Monitoring for a new I3 resistance gene-breaking race of F. oxysporum f. sp. lycopersici (Fusarium wilt) in California processing tomatoes following recent widespread adoption of resistant (F3) cultivars: Challenges with race 3 and 4 differentiation methods

Fusarium wilt, caused by Fusarium oxysporum f. sp. lycopersici (Fol), causes losses in tomato production worldwide, with major impacts on Californian tomato processing. Single-gene resistance is the primary management tool, but its efficacy has been compromised following the emergence of two successive resistance-breaking races, which, in California, emerged within 12 years of resistance deployment. Fol race 3-resistant (F3) processing tomato cultivars (containing the I3 resistance gene) were deployed in the state starting in approximately 2009. The emergence of a new resistance-breaking race (which would be called race 4) is imminent, and early detection will be critical to delay the spread while new resistance is sought. The detection of Fol race 4 is challenged by the lack of validated, rapid, and accurate diagnostic tools. In evaluating in planta phenotyping methods, this study found that rapid seedling phenotyping is not reliable and generates false positives for nonpathogens. Longer (10 weeks) mature plant assays are the most reliable, but may not be sufficiently timely. As an additional challenge, based on field and greenhouse studies, Fol race 3 can cause symptoms in resistant F3 cultivars at frequencies greater (30%) than expected for off-types (<2%). We developed a three-F3 cultivar in planta assay to overcome the challenges this posed to differentiating Fol race 3 and Fol race 4. Using the assay, we determined that all putative resistance-breaking cases were Fol race 3; Fol race 4 was not detected in these early survey efforts. These results highlight the need for developing rapid Fol race 4 detection tools and a better understanding of the factors underlying inconsistent I3 gene expression in Fol race 3.

Phylogenomic Analysis of a 55.1-kb 19-Gene Dataset Resolves a Monophyletic Fusarium that Includes the Fusarium solani Species Complex

Scientific communication is facilitated by a data-driven, scientifically sound taxonomy that considers the end-user's needs and established successful practice. In 2013, the Fusarium community voiced near unanimous support for a concept of Fusarium that represented a clade comprising all agriculturally and clinically important Fusarium species, including the F. solani species complex (FSSC). Subsequently, this concept was challenged in 2015 by one research group who proposed dividing the genus Fusarium into seven genera, including the FSSC described as members of the genus Neocosmospora, with subsequent justification in 2018 based on claims that the 2013 concept of Fusarium is polyphyletic. Here, we test this claim and provide a phylogeny based on exonic nucleotide sequences of 19 orthologous protein-coding genes that strongly support the monophyly of Fusarium including the FSSC. We reassert the practical and scientific argument in support of a genus Fusarium that includes the FSSC and several other basal lineages, consistent with the longstanding use of this name among plant pathologists, medical mycologists, quarantine officials, regulatory agencies, students, and researchers with a stake in its taxonomy. In recognition of this monophyly, 40 species described as genus Neocosmospora were recombined in genus Fusarium, and nine others were renamed Fusarium. Here the global Fusarium community voices strong support for the inclusion of the FSSC in Fusarium, as it remains the best scientific, nomenclatural, and practical taxonomic option available.

Managing Crop Diseases Under Water Scarcity

The significance of water scarcity to crop production and food security has been globally recognized as a pivotal sustainability challenge in the UN Sustainable Development Goals (86). The critical link between water scarcity and sustainability is adaptation. Various changes in water use practices have been employed to alleviate production constraints. However, the potential for these changes to influence crop diseases has received relatively little attention, despite the circumglobal importance of diseases to agricultural sustainability. This article reviews what is known about the realized effects of scarcity-driven alterations in water use practices on diseases in the field in order to raise awareness of the potential for both increased disease risk and possible beneficial effects on crop disease management. This is followed by consideration of the primary mechanistic drivers underlying disease outcomes under various water use adaptation scenarios, concluding with a vision for disease-water co-management options and future research needs.

Effects of Set-Point Substrate Moisture Control on Oomycete Disease Risk in Containerized Annual Crops Based on the Tomato-Phytophthora capsici Pathosystem

In containerized (potted) annual nursery and greenhouse crops, set point-controlled irrigation allows adaptation to increasing water insecurity by precisely reducing water inputs. A key factor influencing adoption is lack of information on disease risk. To facilitate adaptive water use, effects of set-point substrate moisture (SM) control on disease risk and water savings in containerized annual production were evaluated using the Phytophthora capsici-tomato pathosystem (a model system for water stress predisposition to pathogen infection), comparing outcomes of imposing midrange SM (15% volumetric water content [VWC]) and low-range SM (10% VWC) with well-watered (20% VWC) plants. Reducing soil moisture to 10% VWC differentially reduced stem water potential (P < 0.05) and enhanced rate of wilt progress (P = 0.006) and root rot severity (P = 0.03) in P. capsici inoculated plants compared with noninoculated plants. Furthermore, incidence of fine root infections in inoculated asymptomatic plants was greater under reduced SM (10% VWC) compared with in well-watered plants (P < 0.05). Mild reductions to 15% VWC did not influence plant performance (root and shoot weights and plant height) or pathogen infection in either inoculated or noninoculated plants compared with well-watered plants and reduced water inputs by 17%, indicating potential for reducing water usage without increasing disease risk. Furthermore, P. capsici inoculated plants had lower shoot biomass and greater root infection incidence when 15% VWC was applied to older compared with younger plants; the inverse was true for root rot severity, although root rot development was minor overall (P < 0.05). These results indicate that water use reductions pose disease risks, but there is potential to reduce water use and effectively manage plant pathogens in containerized production. Overall, this study indicates that physiological indices should not be solely relied on to develop water reduction methods.

Associations Between Drosophila suzukii (Diptera: Drosophilidae) and Fungi in Raspberries

The invasive vinegar fly, Drosophila suzukii Matsumura, has emerged as one of the most serious arthropod pests of primocane red raspberries (Rubus ideaus L.) in the United States. In raspberries, D. suzukii encounter a diverse community of microbes, including fruit rot pathogens such as Botrytis cinerea Pers and Cladosporium cladosporioides de Vries. In this study, our primary objectives were to evaluate D. suzukii-fungal associations and determine D. suzukii's influence on fungal communities in raspberry fruit. Through culture-based surveys of larval gut microbes, we isolated several yeast fungi (primarily Hanseniaspora spp.), as well as Cladosporium, Botrytis, and several other non-yeast fungi from larval frass, suggesting that D. suzukii larvae encounter and feed on these fungi. Subsequent field surveys confirmed that D. suzukii larvae occurred in berries affected by Botrytis fruit rot and Cladosporium fruit rot. Under laboratory conditions, D. suzukii may facilitate C. cladosporioides infections, likely through the introduction of epiphytic propagules on the fruit surface. We could not detect impacts on B. cinerea infections or establish a clear vectoring relationship for either fruit rot. These studies provide evidence for an association between D. suzukii and fungal fruit rot pathogens. Understanding interactions between raspberry fruit, D. suzukii, and fungal microbes-especially whether D. suzukii facilitates the development of fruit rots or conversely, if fruit rots influence D. suzukii infestation patterns-may improve pest and pathogen management programs.

Characterizing the Brown Spot Pathosystem in Late-Harvest Table Grapes (Vitis vinifera L.) in the California Central Valley

Brown spot, caused by Cladosporium spp., is becoming a problematic postharvest disease of late season table grape (Vitis vinifera) in the California central valley, and management is hindered by knowledge gaps in disease etiology and epidemiology. Brown spot is herein described as a pre- and postharvest dry rot typified by an external brown to black spot or black mycelium which encases the placenta. Isolates in the Cladosporium herbarum and C. cladosporioides species complexes were recovered from 85 and 5% of brown-spot affected berries, respectively. Five isolates in the C. herbarum species complex, representing three phylogenetically distinct species (C. limoniforme, C. ramotenellum, and C. tenellum), and one C. cladosporioides isolate all caused brown spot symptoms under cold-storage conditions, with and without mechanical wounding. Isolate virulence was similar (P > 0.05) based on disease incidence and severity on intact berries but severity varied on wounded berries (P < 0.001). Surface disinfestation reduced severity of cluster rot development following 2 weeks in cold storage (P = 0.027) but incidence was not affected (P = 0.17). This work provides foundational information on brown spot pathosystem etiology and biology in late-harvest table grape, which can be used to improve management.

Evidence for a Hemibiotrophic Association of the Pitch Canker Pathogen Fusarium circinatum with Pinus radiata

Fusarium circinatum can be a cause of mortality in pine seedlings but it is also possible for infected seedlings to remain symptomless. The results of this study documented a biotrophic phase in symptomless Pinus radiata seedlings that can persist for at least 52 weeks. A strain of F. circinatum, transformed to express the green fluorescent protein, was observed to grow intercellularly in the root cortex, with no evidence of damage to surrounding cells. Under experimental conditions, shoot symptoms developed only following collar infection, and root deterioration was seen only in plants that first expressed aboveground symptoms. This sequence of events implies that damage to the root system was a secondary consequence of girdling. If so, root symptoms may not reliably detect seedlings infected by F. circinatum. Supplemental mineral nutrition increased the incidence of infection and symptom development in seedlings but some infected plants remained symptomless, precluding the use of this approach to detect infected seedlings. Overall, our findings suggest that the ecological activities of F. circinatum may not be limited to a necrotrophic association with pine trees. A more comprehensive understanding of the life history of this fungus may yield insights that contribute to more effective management of pitch canker.

First Report of Grass Species (Poaceae) as Naturally Occurring Hosts of the Pine Pathogen Gibberella circinata

Gibberella circinata (anamorph Fusarium circinatum) causes pitch canker in pines and is not known to have any hosts outside the Pinaceae. However, G. circinata is closely related to and interfertile with G. subglutinans, which is associated with grasses both as a pathogen and a commensal endophyte. Furthermore, studies under controlled conditions have shown that G. circinata can colonize corn (Zea mays) without inducing symptoms (4). To determine if G. circinata can also infect grasses under natural conditions, plants were collected in proximity to trees with symptoms of pitch canker in native stands of Pinus radiata (Monterey pine) on the Monterey Peninsula and P. muricata (bishop pine) at Pt. Reyes National Seashore on the California coast during July and August of 2011. Leaves and stems were rinsed in 0.1% Tween 20, immersed in 70% ethanol for 30 s followed by 1 min in 1% NaOCl, and placed on a Fusarium selective medium (FSM) (1). Single-spore subcultures of colonies growing from cultured plant material were transferred to 0.6% KCl agar and identified as G. circinata based on morphological criteria as described by Gordon et al. (2). G. circinata isolates were recovered from Holcus lanatus and Festuca arundinacea on the Monterey Peninsula and H. lanatus at Pt. Reyes National Seashore. Three isolates from each of these sources (nine total) and one known G. circinata isolate from pines (GL 17) were tested for virulence by inoculating 1-year-old, greenhouse-grown Monterey pine trees; three trees were inoculated, once for each isolate. Trees were inoculated by depositing 250 spores in a wound on the main stem by the method described by Gordon et al. (3). Two weeks later, all grass isolates had induced resinous branch cankers with lesions comparable in length (17 to 24 mm) and appearance to those caused by GL 17. Similar results were obtained when inoculations were repeated. One isolate from F. arundinacea and one from H. lanatus (collected at Pt. Reyes National Seashore) were tested and shown to be somatically compatible with tester strains for vegetative compatibility groups C6 and C1, respectively, both of which are associated with isolates previously recovered from diseased pines (2). GL 17 and one isolate each from F. arundinaceae and H. lanatus were tested for their ability to infect F. arundinaceae cv. Fawn. For each isolate, 20 14-day-old seedlings (10 pots with two plants per pot) were sprayed to runoff with an aqueous suspension of 10⁶ spores per ml. All inoculations were repeated once. Two weeks after inoculation, leaves and stems were rinsed briefly in 0.1% Tween 20, immersed for 10 s in 70% ethanol, followed by 30 s in 1% NaOCl, and cultured on FSM. All tested isolates were recovered from at least some of the inoculated plants (range 20 to 100%), from living stems and leaves, as well as from senescing tissue. These results show that grass species can be symptomless hosts for G. circinata, constituting the first documentation of any host for this pathogen outside the Pinaceae. Studies are underway to further characterize the host range of G. circinata and assess the epidemiological implications of grasses as alternate hosts for the pitch canker pathogen. References: (1) B. J. Aegerter and T. R. Gordon. For. Ecol. Manag. 235:14, 2006. (2) T. R. Gordon et al. Mycol. Res. 100:850, 1996. (3) T. R. Gordon et al. Hortscience 33:868, 1998. (4) C. L. Swett and T. R. Gordon. Phytopathology (Abstr.) 89:S126, 2009.