Phosphite compounds reduce disease severity in potato seed tubers and foliage

Phosphites (Phi) are alkali metal salts of phosphorous acid, with the ability to protect plants against different pathogens. In this research, the effect of Phi applied to potato plants on severity of three important potato diseases in Argentina was assessed. Seed tubers and foliage of potato cvs Shepody and Kennebec were treated with Phi to assess effects on resistance against Phytophthora infestans, Fusarium solani and Rhizoctonia solani. Protection resulting from Phi treatment in seed tubers was high against P. infestans, intermediate against F. solani, and low against R. solani. In addition, seed tubers treated with calcium or potassium phosphites (CaPhi and KPhi, respectively) at 1% of commercial product emerged earlier than untreated ones. When Phi were foliarly applied two or four times at different doses, high levels of protection against P. infestans were achieved in both cultivars. Higher protection was observed in Kennebec when CaPhi was applied, while in Shepody this was true for KPhi. Expression of β-1,3-glucanases was induced at different times after treatment but no correlation between β-1,3-glucanases expression and foliar protection level was found. On the other hand, Phi positive protection effects did not produce negative effects in plant growth. Leaves from CaPhi-treated plants showed a darker green colour than leaves from control plants; also an increase in Rubisco protein and a delay in crop senescence was observed.     

BABA effects on the behaviour of potato cultivars infected by <Emphasis Type="Italic">Phytophthora infestans</Emphasis> and <Emphasis Type="Italic">Fusarium solani</Emphasis>

Since most plants possess resistance mechanisms which can be induced upon pre-treatment with a variety of chemical compounds, the use of β-aminobutyric acid (BABA) as a defence inducer without reported toxic effect on the environment was studied. The aim of this work was to analyse the effectiveness of BABA to induce resistance against Phytophthora infestans and Fusarium solani in potato cultivars differing in their level of resistance to late blight. The behaviour of some components of biochemical mechanisms by which BABA increases resistance against P. infestans, as well as the effect of BABA on the activity of a potential pathogenic factor of F. solani, were studied. Plants with four applications of BABA throughout the crop cycle produced tubers more resistant to P. infestans and F. solani than non-treated plants. In addition, tuber slices from treated plants, inoculated with P. infestans, showed an increase in phenol and phytoalexin content. The aspartyl protease StAP1 accumulation was also higher in tubers obtained from treated plants and inoculated with P. infestans. This result was observed only in the more resistant potato cv. Pampeana, early after infection. In the potato–F. solani interaction, infected tubers coming from BABA-treated plants showed minor fungal proteolytic activity than infected, non-treated ones. For potato cvs Pampeana and Bintje, the BABA treatment improved the yield of harvested tubers. The number of tubers per plant and total weight of harvested tubers was greater for those obtained from treated plants with two early or four applications of BABA. The results show that the BABA treatment increases the resistance of potatoes but the degree of increase depends on the original level of resistance present in each cultivar.     

Evidence for presence of the founder Ia mtDNA haplotype of Phytophthora infestans in 19th century potato tubers from the Rothamsted archives

Late blight remained a significant disease for potato growers in Europe long after the famine of the 1840s. Of the four mitochondrial haplotypes of Phytophthora infestans, only the Ia mitochondrial DNA (mtDNA) haplotype has been identified previously in infected potato leaves from famine‐era herbarium specimens collected in England, Ireland and Europe in the 19th century. Long‐term soil fertility experiments were conducted on potato between 1876 and 1901 in Rothamsted to investigate effects of combinations of organic manures and mineral fertilizers on disease and yield. This report identifies for the first time the same Ia mtDNA haplotype of P. infestans in three diseased tubers from 1877 from the long‐term Rothamsted trials, thus providing the earliest evidence of the presence of the founder Ia mtDNA haplotype of P. infestans in potato tubers in England. Soil amendments had a significant impact on disease and yield. A real‐time PCR assay was used to detect and quantify P. infestans in tubers. The level of pathogen DNA was greatest in tubers from highest yielding plots that received combinations of inorganic nitrogenous and mineral fertilizers and least in tubers from plots with organic farmyard manures or non‐nitrogenous mineral fertilizers. The Ia mtDNA haplotype was also confirmed from diseased potato leaves during the same time period. Thus, the founder Ia mtDNA haplotype survived in potato tubers after 1846 and was present over 30 years later in the UK.     

The novel elicitor COS‐OGA enhances potato resistance to late blight

Phytophthora infestans is the causal agent of potato late blight. This pathogen is usually controlled by fungicides, but new European regulations have imposed changes in crop protection management that have led to a search for alternative control measures. The induction of plant defence responses by elicitors is a promising new strategy compatible with sustainable agriculture. This study investigated the effect of eliciting a defence response in potato against P. infestans using a formulation of the COS‐OGA elicitor that combines cationic chitosan oligomers (COS) and anionic pectin oligomers (OGA). Trials were conducted under greenhouse conditions to assess the ability of COS‐OGA to control P. infestans. The results showed that three foliar applications with this elicitor significantly increased potato protection against late blight in controlled conditions. The activation of potato defence genes was also evaluated by RT‐qPCR during these trials. Two pathogenesis‐related proteins, basic PR‐1 and acidic PR‐2, were rapidly and significantly up‐regulated by the elicitor treatment. Therefore, these results suggest that the COS‐OGA elicitor increases the protection of potato against P. infestans and that this protection could be explained by an increase in the expression of potato defence genes rather than by biocide activity.     

Enhancement of natural disease resistance in potatoes by chemicals

The mechanism involved in systemic acquired resistance (SAR) can be non-specifically induced in susceptible plants. In response to pathogens, plants' natural defence mechanisms include the production of lignin and phytoalexins and the induction of plant enzymes. The aim of this research was to study the induction of SAR mediated by the chemical activator DL-3-aminobutyric acid (BABA) and the fungicide fosetyl-aluminium in potato cultivars with different levels of resistance against Phytophthora infestans (Mont) de Bary. To study the chemical induction of the resistance, the foliage of several potato cultivars was sprayed with BABA, fosetyl-aluminium or water (as a control treatment). After 3 days the foliage was inoculated with P. infestans. Seven days after inoculation, development of disease symptoms in the foliage was assessed. In postharvest tuber samples, evidence for enhancement of the defence response was evaluated by measuring the protein content of several hydrolytic enzymes as well as the phenol and phytoalexin content. The highest level of protection against late blight was observed when the chemicals were applied at early stages of crop development. An increase in resistance to late blight was also detected in tubers after harvest. There was also an increase in the protein level of beta-1,3-glucanase and aspartic protease as well as in the phenol and phytoalexin content of potato tuber discs obtained from postharvest tubers of treated plants. Thus the protective effect seemed to persist throughout the whole crop cycle. This treatment may offer the possibility of controlling both foliage and tuber blight and could have a major impact in reducing over-winter survival of P. infestans in tubers.     

Functional imaging of biophoton responses of plants to fungal infection

Each living cell of a plant produces photons in certain conditions. Under normal physiological conditions, cell photon emission is stationary and minimal. Disturbance in the oxidative homeostasis by biotic stress is manifested by increased ‘biophoton’ production. Such biophoton responses of plants may be used as an integral indicator of the degree of oxidative homeostasis misbalance. Our results demonstrate that biophoton generation has been much higher in a resistant potato variety than in a susceptible one till 10 h after Phytophthora infestans inoculation. In contrast, ultra-weak luminescence from detached susceptible potato and moderately resistant pelargonium leaves increased from 1–4 to 4–5 days after inoculation with Phytophthora infestans or Botrytis cinerea, respectively. Pre-treatment of susceptible potato leaves with a defence inducer, arachidonic acid, resulted in a transient burst of light in response to P. infestans lasting for 30–45 h post inoculation (hpi). This study presents the potential adaptation of functional imaging of ultra-weak luminescence to monitor time-dependent free radical processes during disease development and its application to draw conclusions on plant resistance to pathogens of different lifestyle. Moreover, it has been shown that imaging of temporal biophoton generation from potato leaves treated with arachidonic acid might be a helpful marker in mapping oxidative changes leading to systemic acquired resistance (SAR).     

Characterization of Pectobacterium brasiliense strains from potato and vegetables in Israel

Pectobacterium brasiliense (Pbr) infects a wide range of crops worldwide, causing potato blackleg and soft rot and vegetable soft rots. This study aimed to characterize the genetic diversity and virulence variability among 68 Pbr strains isolated from either symptomless potato progeny tubers, diseased potato plants, ware potatoes wash water, or vegetables grown in Israel, as well as strains isolated from symptomless seed tubers grown in Europe, or diseased potato plants grown in France. The collection was typed using PCR and TaqMan real-time PCR analyses, dnaX sequence analysis, pulsed-field gel electrophoresis (PFGE), and pectolytic activity. dnaX phylogeny grouped almost all strains in a common genetic clade related to Pbr, which was distinct from the other Pectobacterium species. PFGE analysis identified two main clusters, including one major group of 47 strains with 95%–100% similarity. Maceration assays on two potato cultivars showed significant differences between strains but with no correlations with the source of the strains nor the status of the host (with/without symptoms). Molecular (dnaX sequences and PFGE profiles) and phenotypic analyses (tuber maceration tests) showed that the tested Pbr strains are not a homogeneous group. Analysis of the tested Pbr strains isolated from potato and vegetables grown in fields with a history of potato cultivation suggests that seed tubers imported from Europe may be the main source for Pbr in Israel. To the best of our knowledge, this is the first study that describes biodiversity and population structure of P. brasiliense isolated from potato and vegetables under hot climate conditions.     

Involvement of Iron and Ferritin in the Potato–Phytophthora infestans Interaction

This work shows that the infection of potato (Solanum tuberosum) detached leaves by the late blight pathogen Phytophthora infestans, was drastically reduced by adding deferoxamine, an exogenous iron chelator. Reactive oxygen species in leaves inoculated with P. infestans were also reduced after adding deferoxamine. A leaf ferritin cDNA fragment was obtained by PCR and used as probe for screening a tuber cDNA library. A cDNA (named StF1) encoding the iron-storing potato ferritin was cloned. StF1 is 915 bp in length and has an open reading frame of 230 amino acids that contains the information for the mature 28 kDa subunit of potato ferritin. StF1 was used as probe in northern blot hybridizations to analyze expression of the ferritin gene. In leaves, ferritin mRNA accumulated in response to pathogen attack. In tubers, ferritin mRNA increased upon treatment with the elicitor eicosapentaenoic acid. These results suggest that iron plays a role in the potato-P. infestans interaction.     

PCR-based determination of colonization patterns during potato tuber infection by single and multiple pathogens

Potato tubers piled in storage are prone to infection by numerous pathogens. Each pathogen can cause damage alone, but severe losses often arise when more than one pathogen is involved. Currently, only a visual diagnosis is practiced on potato tubers before storing them, which does not allow any prediction of further disease spread. The aim of the present study was to determine differences in patterns of tissue colonization by several tuber decay pathogens and how late blight infection affects further tuber colonization by other important tuber pathogens. This study was conducted using artificial inoculation of potato tubers and PCR to provide an early and accurate diagnosis of disease development for major potato tuber rots, and to assess potential synergism/antagonism between Phytophthora infestans and other pathogens in stored tubers. In order to accurately follow the progress of each pathogen in tuber tissues, samples were collected over time from both the surface (peel, 0–2 mm depth) and internal tissues (flesh, depth > 2 mm) of the tubers at various distances from the inoculation site, at 3, 5, 7, 10, 12, 14, 17, and 19 days after inoculation. Successful detection of single or multiple pathogens was achieved using specific PCR-primers for each pathogen. Pathogens were always detected several centimeters ahead of the visible lesions. This tracking enabled us to determine the extent of colonization both on the tuber’s surface and in internal tissues by each tested pathogen, either after single or multiple infections involving P. infestans as the primary pathogen. The presence of P. infestans was shown to enhance the development of Pectobacterium atrosepticum and to slow down that of P. erythrospetica and Pythium ultimum. No noticeable effect on further tuber colonization by F. sambucinum, V. dahliae or V. albo-atrum was observed in the presence of P. infestans. This approach involving more than one pathogen is more realistic than classical studies considering single pathogens, and may be helpful in monitoring the sanitary status of stored tubers. Our results make the outcome of certain combinations of pathogens in potato tubers more predictable and may result in more efficient preventive measures.     

Development of a quantitative real‐time PCR assay for Phytophthora infestans and its applicability to leaf,tuber and soil samples

A sensitive real‐time polymerase chain reaction (PCR) assay was developed for the quantification of Phytophthora infestans, the cause of foliar and tuber late blight in potato. A primer pair (PinfTQF/PinfTQR) and a fluorogenic probe (PinfTQPR) were designed to perform a quantitative assay for the detection of P. infestans in leaves, tubers and soils. The assay was shown to be specific to P. infestans and the very closely taxonomically related non‐potato pathogen species P. mirabilis, P. phaseoli and P. ipomoea, but did not detect the potato pathogens P. erythroseptica and P. nicotianae. The assay was able to reliably detect P. infestans DNA at 100 fg per reaction and was effective in quantifying P. infestans in infected leaf tissue from 24 h after inoculation and also in infected symptomless tubers and diseased tubers. Attempts to detect oospores of P. infestans in naturally and artificially infested soil samples are described and compared with baiting tests and previous literature. It was not possible to detect oospores in soil samples due to problems with DNA extraction from the oospores themselves. However, the assay was shown to detect even very low levels of asexual inoculum (sporangia and mycelium) in soil. This work assembles all the necessary features of a quantitative P. infestans assay, which have previously been somewhat disparate: the sensitivity, specificity and quantitation are fully validated, the assay is shown to work in common applications in leaf and tuber tissue and the problems with P. infestans oospore detection are explored and tested experimentally.     

Activation of defence responses to Phytophthora infestans in potato by BABA

Late blight caused by Phytophthora infestans is one of the most devastating diseases of the potato crop. Resistance breeding and current fungicides are unable to control the rapidly evolving P. infestans and new control strategies are urgently needed. This study examined mechanisms of dl ‐β‐aminobutyric acid (BABA)‐induced resistance (IR) in the potato–P. infestans system. Leaves from two cultivars that differ in their degree of resistance, Bintje and Ovatio, were analysed after foliar treatment with BABA. Rapid activation of various defence responses and a significant reduction in P. infestans growth were observed in leaves treated with BABA. In the more resistant cultivar, Ovatio, the activation was both faster and stronger than in Bintje. Microscopic analysis of leaves treated with BABA revealed induction of small hypersensitive response (HR)‐like lesions surrounded by callose, as well as production of hydrogen peroxide (H₂O₂). Molecular and chemical analyses revealed soluble phenols such as arbutin and chlorogenic acid and activation of PR‐1. These results show a direct activation of defence responses in potato, rather than priming as reported for other plant species. They also show that the efficiency of BABA‐IR differs between cultivars, which highlights the importance of taking all aspects into consideration when establishing new methods for disease management.     

Sugar beet extract induces defence against Phytophthora infestans in potato plants

The aim of this study was to find a natural and cheap agent that could induce defence responses in potato plants to combat Phytophthora infestans, which causes late blight disease that is one of the most devastating plant pathogens in agriculture. We tested whether a sugar beet extract (SBE), derived through a simple extraction procedure from a large-scale plant waste product, induced resistance under green-house conditions. In three potato genotypes differing in their level of resistance to P. infestans (two susceptible genotypes: Desiree and Bintje and one partially resistant: Ovatio), treatment with SBE resulted in significant reduction of the size of the infection lesions in a pattern similar to that seen with application of a known defence-inducing compound, β-aminobutyric acid (BABA). Lower sporangial production was also observed on SBE-treated leaves, but the reduction in sporangial production was more pronounced after BABA treatment. SBE had no apparent toxic effect on the hyphal growth of the pathogen or on the germination of sporangia. Instead, SBE triggered pathogenesis-related protein (PR-1 and PR-2) induction which suggests that the protection conferred by SBE could be via induced resistance. An array of phenolic metabolites was found in the SBE that may contribute to the defence response.     

Development of symptoms caused by Erwinia carotovora ssp. atroseptica under field conditions and their effects on the yield of individual potato plants

Infection of seed tubers by pectinolytic Erwinia species can lead to the development of various symptoms during vegetative growth of potato crops, including non-emergence of plants, chlorosis, wilting, haulm desiccation and typical blackleg. The relationships between types of symptoms and yield are poorly documented, and are investigated by following the development of symptoms in potato plants grown under field conditions from seed tubers artificially inoculated with E. carotovora ssp. atroseptica ( Eca ) , and measuring the yield of each plant. Symptoms were classified into five main types (non-emergence, wilting/chlorosis, blackleg, haulm desiccation and plant death). Each plant was scored for types of symptom on four successive dates; plants without visible symptoms were scored as healthy. The method of inoculation and inoculum concentration proved major factors for the subsequent development of symptoms. Disease development was more severe after vacuum infiltration of bacteria into seed tubers than after shaking tubers in contaminated sand. Disease usually progressed from chlorosis and/or wilting to partial or total desiccation on a given plant. Yield losses varied according to symptom type, but the relationship between symptoms recorded and yield also depended on scoring dates. The data suggest that the beginning of tuber growth might be the most suitable stage for predicting yield losses from symptom observations. In both cultivars studied (Bintje, highly susceptible, and Désirée, moderately resistant), the yield of symptomless plants growing from inoculated seed tubers was significantly less than that of control plants, indicating that the presence of bacteria on the seed tuber was detrimental, even in the absence of visible symptoms. Differences in symptom expression in the field between cultivars matched the level of visible infection of tubers at harvest, as Bintje tubers showed a higher incidence of rot than Désirée tubers.     

Biological control of potato soft rot caused by Dickeya solani and the survival of bacterial antagonists under cold storage conditions

Dickeya and Pectobacterium are responsible for causing blackleg of plants and soft rot of tubers in storage and in the field, giving rise to losses in seed potato production. In an attempt to improve potato health, biocontrol activity of known and putative antagonists was screened using in vitro and in planta assays, followed by analysis of their persistence at various storage temperatures. Most antagonists had low survival on potato tuber surfaces at 4 °C. The population dynamics of the best low-temperature tolerant strain and also the most efficient antagonist, Serratia plymuthica A30, along with Dickeya solani as target pathogen, was studied with TaqMan real-time PCR throughout the storage period. Tubers of three potato cultivars were treated in the autumn with the antagonist and then inoculated with D. solani. Although the cell densities of both strains decreased during the storage period in inoculated tubers, the pathogen population was always lower in the presence of the antagonist. The treated tubers were planted in the field the following growing season to evaluate the efficiency of the bacterial antagonist for controlling disease incidence. The potato endophyte S. plymuthica A30 protected potato plants by reducing blackleg development on average by 58.5% and transmission to tuber progeny as latent infection by 47–75%. These results suggest that treatment of potato tubers with biocontrol agents after harvest can reduce the severity of soft rot disease during storage and affect the transmission of soft rot bacteria from mother tubers to progeny tubers during field cultivation.     

Tuber transmission of ‘Candidatus Liberibacter solanacearum’ and its association with zebra chip on potato in New Zealand

Zebra chip, an emerging disease of potatoes, has recently been associated with ‘Candidatus Liberibacter solanacearum’ in New Zealand. The phloem-limited bacterium is known to be vectored by the tomato potato psyllid (Bactericera cockerelli). In this study, the role of tuber transmission in the spread of Ca. L. solanacearum was investigated by re-planting potato tubers infected with Ca. L. solanacearum in the absence of the psyllid. Nested PCR demonstrated that Ca. L. solanacearum could be transmitted from the mother tubers both to the foliage of growing plants and to progeny tubers, resulting in symptomatic and asymptomatic plants. Of 62 Ca. L. solanacearum-infected tubers four did not sprout symptomatic of zebra chip. A further two plants developed foliar symptoms associated with zebra chip during the growing season and died prematurely. Fifty-six of the infected tubers produced asymptomatic plants, although Ca. L. solanacearum was detected in the foliage of 39 of them indicative of transmission into asymptomatic progeny plants. At harvest, Ca. L. solanacearum was found in the daughter tubers of only five of the 39 asymptomatic plants, and only one of these plants was found to have zebra chip symptoms in the daughter tubers. Our results show that tuber transmission of Ca. L. solanacearum could play a role in the life cycle of this pathogen, providing a source for acquisition by Bactericera cockerelli and for movement of the pathogen to other regions of New Zealand via transport of seed tubers.     

Mycorrhiza induced resistance in potato plantlets challenged by Phytophthora infestans

Biological control of soil-borne pathogens by arbuscular mycorrhizal (AM) fungi has been repeatedly demonstrated. However, their role in the control of above-ground hemibiotrophic pathogens is less conclusive. Here, we investigated in vitro the impact of an AM fungus on Phytophthora infestans in potato plants. The leaf infection index was decreased in mycorrhizal potato plants. Real-Time Quantitative PCR revealed the induction of two pathogenesis related genes (PR1 and PR2) in the leaves of mycorrhizal plants shortly after infection with P. infestans. These results suggested a systemic resistance in mycorrhizal plants, related to the priming of the two PR genes in potato.     

Population structure of Phytophthora infestans collected on potato and tomato in Italy

Late blight caused by the oomycete Phytophthora infestans is a disease of potato and tomato of worldwide relevance and is widespread throughout Europe and the Mediterranean region. While pathogen populations in northern Europe have been sampled and characterized for many years, the genetic structure of populations from southern Europe, including Italy, has been less studied. Between 2018 and 2019, we collected 91 samples of P. infestans from potato and tomato crops in Italy, Algeria, and Tunisia on FTA cards and genotyped them using 12-plex microsatellites. These samples were compared to genotypes of P. infestans previously collected within the framework of the EuroBlight network and from published sources. Four clonal lineages were identified: 13_A2 (Blue 13), 2_A1, 23_A1, and 36_A2. Two other isolates collected could not be matched to any currently known clonal lineage. The 13_A2 and 36_A2 lineages were found exclusively in southern Italy and Algeria, while 2_A1 was only found in Algeria. This is the first report of the 36_A2 lineage in Italy. Two isolates from Solanum nigrum were 13_A2, suggesting this weed host could be a reservoir of inoculum. The 23_A1 lineage was found widely on infected tomato crops in Italy and is the same as the lineage US-23 that is widespread in North America. Differences in genotypes across the country suggests that there may be different sources of introduction into Italy, possibly via infected seed tubers from other countries in Europe, tubers for consumption from North Africa, or tomatoes.