Abstract Pseudomonas species are Gram-negative bacterial pathogens that affect a wide range of economically important crops, including almond. Increasing resistance of Pseudomonas syringae to conventional management strategies highlights the need for alternative disease control options. In this study, we isolated and characterized three lytic bacteriophages, including vB_PsyP_Mobley, vB_PsyP_Plaza, and vB_PsyP_Mission, targeting almond-infecting Pseudomonas strains. Host-range analysis across 36 isolates revealed partially overlapping infectivity profiles, with strongest activity against phylogroup 2 (PG2) almond-associated P. syringae pv. syringae and reduced infectivity against phylogenetically distinct isolates, including PG7 P. viridiflava and other crop-associated pathovars. Efficiency-of-plating assays quantitatively supported these host-range patterns. Plaque morphology and transmission electron microscopy demonstrated icosahedral capsids with short, non-contractile tails consistent with tailed dsDNA bacteriophages of the class Caudoviricetes, with vB_PsyP_Mission producing halo-associated plaques consistent with predicted extracellular polysaccharide-modifying proteins. Genomic analyses revealed compact (~ 40 kb) genomes lacking integrases, tRNAs, lysogeny-associated genes, and known virulence or antimicrobial resistance determinants. Comparative genomics showed that vB_PsyP_Plaza and vB_PsyP_Mission likely represent novel isolates within previously described species-level groups, whereas vB_PsyP_Mobley is more genomically divergent. Collectively, these results define the phenotypic and genomic characteristics of three related but distinguishable Pseudomonas phages and provide a basis for future in planta evaluation of their potential utility against almond bacterial blast.

Phytophthora diseases have become increasingly important in California almond production and often seriously impact orchard establishment. Phytophthora mediterranea and P. niederhauserii, isolated from crown and trunk cankers, and P. syringae, mostly obtained from aerial Phytophthora infections, were common species recovered. This represents a shift in species occurring on almond. Mycelial growth of a total of 195 isolates of these species was effectively inhibited by ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin, with effective concentration of fungicide required for a 50% effective concentration (EC50) values of 0.006 to 0.190 μg/ml, 0.006 to 0.317 μg/ml, 0.001 to 0.011 μg/ml, and 0.00011 to 0.00041 μg/ml, respectively, with similar ranges for each species. All isolates were sensitive to mefenoxam, with EC50 values ≤ 0.289 μg/ml. Phytophthora niederhauserii and P. syringae were also sensitive to potassium phosphite (KPO3; EC50 values 2.8 μg/ml to 60.8 μg/ml); however, all but 4 of the 63 isolates of P. mediterranea were resistant (EC50 values between 50.9 μg/ml and 464.3 μg/ml). Fungicidal versus fungistatic activity of the six fungicides at concentrations proportionally scaled based on field rates was determined by vital staining and germination of zoospore cysts of P. citrophthora that were fungicide-treated and then washed. Each fungicide significantly reduced cyst viability. Fluopicolide and oxathiapiprolin had the highest fungicidal activity, whereas KPO3 was mostly fungistatic. With high in vitro toxicity and high direct fungicidal activity at low concentrations, the new ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin have the potential to effectively manage Phytophthora diseases and also reduce pathogen populations. Oxathiapiprolin has been registered for almond, and registration for the other three fungicides is planned.

Xanthomonas arboricola (Xar) is a phytopathogenic bacterial species responsible for economically significant diseases in a wide range of plants, including agricultural, ornamental, and forest species. This study aimed to investigate the genomic basis of host specificity, adaptation, and virulence in Xar through comprehensive comparative genomics. A total of 177 genomes from nine Xar pathovars were analyzed for evolutionary relationships and effector repertoires. From these, 30 genetically diverse genomes were selected for in-depth comparison. Core, unique, and shared genes were identified and functionally annotated, focusing on their potential roles in adaptation and pathogenicity. Nineteen of the genomes were originally misclassified and did not belong to the Xar species. The remaining 158 genomes clustered into three major clades: I (Xar. pv. juglandis), II (Xar. pv. pruni + Xar. pv. corylina), and III (miscellaneous Xar). Clades I and II exhibited high effector diversity, ranging from 38 to 54 genes, with Xar. pv. corylina harboring the most. In contrast, Clade III genomes had significantly fewer effectors, with subclade IIIa containing only 5 and IIIb up to 15. Only one TAL effector was found in nine Xar. pv. corylina strains (with no conserved RVD patterns) and in both Xar. pv. guizotiae strains (up to 31 RVDs identified). Phylogenomic and effectorome analyses revealed potential genomic islands acquired via horizontal gene transfer, encoding metal metabolism genes, type II/IV secretion systems, and DNA modification enzymes. Additionally, several gene losses were observed: 19 genomes lacked flagellar assembly genes, 15 lacked nitrate metabolism genes, and 9 lacked cellulose biosynthesis and secretion genes. In contrast, all genomes possessed a lasso peptide biosynthetic cluster, highlighting recurrent genomic rearrangements through insertions and deletions. This study provides a refined understanding of the genetic diversity and adaptive mechanisms in X. arboricola, emphasizing gene gain/loss events as central to pathovar-specific metabolic and virulence traits. These findings identify novel molecular markers with potential applications in diagnostics and targeted disease control strategies. In particular, the characterization of conserved and lineage-specific effector repertoires provides a framework to inform strategies for breeding resistance through the identification of candidate targets for durable host immunity.

Fire blight, caused by Erwinia amylovora, severely impacts global apple and pear production. Current control measures rely heavily on conventional antibiotics like streptomycin, oxytetracycline, and kasugamycin, which raise concerns regarding resistance development and environmental impacts. This research introduces RejuAgro A (RAA), an antimicrobial produced by Pseudomonas soli 0617-T307, showing potent activity against E. amylovora, including streptomycin-resistant strains. RAA demonstrates efficacy comparable to streptomycin in field trials, effectively reducing fire blight incidence. Studies on the antimicrobial mechanism reveal that RAA inhibits RNA, DNA, and protein synthesis, distinguishing from that of conventional antibiotics. Furthermore, RAA displays broad-spectrum activity against diverse plant bacterial and fungal pathogens. The RAA biosynthesis gene cluster in P. soli is identified, revealing key genes essential for its production. RAA presents an alternative to traditional antibiotics, potentially enhancing sustainable apple and pear production and addressing antibiotic resistance concerns.

The increasing prevalence of Phytophthora crown and trunk cankers on almond in California, especially in newly planted orchards, and pathogen resistance to older fungicides warranted the evaluation of new treatments for improved management. In two orchard trials, Nonpareil almond trees grafted on two rootstocks were soil-inoculated with Phytophthora cactorum, and the soil was treated with ethaboxam, fluopicolide, mandipropamid, oxathiapiprolin, or selected mixtures that were compared with potassium phosphite (KPO3) and mefenoxam. All fungicides except mefenoxam significantly reduced the incidence of disease. Overall, oxathiapiprolin resulted in the highest reduction with few or no trees dying. Fluopicolide ranked second, with efficacy similar to KPO3. Ethaboxam was the least effective. Binding of oxathiapiprolin to dry soil was demonstrated in field and laboratory studies, stressing the importance of application to prewetted soil followed by irrigation. In greenhouse and field studies where trunks of soil-treated trees were wound-inoculated, systemic uptake of the new Oomycota fungicides and mefenoxam and KPO3 into rootstocks and scions of almond trees was evident. Our studies demonstrate that oxathiapiprolin and fluopicolide can provide highly effective control of Phytophthora crown and trunk cankers in young almond orchards. The less effective ethaboxam should be used in mixture with another fungicide such as fluopicolide. Based in part on our studies, oxathiapiprolin was registered on almond in 2021, and registration of fluopicolide, ethaboxam, and mandipropamid (nursery use only) is ongoing. With a range of modes of action available in the future, resistance management can be practiced with fungicide mixtures and rotations.

Phytophthora cinnamomi, the causal agent of Phytophthora root rot (PRR), poses a persistent threat to the United States avocado industry, the top domestic producer and consumer. Avocado growers are facing clonal A2 P. cinnamomi populations challenging their current PRR control methods. In this study, we characterized 125 isolates collected from orchards in California, Florida, Hawaii, Texas, and Puerto Rico for radial growth per day, optimal growth temperature, in vitro fungicide sensitivity, and virulence on D’Anjou pear fruit and UC2001 avocado seedlings. Across all isolates, optimal growth occurred most frequently at a range from 22 to 25 °C; however, a subset of isolates from Hawaii, Florida, and California exhibited higher optimal growth temperatures (28˚C and 30˚C) suggesting thermal adaptation in warmer regions. Potassium phosphite EC50 values spanned from 4.61 to 763.13 µg/ml, with significantly higher insensitivity in isolates from California and Florida, reflecting the continued overuse of this fungicide in these major production states. In contrast, baseline sensitivities to ethaboxam, mandipropamid, mefenoxam, fluopicolide, and oxathiapiprolin were uniformly high, with narrow, unimodal EC50 distributions across states. Finally, a wide range of virulence among isolates was detected using avocado seedlings and D’Anjou pear fruits with isolates from California and Puerto Rico being the most virulent. Together, this data documents extensive phenotypic diversity within clonal A2 P. cinnamomi populations including heat−adapted and phosphite−insensitive lineages, establishes multi−state fungicide sensitivity baselines, and underscores the need for continued surveillance, integrated fungicide stewardship (especially phosphonates), and rootstock screening against phenotypically diverse populations to sustain avocado PRR management and ensure the United States’ avocado industry sustainability and profitability.

A limited number of effective fungicides available warranted the evaluation of new compounds to manage Phytophthora diseases of sweet cherry. In multiyear greenhouse and field studies with Phytophthora cambivora-, P. cactorum-, and P. citricola-inoculated soils, soil applications with the Oomycota fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin significantly reduced incidence and severity of Phytophthora root and crown rots with comparable or higher efficacy than the registered mefenoxam or potassium phosphite. Mid-day stem water potential measurements in the field to determine water stress resulting from root and crown infections agreed with visual tree health ratings. Fluopicolide caused leaf phytotoxicity at a field site with a Monserate sandy loam soil but not at a site with a Yolo silty clay loam soil. Injury was significantly reduced when fluopicolide was rotated with oxathiapiprolin. Soil-applied ethaboxam, fluopicolide, and oxathiapiprolin moved into roots and then acropetally into rootstocks and scions at different amounts where they reduced canker development after wound inoculation with P. citricola. Fluopicolide showed higher mobility than ethaboxam or oxathiapiprolin. Fungicide mobility was also empirically predicted by the adaptive Bromilow model, which uses the partition coefficient logP (indicating lipophilicity or hydrophilicity) and the acid dissociation constant pKa of a compound. Results of our studies support registration of these fungicides and will help to optimize their use on cherry and other tree crops for disease prevention and management.

Cyproconazole (CPZ) was identified as a new postharvest fungicide with incomplete cross-resistance to other demethylation inhibitor fungicides registered for managing major decays of citrus. CPZ was effective on lemons, oranges, grapefruit, and mandarins, significantly reducing sour rot (Geotrichum citri-aurantii) and green mold (Penicillium digitatum). In laboratory and experimental packing line studies, CPZ was compatible with other fungicides, performed well in fruit coatings, and was effective using different application systems. CPZ at 300 to 1,250 μg/ml reduced sour rot incidence of lemons inoculated with a propiconazole (PPZ)-moderately resistant (MR) isolate of G. citri-aurantii from 87.5% in the control to between 54.6 and 13.3%, whereas. green mold caused by an imazalil (IMZ)-resistant (R) isolate of P. digitatum was reduced from 99.0% in the control to between 59.9 and 20.8%. CPZ at 1,000, 2,000, and 4,000 μg/ml in aqueous or storage coating preparations generally performed statistically similar against PPZ-sensitive (S), MR, and highly resistant (HR) isolates of G. citri-aurantii. Sour rot was reduced to zero levels with increasing concentrations for S, and MR isolates, whereas decay caused by the HR isolate was reduced from 97.1% in the control to 26.0% in storage wax treatments. For green mold and sporulation control, storage and pack coatings generally reduced the performance of CPZ, PPZ, IMZ, fludioxonil + azoxystrobin, and mixtures compared with aqueous applications. CPZ was not as effective as IMZ or fludioxonil + azoxystrobin in reducing sporulation. There was no significant difference in efficacy between heated and ambient-temperature flooder applications of CPZ or PPZ in controlling sour rot or green mold. CPZ is currently undergoing registration for postharvest use on citrus in the United States and will be an effective treatment by itself and in mixtures with other fungicides for major decays caused by S and R pathogens.

Scab of peach and almond is caused by Venturia carpophila, but little is known regarding the population genetics of the pathogen, which aids in understanding the epidemiology of the disease and guiding its management. Isolates (735) of V. carpophila were sampled from peach and almond orchards in the Eastern United States and California, respectively. The aims were to determine population genetic diversity and structure of V. carpophila, compare populations of the pathogen from each host, and determine mating type ratios. Measures of gene and genotypic diversity indicated greater diversity among peach as compared with almond populations of the pathogen. Mating types were consistently at equilibrium (clone corrected) for populations from peach, but none of the populations from almond were at equilibrium. The MAT1-1-1 idiomorph was at low frequency or absent from almond populations. All populations on both peach and almond exhibited linkage disequilibrium. Analysis of molecular variance showed structure (71.8% of variance at the scale of the tree and 25.3% at the scale of the geographic region or crop). A discriminant analysis of principal components showed that the peach and almond populations clustered as independent groups. The results suggest that on peach in the Southeastern United States, V. carpophila is a sexually reproducing pathogen, but on almond in California, it is likely asexual. The confounded factors of geographic location and crop host may have both contributed to population differentiation. Although more research is needed, these results have implications for our understanding of the disease epidemiology and management in the two regions.

Moderate (MR) and high resistance (HR) in Geotrichum citri-aurantii to propiconazole (PPZ) have been detected in California citrus packinghouses. Fitness cost and resistance mechanisms were characterized in this study. No fitness cost was identified in resistant isolates, and they were highly virulent and competitive in the presence of sensitive (S) isolates. F1 ascospore progeny from crosses between MR or HR isolates and an S isolate mostly segregated in a 1:1 ratio of S to R isolates, indicating involvement of a single gene in resistance. Resistance was associated with point mutations in GcaCYP51A resulting in Y143F and L389V amino acid substitutions in HR isolates and an A125V substitution in MR isolates. There were no sequence differences in the GcaCYP51B paralog between S and MR isolates, but missense and silent mutations were identified in HR isolates. Segregation of GcaCYP51B alleles was observed in crosses between S and HR isolates: some PPZ-S progeny carried the GcaCYP51B-HR allele, whereas some PPZ-HR isolates carried the GcaCYP51B-S allele. Therefore, mutations in GcaCYP51B have no major role in PPZ resistance. The direct involvement of mutations in GcaCYP51A in resistance was demonstrated by transformation of a PPZ-S isolate with cloned alleles from MR or HR isolates, where transformants exhibited the expected resistance phenotype. Transformants carrying the GcaCYP51B allele of HR isolates, however, remained sensitive. No differences between S and R phenotypes were detected in the promoter sequences of GcaCYP51A or GcaCYP51B. Our data indicate that PPZ resistance in G. citri-aurantii is solely determined by point mutations in GcaCYP51A.

High levels of sour rot on propiconazole-treated lemon fruit that was stored for extended times in some California packinghouses in 2020 and 2021 initiated surveys on fungicide sensitivity of the causal pathogen. In isolations from diseased fruit in 2020 to 2023, 157 isolates of Geotrichum spp. were obtained. Using species-specific primers, 143 were determined to be G. citri-aurantii and 15 were G. candidum. Isolates of G. citri-aurantii were either sensitive (effective concentration of fungicide required for 50% growth inhibition [EC50] 0.06 to 0.34 mu g/ml), moderately resistant (EC50 1.20 to 2.34 mu g/ml), or highly resistant (EC50 >= 17.68 mu g/ml) to propiconazole. There was incomplete cross-resistance to cyproconazole, another demethylation inhibitor fungicide, pending postharvest registration on citrus in the United States. Isolates sensitive to propiconazole were sensitive, isolates moderately resistant to propiconazole were sensitive, and isolates highly resistant were moderately resistant to cyproconazole (EC50 0.11 to 0.63 mu g/ml, 0.19 to 0.73 mu g/ml, and 2.66 to 6.79 mu g/ml, respectively). All except one isolate of G. candidum were highly resistant to both fungicides (EC50 > 9.55). Isolates of both species were all considered sensitive to natamycin (EC50 1.18 to 5.01 mu g/ml). In lemon fruit inoculations with G. candidum, the incidence of typical sour rot increased from 4.7 to 68.2% when inoculum was amended with 2 mu g/ml or 100 mu g/ml cycloheximide, respectively, a compound known to suppress host defenses. In coinoculations with 1:1 mixtures of the two Geotrichum spp., G. candidum was only recovered from the centers of decay lesions, whereas G. citri-aurantii was also obtained from the advancing margins. We conclude that G. candidum is a secondary pathogen of lemons, and its presence was favored by extended late-season storage of senescent fruit with reduced defense mechanisms.

We sequenced and comprehensively analysed the genomic architecture of 98 fluorescent pseudomonads isolated from different symptomatic and asymptomatic tissues of almond and a few other Prunus spp. Phylogenomic analyses, genome mining, field pathogenicity tests, and in vitro ice nucleation and antibiotic sensitivity tests were integrated to improve knowledge of the biology and management of bacterial blast and bacterial canker of almond. We identified Pseudomonas syringae pv. syringae, P. cerasi, and P. viridiflava as almond canker pathogens. P. syringae pv. syringae caused both canker and foliar (blast) symptoms. In contrast, P. cerasi and P. viridiflava only caused cankers, and P. viridiflava appeared to be a weak pathogen of almond. Isolates belonging to P. syringae pv. syringae were the most frequently isolated among the pathogenic species/pathovars, composing 75% of all pathogenic isolates. P. cerasi and P. viridiflava isolates composed 8.3 and 16.7% of the pathogenic isolates, respectively. Laboratory leaf infiltration bioassays produced results distinct from experiments in the field with both P. cerasi and P. syringae pv. syringae, causing significant necrosis and browning of detached leaves, whereas P. viridiflava conferred moderate effects. Genome mining revealed the absence of key epiphytic fitness-related genes in P. cerasi and P. viridiflava genomic sequences, which could explain the contrasting field and laboratory bioassay results. P. syringae pv. syringae and P. cerasi isolates harboured the ice nucleation protein, which correlated with the ice nucleation phenotype. Results of sensitivity tests to copper and kasugamycin showed a strong linkage to putative resistance genes. Isolates harbouring the ctpV gene showed resistance to copper up to 600 μg/ml. In contrast, isolates without the ctpV gene could not grow on nutrient agar amended with 200 μg/ml copper, suggesting ctpV can be used to phenotype copper resistance. All isolates were sensitive to kasugamycin at the label-recommended rate of 100μg/ml.

Isolates of the citrus brown rot pathogens Phytophthora citrophthora and P. syringae from the Inland Empire (IE) and Ventura Co. (VE) regions of southern California were evaluated for their sensitivity to ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin, and the previously published baselines that were generated for Central Valley (CV) isolates of California were expanded. Fungicides were generally more toxic to CV isolates of both species for all four fungicides. Specific differences were found in the toxicity of ethaboxam to P. syringae where CV isolates on average were 6.8 or 8.2 times more sensitive than those from the VE or IE regions, respectively. Based on the grouping of isolates in an unweighted pair-group method with arithmetic mean (UPGMA) dendrogram, as well as fastStructure analyses and plotting of principal component analyses (PCAs), differences in ethaboxam sensitivity could be related to differences in genetic background of the isolates. Isolates of P. citrophthora from the IE and VE had slightly reduced (i.e., 1.5×) sensitivity to mandipropamid as compared with isolates from the CV and were found on distinct branches in the UPGMA dendrogram. Differences in genetic background of less sensitive isolates within each species indicate that these two phenotypes emerged multiple times independently. IE and VE isolates of both species were sensitive to mefenoxam. Moderate resistance to potassium phosphite (EC50 values of 25 to 75 μg/ml) was present in IE and VE isolates of P. syringae, whereas some IE isolates of P. citrophthora were considered resistant with EC50 values of up to 113.69 μg/ml. Resistance to potassium phosphite did not relate to distinct genotypes.

Phytophthora citrophthora and P. syringae are currently the primary causal organisms of brown rot of citrus fruits in California. To possibly find an explanation for the prevalence of the previously minor species P. syringae, we determined the population structures of both pathogens in California using next-generation sequencing and population genomics analyses. Whole-genome sequencing and aligning with newly assembled reference genomes identified 972,266 variants in 132 isolates of P. citrophthora and 422,208 variants in 154 isolates (including 24 from noncitrus tree crops) of P. syringae originating from three major growing regions. The resulting data sets were visualized using principal component analysis, discriminant analysis of principal components, unweighted pair-group method with arithmetic mean dendrograms, fastStructure, and minimum spanning networks, and we obtained the index of association, diversity summary statistics, and genetic distance statistics values GST, G''ST, and Jost's D. Subpopulations of both species were mostly defined by their geographic origin indicating restricted dispersal of inoculum. Except for five isolates, the population structure of P. citrophthora (that is heterothallic and unlikely to reproduce sexually) was clonal to semi-clonal, with very little genetic diversity within and among subgroups. In contrast, the population structure of P. syringae was also clonal to semi-clonal, but isolates were placed into four main clusters of much higher diversity. Clonality in both species can be explained by a high level of asexual reproduction. The higher diversity in the homothallic P. syringae is likely due to commonly occurring sexual reproduction. One distinct cluster of P. syringae consisted solely of isolates from noncitrus hosts; therefore, the origin of P. syringae in citrus could not be resolved.

Four new modes of action, oxathiapiprolin, fluopicolide, mandipropamid, and ethaboxam, have or are pending registrations on multiple tree crops in the USA including avocado, citrus, almond (and other tree nuts), and stone fruits to be used in sustainable anti-resistance rotation and mixture programs for managing Phytophthora diseases. All four fungicides are highly toxic in vitro against species of Phytophthora from different tree crops. Efficacy of foliar applications of oxathiapiprolin and mandipropamid for the management of citrus brown rot persisted for over 10 weeks. Still, residues could be removed to non-detectable levels by postharvest washing and brushing of fresh-market fruit. The four fungicides were highly effective in reducing root rot and crown/trunk cankers on tree crops and demonstrated acropetal systemic movement through the xylem after soil application and absorption by almond, citrus, and cherry roots. This was substantiated by reduced lesion size in stem inoculations of potted trees that were treated by soil applications or by in vitro bioassays and HPLC–MS–MS analyses of root, stem, and leaf extracts. Using an adapted Bromilow model that is based on the partition coefficient logP that indicates the lipo- or hydrophilicity and the acid dissociation constant pKa, mobility of ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin in plants was predicted, and this was supported by our studies. Mandipropamid and a mandipropamid–oxathiapiprolin pre-mixture are now registered as foliar applications for managing citrus brown rot, whereas oxathiapiprolin and fluopicolide are registered, and ethaboxam is pending registration as soil applications to control citrus root rot. Oxathiapiprolin has also been registered for soil use on almond and other nut crops, as well as avocado, and soil applications of mandipropamid are planned to be restricted to nursery use of multiple crops.

Brown rot caused by Phytophthora citrophthora, P. nicotianae, P. syringae, and P. hibernalis is an important fruit disease of citrus in California, and the latter two species are quarantine pathogens in some important export markets. The newly registered fungicides oxathiapiprolin (OXA) and mandipropamid (MAN), as well as a premixture of the two (MAN + OXA) were compared with standard fixed copper and potassium phosphite (KPO3) treatments (all with different modes of action) under field conditions in two citrus production regions of California. Fruit were sampled periodically over 8 weeks after application in winter or spring seasons, inoculated with zoospores of P. citrophthora or P. syringae, and brown rot incidence was evaluated. Single applications with all fungicides significantly reduced brown rot incidence of fruit harvested after 8 weeks as compared with the control in seasons with different amounts of precipitation (i.e., 17.2 to 153.9 mm between application and the 8-week sampling). MAN and OXA were similarly or significantly more effective than copper or KPO3. Two applications done in November and January significantly improved the efficacy of KPO3 and copper when compared with a single application of each fungicide done in January. For MAN and OXA, however, a single application was similar in efficacy as two applications. Two-application rotations of MAN, OXA, MAN + OXA, or copper significantly reduced the disease incidence by >84% from the control for at least 8 weeks after the second application. Low-volume (935 liters/ha) applications of MAN, OXA, MAN + OXA, or KPO3, but not copper, were significantly more effective than industry standard high-volume (3,740 liters/ha) applications. Thus, our studies identified and supported registration of new preharvest fungicide treatments to manage brown rot of citrus that are highly effective and persistent, and we optimized treatment strategies. Additionally, rotational programs with fungicides with different modes of action will minimize resistance development in pathogen populations and extend the usage of these fungicides.

Phytophthora root rot can greatly impact citrus production worldwide, especially in newly established orchards by reducing crop yield and increasing the cost of disease management. Mandipropamid is an Oomycota fungicide that is currently registered as a soil treatment for citrus nursery container plants to manage Phytophthora root rot. In this study, we investigated the uptake of mandipropamid into citrus roots and its translocation to stems and leaves after soil application and evaluated its mobility in roots as compared to oxathiapiprolin and mefenoxam using split-root potted plants and trees in the field. A bioassay and liquid chromatography-tandem mass spectrometry were used to detect and quantify fungicides in citrus tissues, and overall, similar results were obtained using the two methods. When applied to the soil of potted, 6- to 7-month-old citrus plants using labeled rates, the majority of mandipropamid was found in root tissues (4.9 to 18.1 μg/g), but small amounts were also present in stems (0.18 to 0.32 μg/g) and leaves (0.03 to 0.22 μg/g). There was no significant increase in concentrations in all three tissues between 1 and 4 weeks after application. Concentrations in all tissues exceeded established EC50 values for mycelial growth inhibition of P. citrophthora and P. nicotianae, the main citrus root rot pathogens in California. In a split-root study where the root systems of single plants were separated, no basipetal phloem-based mobility of mandipropamid or oxathiapiprolin was observed, but relative uptake into roots was higher for mandipropamid. In contrast, low amounts of mefenoxam were also present in roots in the untreated soil. Similar results were obtained in a field study where part of the root system was treated, and fungicides were extracted from nontreated roots. All three fungicides persisted inside roots over the 8-week period of this study. Uptake and persistence inside roots, as well as the previously reported high efficacy against citrus root rot in greenhouse and field studies support the use of mandipropamid in citrus nurseries and potentially in the orchard.