Risk of epidemic development in nurseries from soil inoculum of Phytophthora ramorum

Soilborne inoculum arising from buried, infested leaf debris may contribute to the persistence of Phytophthora ramorum at recurrently positive nurseries. To initiate new epidemics, inoculum must not only survive, but produce sporangia during times conducive to infection at the soil surface. To assess this risk, we performed two year-long experiments in a soil plot at the National Ornamentals Research Site at Dominican University of California. Inoculated rhododendron leaf disks were buried at a depth of 5 or 15 cm in the early summer of 2014 or 2015. Inoculum was baited at the soil surface with non-infested leaf disks (2014 only), then retrieved to assess pathogen viability and sporulation capacity every five weeks. Two 14-week-long trials were conducted in 2016. We were able to consistently culture P. ramorum over all time periods. Soil incubation rapidly reduced the capacity of inoculum to sporulate, especially at 5 cm; however, sporulation capacity increased with the onset of seasonally cooler temperatures. P. ramorum was baited most frequently between November and January, especially from inoculum buried at 5 cm 1-day before the baiting period; in January we also baited P. ramorum from inoculum buried at 15 cm the previous June. We validate prior observations that P. ramorum poses a greater risk after exposure to cooler temperatures and provide evidence that infested leaf debris plays a role in the perpetuation of P. ramorum in nurseries. This work provides novel insights into the survival and epidemic behavior of P. ramorum in nursery soils.

Direct induction of adventitious root in Schefflera octophylla (Lour.) Harms from leaf explants cultured in vitro

Schefflera octophylla (Lour.) Harms is a precious plant species belonging to the Araliaceae family. All parts of plant have been used to create products for human health. In tissue culture of medicinal plants, the induction and multiplication of adventitious root of Schefflera octophylla for biomass collection have been studied. In this report, results on induction of adventitious root from leaf explants cultured in vitro of this plant species were presented. Leaf disks (~ 10 x 10 mm), leaf transverse - thin cell layers (t-TCLs) (~ 3 x 10 mm) were cultured on different mineral media MS, ½MS, B5, SH with NAA (0 - 5 mg/L), sucrose (0 - 50 g/L) and light intensity (0 - 4,000 lux). The results showed that, 30 days after culturing on ½MS solid medium plus 3 mg/L NAA, and 30 g/L sucrose in 4,000 lux light condition, direct formation of adventitious root was best from leaf disks, t-TCLs with rooting rate (%) 100, 100; root number/sample 68.80, 21.96; root lenght (mm) 16.53, 15.53, respectively. Leaf disk culture resulted in better rooting than t-TCL culture in two criteria of root number and root length. Morphological and histological observations of adventitious root primordia formation in the leaf disk were also performed. This is the first report on direct formation of adventitious root by in vitro culture of leaf disks/t-TCLs in Schefflera octophylla with very high efficiency, creating basis for further studies on root biomass multiplication for production of bioactive compounds.

Potential susceptibility of six aquatic plant species to infection by five species of Phytophthora

Investigations of the susceptibility of aquatic plants to species of Phytophthora are limited. Therefore, the objective of this study was to assess the potential susceptibility of six aquatic plant species, frequently used in constructed wetlands or vegetated channels, to infection by five species of Phytophthora commonly found at nurseries in the southeastern United States. In a greenhouse experiment, roots of each plant species (Agrostis alba, Carex stricta, Iris ensata ‘Rising Sun’, Panicum virgatum, Pontederia cordata, and Typha latifolia) growing in aqueous solutions were exposed to zoospores of each of the species of Phytophthora (P. cinnamomi, P. citrophthora, P. cryptogea, P. nicotianae, and P. palmivora). Zoospore presence and activity in solution were monitored using a standard baiting bioassay with rhododendron leaf disks as baits. Experiments were initiated in 2016 and repeated in 2017 and 2018. During the 2016 trials, Phytophthora spp. were not isolated from the roots of any of the plants, but some roots of C. stricta, P. virgatum, and T. latifolia were infected with multiple species of Phytophthora during trials in 2017 and 2018. Presence of plant roots reduced the percentage of rhododendron leaf disks infected by zoospores of four of the species of Phytophthora, but not those infected by P. cinnamomi, which suggested that roots of these plants negatively affected the presence or activity of zoospores of these four species of Phytophthora in the aqueous growing solution. Results from this study demonstrated that certain aquatic plant species may serve as sources of inoculum at ornamental plant nurseries if these plants are present naturally in waterways or used in constructed wetlands treating water flowing off production areas, which could be of concern to plant producers who recycle irrigation water.

Production of Betacyanins in Transgenic Nicotiana tabacum Increases Tolerance to Salinity

Although red betalain pigments (betacyanins) have been associated with salinity tolerance in some halophytes like Disphyma australe, efforts to determine whether they have a causal role and the underlying mechanisms have been hampered by a lack of a model system. To address this, we engineered betalain-producing Nicotiana tabacum, by the introduction of three betalain biosynthetic genes. The plants were violet-red due to the accumulation of three betacyanins: betanin, isobetanin, and betanidin. Under salt stress, betacyanic seedlings had increased survivability and leaves of mature plants had higher photochemical quantum yields of photosystem II (Fv/Fm) and faster photosynthetic recovery after saturating light treatment. Under salt stress, compared to controls betacyanic leaf disks had no loss of carotenoids, a slower rate of chlorophyll degradation, and higher Fv/Fm values. Furthermore, simulation of betacyanin pigmentation by using a red filter cover improved Fv/Fm value of green tissue under salt stress. Our results confirm a direct causal role of betacyanins in plant salinity tolerance and indicate a key mechanism is photoprotection. A role in delaying leaf senescence was also indicated, and the enhanced antioxidant capability of the betacyanic leaves suggested a potential contribution to scavenging reactive oxygen species. The study can inform the development of novel biotechnological approaches to improving agricultural productivity in saline-affected areas.

The Effect of a Biomass Increase on The Carbon Fixation Rates of A. Thaliana

Arabidopsis Thaliana, a model organism for genetic modification, was tested to discover whether or not a genetic increase in biomass affects the plant’s rate of carbon fixation. An experimental design was employed, involving the testing of the carbon fixation rates from 4 separate experimental groups, genetically modified and a control group, both sterilized and unsterilized. Sterile water and a 50% bleach solution were used for seed sterilization, and once complete, all types of seeds were given a week long cold treatment. After the cold treatment had concluded, these plants were grown on moist fertilized soil, and were watered using a glass tray. Water was added into the flat surface every 2 days, and this nourishment was absorbed by the soil through holes in 4.409 x 3.937 terracotta pots. This absorption allowed for dry soil to remoisten, ultimately providing hydration to A. Thaliana. Growth measurements of the observed largest leaf were taken each week for a 4 week growth period. Once fully grown, a procedure known as photosynthetic flotation was used to uncover carbon fixation rates between the experimental groups, involving 0.5 cm leaf disks rising in a bicarbonate solution as carbon dioxide was converted to oxygen. The faster the leaf disk rose, the more rapidly carbon dioxide was fixated. Overall, data gathered from this experiment showed little difference between carbon fixation rates of biomass genetic modification and a control group, causing for this genetic modification only being able to be used to combat overpopulation with larger food being produced, but not that of reversing climate change. However, there was a significant gap in terms of the time to rise of leaf disks between sterilized and unsterilized groups, revealing that a more specific study in regards to this variable must be conducted.

Responses to Drought Stress Modulate the Susceptibility to Plasmopara Viticola in Grapevine

Climate change will increase the occurrence of plants simultaneously suffering drought and pathogen stress. Although it is well-known that drought can alter the way plants respond to pathogens, the knowledge about the effect of concurrent drought and biotic stress in grapevine is scarce. This is especially true for Plasmopara viticola, the causal agent of grapevine downy mildew. This research addresses how vines with different drought tolerance respond to the challenge with P. viticola, drought stress or their combination, and how one stress affects the other. An artificial inoculation was performed on two cultivars, exposed to full or deficit irrigation, in the Mediterranean climate of Cyprus. In parallel, leaf disks from these plants were inoculated in controlled conditions. Leaves were sampled at an early infection stage to determine the influence of the single and combined stresses on oxidative parameters, chlorophyll and phytohormones. Under irrigation, the local Cypriot cultivar Xynisteri was more susceptible to P. viticola than the drought-sensitive Chardonnay. Drought stress increased their susceptibility in leaves inoculated in controlled conditions. Conversely, both cultivars showed resistance against P. viticola when inoculated in planta under continued deficit irrigation. Despite their resistance, the pathogen-associated responses in auxin, antioxidant enzyme activity and proline still occurred in these drought-stressed plants. Surprisingly, abscisic acid, rather than the generally implicated jasmonic and salicylic acid, seemed to play a prominent role in this resistance. The irrigation-dependent susceptibility highlights that the changing climate and the practices used to mitigate its effects, may have a profound impact on plant pathogens.

Plant Preferences of Z-Pheromone Race Ostrinia nubilalis (Lepidoptera: Crambidae) Based on Leaf Tissue Consumption Rates

European corn borer, Ostrinia nubilalis Hübner (Lepidoptera: Crambidae), has been present in the United States for over 100 yr and documented on >200 plant species, including economically valuable crops. The reported preferred host of O. nubilalis is corn, Zea mays L. (Cyperales: Poaceae), although it is considered to be a generalist agricultural pest. Life cycles of the two pheromone races, E and Z, align with the seasonality of different agricultural plants. Since the introduction of Bt corn in 1996, overall O. nubilalis presence has declined and suggests that alternative crop plants might not be suitable hosts. We investigated plant vegetation preference of third-instar Z-race O. nubilalis for leaf disks of corn and a variety of other crops using 48 h no-choice and choice tests. Z-race larvae gained more mass on V6 non-Bt field corn leaf disks in comparison to other plant species options. Additionally, a preference for non-Bt field corn leaf disks was observed in most comparisons. Higher consumption of cucumber, Cucumis sativus L. (Cucurbitales: Cucurbitaceae), leaf disks as compared to non-Bt field corn leaf disks suggested an ability to feed on excised leaf tissues of a plant species that does not induce defenses to herbivory.

Influence of oxathiapiprolin on preinfectional and early infection stages of Plasmopara halstedii, downy mildew of the sunflower

Oxathiapiprolin is a recently introduced fungicide with particular activity against hemibiotrophic and biotrophic oomycetes. For Plasmopara halstedii, the downy mildew of the sunflower, no detailed studies for the activity of the pure compound on the preinfectional and early infection stages in which the fungicide could most effectively interfere with the life cycle of the pathogen have been reported. The fungicide was shown to be active against all stages of the pathogen starting from the release of the zoospores to the development of the mycelia and the formation of the sporangia. Differences in the sensitivity of the different developmental stages are in accordance with the suggested mechanism of the fungicide activity which targets sterol-binding proteins. The experiments showed that, in preventive treatments against very sensitive stages of the pathogen (such as germination of spores), extremely low concentrations of less than 1 ng/mL can completely protect the plants. Coating the seeds with oxathiapiprolin successfully prevented the seedlings from soil-borne infections. This is of particular importance in sunflower cultivation, because wind-borne infections on plants are much rarer and less harmful than soil-borne infections, which usually become systemic and lead to complete yield loss. The curative effects of oxathiapiprolin were shown on the leaf disks as well as on the infected young plants. However, this seems to be less important in sunflower cultivation than, for instance, in viticulture, because spraying sunflowers in the field after the germination of the seeds is uncommon (except in the ornamental cultivation of cut sunflowers).