S. berthaultii alleles were associated with a decrease in disease severity. Resistance loci reported in the present study co-locate with previously reported R-genes and QRL to P. infestans and other pathogens.

To determine if the desert tomato, Lycopersicon pennellii, possesses resistance to late blight, caused by Phytophthora infestans, two plant populations were analyzed. Resistance was identified through assessments of disease progress in an F2 mapping population (L. esculentum × L. pennellii) and in a series of introgression lines (L. pennellii into L. esculentum). Levels of resistance varied widely among individuals within each population. However, the response of individuals to different strains of P. infestans was consistent. In the mapping population, a quantitative trait locus (QTL) was detected near marker T1556 on chromosome 6. This QTL accounted for 25% of the phenotypic variance in the population. The occurrence of this QTL was confirmed from analysis of the introgression lines (ILs), where IL 6-2 (containing marker T1556) was the most resistant IL in 2002 and the second most resistant IL in 2001. The identification of an additional QTL for resistance to late blight in tomato will aid in the development of durable resistance to this devastating disease.

ABSTRACT A means for determining the rate of release, Q (spores per square meter per second), of spores from a source of inoculum is paramount for quantifying their further dispersal and the potential spread of disease. Values of Q were obtained for Phytophthora infestans sporangia released from an area source of diseased plants in a potato canopy by comparing the concentrations of airborne sporangia measured at several heights above the source, with the concentrations predicted by a Lagrangian Stochastic simulation model. An independent estimate of Q was obtained by quantifying the number of sporangia per unit area of source at the beginning of each sampling day by harvesting diseased plant tissue and enumerating sporangia from these samples. This standing spore crop was the potential number of sporangia released per area of source during the day. The standing spore crop was apportioned into time segments corresponding to sporangia concentration measurement periods using the time trace of sporangia sampled above the source by a Burkard continuous suction spore sampler. This apportionment of the standing spore crop yielded potential release rates that were compared with modeled release rates. The two independent estimates of Q were highly correlated (P = 0.003), indicating that the model has utility for predicting release rates for P. infestans sporangia and the spread of disease between fields.