Abstract Downy mildew, inflicted by Peronospora sparsa , causes yield losses in arctic bramble ( Rubus arcticus ssp. arcticus ), boysenberry ( Rubus spp. hybrid) and blackberry ( Rubus fruticosus ), and is a serious threat to the cultivation. Arctic bramble is a high‐value crop but its commercial potential is not realisable partly due to downy mildew. Although P. sparsa has been associated with yield losses in arctic bramble, this has not been experimentally proven, nor have the downy mildew symptoms in arctic bramble fruits been described in a controlled experiment. In this study, downy mildew was found to affect the fruits and reduce the yield of arctic bramble. There was a direct correlation between the number of infected leaves and hard fruits resulting from the infection. Owing to the new pesticide legislation in the European Union, it is important to find alternatives to fungicides in the control of downy mildew. The products ALIETTE (fungicide), PHOSFIK (leaf fertiliser) and BION (pathogen defence elicitor) were effective in downy mildew control, Aliette and Phosfik being more effective than Bion especially in preventing yield losses. No arctic bramble cultivars known to be resistant to downy mildew are available. Evaluation of cultivars Alli, Mesma and Pima in the field showed that Mesma is more resistant to downy mildew than are Alli and Pima. Currently the main cultivar grown in Finland, Pima, was the most susceptible variety. The results of this study suggest that the yields and profitability of arctic bramble production could be improved by selecting more resistant varieties, combined with the application of agrochemicals that can suppress the development of downy mildew.
▪ Abstract Nineteen single dominant genes (R genes) for resistance to viruses, nematodes, and fungi have been positioned on the molecular map of potato using DNA markers. Fourteen of those genes are located in five “hotspots” for resistance in the potato genome. Quantitative trait loci (QTL) for resistance to late blight caused by the oomycete Phytophthora infestans, to tuber rot caused by the bacterium Erwinia carotovora ssp. atroseptica, and to root cyst nematodes have been identified on all 12 potato chromosomes. Some QTL for resistance to different pathogens are linked to each other and/or to resistance hotspots. Based on the genetic clustering with R genes, we propose that some QTL for resistance have a molecular basis similar to single R genes. Mapping potato genes with sequence similarity to cloned R genes of other plants and other defense-related genes reveals linkage between candidate genes, R genes, and resistance QTL. To explain the molecular basis of polygenic resistance in potato we propose (a) genes having structural similarity with cloned R genes and (b) genes involved in the defense response. The “candidate gene approach” enables the identification of markers highly useful for marker-assisted selection in potato breeding.
On July 24 2011, many participants from all over the world gathered in Oulu, Finland to discuss the latest trends in potato science during the 18th Triennial Conference of the European Association for Potato Research.The area was selected with great care: the surroundings of Oulu are one of the most northern places where potatoes are commercially grown, and the area has been marked as one of the five High Grade Seed Potato Areas of Europe.The programme of the Conference included key note papers, oral presentations, poster sessions, workshops, scientific excursions, social events and the general meeting of the members of the Association.The local organizers had identified five main issues to be discussed during the Conference:
Sweet potato chlorotic stunt virus (SPCSV; genus Crinivirus , family Closteroviridae ) causes heavy yield losses in sweet potato plants co-infected with other viruses. The dsRNA-specific class 1 RNase III–like endoribonuclease (RNase3) encoded by SPCSV suppresses post-transcriptional gene silencing and eliminates antiviral defence in sweet potato plants in an endoribonuclease activity-dependent manner. RNase3 can cleave long dsRNA molecules, synthetic small interfering RNAs (siRNAs), and plant- and virus-derived siRNAs extracted from sweet potato plants. In this study, conditions for efficient expression and purification of enzymically active recombinant RNase3 were established. Similar to bacterial class 1 RNase III enzymes, RNase3-Ala (a dsRNA cleavage-deficient mutant) bound to and processed double-stranded siRNA (ds-siRNA) as a dimer. The results support the classification of SPCSV RNase3 as a class 1 RNase III enzyme. There is little information about the specificity of RNase III enzymes on small dsRNAs. In vitro assays indicated that ds-siRNAs and microRNAs (miRNAs) with a regular A-form conformation were cleaved by RNase3, but asymmetrical bulges, extensive mismatches and 2′- O -methylation of ds-siRNA and miRNA interfered with processing. Whereas Mg 2+ was the cation that best supported the catalytic activity of RNase3, binding of 21 nt small dsRNA molecules was most efficient in the presence of Mn 2+ . Processing of long dsRNA by RNase3 was efficient at pH 7.5 and 8.5, whereas ds-siRNA was processed more efficiently at pH 8.5. The results revealed factors that influence binding and processing of small dsRNA substrates by class 1 RNase III in vitro or make them unsuitable for processing by the enzyme.
Abstract Cryotherapy of shoot tips is a new method for pathogen eradication based on cryopreservation techniques. Cryopreservation refers to the storage of biological samples at ultra‐low temperature, usually that of liquid nitrogen (−196°C), and is considered as an ideal means for long‐term storage of plant germplasm. In cryotherapy, plant pathogens such as viruses, phytoplasmas and bacteria are eradicated from shoot tips by exposing them briefly to liquid nitrogen. Uneven distribution of viruses and obligate vasculature‐limited microbes in shoot tips allows elimination of the infected cells by injuring them with the cryo‐treatment and regeneration of healthy shoots from the surviving pathogen‐free meristematic cells. Thermotherapy followed by cryotherapy of shoot tips can be used to enhance virus eradication. Cryotherapy of shoot tips is easy to implement. It allows treatment of large numbers of samples and results in a high frequency of pathogen‐free regenerants. Difficulties related to excision and regeneration of small meristems are largely circumvented. To date, severe pathogens in banana ( Musa spp.), Citrus spp., grapevine ( Vitis vinifera ), Prunus spp., raspberry ( Rubus idaeus ), potato ( Solanum tuberosum ) and sweet potato ( Ipomoea batatas ) have been eradicated using cryotherapy. These pathogens include nine viruses (banana streak virus, cucumber mosaic virus, grapevine virus A, plum pox virus, potato leaf roll virus, potato virus Y, raspberry bushy dwarf virus, sweet potato feathery mottle virus and sweet potato chlorotic stunt virus), sweet potato little leaf phytoplasma and Huanglongbing bacterium causing ‘citrus greening’. Cryopreservation protocols have been developed for a wide variety of plant species, including agricultural and horticultural crops and ornamental plants, and can be used as such or adjusted for the purpose of cryotherapy.
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