Search Results (18)

Avocado tree wilt is a disease caused by Phytophthora cinnamomi Rands. Recently, this disease has been associated to Pythium amazonianum, another causal agent. Avocado tree wilt is being currently controlled with synthetic fungicides that kill beneficial microorganisms, polluting the environment and leading to resistance problems in plant pathogens. The current research work aims to provide alternative management using extracts from Proboscidea parviflora W. and Phaseolus lunatus L. to control the development of mycelia in P. amazonianum in vitro. Raw extracts were prepared at UAAAN Toxicology Laboratory, determining the inhibition percentages, inhibition concentrations and inhibition lethal times. Several concentrations of the plant extracts were evaluated using the poisoned medium methodology, showing that both extracts control and inhibit mycelial development, in particular P. lutatus, which inhibits mycelial growth at concentrations lower than 80 mg/L, being lower than P. parviflora extracts. These extracts are promising candidates for excellent control of Pythium amazonianum. Full article

Holm oaks (Quercus ilex L.) can suffer severe infection by the oomycete Phytophthora cinnamomi Rands; the production of more tolerant plants is, therefore, required. Embryo formation is a key period in the establishment of epigenetic memory. Somatic embryos from three holm oak genotypes were elicited, either over 3 days or 60 days, with methyl-jasmonate, salicylic acid (SA), β-aminobutyric acid (BABA), or benzothiadiazole (all at 50 μM and 100 μM), or 10% and 30% of a filtered oomycete extract (FILT10 and FILT30) to activate plant immune responses. The number of embryos produced and conversion rate under all conditions were recorded. Some elicited embryos were then exposed to P. cinnamomi in dual culture, and differential mycelial growth and the progression of necrosis were measured. The same was performed with the roots of germinated embryos. Within genotypes, significant differences were seen among the elicitation treatments in terms of both variables. Embryos and roots of 60-day BABA, SA, or FILT10 treatments inhibited mycelium growth. The 3-day BABA (either concentration) and 60-day FILT10 induced the greatest inhibition of necrosis. Mycelium and necrosis inhibition were compared with those of tolerant trees. Both inhibitions might be a defense response maintained after primed embryo germination, thus increasing the likelihood of tolerance to infection. Full article

Root rot affecting holm oak is a cause of high ecological and economic losses in the Iberian Peninsula, highlighting the relevance of developing disease control methods. The aim of this work was to assess the effect of a biological treatment composed of beneficial organisms (Trichoderma complex, T-complex) on holm oak seedlings infected by Phytophthora cinnamomi in two contrasted holm oak ecotypes, one considered highly susceptible (HU) and another considered tolerant to the pathogen (GR). For this purpose, a complete multifactorial test was carried out in a greenhouse, and seedlings were monitored for survival analysis and morphological and physiological attribute evaluation. Mortality began earlier in the susceptible (HU) than in the tolerant (GR) ecotype, and survival showed different trends due to the inoculation with beneficial microorganisms depending on the ecotype of the plants. The tolerant ecotype showed a high survival rate and better response to the treatment with beneficial microorganisms. GLM showed that the main reason for differences between treatments was ecotype, followed by T-complex and irrigation, and a weak interaction between ecotype and P. cinnamomi was found. The linear relationship between photosynthesis (A) and transpiration (Tr) showed an increase in the A/Tr rates for infected and inoculated plants under drought conditions for the GR ecotype. The tolerant ecotype was benefited more by the beneficial microorganism treatment. The understanding of the genetic diversity of Q. ilex and water stress influence on the efficacy of biological treatments against root rot provides useful information to develop environmentally friendly disease control methods to address the holm oak decline. Full article

Stem diameter growth in living trees refers to the invariably increase in dimension over a given period. However, reversible stem diameter decrease could occur, related to water movement in the vessels within the wood (on a daily basis) and to tree water deficit and depletion of stored water (on a seasonal basis). Recently, the perennial decrement in stem diameter size has been related to a tree decline pathway, and eventually resulting in tree death. In this study, we analysed stem diameter intra-annual growth dynamics of holm oak (Quercus rotundifolia Lam.) trees for two full growing seasons in distinct woodlands, Laborela and Aldeia dos Fernandes, in southern Portugal (Ourique district). Our focus was to assess stem diameter growth patterns and trends in holm oak trees in endangered woodlands with high tree mortality and to question if perennial decrement. Holm oaks in Laborela were much more sensitive in their stem diameter variations than in Aldeia dos Fernandes and, on average, their stem diameter decreased continuously by 1.0 mm along the 2-year study period, with a slightly higher annual decrease in the first study year (0.6 mm.yr⁻¹). In addition, on average, trees had a higher decrease in stem sectional area of 5.8 cm² in Laborela against a decrease of 3.7 cm² in Aldeia dos Fernandes, where the stem diameter of holm oaks only decreased in the first study year (0.1 mm.yr⁻¹). In each study area, the repeated- measures ANOVA showed that tree size effect influenced the stem diameter variations in contrast to tree crown defoliation. Trees were, on average, relatively smaller in Laborela, with DBH = 35.5 cm against DBH = 40.6 cm in Aldeia dos Fernandes, and were highly sensitive in their stem diameter decrement along two consecutives full growing seasons. This is the first exploratory study on assessing the stem diameter fluctuations on holm oaks to address a decline pathway. Our results revealed that holm oaks can survive during two consecutive growing seasons, with a continuous decrease in their stem diameters, which might indicate one tree’s decline pathway. Full article

The main threats to chestnut in Europe are climate change and emerging pathogens. Although many works have separately addressed the impacts on chestnut of elevated temperatures and Phytophthora cinnamomi Rands (Pc) infection, none have studied their combined effect. The objectives of this work were to describe the physiology, secondary metabolism and survival of 6-month-old C. sativa seedlings after plants were exposed to ambient temperature, high ambient temperature and heat wave events, and subsequent infection by Pc. Ten days after the warming scenarios, the biochemistry of plant leaves and roots was quantified and the recovery effect assessed. Plant growth and root biomass under high ambient temperature were significantly higher than in plants under ambient temperature and heat wave event. Seven secondary metabolite compounds in leaves and three in roots were altered significantly with temperature. Phenolic compounds typically decreased in response to increased temperature, whereas ellagic acid in roots was significantly more abundant in plants exposed to ambient and high ambient temperature than in plants subjected to heat waves. At recovery, leaf procyanidin and catechin remained downregulated in plants exposed to high ambient temperature. Mortality by Pc was fastest and highest in plants exposed to ambient temperature and lowest in plants under high ambient temperature. Changes in the secondary metabolite profile of plants in response to Pc were dependent on the warming scenarios plants were exposed to, with five compounds in leaves and three in roots showing a significant ‘warming scenario’ × ‘Pc’ interaction. The group of trees that best survived Pc infection was characterised by increased quercetin 3-O-glucuronide, 3-feruloylquinic acid, gallic acid ethyl ester and ellagic acid. To the best of our knowledge, this is the first study addressing the combined effects of global warming and Pc infection in chestnut. Full article

Oak forests are facing multiple threats due to global change, with the introduction and expansion of invasive pathogens as one of the most detrimental. Here, we evaluated the use of soil biological fertiliser Biohumin^® to improve the response of Quercus ilex L. to the soil-borne pathogen Phytophthora cinnamomi Rands by using one-year-old seedlings fertilised at 0, 12.5, and 25% concentrations of Biohumin^® (v/v). Our hypothesis was that plant vigour and response to the pathogen would improve with Biohumin^®. The effects of soil infestation and fertilisation were tested by assessing plant survival, growth, and physiology. The soil infested with P. cinnamomi negatively affected all the studied traits. We observed that a moderate concentration of Biohumin^® (12.5%) increased plant survival. However, a high concentration (25%) reduced the survival compared with the control, probably as a result of the stress caused by both biotic (infection) and abiotic (soil toxicity) factors. Biohumin^® at the highest concentration reduced the plant height-to-stem diameter ratio (H/D) and negatively affected plant biomass and physiological activity. Combined biofertilisation and infection induced synergistic negative effects in the leaf water potential compared with infection and fertilisation applied alone. A higher concentration of Biohumin^® may favour pathogens more than plants. Further studies should explore the causes of the negative effect of the high concentration of Biohumin^® observed here and evaluate if lower concentrations may benefit plant survival and physiology against soil pathogens. Full article

Phytophthora cinnamomi causes one of the most important diseases in avocado crop and its chemical management represents 25% of the production cost per year. Induction of plant defense responses by elicitors is a promising strategy that is compatible with sustainable agriculture. This study aimed to evaluate the effect of curdlan application on the induction of defense responses in avocado plants against P. cinnamomi. The trials were conducted under greenhouse conditions, and curdlan leaf spraying was performed one day before the inoculation of the pathogen. The results showed that the application of elicitor significantly increased the protection of avocado plants against P. cinnamomi, decreasing the injury and wilting. The Curd + Phy treatment improved the defenses of plants by increasing the enzymes peroxidase (POD) and polyphenol oxidase (PPO) in the first 3 h after inoculation and increasing the enzymes superoxide dismutase (SOD) and phenylalanine ammonium lyase (PAL) 144 h after inoculation (p < 0.05). Also, chlorophyll and carotenoid content increased or remained stable in Curd + Phy treatment. Therefore, these results suggest that curdlan increases the protection against P. cinnamomi and its protection could be due to an increase in the activity of the enzymes related to the phenylpropanoid pathway as well as the effect on chlorophyll and carotenoids. Full article

The ability of three herbaceous plants (Diplotaxis tenuifolia (L.) DC., Eruca vesicaria L. and Raphanus raphanistrum L.) from Iberian wood pastures to reduce Phytophthora cinnamomi Rands pathogen populations through allelopathic relationships is studied. The inhibitory capacity of their aqueous root extracts (AREs) on mycelial growth and production of P. cinnamomi reproductive structures is analysed in vitro. In addition, Quercus seedlings were grown in infested by P. cinnamomi-soils and with the presence or absence of allelopathic and susceptible herb species to the pathogen to assess the defensive chemical response of Quercus seedlings through their leaf phenolic compounds. Results show a strong inhibitory capacity of AREs on P. cinnamomi activity in vitro and a protective effect of these herb species on Quercus plants against P. cinnamomi in vivo. D. tenuifolia would be especially suited for biological control in the pathogen suppression. Full article

The maternal environment of a tree species can influence the development and resistance of its offspring. Transgenerational induction of resistance is well known in plants but its occurrence in forest tree species has been less reported. Quercus ilex L. (holm oak) is a widespread Mediterranean tree species threatened by the invasive Phytophthora cinnamomi Rands pathogen. The influence of P. cinnamomi on the offspring of infected Q. ilex mother trees has not been studied. This study compared the performance and tolerance to P. cinnamomi of seedlings from non-infected and P. cinnamomi-infected trees. Acorns from Q. ilex trees were collected from five forests. After isolations were conducted in the rhizosphere of several trees, in each forest, three trees were selected as non-infected and three were selected as P. cinnamomi-infected. Forty acorns per tree were weighed and sown under greenhouse conditions, and when plants were aged ~9 months they were challenged with P. cinnamomi. Plant mortality was higher in the offspring of non-infected trees than in the offspring of P. cinnamomi-infected trees (26.2% vs. 21.1%, respectively). Consistently, survival probabilities of seedlings from P. cinnamomi-infected trees were higher than those of seedlings from non-infected trees, particularly in seedlings with reduced growth. Although acorns from healthy Q. ilex trees were heavier than acorns from P. cinnamomi-infected trees, the time to death of inoculated seedlings was not influenced by seed weight. The time to death of seedlings was positively related to belowground mass, particularly to an increased proportion of fine secondary roots. We report transgenerational-induced resistance to P. cinnamomi in Q. ilex triggered by an unknown mechanism independent of acorn mass. Information about the persistence of transgenerational effects in Q. ilex offspring and the influence of these effects on plant fitness is crucial to improve the management and regeneration of this declining species. Full article

American chestnut (Castanea dentata (Marsh.) Borkh) was almost completely wiped out by the fungal pathogen, Cryphonectria parasitica (Murrill) M.E. Barr. Another invasive pathogen, Phytophthora cinnamomi Rands, is devastating American chestnuts in the southern region of the United States. An alternative approach for controlling these pathogens is to use genetic engineering or gene editing. We successfully transformed American chestnut with a detoxifying enzyme, oxalate oxidase, to enhance blight tolerance and more recently with the Cast_Gnk2-like gene, which encodes for an antifungal protein, to be tested for P. cinnamomi putative tolerance. Eight somatic embryo lines were transformed using three methods of selection: semisolid medium in Petri plates, liquid medium in RITA^® temporary immersion bioreactors, or liquid medium in We Vitro containers. No significant differences were found between the treatments. These methods will allow for further testing of transgenes and the development of enhanced pathogen resistance in chestnut. It can serve as a model for other tree species threatened by invasive pests and pathogens. Full article

There is, at the present time, a great demand for chestnut rootstocks with improved resistance to Phytophthora cinnamomi Rands in the nurseries. New genotypes are emerging from European chestnut breeding programs and the production of thriving plants to restore old orchards with low yields due to a high incidence of diseases, namely root rot, is necessary. Micropropagation is a useful technique for clonal propagation. Nevertheless, in vitro culture propagation is genotype-dependent. Consequently, the existing protocols may demonstrate poor reproducibility and low efficacy. Thus, the need to contribute to the development of new micropropagation protocols suitable for large production of emerging genotypes. As a contribution to fill this gap, a three-step protocol was developed by using new combinations of Murashige & Skoog, Woody Plant, and adapted modified Melin-Norkrans media in different stages of the propagation process. About 90% of shoots were rooted, and after three months of acclimatization, 85% of these plants survived and were capable of continuous growth in the field. Currently, this protocol is being used in the production of several hybrid genotypes (with improved resistance to P. cinnamomi), selected from our ongoing breeding program and also in Castanea sativa Mill. and Castanea crenata Siebold and Zucc. species. Full article

Rangeland (known as Dehesas or Montados) is a characteristic ecosystem of the southwestern part of the Iberian Peninsula that occupies approximately 3.5 million ha, representing the most important agrosilvopastoral system in Europe. Nowadays, this situation is changing, being under circumstances of threat due to different aspects that are causing degradation of holm oaks and cork oaks throughout the Iberian Peninsula. These problems are of various kinds, accentuating the disease or syndrome of seca, tree death caused by Phytophthora cinnamomi. For the development of death susceptibility models, maximum entropy algorithms (MAXENT) were used, often widely used in ecological niche models. In the development of models, a wide range of variables (dependents and predictive), both climatic or bioclimatic, geological or soil, vegetation and economic and geographical characteristics were used. The study was carried out at two scales, the Autonomous Community of Extremadura in its entirety, and another more specific work scale, such as seca focus in protected natural areas within the Natura 2000 Network. The regional model showed a total of 1,179,639 ha prone to be affected by this condition, among which, 383,339 ha showed a high potential risk level of seca presence. These models, carried out at local scale in 4 polygons selected within the Natura 2000 Network, showed more than 70% of the land surface studied as areas with risk of suffering seca. Full article

Phytophthora root rot is considered one of the main factors associated with holm oak (Quercus ilex L.) mortality. The effectiveness and accuracy of soilborne pathogen and management could be influenced by soil spatial heterogeneity. This factor is of special relevance in many afforestation of southwestern Spain, which were carried out without phytosanitary control of the nursery seedlings. We selected a study area located in a 15 year-old afforestation of Q. ilex, known to be infested by Phytophthora cinnamomi Rands. Soil samples (n_total = 132) were taken systematically from a grid under 4 trees, and analysed to quantify 12 variables, the colony forming units (cfu) of P. cinnamomi plus 11 physical and chemical soil properties. The combined analysis of all variables was performed with linear mixed models (GLMM), and the spatial patterns of cfu were characterised using an aggregation index (I_a) and a clustering index (ν) by SADIE. Cfu values ranged from 0 to 211 cfu g⁻¹, and the GLMM built with the variables silt, P, K and soil moisture explained the cfu distribution to the greatest extent. The spatial analysis showed that 9 of the 12 variables presented spatial aggregation (I_a > 1), and the clustering of local patches (ν_i ≥ 1.5) for organic matter, silt, and Ca. The spatial patterns of the P. cinnamomi cfu under planted holm oak trees are related to edaphic variables and canopy cover. Small-scale spatial analysis of microsite variability can predict which areas surrounding trees can influence lower oomycetes cfu availability. Full article

The sustainability of “dehesas” is threatened by the Holm oak decline. It is thought that the effects of root rot on plant physiology vary depending on external stress factors. Plant growth and biomass allocation are useful tools to characterize differences in the response to drought and infection. The study of physiological responses together with growth patterns will clarify how and to what extent root rot is able to damage the plant. A fully factorial experiment, including drought and Phytophtora cinnamomi Rands infection as factors, was carried out with Quercus ilex L. seedlings. Photosynthesis, biomass allocation and root traits were assessed. Photosynthetic variables responded differently to drought and infection over time. The root mass fraction showed a significant reduction due to infection. P. cinnamomi root rot altered the growth patterns. Plants could not recover from the physiological effects of infection only when the root rot coincided with water stress. Without additional stressors, the strategy of our seedlings in the face of root rot was to reduce the biomass increment and reallocate resources. Underlying mechanisms involved in plant-pathogen interactions should be considered in the study of holm oak decline, beyond the consideration of water stress as the primary cause of tree mortality. Full article

A series of new and known geranylated phenol/methoxyphenol derivatives has been tested in vitro as inhibitor agents of mycelial growth of Phytophthora cinnamomi. The activity of tested compounds is correlated with the nature, number, and position of the substituent group on the aromatic ring. Results indicate that the most active geranylated derivatives are those having two hydroxyl groups (or one –OH and one –OCH₃) attached to the aromatic ring. Interestingly, these derivatives are as active as Metalaxil^®, a commonly used commercial fungicide. Thus, our results suggest that some of these compounds might be of agricultural interest due to their potential use as fungicides against P. cinnamomi. The effect of structure on fungicide activity is discussed in terms of electronic distribution on both the aromatic ring and side geranyl chain. All tested compounds have been synthesized by direct coupling of geraniol and the respective phenol. Interestingly, new digeranylated derivatives were obtained by increasing the reaction time. Full article