Soft Rot Erwinia Research Articles

POSTHARVEST EVOLUTION OF PECTINOLYTIC ERWINIA AND ERWINIA SOFT ROT DURING COLD STORAGE OF CONSUMPTION POTATOES PRODUCED IN ALGERIA

In Algeria, potatoes ranks second among the twenty main food crops. It’s characterized by periods of overproduction corresponding to seasonal crops. Except these periods, supply tends to fall. In order for the availability of potatoes to be constant throughout the non-harvest periods, it is imperative to store the surplus production to ensure a constant supply during periods of unavailability of a new harvest. The storage of the potato must maintain a satisfactory quality and minimize losses and the postharvest development of bacteria and fungi. This work aims to identify and follow the evolution of pectinolytic Erwinia and Erwinia soft rot during storage at 4 and 10° C of two varieties of potato grown in Algeria, i.e. Spunta and Désirée. The results indicated the presence of three species of Erwinia: E. carotovora subsp. carotovora, E. carotovora subsp. atroseptica and E. chrysanthemi. Initially, for both varieties, E. carotovora subsp. carotovora predominates followed by E. carotovora subsp. atroseptica and finally E. chrysanthemi. During storage, the three species evolve almost in the same way and their multiplication is more important at 10° C than at 4° C, particularly on the Spunta. About the total bacteria, the highest levels are recorded on Spunta, especially at 10° C. In addition, the Erwinia spp. / total bacteria ratio showed that Erwinia spp. are very most abundant compared to the total bacteria. The assessment of soft rot losses has also shown that they are greater in the Spunta variety whether at 4 or 10° C.

Field application of bio-control agents and aqueous plant extracts for controlling bacterial soft rot and enhancement yield quality of Solanum tuberosum L. cv. Diamond

BackgroundApplication of chemical bactericides, for controlling soft rot bacteria, causes environmental pollution and toxic hazards to human. In addition, it is ineffective, expensive, and limited. Therefore, application of bio-control agents, plant extracts, or safe chemicals may play an important role as safe alternative approaches for controlling phyto-pathogenic bacteria.ObjectiveThis work is aimed to apply bio-control agents (Bacillus subtilis, Bacillus pumilus, Trichoderma harzianum, and Trichoderma virens), aqueous plant extracts (lantana flowers and leaves, lemongrass leaves, and olive cake), and citric acid as pre-sowing treatment for controlling bacterial soft rot pathogen and study their ability for improving yield and quality of potato tubers in field and storage. All treatments were applied as soil treatment and/or foliar spray, except citric acid applied as foliar spray only.ResultsThe cultural filtrates of bio-control agents of B. subtilis, B. pumilus, T. harzianum, and T. virens; aqueous plant extracts of lantana, lemongrass, and olive cake; and citric acid could protect daughter potato tubers against bacterial soft rot disease in field application, except lemongrass (as soil treatment). The bio-control agents highly increased the activities of peroxidase, polyphenol oxidase, and chitinase enzymes, than other treatments. The bio-control agents also improved the shoot parameters, viz, shoot length, number of shoots/pit, and number of leaves/pit and yield parameters, viz, tuber weight/pit, tubers number/pit, and total tubers weight/pit, compared to citric acid and plant extracts, respectively. The treatments as foliar spray have good results for protecting the potato tubers through storage, than soil treatment. The treatments highly enhanced the stored potato tubers quality, viz, dry matter, reducing sugars, total carbohydrates, specific gravity, and starch content.ConclusionsThe tested treatments could protect the potato tubers in field and/or storage against soften development. The treatments could improve the plant growth and yield parameters in field as well as enhanced the stored potato tubers quality and increase the stored time. It is clear that the treatments can be applied as pre-sowing treatment for controlling Erwinia soft rot bacteria.

Potential of Streptrol and Oxolinic Acid Bactericides to Controlling Potato Erwinia Soft Rot Bacteria and their Secondry Effects on Growth, Yield and Tubers Quality

Two successive experiments were conducted. The first was laboratory experiment to examine the best bactericidal effect of streptrol and oxolinic acid according to the inhibition zone on different 10 isolates ofErwiniacarotovora subsp. carotovora. The results showed that the concentration of 100 ppm of each streptrol or oxolinic acid gave the highest inhibition zone with significant superiority of streptrol over oxolinic acidwith percentage estimated by 30.49%. The erwinia isolates E2 and E5 were found to be the most sensitive; while, the isolates of E7 and E9 were the most resistance regardless the used bactericides according to their inhibition zones. The second experiment was conducted in the greenhouse to study the effect of streptrol and oxolinic acidwith 100 ppm on potato plant infected with each of E2, E5, E7 and E9erwinia isolates, in addition to uninfected potato to study the secondary effects of the applied bactericides on the growth, yield and tubers quality of the treated potato plants. The greenhouse experiment revealed that the erwinia isolates of E7 and E9 caused more significant reduction of the studied parameters compared with the isolates of E2 and E5, in 2015-2016 seasons. Also, the using of streptrol in concentration of 100 ppm gave the most bactericidal effect regardless the used erwinia isolate. In addition, the results showed that each of streptrol or oxolinic acid did not show any significant secondary effects on all studied characters except shoot fresh weight, in both seasons of study.

KdgR, an IClR Family Transcriptional Regulator, Inhibits Virulence Mainly by Repression of hrp Genes in Xanthomonas oryzae pv. oryzae

KdgR has been reported to negatively regulate the genes involved in degradation and metabolization of pectic acid and other extracellular enzymes in soft-rotting Erwinia spp. through direct binding to their promoters. The possible involvement of a KdgR orthologue in virulence by affecting the expression of extracellular enzymes in Xanthomonas oryzae pv. oryzae, the causal agent of rice blight disease, was examined by comparing virulence and regulation of extracellular enzymes between the wild type (WT) and a strain carrying a mutation in putative kdgR (ΔXoo0310 mutant). This putative kdgR mutant of X. oryzae pv. oryzae showed increased pathogenicity on rice without affecting the regulation of extracellular enzymes, such as amylase, cellulase, xylanase, and protease. However, the mutant carrying a mutation in an ortholog of xpsL, which encodes the functional secretion machinery for the extracellular enzymes, showed a dramatic decrease in pathogenicity on rice. Both mutants of kdgR and of xpsL orthologs showed higher expression of two major hrp regulatory genes, hrpG and hrpX, and the genes in the hrp operons when grown in hrp-inducing medium. Thus, both genes were shown to be involved in repression of hrp genes. The kdgR ortholog was thought to suppress virulence mainly by repressing the expression of hrp genes without affecting the expression of extracellular enzymes, unlike findings for the kdgR gene in soft-rotting Erwinia spp. On the other hand, xpsL was confirmed to be involved in virulence by promoting the secretion of extracellular enzymes in spite of repressing the expression of the hrp genes.

Mycorrhizal symbiosis enhances Phalaenopsis orchid’s growth and resistance to Erwinia chrysanthemi

Phalaenopsis is the most important potted orchid genus in the world. However, the low seedling survival rate, long vegetative growth period and disease outbreak are problems in production. Orchid micorrhizal fungi (OMF) are their obligate partners in orchid physiology. Orchids use their symbionts to gain access to organic and mineral nutrients by increasing nutrient absorption and translocation to plants under natural conditions. The benefit of orchid mycorrhizal symbiosis using Phalaenopsis as model plants was conducted. We inoculated in vitro grown plantlets of Doritaenopsis Taisuco Wonder ‘King Car Butterfly KC1111’ and Phalaenopsis Tai Lin Redangel ‘V31‘ with two OMF isolates, Ceratobasidium sp. AG-A (R02) and Rizoctonia solan i AG-6 (R04). The effects of OMFs on orchid plant growth and Erwinia soft rot progression were examined after two months of ex vitro growth. The results showed that the presence of OMFs in Phalaenopsis roots significantly increased the growth and soft rot resistance of plants. Selectivity of cultivar type to different OMF was also observed. The relevance of this findings and future work are discussed. Key words : Phalaenopsis, orchid micorrhizal fungi, orchid, cultivar type.

Soft rot inciting Pectobacterium carotovorum (syn. Erwinia carotovora) is unlikely to be transmitted as a latent pathogen in micropropagated banana

This study was undertaken to ascertain if the soft rot inciting Pectobacterium carotovorum/Erwinia carotovora would pass through the micropropagated bananas as a latent pathogen and cause disease during or post acclimatization. In vitro cultures of ‘Grand Naine’ were exposed to the pathogen by providing 100 μl of inoculum (0.001–1.0 at OD600 nm) at the lower leaf axil. These cultures showed a gradual development of soft rot symptoms coupled with obvious bacterial colony growth on banana proliferation medium and consequent plant mortality within a month irrespective of the inoculum level employed. Plants carried forward to acclimatization following inoculation in vitro failed to establish ex vitro. Monitoring the normal field-grown suckers at culture initiation through PCR screening employing soft rot Erwinia primers did not show the amplification of the 119-bp fragment as seen with the pure cultures of pathogen. Further testing of micropropagated banana plants through soil inoculation, in vitro culturing and PCR screening ruled out the possibility of the pathogen surviving in micropropagated stocks in latent form as the organism outgrew and killed the cultures. It emerged that the infection possibly takes place in the nursery. This information will be of particular value for the plant tissue culture industry, plant pathologists and quarantine agencies.

Effect of certain cultural practices on susceptibility of potato tubers to soft rot disease caused by Erwinia carotovora pv. carotovora

Erwinia soft rot causes destructive and serious damage to many vegetable crops including potato in the field, transit and storage periods. The effect of certain cultural practices on the susceptibility of potato tubers to soft rot bacteria was studied and the results of this work can be summarised in the following: potato tubers harvested on 1 May first exhibited the highest disease incidence compared with those harvested on 15 May or 30 May. Harvesting on 15 June resulted in the lowest disease infection. The application of high levels of nitrogen fertiliser as urea (46.5%), ammonium nitrate (31%) and ammonium sulphate (20.5%) resulted in an increase of the susceptibility of potato tubers to bacterial soft rot disease. In contrast, the addition of phosphorous as superphosphate (15.5%) fertiliser caused the reverse effect. The addition of potassium as potassium sulphate (48%) alone at any of the tested levels showed no effect. The susceptibility of potato tubers to bacterial soft rotting disease was increased by increasing storage periods at 4°C for 1, 2, 3 and 4 months. Spraying copper sulphate exhibited the highest decrease in soft rot incidence disease followed by manganese, zinc and iron. However, spraying of boron increased susceptibility to the disease. Potato tubers obtained from plants sprayed with copper and stored for different periods showed the lowest susceptibility to disease incidence. Tuber sprayed with zinc, iron, manganese and finally boron came next.

Role of certain potato tubers constituents in their resistance to bacterial soft rot caused by Erwinia carotovora pv. carotovora

Erwinia soft rot causes destructive and serious damages to many vegetable crops including potato, in the field, transit and storage periods. The role of certain potato tuber constituents in the physiology of disease resistance has been investigated. Pectin substances and calcium contents of potato tuber had a pronounced role in the physiology of disease resistance. Alpha and Santa (less susceptible cultivars) contained the higher amount of pectin and calcium compared by Mirkal, Diamant, Askort, Geganite and Nicola cultivars (more susceptible cultivars). Tubers extracts of all healthy tested potato tubers cultivars contained fructose except Santa cultivar and glucose except Alpha and Mirkal cultivars. Tuber extracts of the more susceptible cultivars (Nicola and Askort) contained a higher concentration of glucose and fructose than those of less susceptible cultivars (Geganite, Mirkal, Santa, Alpha and Diamant).

DYNAMICS OF THE TRANSITION TO PATHOGENICITY INERWINIA CHRYSANTHEMI

The enterobacteria Erwinia chrysanthemi and other soft-rot Erwiniae cause soft-rot disease in plants by secreting extracellular enzymes among which the main virulence factors are pectate lyases (Pels). These pectic enzymes are produced by the activation of the pel genes whose transcription is controlled by a complex regulatory network. Using the knowledge acquired in a previous work, a simplified regulatory network is proposed, keeping only the key variables for the transition to pathogenicity. We identify that the core mechanism for the onset of Pel is governed by a small metabolico-genetic network involving the repressor KdgR and the inductor KDG. Next we consider that the triggering of Pel synthesis is relayed by a quorum sensing (QS) phenomenon describing the ability of bacteria to use the size and density of their colonies to regulate the production of pectate lyases. The simplified network is described by only a few differential equations, thereby allowing the use of standard bifurcation analysis in the phase space. From this modeling emerges a qualitative but generic mechanism for the transition to virulence of a pectinolytic bacterium when it infects a plant.

Assessment of root-associated paenibacillus polymyxa groups on growth promotion and induced systemic resistance in pepper.

Twenty-nine P. polymyxa strains isolated from rhizospheres of various crops were clustered into five genotypic groups on the basis of BOX-PCR analysis. The characteristics of several plant growth-promoting factors among the isolates revealed the distinct attributes in each allocated group. Under gnotobiotic conditions, inoculation of pepper roots with P. polymyxa isolates significantly increased the biomass in 17 of total 29 treated plants with untreated plants. Experiments on induced systemic resistance (ISR) against bacterial spot pathogen Xanthomonas axonopodis pv. vesicatoria in pepper by P. polymyxa strains were conducted and only one isolate (KNUC265) was selected. Further studies into ISR mediation by the KNUC265 strain against the soft-rot pathogen Erwinia carotovora subsp. carotovora in tobacco demonstrated that the tobacco seedlings exposed to either bacterial volatiles or diffusible metabolites exhibited a reduction in disease severity. In conclusion, ISR and plant growth promotion triggered by P. polymyxa isolates were systemically investigated on pepper for the first time. The P. polymyxa KNUC265 strain, which elicited both ISR and plant growth promotion, could be potentially used in improving the yield of pepper and possibly of other crops.

Pre‐PCR Processes in the Molecular Detection of Blackleg and Soft Rot Erwiniae in Seed Potatoes

AbstractThe polymerase chain reaction (PCR) based detection of blackleg and soft rot erwiniae involves pre‐PCR processing steps which may compromise the sensitivity of detection. The aim of this study was to standardize these various steps to develop reproducible diagnostic PCR protocol for the detection of the three known soft rot erwiniae as they occur in the tuber, singly or in combination. Comparison of tuber peel and stolon end tissue as a starting material for enrichment of the bacteria indicated that tuber peel samples resulted in more representative and sensitive detection of the strains than extract from stolon end tissues. Substances of potato origin in the peel extract were found to be highly inhibitory to the PCR. Addition of the antioxidant Dethiotreitol to the samples before enrichment did not have any significant effect on detection during the 24 h period incubation of the peel extract at room temperature. Bulk washing of tubers with one rotten tuber included with the working sample caused surface contamination on 67–91% of the healthy tubers. Washing tubers individually circumvents the problem. The optimum temperature for enrichment of all the three strains was 27°C. At 37°C, Pectobacterium carotovorum failed to be detected while PCR on Pectobacterium atrosepticum and isolates of Dickeya spp. always produced amplification of the specific DNA fragments. Viability test on Nutrient Agar showed that only Dickeya isolates were viable after 48 h of incubation at 37°C suggesting that the detection of P. atrosepticum at 37°C was from dead or non‐viable cells. Post cell death detection experiment further confirmed that DNA was amplified from dead cells of all the strains at 27°C and 33°C whereas at 37°C, only DNA from dead cells of isolates of Dickeya and P. atrosepticum were amplified. There was no amplification from the dead cells of all isolates of P. carotovorum following the 48 h post death incubation at 37°C. The reason for this difference in post death longevity is not clear at this stage.

Premier Russet: A Dual-Purpose, Potato Cultivar with Significant Resistance to Low Temperature Sweetening During Long-Term Storage

Premier Russet is a russet-skinned, high yielding potato cultivar suitable for processing or fresh-pack. Premier Russet is notable for its resistance to the accumulation of reducing sugars in its tubers. This characteristic allows tubers of Premier Russet to be stored at temperatures as low as 5.6°C for 250 days without the need for reconditioning prior to processing. The low temperature sweetening resistance of Premier Russet allows for its storage at colder temperatures which contributes to prolonged tuber dormancy and quality, and to a reduction in the incidence and severity of storage diseases associated with the higher storage temperatures typically used for industry-standard cultivars such as Russet Burbank and Ranger Russet. Premier Russet was released in 2006 by the USDA-ARS and the Agricultural Experiment Stations of Idaho, Oregon, and Washington, and is a product of the Northwest Potato Variety (Tri-State) Development Program. In full-season trials conducted over a 3-year period in the states of Idaho, Oregon, Washington, Colorado, California, New Mexico, and Texas, Premier Russet averaged 5% and 4% higher yields than Ranger Russet and Russet Burbank, respectively, and had a higher percentage of US No. 1 yield than either cultivar. Specific gravity of Premier Russet is consistently greater than that of Ranger Russet or Russet Burbank. Premier Russet is resistant to common and powdery scab, very resistant to the common strain of potato virus Y (PVYO), and moderately resistant to Verticillium wilt, early blight of the foliage and tuber, Pectobacterium (Erwinia) soft rot, and to galling of roots by powdery scab. Blackspot bruise reaction for Premier Russet is similar to that of Ranger Russet. The incidence of hollow heart in Premier Russet is higher than that of Ranger Russet or Russet Burbank. Cultural recommendations to manage hollow heart in Premier Russet are provided.

Blazer russet: An early to Mid-Season potato cultivar with high U.S. no. 1 yields and good processing and culinary qualities

‘Blazer Russet’ is an early to mid-season cultivar notable for its high U.S. No. 1 yield of medium-russeted tubers and its good processing and culinary qualities. It was derived from the cross A7816-14 x ‘NorKing Russet’ and was released in 2005 by the USDA-ARS and the Agricultural Experiment Stations of Idaho, Oregon and Washington. Blazer Russet is very suitable for processing into French fries and other frozen potato products directly from the field or from extended storage, with higher merit scores than ‘Russet Burbank’ and ‘Ranger Russet’ in processing and post-harvest evaluations. Blazer Russet also has high merit for use in the fresh market, with sensory evaluations comparable to those of Russet Burbank. Total yields for Blazer Russet at five early harvest trial locations in the western United States were 12% greater than either ‘Russet Norkotah’ or ‘Shepody’, while U.S. No. 1 yields were 12% greater than Russet Norkotah and 35% greater than Shepody. Specific gravities of Blazer Russet tubers were comparable to tubers of Shepody, and higher than those of Russet Norkotah in early harvest trials. In full-season trials in the western United States, total yields for Blazer Rus set were slightly lower than Ranger Russet and slightly higher than Russet Burbank, but U.S. No. 1 yields were 8% greater than Ranger Russet and 38% greater than Russet Burbank. In these full-season trials, tuber specific gravity of Blazer Russet was similar to Russet Burbank, but lower than Ranger Russet. Blazer Russet is resistant to sugar ends, tuber malformations and most internal and external defects, the exception being its moderate susceptibility to hollow heart. Blazer Russet is resistant to common scab, tuber powdery scab and PVX, and has moderate resistance to blackspot bruise and tuber late blight infections. It is moderately susceptible to powdery scab root galling, PVY0, early blight infection of the tuber, Erwinia soft rot and Fusarium dry rot. Blazer Russet is susceptible to Verticillium wilt, pink rot, PLRV net necrosis, corky ringspot, and foliar early blight and late blight.

Quorum sensing in Erwinia species

The term quorum sensing (QS) refers to the ability of bacteria to regulate gene expression according to the accumulation of signalling molecules that are made by every cell in the population. The erwiniae group of bacteria are often phytopathogens and the expression of a number of their important virulence determinants and secondary metabolites is under QS control. The erwiniae utilise two types of QS signalling molecules: N-acyl homoserine lactones and AI-2-type signalling molecules. Here, we review the regulatory networks involving QS in the soft rot erwiniae.

DETECTION AND CLASSIFICATION OF SOFT ROT ERWINIA OF VEGETABLES IN THAILAND BY DNA POLYMERASE CHAIN REACTION

Detection and classification of soft rot Erwinia of vegetables in Thailand were conducted from ten varieties of vegetables; cauliflower, cabbage, chinese cabbage, pak-choi, spinach, carrot, chinese radish, cucumber, tomato and asparagus. To determine soft rot Erwinia, soft rot bacteria were isolated from rot symptom of these vegetables on pectate assay medium and identified by technique of DNA polymerase chain reaction. Using Y1 and Y2 primers are specific to amplify 434 bp open reading frames of pectate lyase (pel) genes of E. carotovora while ADE1 and ADE2 primers are specific to amplify 420 bp open reading frames of pectate lyase genes of E. chrysanthemi. The result showed that only Y1 and Y2 primers were able to amplify genomic DNA of all isolates. [...]

DNA based detection of blackleg and soft rot disease causing Erwinia strains in seed potatoes

Erwinia carotovora subsp. atroseptica (Eca), Erwinia carotovora subsp. carotovora (Ecc) and Erwinia chrysanthemi (Ech) are the different sub species of Erwinia that cause the diseases commonly known as blackleg, aerial stem rot and soft rot on potato. Blackleg and aerial stem rot affect vines during the growing season, whereas soft rot affects tubers in the field and during transit and storage. The three species can cause soft rot under cool and moist conditions. E. carotovora subsp. atroseptica is the major cause of blackleg, a blackening of the stem base of potato plants, which originates from the mother tuber (Pérombelon and Kelman, 1987). Erwinia carotovora subsp. carotovora mainly causes aerial stem rot (aerial blackleg), but under high temperatures it has been reported to cause blackleg like symptoms. E. chrysanthemi also induces blackleg-like symptoms. Until recently E. chrysanthemi had been mainly confined to warmer climates of Europe, Australia and the tropics. To date the species has been known to occur in cool temperate regions including Finland. In Finland E. chrysanthemi the strain has been reported for the first time in 2004 and it appears to spread fast in certain parts of country (Degefu, unpublished, Pirhonen, personal communication). Although infested crop residues and rotting tubers are among the important sources of inoculum, latent infections in seed tuber provide the major source of infection in potato production (Hannukkala and Segertedt, 2004). At the seed potato laboratory MTT, Ruukki we are carrying out research and services on PCR (DNA) based detection of latent infection of blackleg and soft rot Erwinia strains. The enrichment of the bacteria in semi selective liquid medium prior to PCR ( BIO-PCR) is an important initial step for the success in PCR detection extremely low number of the target bacteria from tubers. The different strains appear to differ in their ability to compete with other saprophytes and reach the target detection limit of bacterial population during the enrichment culture of the potato peel extract. Results of prior PCR enrichment of the bacteria, detection limits of the different strains and preliminary data, from the analysis of some seed lots from the high grade area of north Ostrobothnia and Åland regions, on the occurrence of the strains and the new trends of introduction and spread of E. chrysanthemi in Finland are presented and discussed. Evaluation of the current status of Erwinia diagnostics and areas of future research are highlighted

Defender: A high-yielding, processing potato cultivar with foliar and tuber resistance to late blight

The potato cultivar Defender is high-yielding, white-skinned, and notable for having foliar and tuber resistance to late blight infection caused byPhytophthora infestans (Mont.) de Bary. It was released in 2004 by the USDA-ARS and the agricultural experiment stations of Idaho, Oregon, and Washington. Defender is suitable for processing into french fries and other frozen potato products directly from the field or from storage. Defender also may be used for fresh markets in regions such as California, where cultivars with long tubers and white skin are traditionally grown. Resistances to late blight and other potato diseases make Defender an ideal candidate for organic potato production. Defender consistently produced greater total and U.S. No. 1 yields than ‘Russet Burbank’ in Idaho trials. In early harvest trials conducted in the western U.S., average total yields of Defender were 17% and 23% greater than yields for ‘Ranger Russet’ and ‘Shepody’, respectively. In full-season trials conducted in the western U.S., Defender averaged 10% and 15% higher yields than Ranger Russet and Russet Burbank, respectively. Specific gravity of Defender is consistently high, with values comparable to those of Ranger Russet; tuber ascorbic acid (Vitamin C) levels are also high. In addition to late blight, Defender also is resistant to tuber early blight (Alternaria solani, (Ellis & G. Martin), L.R. Jones & Grout), potato virus X, and net necrosis; it has moderate levels of resistance to Verticillium wilt (Verticillium dahliae, Kleb), pink rot, foliar early blight, corky ringspot, and Erwinia soft rot. Defender is susceptible to scab (common and powdery) and potato leafroll virus; it has moderate susceptibility to dry rot (Fusarium spp.) and potato virus Y. Susceptibilities to internal necrosis, tuber greening, and blackspot bruise also have been noted, but may be minimized through cultural and harvest practices.

Rotting softly and stealthily

The soft rot erwiniae, which are plant pathogens on potato and other crops world-wide, synthesize and secrete large quantities of plant cell wall degrading enzymes that are responsible for the soft rot phenotype, earning them the epithet 'brute force' pathogens. They have been distinguished from classic 'stealth' pathogens, such as Pseudomonas syringae, which possesses an extensive battery of Type III secreted effector proteins and phytotoxins to manipulate and suppress host defences. However, recent studies, including whole-genome sequencing, are revealing many components of stealth pathogenesis within the soft rot erwiniae (SRE), suggesting that 'stealth' and 'brute force' should not be regarded as mutually exclusive modes of pathogenesis.

Resistance to soft rot (Erwinia carotovora subsp.atroseptica) in tetraploid potato families obtained from 4x-2x crosses

Diploid potato clones, interspecific hybrids ofSolanum species, having in their originS. tuberosum,S. chacoense, S. yungasense, S. phureja, S. gourlayi, andS. demissum, with resistance to soft rot, were crossed to tetraploid potato clones in 4x-2x crosses. The 24 tetraploid families obtained in a North Carolina II design were examined for tuber resistance to soft rot in a laboratory test and for basic agronomic traits in field trials conducted for two consecutive years. In addition, one family originating from a 4x-2x cross of two susceptible parents was tested. Statistical analysis of the data revealed significant general (GCA) and specific combining ability (SCA) effects, year, GCA (female) × year, GCA (male) × year, and SCA × year upon the inheritance of resistance to soft rot. About 35% of the progeny was selected as resistant to tuber soft rot, and of these 11% showed high resistance combined with good tuber yield, tuber weight, and tuber appearance. The relationships between resistance to soft rot and chosen agronomic traits were not noted or were weakly significant and sporadic. The resistance to tuber soft rot found in diploid potato hybrids can be transferred to the cultivated tetraploid pool through 4x-2x crosses, and a high frequency of offspring posses resistance.

INFLUENCE OF DRYING CONDITIONS AND STORAGE TEMPERATURE ON DESICCATION AND LOSS OF TUBERS DURING STORAGE AND SUBSEQUENT GROWTH OF ZANTEDESCHIA TUBERS

After lifting Zantedeschia tubers in autumn severe losses of tubers occur during storage and the subsequent growing season caused by soft rot (Erwinia carotovora) and calcification of tubers. In this study the effects of post-harvest drying conditions and storage temperature were investigated. After lifting early November tubers were dried at 17°C without air circulation, with moderate air circulation and with strong air circulation. After one week tubers were placed at different temperatures (9, 13, 17, 20°C) for storage with moderate air circulation during the rest of the storage period until planting in April. No air circulation at 17°C during the first week after lifting caused losses of tubers by bacterial rot during that week and calcification during subsequent storage. Hardly any rot or calcification was observed after moderate and strong air circulation. The temperature during subsequent storage had a great influence on desiccation of the tubers. Remarkably, in the period from cleaning the tubers in December until planting in April, weight loss increased with decreasing temperature. The big loss of weight at 9°C resulted in calcification during storage or severe losses of tubers during the subsequent growing season. It is suggested that drying or storage at temperatures higher than 13°C are necessary to form a skin that prevents desiccation of the tuber.