Abstract: The recent US-8 clonal lineage of Phytophthora infestans, the pathogen causing late blight (LB) disease, is one of the most widespread and difficult to control. Sources of foliar resistance to US-8 were evaluated in 1927 seedling genotypes by sampling 49 plant introductions (PIs)representing 13 Mexican and South American Solanum species having an Endosperm Balance Number (EBN) of 1, 2, or 4, and one species with an mtknown EBN. Species evaluated were 2x(1EBN) S. bulbocastanum, S. cardiophyUum, S. commersonii, S. pinnatisectum, and S. trifidum; 2x(2EBN) S. berthaultii, S. megistacrolobum, S. microdontum, and S. verrucosum; 4x(2EBN) S. fendleri and S. stolon~erum; 6x(4EBN) S. guerreroense; 2x unknown EBN S. polyadenium; as well as 4x S. tuberosum families as controls. Foliar resistance in the field was monitored in 1998 and 1999. Resistance differed relative to species geographic origin, EBN, species themselves, PIs within a species, and genotypes within a PI. In 1998 Mexican germplasm had a lower area under the disease progress curve (AUDPC, 1140) and greater resistance than South American germplasm (1601), while the 1EBN (1119) and unknown EBN (1075) species had greater resistance than 2EBN (1470) and EBN (1888) species. AUDPC ranged among species from 555 (S. cardiophyllum) to 1932 (S. megistacrolobum) and among PIs within a species most widely for S. bulbocastanum (267-1007) and least widely for S. polyadenium (1019-1179). In 1999 LB symptoms were not observed on the species before an early freeze; however, all 1999 S. tuberosum families showed LB symptoms and segregated for resistance, pointing to greater resistance in the species than in cultivated germplasm. Resistance found in diploid 2EBN germplasm is more accessible due to greater crossability with cultivated germplasm; however, though more difficult to introgress, resistance found in some of the more reproductively isolated 1EBN germplasm is worth pursuing.
Abstract: The International Cocoa Genebank, Trinidad (ICG,T) contains about 2000 cacao (Theobroma cacao L.) accessions. The purpose of the collection is to provide a source of genes for the genetic improvement of cacao through breeding. In fulfilling this objective, it is imperative that this collection be evaluated to identify major sources of genes for the genetic improvement of important agronomic–economic traits. In this study, 816 cacao accessions were evaluated for resistance to Phytophthora pod rot (black pod disease), which causes enormous loss of yield in cocoa production throughout the world.The study
identifies promising resistant genotypes that could be exploited in cacao breeding programmes. It further compares the levels of resistance to Phytophthora pod rot among two cacao types (wild and cultivated types), three major groups (Forastero, Trinitario and Refractario) and 11 accession groups (B, AM, CL, ICS, IMC, JA, LP, MOQ, NA, PA and TRD). The distribution of scores for the 816 accessions showed skewness towards the susceptible end of the disease rating scale, indicating that a large proportion (68.9%) of the sample was susceptible (disease rating 6–8) to Phytophthora pod rot. However, 12.9% of the sample population was found to be resistant (disease rating 1–3) and 18.2% moderately resistant (disease rating 4–5). Significant differences were observed between the wild and cultivated accessions and among the Forastero, Refractario and Trinitario groups. Marked differences were also observed among 11 accession groups, each of which was represented by at least 20 genotypes. Higher proportions of resistant (17.7%) and moderately resistant (22.6%) genotypes were observed in the wild accessions than in the cultivated varieties (9.4% resistant and 14.4% moderately resistant). The Forastero group, consisting of many wild accessions, was found to contain more resistant (18.0%) and moderately resistant (23.1%) genotypes than either the Trinitario (4.8% resistant and 13.6% moderately resistant) or Refractario (11.3% resistant and 15.4% moderately resistant). Among 11 accession groups, the largest percentage of resistant (24.2%) and moderately resistant (28.8%) genotypes were found in the PA (Forastero). Different accession groups had varying proportions of resistant and moderately resistant genotypes. This is not unexpected in an out-breeding crop such as cacao with a high level of heterogeneity. This finding reinforces the idea of a pre-breeding programme (germplasm enhancement) to accumulate resistance genes over several populations as a strategy for improving the genetic base of resistance in national cacao breeding programmes. One hundred and five promising resistant genotypes were identified among the 816 accessions evaluated. These accessions are potential sources of resistance genes for the development of high-yielding resistant varieties in cacao.
Abstract: This study reports the occurrence of Age-Related Resistance (ARR) in the potato- P. infestans pathosystem. Six potato varieties with different degrees of resistance to P. infestans were evaluated on the basis of disease severity according to the widely-used Malcolmsons scoring scale at 4, 8 and 12 weeks after plant emergence. The results show that, out of the six varieties, those exhibiting high to moderate levels of resistance to late blight became consistently more resistant as they grew older, while the effect was less pronounced on the two remaining susceptible varieties. Those six varieties exhibited their maximum level of resistance at 12 weeks after plant emergence, when they were just at the bud stage, making the transition to flowering. It is possible that blight resistance in potato plants might be a developmentally regulated response and as the potato plant makes the transition from vegetative to reproductive growth, the re-wiring of certain developmental pathways induces the expression, or results in the accumulation of, certain genes which might also play an important part in defense responses. The data presented in this study emphasizes the importance of considering the age of potato plants when challenged in screening for resistance a well as in determining the most effective timing for use of chemical controls.
Abstract: The potential role in plant defence of the two wheat pathogenesis-related proteins of class 4 Wheatwin1 and Wheatwin2, possessing high in vitro
antimicrobial activity against several pathogens, was investigated through over-expression of their encoding genes wPR4a and wPR4b in transgenic tobacco plants. Several independent transformants were obtained, expressing high levels of either transgene when analysed by northern and western blotting. Accumulation of the wPR4b-encoded protein Wheatwin2 in the apoplast of transgenic plants was also demonstrated. When homozygous transgenic lines in the T4 generation were tested for increased tolerance to Phytophthora nicotianae, they were found to be significantly more resistant than both the wild type and their isogenic, non-wPR4 transgenic lines. These results suggest that both Wheat wins might have in vivo antimicrobial activity, confirming earlier indications from in vitro assays.
Abstract: The mechanisms of induced resistance and susceptibility of potato (Solanum tuberosum L.) tubers to late blight agent (Phytophthora infestans Mont de Bary) were studied using an elicitor chitosan and an immunosuppressor laminarin. It was elucidated that treatment of disks from potato tubers with chitosan resulted in salicylic acid (SA) accumulation due to activation of benzoate-2-hydroxylase and hydrolysis of SA conjugates. Such SA accumulation in potato tissues inhibited one of the antioxidant enzymes, catalase, inducing an oxidative burst and resistance development. The mechanisms of induced susceptibility to the late blight causal agent were studied using an unspecific immunosuppressor, laminarin, an analogue of natural specific suppressor of potato immune responses, β-1,3, β-1,6-glucan. It was established that the development of immunosuppression in tissues treated with laminarin did not affect the SA level in tissues. However, catalase sensitivity to SA reduced in laminarin-treated tissues, and the enzyme activity increased. In its turn, this might result in the reduced level of hydrogen peroxide in the cells and, as a sequence, in the increased potato susceptibility to late blight.
Abstract:Phytophthora infestans (Mont.) de Bary, the cause of late blight, has rapidly overcome major (R) gene resistance in potatoes. A population of short-day adapted tetraploid potatoes with horizontal resistance to late blight was developed at the International Potato Center in Lima, Peru. True seed from this population was obtained from the NRSP-6 Project at Sturgeon Bay, WI for the purpose of evaluating its potential to contribute to the breeding effort for late blight resistance in the United States. In 1996, 552 single hills were
planted on Chapman Farm, Presque Isle, ME; only 448 tuberized. In 1997, these clones were planted on Chapman Farm for increase; 129 were saved, 53 failed to tuberize, and 266 were discarded because they were sprouted at harvest. In 1998 and 1999, 69 clones were tested for their reaction to late blight in replicated plots on Aroostook Farm, Presque Isle, ME. Percent infected foliage was estimated three times toward the end of the season and used to compute area under the disease progress curve. Broad-sense heritability for horizontal resistance to late blight was estimated as 0.78 with a 95% confidence interval f 0.64 to 0.86. Using detached leaflet assays, these clones were inoculated with US-8 strains of P. infestans, and the diameter of the lesion was measured 7, 8, 9, 10 and 11 days after inoculation. The correlation between field resistance and the detached leaflet assay was very low (0.18P. infestans and represent another source of breeding material for developing late blight resistant varieties in the future. However, very late maturity, short dormancy, poor tuberization and low fertility levels may limit the usefulness of this germplasm in northern USA latitudes.
Abstract: Clones of jarrah (Eucalyptus marginata), micropropagated from glasshouse-grown seedlings selected for resistance or susceptibility to
Phytophthora cinnamomi, were planted in a former bauxitemine-site in the jarrah forest and inoculated with P. cinnamomi.Mortality after 13 years
in resistant clones was 0–30%, while that of susceptible clones was 40–100%. Mean heights of resistant clones after 13 years were 7.8–13.6 m, while heights of surviving susceptible clones were 0.9–6.7 m. The resistance character of the seedling ortets was transmitted consistently to the clones. The fieldmortality of clones of some rare, apparently resistant seedlings selected fromsusceptible half-sib families was low after 1 year, but approached that of the susceptible clones after 2 years. The results show that Phytophthora-resistant jarrah ortets can be selected using steminoculation of glasshouse-grown seedlings; the resistance of the resulting clones has been validated in the field in an inoculation trial.
Abstract: Introduction of more durable resistance against Phytophthora infestans causing late blight into the cultivated potato is of importance for sustainable agriculture. We identified a new monogenically inherited resistance locus that is localized on chromosome 4. The resistance is derived from an ABPT clone, which is originally a complex quadruple hybrid in which Solanum acaule, S. bulbocastanum, S. phureja and S. tuberosum were involved. Resistance data of the original resistant accessions of the wild species and analysis of mobility of AFLP markers linked to the resistance locus suggest that the resistance locus is originating from S. bulbocastanum. A population of 1383 genotypes was screened with two AFLP markers flanking the Rpi-abpt locus and 98 recombinants were identified. An accurate high-resolution map was
constructed and the Rpi-abpt locus was localized in a 0.5 cM interval. One AFLP marker was found to cosegregate with the Rpi-abpt locus. Its DNA sequence was highly similar with sequences found on a tomato BAC containing several resistance gene analogues on chromosome 4 and its translated protein sequence appeared to be homologous to several disease resistance related proteins. The results indicated that the Rpi-abpt gene is a member of an R gene cluster.
Abstract: Fifteen Rps genes confer resistance against the oomycete pathogen Phytophthora sojae, which causes root and stem rot disease in soybean. We have isolated a disease resistance gene-like sequence from the genomic region containing Rps1-k. Four classes of cDNA of the sequence were isolated from etiolated hypocotyl tissues that express the Rps1-k-encoded Phytophthora resistance. Sequence analyses of a cDNA clone showed that the sequence is a member of the coiled coil-nucleotide binding site–leucine rich repeat (CC-NBS–LRR)-type of disease resistance genes. It showed 36% identity to the recently cloned soybean resistance gene Rpg1-b, which confers resistance against Pseudomonas syringae pv. glycinea, and 56% and 38% sequence identity to putative resistance gene sequences from lotus and Medicago truncatula, respectively. The soybean genome contains about 38 copies of the sequence. Most of these copies are clustered in approximately 600 kb of contiguous DNA of the Rps1-k region. We have identified a recombinant that carries both Rps1-k- and Rps1-k-haplotype-specific allelomorphs of two Rps1-k-linked molecular markers. An unequal crossover event presumably led to duplication of alleles for these two physically linked molecular markers. We hypothesize that the unequal crossing over was one of the mechanisms involved in tandem duplication of CC-NBS–LRR sequences in the Rps1-k region.
Abstract: Under epiphytotic conditions for late blight in spring seasons, data were recorded on its intensity four times at 4 days intervals from the start of the disease in the field, in 114 (19 females · 6 males) progenies and their parents
planted in randomized complete block design in the years 2005 and 2006. Regression and stepwise regression analysis showed that observations during the rapidly increasing phase of disease between initial and last phase of disease are more important than the observations at initial and last phase of the disease. Combining ability analysis on a sub-set of 68 progenies showed that the
additive component of genetic variance was more important than the non-additive component of variance in inheritance of quantitative resistance to late blight. The per se performance of the parents does give an idea about their general
combining ability (GCA). However, selection of parents based on their GCA will be very useful for breeding for quantitative resistance to late blight. Parents JX 90, JF 4841, CP 3356, CP 1358, CP 3290, JN 1197 and CP 3125 were found to
have good GCA for quantitative resistance for late blight and the best six crosses for late bilght resistance based on mean performance involved parents with good combining ability only.
Abstract