Zoospore Survival Research Articles

An improved method to study Phytophthora cinnamomi Rands zoospores interactions with host

Phytophthora cinnamomi Rands is a highly prevalent phytopathogen worldwide, ranking among the top ten in terms of distribution. It inflicts crown rot, canker, and root rot on numerous plant species, significantly impacting the biodiversity of both flora and fauna within affected environments. With a host range spanning over 5,000 species, including important plants like Quercus suber, Quercus ilex, Castanea sativa, and commercially significant crops such as avocado (Persea americana), maize (Zea mays), and tomato (Solanum lycopersicum), Phytophthora cinnamomi poses a substantial threat to agriculture and ecosystems. The efficient dissemination of the oomycete relies on its short-lived asexually motile zoospores, which depend on water currents to infect host roots. However, managing these zoospores in the laboratory has long been challenging due to the complexity of the life cycle. Current protocols involve intricate procedures, including alternating cycles of growth, drought, and flooding. Unfortunately, these artificial conditions often result in a rapid decline in virulence, necessitating additional steps to maintain infectivity during cultivation. In our research, we sought to address this challenge by investigating zoospore survival under various conditions. Our goal was to develop a stable stock of zoospores that is both easily deployable and highly infective. Through direct freezing in liquid nitrogen, we have successfully preserved their virulence. This breakthrough eliminates the need for repeated culture transfers, simplifying the process of plant inoculation. Moreover, it enables more comprehensive studies of Phytophthora cinnamomi and its interactions with host plants.

Comparative biological traits of perkinsozoan parasitoids infecting marine dinoflagellates

The number of perkinsozoan parasitoid species known to infect dinoflagellates has increased to 11 over the last two decades. However, most of the current knowledge about the autecology of perkinsozoan parasitoids of dinoflagellates has derived from studies of one or two species, thereby making it difficult to directly compare their biological traits at the same time and even their potentials as biological control agents if they are to be exploited to mitigate harmful dinoflagellate blooms in the field. This study investigated total generation time, the number of zoospores produced per sporangium, zoospore size, swimming speed, parasite prevalence, zoospore survival and success rate, and host range and susceptibility for five perkinsozoan parasitoids. Four of the species (Dinovorax pyriformis, Tuberlatum coatsi, Parvilucifera infectans, and P. multicavata) were from the family Parviluciferaceae and one (Pararosarium dinoexitiosum) was from the family Pararosariidae, with dinoflagellate Alexandrium pacificum employed as a common host. Distinct differences in the biological traits of the five perkinsozoan parasitoid species were found, suggesting that the fitness of these parasitoids for the common host species differs. These results thus offer useful background information for the understanding of the impacts of parasitoids on the natural host population and for the design of numerical modeling including the host-parasitoid systems and biocontrol experiments in the field.

The zoospores of the thraustochytrid Aurantiochytrium limacinum: Transcriptional reprogramming and lipid metabolism associated to their specific functions.

Aurantiochytrium limacinum (Thraustochytriaceae, class Labyrinthulomycetes) is a marine Stramenopile and a pioneering mangrove decomposer. Its life cycle involves a non-motile stage and zoospore production. We observed that the composition of the medium, the presence of amino acids in particular, affects the release of zoospores. Two opposite conditions were defined, one with a cell population mainly composed of zoospores and another one with almost only non-motile cells. In silico allelic frequency analysis and flow cytometry suggest that zoospores and non-motile cells share the same ploidy level and are diploid. Through an RNA-seq approach, the transcriptional reprogramming accompanying the formation of zoospores was investigated, with a particular focus on their lipid metabolism. Based on a differential expression analysis, zoospores are characterized by high motility, very active signal transduction, an arrest of the cell division, a low amino acid metabolism and low glycolysis. Focusing on lipid metabolism, genes involved in lipase activities and peroxisomal β-oxidation are upregulated. qRT-PCR of selected lipid genes and lipid analyses during the life span of zoospores confirmed these observations. These results highlight the importance of the lipid dynamics in zoospores and show the metabolic processes required to use these energy-dense molecules as fuel for zoospore survival during their quest of new territories.

Efficacy of a bacterial preparation of Aneurinibacillus migulanus against downy mildew of cucumber (Pseudoperonospora cubensis)

A bacterial preparation of Aneurinibacillus migulanus white (a phenotype isolated in our laboratory after subculturing of A. migulanus WT in TSB medium) was investigated in the host-pathogen system cucumber (Cucumis sativus) and downy mildew (Pseudoperonospora cubensis). With a 1:1 diluted non-separated liquid culture (containing gramicidin S attached to the spores and a biosurfactant released into the broth), an efficacy above 90% was achieved, when applied protectively. Curative or systemic effects were not found. It appeared, that the culture had no influence on the pathogen once it had established in the plant. In vitro, the non-separated liquid preparation of A. migulanus white and the separated spores (both 1:1 diluted) almost completely inhibited the release and the survival of zoospores of another oomycete, Phytophthora infestans. On leaf discs, released and encysted zoospores of P. cubensis were found, but only few germ tubes were formed. In vitro growth of mycelia of P. infestans was inhibited by pure gramicidin S from a concentration of 25 μg ml−1. With 100 μg ml−1 an inhibitory effect of about 85% compared with the water control was recorded. The treatment of cucumber plants with liquid culture of A. migulanus E1 (1:1 diluted; no production of gramicidin S, only biosurfactant) significantly controlled P. cubensis by reducing the time of leaf wetness. For a drying time of plants of 30 min before incubation at 100% relative humidity, the efficacy was 71% when calculated against the water control of the same drying time. Overall, we demonstrated that both, gramicidin S containing spores and the biosurfactant of A. migulanus white liquid culture contributed to the control of P. cubensis on cucumber, based on different modes of action. Spores carrying gramicidin S exhibit a direct effect both, on mycelia as well as on sporangia and zoospores of the oomycetes P. cubensis and P. infestans. The supernatant worked mainly by the effect of faster drying of the leaf surface. We conclude that a liquid culture of A. migulanus has a high potential to control downy mildew on cucumbers and other oomycete diseases.

Efficacy of a plant extract of Macleaya cordata (Willd.) R. Br. against downy mildew of cucumber (Pseudoperonospora cubensis)

Phytopathogenic oomycetes are present worldwide and cause large losses in important crops. Because of strict regulation of copper based fungicides there is a need of alternative plant protecting agents. On this background the efficacy of a plant extract of Macleaya cordata (Willd.) R. Br. was assessed against downy mildew of cucumber (Pseudoperonospora cubensis). The plant extract of M. cordata was produced by using an optimized process including the upgrading of the active ingredients (sanguinarine and chelerythrine). At very low concentrations of 40–50 μg/ml the upgraded extract gave rise to a 90 % reduction of the infestation of cucumber with P. cubensis in bioassays after protective treatment. The plant extract reduced the release and the survival of zoospores of the in vitro cultivable oomycete Phytophthora infestans. The growth of mycelia of P. infestans was also inhibited. In an assay on leaf discs, released and encysted zoospores of P. cubensis could be found, but only few germ tubes were formed.

Oxygen stress reduces zoospore survival of Phytophthora species in a simulated aquatic system.

BackgroundThe genus Phytophthora includes a group of agriculturally important pathogens and they are commonly regarded as water molds. They produce motile zoospores that can move via water currents and on their own locomotion in aquatic environments. However, zoosporic response to dissolved oxygen, an important water quality parameter, is not known. Like other water quality parameters, dissolved oxygen concentration in irrigation reservoirs fluctuates dramatically over time. The aim of this study was to determine whether and how zoospore survival may be affected by elevated and low concentrations of dissolved oxygen in water to better understand the aquatic biology of these pathogens in irrigation reservoirs.ResultsZoospores of P. megasperma, P. nicotianae, P. pini and P. tropicalis were assessed for survival in 10% Hoagland’s solution at a range of dissolved concentrations from 0.9 to 20.1 mg L-1 for up to seven exposure times from 0 to 72 h. Zoospore survival was measured by resultant colony counts per ml. Zoospores of these species survived the best in control Hoagland’s solution at dissolved oxygen concentrations of 5.3 to 5.6 mg L-1. Zoospore survival rates decreased with increasing and decreasing concentration of dissolved oxygen, depending upon Phytophthora species and exposure time. Overall, P. megasperma and P. pini are less sensitive than P. nicotianae and P. tropicalis to hyperoxia and hypoxia conditions.ConclusionZoospores in the control solution declined over time and this natural decline process was enhanced under hyperoxia and hypoxia conditions. These findings suggest that dramatic fluctuations of dissolved oxygen in irrigation reservoirs contribute to the population decline of Phytophthora species along the water path in the same reservoirs. These findings advanced our understanding of the aquatic ecology of these pathogens in irrigation reservoirs. They also provided a basis for pathogen risk mitigation by prolonging the turnover time of runoff water in recycling irrigation systems via better system designs.

Genetic and experimental evidence for a mixed-age, mixed-origin bank of kelp microscopic stages in southern California.

Laboratory studies have demonstrated that the microscopic stages of kelps can rapidly resume development from a delayed state. Like terrestrial seeds or aquatic resting eggs, banks of delayed kelp stages may supplement population recovery after periods of stress, playing an important role for kelp populations that experience adult sporophyte absences due to seasonal or interannual disturbances. We found that removing the microscopic stages from natural rock substratum could prevent the appearance of juvenile kelp sporophytes for three months and the establishment of a diverse kelp assemblage for over four months within a southern California kelp forest. Juveniles were observed within one month in plots where microscopic stages were left intact, which may confer an advantage for the resulting sporophytes as they attain larger sizes before later recruiting neighbors. Microsatellite diversity was high (expected heterozygosity HE approximately 0.9) for juveniles and adults within our sites. Using a microsatellite-based parentage analysis for the dominant kelp, Macrocystis pyrifera, we estimated that a portion of the new M. pyrifera sporophyte recruits had originated from their parents at least seven months after their parents had disappeared. Similar delay durations have been demonstrated in recent laboratory studies. Additionally, our results suggest that zoospore dispersal distances > 50 m may be supported by including additional microsatellite loci in the analysis. We propose a mixed-age and, potentially, a mixed-origin bank of M. pyrifera gametophytes promotes maximal genetic diversity in recovering populations and reduces population genetic subdivision and self-fertilization rates for intact populations by promoting the survival of zoospores dispersed > 10 m and during inhospitable environmental conditions.

The effects of mysid grazing on kelp zoospore survival and settlement.

Recent studies have indicated that long-distance dispersal by kelp zoospores may play an important role in the colonization of newly exposed rocky habitats and in the recovery of recently disturbed kelp forests. This may be facilitated by the vertical transport of zoospores into the shallower portions of the water column where they are exposed to greater alongshore currents that increase their dispersal potential. However, this vertical transport can also expose them to elevated irradiances and enhanced grazing by zooplankton, both of which negatively impact zoospore survival and settlement. In this study, we used plankton tows to show that zooplankton (mysids) were at least seven times more abundant in the surface waters than near the benthos along the edge of a large kelp forest at the time of our spring sampling. We then used feeding experiments and epifluorescence microscopy to verify that these mysids grazed on kelp zoospores. Finally, we conducted laboratory experiments to show that grazing by these mysids over a 12h period reduced kelp zoospore settlement by at least 50% relative to treatments without grazing. Together with previous studies that have revealed the impacts of high irradiance on zoospore survival and settlement, our study indicates that the vertical transport of kelp zoospores into the shallower portions of the water can also expose them to significantly increased mortality from mysid grazing. Thus, if these patterns are consistent over broader temporal and geographic scales, vertical transport may not be a viable method for sustained long-distance zoospore dispersal.

Carbon Dioxide as a Potential Water Disinfestant for Phytophthora Disease Risk Mitigation.

The spread of Phytophthora spp. through irrigation systems and natural waterways can have a significant impact on plant health and requires mitigation. Pressurized carbon dioxide (CO2) can inactivate Phytophthora nicotianae zoospores but its effectiveness at low pressure and on other species was unknown. This study evaluated the effect of injected CO2 at 63 to 4,000 ppm in irrigation water on zoospore survival of four Phytophthora spp. and infectivity of P. nicotianae zoospores. Zoospore survival of P. nicotianae, P. tropicalis, and P. pini was reduced by over 90% at 4,000 ppm and was reduced by 40% at 125 to 2,000 ppm after a 2-h exposure. Survival of P. megasperma was less affected by injected CO2, with a reduction of 37.1% at ≤4,000 ppm. CO2 treatments at 4,000 ppm for 30 or 120 min of water infested with P. nicotianae at zoospore concentrations of 1,000 and 5,000 ml-1 reduced disease incidence of annual vinca (Catharanthus roseus) by 92 and 75%. Comparable efficacy was observed in the CO2 treatment at 2,000 ppm. The CO2 treatments at <2,000 ppm also significantly reduced disease caused by water infested at 1,000 zoospores ml-1. These results indicate that CO2 may have potential as a safe and effective water disinfestant for Phytophthora spp.

Survival of Phytophthora alni,Phytophthora kernoviae, and Phytophthora ramorum in a simulated aquatic environment at different levels of pH

Phytophthora ramorum, Phytophthora alni, and Phytophthora kernoviae present significant threats to biosecurity. As zoosporic oomycetes, these plant pathogens may spread through natural waterways and irrigation systems. However, survival of these pathogens in aquatic systems in response to water quality is not well understood. In this study, we investigated their zoospore survival at pH 3-11 in a 10% Hoagland's solution over a 14-day period. The results showed that all three pathogens were most stable at pH 7, although the populations declined overnight irrespective of pH. Extended survival of these species depended on the tolerance of pH of their germinants. Germinants of P. alni ssp. alni and P. ramorum were more basic tolerant (pH 5-11), while those of P. kernoviae were more acidic tolerant (pH 3-9). These tolerant germinants formed compact hyphae or secondary sporangia to allow longer survival of these pathogens. Long-term survival at a broad pH range suggests that these pathogens, especially P. ramorum, are adapted to an aquatic environment and pose a threat to new production areas through water dispersal.

Effect of electrical conductivity on survival of Phytophthora alni, P. kernoviae and P. ramorum in a simulated aquatic environment

This study investigated survival of the pathogens Phytophthora ramorum, P. alni and P. kernoviae as zoospores or sporangia in response to an important water quality parameter, electrical conductivity (EC), at its range in irrigation water reservoirs and irrigated cropping systems. Experiments with different strengths of Hoagland’s solution showed that all three pathogens survived at a broad range of EC levels for at least 3 days and were stimulated to grow and sporulate at ECs &gt; 1·89 dS m−1. Recovery of initial populations after a 14‐day exposure was over 20% for P. alni subsp. alni and P. kernoviae, and 61·3% and 130% for zoospores and sporangia of P. ramorum, respectively. Zoospore survival of these pathogens at ECs &lt; 0·41 dS m−1 was poor, barely beyond 3 days in pure water; only 0·3% (P. alni), 2·9% (P. kernoviae) and 15·1% (P. ramorum) of the initial population survived after 14 days at EC = 0·21 dS m−1. The variation in rates of survival at different EC levels suggests that these pathogens survive better in cropping systems than in irrigation water. Containment of run‐off and reduction in EC levels may therefore be non‐chemical control options to reduce the risk of pathogen spread through natural waterways and irrigation systems.

Effects of hydrostatic pressure, agitation and CO2 stress on Phytophthora nicotianae zoospore survival

Phytophthora nicotianae Breda de Haan is a common pathogen of ornamental plants in recycled irrigation systems. In a previous study, annual vinca (Catharanthus roseus Don) inoculated with zoospore suspensions using a CO(2)-pressurized sprayer had less foliage blight than plants inoculated using a hand sprayer. Here, the impact of hydrostatic pressure, agitation and aeration with CO(2) on the survival of P. nicotianae zoospores was examined. Exposure of zoospores to 840 kPa hydrostatic pressure for 8 min or agitation at a mixing intensity (G) of 6483 s(-1) for 4 min at 22-23 degrees C did not kill zoospores, but resulted in viable cysts. Motile and forcefully encysted zoospores of P. nicotianae were equally infectious on vinca or lupine (Lupinus polyphylus Lindl.). Bubbling CO(2) into zoospore-infested water at 110.4 mL (0.2 g) min(-1) for 5 min caused 81% reduction in the number of germinated zoospores. Pressure at 630 kPa (16.3 g CO(2)) or 70 kPa (3.85 g CO(2)) facilitated CO(2) injection and shortened the zoospore inactivation time to 30 s. When air was bubbled through the suspension, germination was similar to the control. Exposure to CO(2) killed P. nicotianae zoospores in water. Neither pressure nor agitation had an effect on zoospore viability or infectivity. Based on results of this study, the authors designed a recycling CO(2) water treatment system that is currently under evaluation.

A model predicting primary infections of Plasmopara viticola in different grapevine-growing areas of Italy

A dynamic model for Plasmopara viticola primary infections was evaluated by comparing model predictions with disease onset in: (i) 100 vineyards of northern, southern and insular Italy (1995 to 2007); (ii) 42 groups of potted grapevine plants exposed to inoculum (2006 to 2008). The model simulates the development of any oospore cohort during the primary inoculum season, including oospore germination, production and survival of sporangia, release, survival and dispersal of zoospores, and infection and incubation. The model showed high sensitivity, specificity, and accuracy both in vineyards and in potted plants. The true positive and negative proportions were TPP=0.99 and TNP=0.87, respectively. Because of a certain proportion of false positive predictions (FPP=0.13), confidence in prediction of non-infections was higher than in prediction of infections. These wrong predictions occurred early in the season or when oospore inoculum was low, or were triggered by isolated weak rain events. In only one case (a group of potted plants) there was infection when infection was not predicted (FNP=0.005). The model can be considered an accurate and robust predictor of P. viticola oospore infections and could be used to reduce or improve the timing of fungicide sprays.

Zoosporic Tolerance to pH Stress and Its Implications for Phytophthora Species in Aquatic Ecosystems

Phytophthora species, a group of destructive plant pathogens, are commonly referred to as water molds, but little is known about their aquatic ecology. Here we show the effect of pH on zoospore survival of seven Phytophthora species commonly isolated from irrigation reservoirs and natural waterways and dissect zoospore survival strategy. Zoospores were incubated in a basal salt liquid medium at pH 3 to 11 for up to 7 days and then plated on a selective medium to determine their survival. The optimal pHs differed among Phytophthora species, with the optimal pH for P. citricola at pH 9, the optimal pH for P. tropicalis at pH 5, and the optimal pH for the five other species, P. citrophthora, P. insolita, P. irrigata, P. megasperma, and P. nicotianae, at pH 7. The greatest number of colonies was recovered from zoospores of all species plated immediately after being exposed to different levels of pH. At pH 5 to 11, the recovery rate decreased sharply (P < or = 0.0472) after 1-day exposure for five of the seven species. In contrast, no change occurred (P > or = 0.1125) in the recovery of any species even after a 7-day exposure at pH 3. Overall, P. megasperma and P. citricola survived longer at higher rates in a wider range of pHs than other species did. These results are generally applicable to field conditions as indicated by additional examination of P. citrophthora and P. megasperma in irrigation water at different levels of pH. These results challenge the notion that all Phytophthora species inhabit aquatic environments as water molds and have significant implications in the management of plant diseases resulting from waterborne microbial contamination.

Comparative epidemiology of zoosporic plant pathogens

Loss of zoospores has happened independently several times in different phylogenic lines and has, it is claimed, no major phylogenetic significance. But whether or not, how, and under which conditions plant pathogens retain the ability to produce motile asexual spores has fundamental importance from an ecological and epidemiological perspective. Recent molecular investigations of the early evolution of fungi and oomycetes are shedding light on the issue of zoospore loss in organisms able to cause plant diseases. Zoospore loss may have accompanied the development of new forms of dispersal adapted to the terrestrial environment, or the simplification processes which often follow the shift to parasitic or biotrophic life-forms. In this review we consider hybridisation events between Phytophthora species, long distance dispersal of oomycetes, sporangia and zoospore survival, direct and indirect infection processes and newly observed sporulating structures. These aspects are all relevant features for an understanding of the epidemiology of zoosporic plant pathogens. Disease management should not be based on the presumption that the zoosporic stage is a weak link in the life cycle. Oomycete plant pathogens show remarkable flexibility in their life cycles and ability to adapt to changing environmental circumstances.

LIFE-HISTORY TRADE-OFFS INFLUENCE DISEASE IN CHANGING CLIMATES: STRATEGIES OF AN AMPHIBIAN PATHOGEN

Life-history trade-offs allow many animals to maintain reproductive fitness across a range of climatic conditions. When used by parasites and pathogens, these strategies may influence patterns of disease in changing climates. The chytrid fungus, Batrachochytrium dendrobatidis, is linked to global declines of amphibian populations. Short-term growth in culture is maximal at 17 degrees-25 degrees C. This has been used in an argument that global warming, which increases the time that amphibians spend at these temperatures in cloud-covered montane environments, has led to extinctions. Here we show that the amphibian chytrid responds to decreasing temperatures with trade-offs that increase fecundity as maturation rate slows and increase infectivity as growth decreases. At 17 degrees-25 degrees C, infectious zoospores encyst (settle and develop a cell wall) and develop into the zoospore-producing stage (zoosporangium) faster, while at 7 degrees-10 degrees C, greater numbers of zoospores are produced per zoosporangium; these remain infectious for a longer period of time. We modeled the population growth of B. dendrobatidis through time at various temperatures using delayed differential equations and observational data for four parameters: developmental rate of thalli, fecundity, rate of zoospore encystment, and rate of zoospore survival. From the models, it is clear that life-history trade-offs allow B. dendrobatidis to maintain a relatively high long-term growth rate at low temperatures, so that it maintains high fitness across a range of temperatures. When a seven-day cold shock is simulated, the outcome is intermediate between the two constant temperature regimes, and in culture, a sudden drop in temperature induces zoospore release. These trade-offs can be ecologically important for a variety of organisms with complex life histories, including pathogenic microorganisms. The effect of temperature on amphibian mortality will depend on the interaction between fungal growth and host immune function and will be modified by host ecology, behavior, and life history. These results demonstrate that B. dendrobatidis populations can grow at high rates across a broad range of environmental temperatures and help to explain why it is so successful in cold montane environments.

Further research on the production, longevity and infectivity of the zoospores of Leptolegnia chapmanii Seymour (Oomycota: Peronosporomycetes)

The effect of temperature on the production, survival and infectivity of zoospores of an Argentinean isolate of Leptolegnia chapmanii was determined under laboratory conditions. Production of zoospores of L. chapmanii in vitro and in vivo upon first and fourth instars larvae of the mosquito Aedes aegypti was studied at three different temperatures. Zoospores from infected larvae were infective to mosquito larvae for 51, 12, and 5 consecutive days when maintained at 25, 35, and 10 °C, respectively. Maximum zoospore production in infected fourth-instar larvae was 9.6 ± 1.4 × 10 4 zoosp/larva at 48 h at 25 °C. The average number of zoospores produced by individual fourth-instar Ae. aegypti larvae infected with L. chapmanii was 3.57 ± 0.46 × 10 5 zoospores during 6 consecutive days at 25 °C. Zoospore production in vitro was also affected by temperature with a maximum of zoospores ( n = 47,666/ml) produced at 25 °C. When zoospores produced in vitro were used as inoculum against Ae. aegypti larvae at 25 °C, larval mortality was recorded for 5 consecutive weeks. The encystment process for zoospores took 17–20 min; the germination of cysts (excystment) occurred 5 min after exposure in water to mosquito larvae. The minimal time of contact between zoospores and mosquito larvae to develop infection was two minutes. Infection took place by zoospore attachment onto and then penetration through the larval cuticle or by ingestion of cysts as was confirmed by histological studies. Temperature directly affected infectivity and production of zoospores in vivo and in vitro although L. chapmanii zoospores tolerate a wide range of temperatures.

THE EFFECTS OF HIGH IRRADIANCE ON THE SETTLEMENT COMPETENCY AND VIABILITY OF KELP ZOOSPORES(1).

Elevated irradiance has a profound effect on the successful dispersal and establishment of kelp zoospores, affecting their physiology and viability. The research to date, however, has been on zoospores localized near the benthos, with little attention on the importance of vertical transportation and subsequent exposure to increased irradiance. Therefore, we wanted to investigate the effects of exposure to high irradiance on the reproductive planktonic life-history stages of kelps Macrocystis pyrifera (L.) C. Agardh and Pterygophora californica Rupr. Zoospores of both species were exposed to different irradiances (75, 275, 575, 1,025 μmol photons · m(-2) · s(-1) ) over varying durations (1, 2, 4, 8, 12 h) and subsequently monitored for settlement competency, gametophyte development, and reproductive viability. Settlement success for M.pyrifera was uniform throughout all irradiance × time treatments, while settlement for P.californica decreased with increasing exposure time but not irradiance, although settlement was generally reduced at the highest irradiance level. Following zoospore settlement, germ tube development was visible in the gametophytes of both species within 1 week, although a significant decline of germ tube density in P.californica was observed with increasing irradiance. Similarly, a decrease in germ tube development with increasing exposure was observed across all irradiance levels for M.pyrifera, but irradiance itself was not significant. Further development into embryonic sporophytes was remarkably similar to gametophyte development, suggesting that the effect of exposure of kelp zoospores to high irradiance on subsequent sporophyte production is mediated through gametophyte development as well as zoospore survival.

A mechanistic model simulating primary infections of downy mildew in grapevine

A dynamic model for Plasmopara viticola primary infections on grapevine was elaborated according to a mechanistic approach. Development of the sexual stage of the pathogen was split into different state variables, in which changes from one state to another were regulated by rates depending on environmental conditions. The conceptual model was based on the definition of a primary inoculum season, a seasonal oospore (inoculum) dose, and its division into many coeval cohorts. Each cohort progresses along the primary infection cycle (production and survival of sporangia, release, survival and dispersal of zoospores, infection, appearance of disease symptoms) simultaneously, with a time step of one hour. The model was evaluated over a 12-year period (1995–2006) in Emilia-Romagna (Northern Italy) by comparing simulations with actual observations of the first seasonal symptoms of downy mildew in 43 vineyards. This data set was not used in model building and represents different environmental conditions, with both early and late primary infections. The model showed high sensitivity, specificity and accuracy. The achieved probabilities that an oospore cohort produce or not an infection were 0.994 and 0.999, respectively, while there was practically no probability that an oospore cohort produce infection when infection is not predicted by the model. Because of a small probability (<0.01) that a predicted infection is not a true infection, confidence in prediction of non-infections was higher than in prediction of infections. Most of the wrong positive predictions occurred in early season, when the host was in the earlier growth stages, or when the oospore germination was triggered by isolated weak rain events. Considering that neither calibration nor empirical adjustment of model parameters were necessary to obtain accurate simulation, it was concluded that this model produces a reasonable approximation of the primary infection processes underlying oospore development. The model showed that 86% of the oospore cohorts failed infection, because sporangia did not survive till zoospore release (5.6% of these cohorts), zoospores released from sporangia did not survive until dispersal (83.6%), or the zoospores that dispersed to leaves did not cause infection (10.8%).

Assessment of the control of Phytophthora root rot disease spread by Spin Out ®-treated fabrics in container-grown hardy nursery-stock

A non-woven capillary matting fabric (Tex-R ® Pro), coated with a latex polymer-based formulation of cupric hydroxide (Spin Out ®), developed to cover standing-out areas for weed control and to prevent rooting through, was assessed for its potential to reduce the spread of Phytophthora root rot in container-grown hardy nursery-stock. As well as production bed covers, the fabric was cut into discs and (a) used to cover the tops of plant containers (pot toppers) and (b) inserted to cover the holes in the bottoms of plant containers (disc inserts). These were all tested as barriers to the passage of infective zoospores of Phytophthora cryptogea in enclosed re-circulating irrigation systems growing test plants of Chamaecyparis lawsoniana. Bed covers and disc inserts significantly reduced disease spread, as indicated by the incidence of symptoms and infection, and by bait and colony-forming unit tests of re-circulating water, both in overhead- and trickle-irrigated systems. Pot toppers were not effective. This may be explained by the mode of spread of Phytophthora in such irrigation systems, which appears to be largely via capillary water and therefore not intercepted by toppers. These results were supported by in vitro studies on the passage of zoospores through fresh and used fabric samples which showed that all Tex-R Pro fabric materials significantly reduced the survival of zoospores and zoospore cysts (to between 0% and 4.8%) compared with untreated fabrics (between 32.3% and 42.8%). Analysis of Cu 2+ concentrations in re-circulating irrigation water and in matting samples at the end of the field experiments showed that the majority of the active ingredient stayed bound to the fabric. These results demonstrate that it is possible to deploy Spin Out-treated fabrics in nurseries to help prevent the spread of Phytophthora propagules in addition to their primary use for controlling weeds and rooting through.