Management of Plant Pathogens
Grapevine pruning strategy affects trunk disease symptoms, wood pathobiome and mycobiome
Vine training and pruning are cultural strategies that can be deployed to manage grapevine trunk diseases (GTDs). Forty-year-old commercial vineyards in the Cognac region, France, trained to either Guyot-Arcure (severe pruning) or Guyot-Poussard (minimal pruning), were studied to determine how the two systems affected trunk disease symptomatology. Effects of pruning practices on the pathobiome and mycobiome of asymptomatic grapevines were also assessed, using culture- and amplicon-based Illumina sequencing approaches. The hypothesis examined was that severe pruning of Guyot-Arcure increases trunk diseases incidence and severity, and causes higher pathogen load and microbial diversity, compared to Guyot-Poussard. Numbers of symptomatic and asymptomatic vines for the two training systems were recorded over 3 years, including numbers of vines with esca foliar symptoms, and partially unproductive and dead vines. Six asymptomatic vines from each pruning method were selected, and culturing and sequencing data were obtained from 27 samples per vine. Fungi in the Phaeomoniellaceae, Togniniaceae, and Botryosphaeriaceae were the most frequently identified. The data indicated that severe pruning increased risk of pathogen infections, with Phaeomoniella chlamydospora, Phaeoacremonium minimum and Diplodia sp. being the most commonly identified fungi. Greater numbers of dead or dying vines were recorded in the severely pruned vineyard, indicating that this strategy shortens vine longevity. Results also showed that severe pruning increased endophytic microbial diversity, and that the pruning methods influenced mycobiome community composition. This knowledge will improve recommendations to growers for practical and cost-effective ways to manage GTDs.
Delivery, uptake, fate, and transport of engineered nanoparticles in plants: a critical review and data analysis
The increasing demand for food coupled to various environmental pressures, is increasing the importance of sustainable agricultural practices. Based on results published across a wide range of disciplines, it is becoming evident that nanotechnology can play a crucial role in increasing the sustainability of agriculture, particularly in the area of fertilizer delivery, gene modification, and pest control. In this paper, we review critical plant morphological and physiological indices (pore size in xylem and phloem, xylem/phloem sap composition, xylem/phloem sap flow rate and flow conducting area) for nanoparticle (NP) transport, and examine the efficacy of various delivery methods for NPs (foliar application, root application, and feeding/injecting directly into plant tissue) with an emphasis on NP transport efficiency throughout the entire plant. While only few studies have explored the feeding/injection of NPs, these application pathways are the most efficient in terms of transport, indicating their practical potential (e.g., for agrochemical delivery). In contrast, when applied via soil drenching or foliar spraying, the majority of the applied NPs are not taken up by the plants. However, those NPs that do penetrate the plant exhibit efficient transport from leaf to root, and vice versa. Of these two application methods, foliar application appears to be more effective in both NP delivery and transport than soil drenching. To further explain the data reported in the literature and to study the transport processes of NPs throughout the plant, we applied the Derjaguin-Landau-Verwey-Overbeek model to study the interactions of NPs with the surfaces of the plant vascular system (xylem and phloem), by which these NPs are transported throughout the plant structure. We found that the interaction energy between negatively charged NPs and plant tissue is positive, indicating that these NPs can effectively transport. We discuss future research needs regarding NP transport, which will enable effective utilization of NPs for different agricultural applications.
Radicinin from Cochliobolus sp. inhibits Xylella fastidiosa, the causal agent of Pierce’s Disease of grapevine
The fastidious phytopathogenic bacterium, Xylella fastidiosa, poses a substantial threat to many economically important crops, causing devastating diseases including Pierce’s Disease of grapevine. Grapevines (Vitis vinifera L.) planted in an area under Pierce’s Disease pressure often display differences in disease severity and symptom expression, with apparently healthy vines growing alongside the dying ones, despite the fact that all the vines are genetic clones of one another. Under the hypothesis that endophytic microbes might be responsible for this non-genetic resistance to X. fastidiosa, endophytic fungi were isolated from vineyard cvs. ‘Chardonnay’ and ‘Cabernet Sauvignon’ grown under high Pierce’s Disease pressure. A Cochliobolus sp. isolated from a Cabernet Sauvignon grapevine inhibited the growth of X. fastidiosa in vitro. Bioassay-guided isolation of an organic extract of Cochliobolus sp. yielded the natural product radicinin as the major active compound. Radicinin also inhibited proteases isolated from the culture supernatant of X. fastidiosa. In order to assess structure–activity relationships, three semi-synthetic derivatives of radicinin were prepared and tested for activity against X. fastidiosa in vitro. Assay results of these derivatives are consistent with enzyme inactivation by conjugate addition to carbon-10 of radicinin, as proposed previously.
Evaluation of Pruning Wound Susceptibility and Protection Against Fungi Associated with Grapevine Trunk Diseases
Trunk diseases diminish vineyard longevity and productivity in nearly every raisin, table, and wine grape production region worldwide. Fungi causing these diseases infect primarily through pruning wounds. One way to control these diseases is to protect pruning wounds with fungicide applications, which can be problematic because of the limited number of registered products; the difficulty for these products to control numerous taxonomically unrelated organisms; the challenge of these products to protect for the entire period of wound susceptibility; and the difficulties and costs associated with hand application of protection treatments. Our goal was to compare the susceptibility of grapevine pruning wounds to various fungi associated with trunk diseases and to evaluate the efficacy of selected fungicides to control these pathogens when applied as pruning wound protectants. The study was conducted over two consecutive years in two separate vineyards in Sonoma and Colusa counties, California. Nine pathogenic fungi were tested: Eutypa lata, Botryosphaeria dothidea, Diplodia seriata, Dothiorella viticola, Lasodiplodia theobromae, Phaeomoniella chlamydospora, Pleurostomophora richardsiae, Togninia minima, and Phaeoacremonium parasiticum. Results showed differences in the infection rates of pruning wounds by these fungi. Species of Botryosphaeriaceae were the most infectious, T. minima, P. parasiticum, P. richardsiae, and E. lata were less infectious, and Pa. chlamydospora was intermediate. Four selected fungicides were tested: 1% Topsin M, Biopaste (5% boric acid in a wound-sealing paste), 1% Cabrio EG, and Garrison. Although results highlight the difficulty of these products to control the entire spectrum of pathogens efficiently, Topsin M was overall the most efficacious product.
Use of Boron for the Control of Eutypa Dieback of Grapevines
Eutypa dieback is a perennial canker disease of grapevine (Vitis vinifera) caused by Eutypa lata. The fungus produces ascospores, which infect grapevines through pruning wounds during the dormant season. Management of the disease has been achieved with fungicide applications during the dormant period. However, no effective fungicide was available for this purpose after Benlate was withdrawn from the market. Boric acid (17.5% a.i. boron), a potential alternative to Benlate, was tested in the present study against E. lata. The EC50 values for inhibition of mycelial growth and ascospore germination were 125 and 475 μg of boric acid per ml (22 and 83 μg a.i./ml), respectively. Two boron-based treatments were developed and tested in vitro and in four field trials during 2001 to 2003. One product, biopaste, contained 5% boric acid (8.75 mg a.i./ml) in a commercial paste. The second product, bioshield, contained 5% boric acid in a spore suspension of Cladosporium herbarum. Both products significantly reduced disease in vitro and in field trials in comparison with a water control treatment. Boron was not found to accumulate in leaves and shoots, but bud failure at the first node below the treated wound occurred at a higher rate than in untreated vines.
Delivery, Fate, and Mobility of Silver Nanoparticles in Citrus Trees
Crop disease control is crucial for the sustainable development of agriculture, with recent advances in nanotechnology offering a promising solution to this pressing problem. However, the efficacy of nanoparticle (NP) delivery methods has not been fully explored, and knowledge regarding the
fate and mobility of NPs within trees is still largely unknown. In this study, we evaluate the efficiency of NP delivery methods and investigate the mobility and distribution of NPs with different surface coatings (citrate (Ct), poly-vinylpyrrolidone (PVP), and gum Arabic (GA)) within Mexican lime citrus
trees. In contrast to the limited delivery efficiency reported for foliar and root delivery methods, petiole feeding and trunk injection are able to deliver a large amount of NPs into trees, although petiole feeding takes much longer time than trunk injection (7 days vs 2 h in citrus trees). Once NPs enter plants, steric repulsive interactions between NPs and conducting tube surfaces are predicted to facilitate NP transport throughout the plant. Compared to PVP and Ct, GA is highly effective in inhibiting the aggregation of NPs in synthetic sap and enhancing the mobility of NPs in trees. Over a 7 day experimental period, the majority of the Ag recovered from trees (10 mL, 10 ppm GA-AgNP suspension) remain throughout the trunk (81.0% on average), with a considerable amount in the roots (11.7% on average), some in branches (4.4% on average), and a limited amount in leaves (2.9% on average). Furthermore, NP concentrations during injection and tree incubation time postinjection are found to impact the distribution of Ag in tree. We also present evidence for a transport pathway that allows NPs to move from the xylem to the phloem, which disperses the NPs throughout the plant architecture, including to the roots.
Cost–benefit analysis of nanofertilizers and nanopesticides emphasizes the need to improve the efficiency of nanoformulations for widescale adoption
Nanotechnology-based approaches have demonstrated encouraging results for sustainable agriculture production, particularly in the field of fertilizers and pesticide innovation. It is essential to evaluate the economic and environmental benefits of these nanoformulations. Here we estimate the potential revenue gain/loss associated with nanofertilizer and/or nanopesticide use, calculate the greenhouse gas emissions change from the use of nanofertilizer and identify feasible applications and critical issues. The cost–benefit analysis demonstrates that, while current nanoformulations show promise in increasing the net revenue from crops and lowering the environmental impact, further improving the efficiency of nanoformulations is necessary for their widescale adoption. Innovating nanoformulation for targeted delivery, lowering the greenhouse gas emissions associated with nanomaterials and minimizing the content of nanomaterials in the derived nanofertilizers or pesticides can substantially improve both economic and environmental benefits.
Long-Term Benefits of Protecting Table Grape Vineyards against Trunk Diseases in the California Desert
Grapevine trunk diseases (GTD) are caused by several fungal species and are major limiting factors to vineyard productivity and profitability in all viticulture areas. This study is aimed at addressing the gap in the knowledge with regards to measuring the long-term benefits of post-pruning fungicide application on trunk diseases incidence and crop yield in grape production systems. It also calculated the net economic benefit of implementing such practice over the vineyard lifespan. We selected a newly planted commercial table grape vineyard in the California desert and divided it in two blocks. In one block, the registered fungicide thiophanate-methyl was mechanically applied on pruning wounds for six consecutive years, while the other half remained untreated. Our results showed a significant lower GTD incidence and vine replants in treated blocks combined with a significant increase of total and marketable fruit. Potential annual economic benefits of applying fungicide on pruning wounds appear to be in the range of $8500–$12,500 per hectare annually in a 50–75% disease control scenario.
Synthesis of Deoxyradicinin, an Inhibitor of Xylella fastidiosa and Liberibacter crescens, a Culturable Surrogate for Candidatus Liberibacter asiaticus
Pierce’s disease of grapevine and citrus huanglongbing are caused by the
bacterial pathogens Xylella fastidiosa and Candidatus Liberibacter asiaticus (CLas), respectively.
Both pathogens reside within the plant vascular system, occluding water and nutrient transport,
leading to a decrease in productivity and fruit marketability and ultimately death of their hosts.
Field observations of apparently healthy plants in disease-affected vineyards and groves led to
the hypothesis that natural products from endophytes may inhibit these bacterial pathogens.
Previously, we showed that the natural product radicinin from Cochliobolus sp. inhibits X.
fastidiosa. Herein we describe a chemical synthesis of deoxyradicinin and establish it as an inhibitor of both X. fastidiosa and Liberibacter crescens, a culturable surrogate for CLas. The key to this three-step route is a zinc-mediated enolate C-acylation, which allows for direct introduction of the propenyl side chain without extraneous redox manipulations.
Quantitative Trait Locus Analysis in Avocado: The Challenge of a Slow-maturing Horticultural Tree Crop
The glossy, green-fleshed fruit of the avocado (Persea americana) has been the object of human selection for thousands of years. Recent interest in healthy nutrition has singled out the avocado as an excellent source of several phytonutrients. Yet as a sizeable, slow-maturing tree crop, it has been largely neglected by genetic studies, owing to a long breeding cycle and costly field trials. We use a small, replicated experimental population of 50 progeny, grown at two locations in two successive years, to explore the feasibility of developing a dense genetic linkage map and to implement quantitative trait locus (QTL) analysis for seven phenotypic traits. Additionally, we test the utility of candidate-gene single-nucleotide polymorphisms developed to genes from biosynthetic pathways of phytonutrients beneficial to human health. The resulting linkage map consisted of 1346 markers (1044.7 cM) distributed across 12 linkage groups. Numerous markers on Linkage Group 10 were associated with a QTL for flowering type. One marker on Linkage Group 1 tracked a QTL for β-sitosterol content of the fruit. A region on Linkage Group 3 tracked vitamin E (α-tocopherol) content of the fruit, and several markers were stable across both locations and study years. We argue that the pursuit of linkage mapping and QTL analysis is worthwhile, even when population size is small.
Advancing Biopesticide Technologies for Managing Pierce’s Disease - 2022 field trial update
The objective of this trial is to develop a range of efficacious biopesticides for commercial
application against Pierce’s disease (PD). This project focuses on field testing of natural biological products for control of PD. Biological products are microbes that live in association with plants. Our research groups isolated biocontrols, both bacterial and bacteriophage-based, from agricultural samples, including grapevine wood. Bacteriophages are viruses that selectively infect and kill bacteria but do not infect plant or animal cells. This research aimed to generate the data needed for commercialization of new biopesticides that can be used by grape growers to manage PD.