Susceptibility to Phytophthora ramorum and Inoculum Production Potential of Some Common Eastern Forest Understory Plant Species

Trichoderma guizhouense NJAU4742 augments morphophysiological responses, nutrient availability and photosynthetic efficacy of ornamental Ilex verticillata

Winterberry holly (Ilex verticillata [L.] A. Gray), a deciduous shrub producing glossy bright red berries, is a valuable ornamental and medicinal plant with good market prospects. However, the growth and development of I. verticillata are significantly affected by various stresses, and environmentally hazardous agrochemicals are often used to mitigate them. Trichoderma spp., ubiquitous soil-borne eco-friendly plant growth-promoting fungi, are potent biostimulants and biofertilizers and viable alternatives to agrochemicals for healthy and sustainable agriculture. In this study, the temporal efficacy of different dosages of the filamentous fungus Trichoderma guizhouense NJAU4742 in promoting morphophysiological responses of I. verticillata and the physicochemical properties and enzymatic activities of the substrate were investigated. Different concentrations of the strain T. guizhouense NJAU4742 spore suspension (C [0%], T1 [5%, v/m], T2 [10%, v/m] and T3 [15%, v/m]) were injected in the substrate contained in a pot in which 1-year-old I. verticillata was planted for temporal treatment (15, 45 and 75 days) under open-air conditions. The beneficial effects of T2 and/or T3 treatment for a long duration (75 days) were evident on the different root, aerial and photosynthetic traits; total contents of nitrogen (N), phosphorus (P) and potassium (K) in different tissues and the physicochemical properties of the substrate and its enzymatic activities (urease and invertase). Overall, the study revealed the potency of strain T. guizhouense NJAU4742 as a sustainable solution to improve the growth and development and ornamental value of I. verticillata.

Sporulation Potential of Phytophthora ramorum Differs Among Common California Plant Species in the Big Sur Region

Sudden oak death (SOD), caused by the generalist pathogen Phytophthora ramorum, has profoundly impacted California coastal ecosystems. SOD has largely been treated as a two-host system, with Umbellularia californica as the most transmissive host, Notholithocarpus densiflorus less so, and remaining species as epidemiologically unimportant. However, this understanding of transmission potential primarily stems from observational field studies rather than direct measurements on the diverse assemblage of plant species. Here, we formally quantify the sporulation potential of common plant species inhabiting SOD-endemic ecosystems on the California coast in the Big Sur region. This study allows us to better understand the pathogen's basic biology, trajectory of SOD in a changing environment, and how the entire host community contributes to disease risk. Leaves were inoculated in a controlled laboratory environment and assessed for production of sporangia and chlamydospores, the infectious and resistant propagules, respectively. P. ramorum was capable of infecting every species in our study and almost all species produced spores to some extent. Sporangia production was greatest in N. densiflorus and U. californica and the difference was insignificant. Even though other species produced much less, quantities were nonzero. Thus, additional species may play a previously unrecognized role in local transmission. Chlamydospore production was highest in Acer macrophyllum and Ceanothus oliganthus, raising questions about the role they play in pathogen persistence. Lesion size did not consistently correlate with the production of either sporangia or chlamydospores. Overall, we achieved an empirical foundation to better understand how community composition affects transmission of P. ramorum.

Commodity risk assessment of Robinia pseudoacacia plants from Turkey

The European Commission requested the EFSA Panel on Plant Health to prepare and deliver risk assessments for commodities listed in Commission Implementing Regulation EU/2018/2019 as 'High risk plants, plant products and other objects'. This Scientific Opinion covers plant health risks posed by bare rooted and potted plants of Robinia pseudoacacia that are imported from Turkey, taking into account the available scientific information, including the technical information provided by the Turkish NPPO. The relevance of any pest for this opinion was based on evidence following defined criteria. Three species, the EU-quarantine pests Anoplophora chinensis and Lopholeucapsis japonica, and the EU non-regulated pest Phocazia shantungensis fulfilled all relevant criteria and were selected for further evaluation. For these pests, the risk mitigation measures proposed in the technical dossier from Turkey were evaluated taking into account the possible limiting factors. For these pests, an expert judgement is given on the likelihood of pest freedom taking into consideration the risk mitigation measures acting on the pest, including uncertainties associated with the assessment. The estimated degree of pest freedom varies among the pests evaluated, with L. japonica being the pest most frequently expected on the imported plants. The Expert Knowledge Elicitation indicated, with 95% certainty, that between 9,521 and 10,000 plants per 10,000 would be free of L. japonica.

Indications of Susceptibility to Calonectria pseudonaviculata in Some Common Groundcovers and Boxwood Companion Plants

Knowing the host range of a pathogen is critical to developing and implementing effective disease management programs. Calonectria pseudonaviculata (Cps) is known to attack a number of species, varieties, and cultivars in the genus Buxus as well as three Pachysandra species (Pachysandra terminalis, Pachysandra procumbens, and Pachysandra axillaris) and several Sarcococca species, all in the Buxaceae family. The objective of this study was to evaluate non-Buxaceae groundcovers and companion plants commonly associated with boxwood plantings for their susceptibility to Cps. Twenty-seven plant species belonging to 21 families were exposed to different levels of inoculum: 50 to 300 conidia per drop for detached leaf assays and 30,000 to 120,000 conidia per 1 ml for whole-plant assays. Inoculated plants were incubated in humid environments for at least 48 h to facilitate infection. Cps infection and sporulation were observed on 12 plant species: Alchemilla mollis, Arctostaphylos uva-ursi, Brunnera macrophylla, Epimedium × youngianum, Galium odoratum, Geranium sanguineum, Phlox subulata, Tiarella cordifolia, Callirhoe involucrata, Iberis sempervirens, Mazus reptans, and Vinca minor. These results suggest that there may be more hosts of Cps commonly grown in nurseries and landscapes. If corroborated by observations of natural infection, these findings have implications for the Boxwood Blight Cleanliness Program instituted by the National Plant Board and for planning disease mitigation at production and in the landscape.

The Effect of Leaf Wetness on Phytophthora ramorum Zoospore Infection of Rhododendron 'Cunningham's White' and Viburnum tinus

We performed studies using zoospore inoculum combined from nine isolates of Phytophthora ramorum and determined the effect of leaf wetness on infection of whole plants of Rhododendron 'Cunningham's White' and Viburnum tinus. The mean percentage of infected leaves for both host species increased gradually across a dew chamber moisture period of 1 to 6 h, reaching approximately 80% infection by 6 h. We also evaluated the effect of a postinoculation drying period on infectivity of the two host species with zoospore inoculum. With a 30-min postinoculation drying period, Rhododendron 'Cunningham's White' sustained less than 40% infected leaves, whereas V. tinus had an infection rate of almost 75% infected leaves. Disease percentages for both host species declined sharply with drying periods longer than 30 min. Knowledge of infectivity parameters for P. ramorum will provide a better understanding of epidemic development and lead to improved recommendations for control.

An Overview of Canadian Research Activities on Diseases Caused by Phytophthora ramorum: Results, Progress, and Challenges

International trade and travel are the driving forces behind the spread of invasive plant pathogens around the world, and human-mediated movement of plants and plant products is now generally accepted as the primary mode of their introduction, resulting in huge disturbance to ecosystems and severe socio-economic impact. These problems are exacerbated under the present conditions of rapid climatic change. We report an overview of the Canadian research activities on Phytophthora ramorum. Since the first discovery and subsequent eradication of P. ramorum on infected ornamentals in nurseries in Vancouver, British Columbia, in 2003, a research team of Canadian government scientists representing the Canadian Forest Service, Canadian Food Inspection Agency, and Agriculture and Agri-Food Canada worked together over a 10-year period and have significantly contributed to many aspects of research and risk assessment on this pathogen. The overall objectives of the Canadian research efforts were to gain a better understanding of the molecular diagnostics of P. ramorum, its biology, host-pathogen interactions, and management options. With this information, it was possible to develop pest risk assessments and evaluate the environmental and economic impact and future research needs and challenges relevant to P. ramorum and other emerging forest Phytophthora spp.

Sporangia Production Over Time by Phytophthora ramorum on Rhododendron 'Cunningham's White' After Placement at Different Relative Humidities

We examined the impact of relative humidity (RH) on Phytophthora ramorum sporangia production on Rhododendron 'Cunningham's White'. When diseased plants were maintained under continuous moisture in a mist tent, sporangia were collected from some plants for 22 weeks. More than 3,000 sporangia/leaf/week were collected over the first 3 weeks but levels declined to <100 sporangia/leaf/week after 7 weeks. We also examined the impact of drying on P. ramorum sporangia production. Diseased, detached leaves were maintained in humidity chambers (100, 96.2, 84.5, 74.9, and 56.2% RH) for up to 9 weeks and removed weekly to assess sporulation. For comparison, diseased leaves were harvested from plants maintained with dry foliage or subjected to 10 h of simulated dew nightly. All leaves supported sporulation following 5 weeks at 100% RH, 3 weeks at 96.2% RH, and 1 week at 84.5% RH. All leaves collected from plants subjected to nightly dew supported sporulation for 3 weeks; however, only 66.7% of leaves collected from plants with dry foliage supported sporulation after 1 week. Knowledge of the effects of RH levels on P. ramorum sporulation capacity will prove useful in terms of disease management recommendations and for development of predictive models and pest risk assessments.

Pyracantha 'Mohave' Fruit Infection by Phytophthora ramorum and Transmission of the Pathogen from Infected Fruit to Roots of Viburnum tinus

Colonization of the fleshy fruit of Cornus florida, C. kousa, Laurus nobilis, Malus hupehensis, and Pyracantha 'Mohave' was observed following inoculation with sporangia of Phytophthora ramorum. However, abundant production of chlamydospores was only observed in the fruit of Pyracantha 'Mohave'. Pyracantha 'Mohave' fruit that had been inoculated with a P. ramorum sporangia suspension were placed in pots containing rooted cuttings of Viburnum tinus in a misting tent or in water-filled trays in a climate-controlled greenhouse. Runoff was collected for 24 to 30 days, and roots were plated after the final collection. Mean percent recovery from infected roots was not significantly different (P = 0.05, Tukey's test) between bottom-watered treatments in trays and misted treatments, averaging 58% for bottom-watered and 54% for mist treatments. The number of CFU collected in runoff from bottom-watered plants was consistently lower than that obtained from plants held under mist, likely due to desiccation of the fruit. The results show that root infection of V. tinus can occur by P. ramorum via infected fruit of Pyracantha 'Mohave'. This phenomenon represents a pathway of infection for P. ramorum not previously reported, which may play a role in disease epidemiology.

Differences in Virulence and Sporulation of Phytophthora kernoviae Isolates Originating From Two Distinct Geographical Regions

Phytophthora kernoviae has only been isolated from the United Kingdom and New Zealand. To understand what differences may exist between isolates from these two distinct geographical regions, virulence studies on three host plants and sporulation on host leaves were conducted on select isolates. Three host plant species (Rhododendron ponticum, Magnolia stellata, and Annona cherimola) were inoculated individually with sporangia of six different isolates from each geographical region. Results showed an overall higher virulence on all three hosts from isolates originating from the United Kingdom. After inoculation, P. kernoviae sporangia and oospore formation on different host leaves were observed and compared with P. cactorum and P. syringae. Results were host dependent, with P. kernoviae producing generally similar or higher amounts of both propagules compared with the other U.S. indigenous species. These results have implications for regulatory agencies and scientists who are interested in preventing its entrance into the United States and learning more about its potential spread.

Combining inferential and deductive approaches to estimate the potential geographical range of the invasive plant pathogen, Phytophthora ramorum

Phytophthora ramorum, an invasive plant pathogen of unknown origin, causes considerable and widespread damage in plant industries and natural ecosystems of the USA and Europe. Estimating the potential geographical range of P. ramorum has been complicated by a lack of biological and geographical data with which to calibrate climatic models. Previous attempts to do so, using either invaded range data or surrogate species approaches, have delivered varying results. A simulation model was developed using CLIMEX to estimate the global climate suitability patterns for establishment of P. ramorum. Growth requirements and stress response parameters were derived from ecophysiological laboratory observations and site-level transmission and disease factors related to climate data in the field. Geographical distribution data from the USA (California and Oregon) and Norway were reserved from model-fitting and used to validate the models. The model suggests that the invasion of P. ramorum in both North America and Europe is still in its infancy and that it is presently occupying a small fraction of its potential range. Phytophthora ramorum appears to be climatically suited to large areas of Africa, Australasia and South America, where it could cause biodiversity and economic losses in plant industries and natural ecosystems with susceptible hosts if introduced.

Infectivity and Inoculum Production of Phytophthora ramorum on Roots of Eastern United States Oak Species

Little is known about colonization of roots of trees by Phytophthora ramorum. We examined zoospore concentration and exposure time needed to infect six Quercus (oak) species and the inoculum produced from their roots. Sprouted acorns, exposed to zoospores (3,000/ml) for different times and transplanted to potting soil, were susceptible to infection within 1 h of exposure but root weights were not impacted after 4 weeks (P = 0.952). Roots of Quercus prinus seedlings, inoculated with sporangia, had 0.6 to 3.2% colonization of the total root mass after 5 months. Neither root lesions nor obvious root sloughing were observed. Inoculum threshold levels were tested by exposing radicles to varying zoospore concentrations for 24 h. Results showed that radicle infection occurred even at 1 zoospore/ml. To test inoculum production, roots were inoculated with sporangia and transplanted into pots. Periodically, samples of runoff were collected and plated on selective medium. Afterward, root segments were plated to calculate percent colonization. After 16 and 35 days, root colonization and inoculum production from oak was lower than that of Viburnum tinus, a positive control. This study shows that P. ramorum is able to infect sprouted oak acorns and produce secondary inoculum, which may be important epidemiologically.

Phytophthora ramorum Occurrence in Ornamentals in Serbia

In a survey to determine the presence of Phytophthora ramorum in Serbia, ornamentals from garden centers, nurseries, and private and public gardens, as well as imported plant material, were inspected. In total, 577 plant, soil, and potting media samples were tested using various detection methods: lateral flow diagnostic test, enzyme-linked immunosorbent assay, conventional polymerase chain reaction, and isolation, followed by identification based on growth characteristics in culture and morphological features. P. ramorum was not detected in any of the 162 soil or potting media tested by the baiting method. P. ramorum was detected in 12 Rhododendron samples from one private garden in Zemun (City of Belgrade District) exhibiting symptoms of leaf necrosis and blight and petiole necrosis, and in three samples of Pieris spp. from one garden center exhibiting symptoms of leaf necrosis. Eight Phytophthora isolates were obtained from the positive Rhododendron plants and three isolates from Pieris plants, and all were identified as P. ramorum on the basis of their uniform morphological and growth characteristics. P. ramorum conformation was also made by sequencing of the internal transcribed spacer regions for a single isolate taken from one infected rhododendron and one pieris plant. Serbian isolates were determined as A1 mating type, due to formation of a few typical sexual structures when crossed with the A2 mating type of P. cinnamomi and P. cryptogea. Pathogenicity test on nonwounded detached leaves of 19 popular ornamentals, as well as the most frequently imported ones, revealed that 10 host species were susceptible, including Robinia pseudoacacia, which is widely distributed in Serbia. During this study, Cotoneaster horizontalis and C. dammeri were determined to be new experimental hosts of P. ramorum. This article provides evidence of P. ramorum introduction into Serbia. Although P. ramorum has not been detected in Serbian production nurseries, its presence outdoors might cause severe damages on susceptible common urban plants in public green and natural ecosystems.