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.

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.

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.

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.

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.

Alternaria leaf spot caused by Alternaria alternata and A. arborescens is a common disease of almond in California. Succinate dehydrogenase inhibitors (SDHIs) are widely used for its management; however, we observed reduced performance of SDHI fungicides at some field sites. Thus, we evaluated the sensitivity to boscalid of 520 isolates of the main pathogen A. alternata collected from major production areas between 2006 and 2019, and also evaluated the sensitivity of a subset of 204 isolates to six members of the SDHIs belonging to six subgroups. Additionally, 97 isolates (14 sensitive and 83 with reduced sensitivity) of the 204 were used to determine the molecular mechanisms of resistance. A wide range of in vitro concentrations to effectively inhibit mycelial growth by 50% (EC50 values) was determined for each fungicide using the spiral gradient dilution method. Some isolates were highly resistant (EC50 values >10 μg/ml) to boscalid (a pyridine-carboxamide), pyraziflumid (a pyrazine-carboxamide), and fluxapyroxad (a pyrazole-4-carboxamide), but not to fluopyram (a pyridinyl-ethyl-benzamide), isofetamid (a phenyl-oxo-ethyl thiophene amide), and pydiflumetofen (a N-methoxy-(phenyl-ethyl)-pyrazole-carboxamide). There was no strong cross resistance among the fungicides tested, including for the two pyrazole-4-carboxamides fluxapyroxad and penthiopyrad (tested for 33 of the 204 isolates). The comparison of EC50 values for fluopyram and isofetamid resulted in the highest coefficient of determination (R2 = 0.582) among 10 pairwise comparisons between subgroups. Sequence analyses of the 97 isolates revealed five mutations in SdhB, SdhC, or SdhD subunits of the Sdh target gene among 73 isolates with reduced sensitivity to at least one SDHI. No mutations were detected in the 14 sensitive isolates and in 10 of the 83 isolates with reduced sensitivity. The most common mutation (59 isolates) was H134R in SdhC. Other mutations included H277Y (eight isolates) and H277L (two isolates) in SdhB, as well as G79R (two isolates) and S135R (two isolates) in SdhC. Mutations H277Y in SdhB and S135R in SdhC were only present in isolates collected in 2012 or earlier. Both conferred mostly high levels of resistance to boscalid and also reduced sensitivity to pyraziflumid, fluxapyroxad, and isofetamid with intermediate EC50 levels. Mutations H277L in SdhB, as well as H134R and G79R in SdhC, found in isolates obtained after 2012 had very similar resistance phenotypes with different levels of resistance to boscalid, pyraziflumid, and fluxapyroxad, whereas sensitivity to fluopyram, isofetamid, and pydiflumetofen was mostly less affected. Our data for SDHI fungicides do not support the classical concept of positive cross resistance within a single mode of action. Because some mutations conferred resistance to multiple SDHI subgroups, however, resistance management needs to consider all SDHIs as a homogenous group that should be mixed or rotated with other modes of action to delay development of resistance.

Natamycin is a new postharvest biofungicide for citrus and some other fruit crops in the United States that can be effectively used in recycling drench or flooder treatments. These applications necessitate sanitation of the fungicide solution to ensure that it remains free from contamination by bacteria that are potentially human pathogens. During in vitro experiments, heated (48°C) citric acid (1,100 or 2,200 μg/ml) amended with sodium dodecylbenzenesulfonate (SDBS) (60 or 120 μg/ml, respectively) significantly reduced the viability of a nonpathogenic strain of Escherichia coli in natamycin solutions by >5 log10 compared with the control. During laboratory studies with Penicillium digitatum-inoculated lemon fruit, 1,000 μg/ml of natamycin mixed with 1,000 μg/ml of lactic acid or citric acid and with or without SDBS (55 μg/ml) effectively and significantly reduced green mold. Natamycin mixed with lactic acid at ≥2,000 μg/ml, however, caused fruit injury, resulting in browning and rind pitting. Natamycin was incompatible with peroxyacetic acid, resulting in reduced efficacy against green mold. Sodium hypochlorite mixed with natamycin lost its toxicity to E. coli; however, the performance of natamycin was not affected. With heated (average 49°C) drench treatments on an experimental packing line, natamycin (1,000 μg/ml), fludioxonil (300 μg/ml), or azoxystrobin (300 μg/ml) mixed with citric acid (1,000 μg/ml) and SDBS (55 μg/ml) were effective against green mold without fruit injury. At a pH between 3.6 and 3.8, citric acid-SDBS significantly reduced the viability of E. coli by approximately 4 log10 in mixtures with fludioxonil or azoxystrobin, but not with natamycin. However, natamycin at 1,000 μg/ml mixed with 2,000 μg/ml of citric acid and SDBS (55 μg/ml) significantly reduced E. coli counts by >4 log10 within 4 min when the pH was maintained between 3.0 and 3.3, and the efficacy of the fungicide was retained. The use of citric acid with a surfactant can be a viable alternative sanitation method for natamycin in citrus packinghouses utilizing heated recirculating fungicide systems.

Natamycin is a biofungicide that was registered in the United States in 2016 and approved in California in 2017 for postharvest use on citrus and stone fruits. It has been used as a food preservative for many decades, with no resistance ever observed to date. The objective of this study was to determine baseline sensitivities for mycelial growth of 43 to 72 isolates of seven postharvest pathogens to natamycin and the resistance potential of Penicillium digitatum. Mean effective concentrations to inhibit mycelial growth by 50% (EC50 values), as determined by the spiral gradient method, were 0.90 μg/ml for Alternaria alternata, 0.76 μg/ml for Botrytis cinerea, 3.20 μg/ml for Geotrichum citri-aurantii, 0.17 μg/ml for Monilinia fructicola, 1.54 μg/ml for P. digitatum, 1.14 μg/ml for P. expansum, and 0.48 μg/ml for Rhizopus stolonifer. Distributions of EC50 values for each pathogen were unimodal and mostly normal with no outliers detected. Natamycin was also inhibitory to spore germination with values for five of the species similar to those for mycelial growth. Microscopically, natamycin generally arrested spores at the pregermination swelling stage. Mass platings of a conidial mixture of 10 isolates of P. digitatum were inoculated on agar media with 2.5-log radial concentration gradients of natamycin or fludioxonil, and a conidial mixture of 10 isolates of G. citri-aurantii were plated on media amended with natamycin or propiconazole. No resistant isolates were observed for both species to natamycin or for G. citri-aurantii to propiconazole, whereas a resistance frequency of 4.5 × 10-6 to 3.1 × 10-6 was calculated for P. digitatum to fludioxonil. The wide spectrum of activity against different fungal pathogens and a low resistance potential support the registration of natamycin as a postharvest treatment and its integration into an integrated pest management program with other practices including sanitation and rotation of other fungicides with different modes of action.

Isolates of the fire blight pathogen Erwinia amylovora with high-level resistance to oxytetracycline (minimal inhibitory concentration [MIC] > 100 μg/ml) and to streptomycin (MIC > 100 μg/ml) were recovered from four commercial pear orchards in California between 2018 and 2020. The two representative oxytetracycline- and streptomycin-resistant (OxyTcR-SmR) strains 32-10 and 33-1 were as virulent as the antibiotic susceptible strain 13-1 in causing blossom blight of pear and were recovered more than 50% of the time 7 days after co-inoculation to pear flowers with strain 13-1. In the field, inoculation of strain 32-10 to pear flowers that were pretreated with oxytetracycline at 200 μg/ml did not reduce disease compared with an untreated control. Four OxyTcR-SmR strains were subjected to draft genome sequencing to identify the genetic determinants of antibiotic resistance and their location. A 43.6-kb IncX plasmid, designated pX11-7, was detected in each of the four strains, and this plasmid encoded the tetracycline-resistance gene tetB and the streptomycin-resistance gene pair strAB within a large putatively mobile genetic element consisting of the transposon Tn10 that had inserted within the streptomycin-resistance transposon Tn6082. We also determined that pX11-7 was conjugative and was transferred at a rate that was 104 to 105 higher into an E. amylovora strain isolated in California compared with an E. amylovora strain that was isolated in Michigan. The occurrence of high levels of resistance to both oxytetracycline and streptomycin in E. amylovora strains from commercial pear orchards in California significantly limits the options for blossom blight management in these locations.

Oxathiapiprolin is highly effective in the management of Phytophthora root rot of citrus; however, its uptake into plants after soil application is not known. This was investigated and compared with mefenoxam using potted citrus seedlings sampled 7, 10, 13, and 16 days after soil treatments. Bioassays and high-performance liquid chromatography tandem mass spectroscopy (HPLC-MS/MS) were used to quantify fungicide amounts in plant extracts. Distinct inhibition zones of mycelial growth of Phytophthora citrophthora were observed in bioassays when root, stem, or leaf extracts were added to filter paper disks on agar plates. Based on the two quantification methods, concentrations of both fungicides in the three tissue types and at all sampling times were above the mean effective concentration that provides 50% growth reduction values of the baseline sensitivities. Relative concentrations at the four sampling times sometimes varied between the two methods but, for both methods, concentrations of oxathiapiprolin were significantly higher in roots and leaves as compared with stems 10 days after treatment and statistically similar in the three tissues after 7 days. For mefenoxam, concentrations significantly increased in roots between 7 and 16 days after treatment and were significantly the highest in roots as compared with stems or leaves 16 days after treatment. Regressions of oxathiapiprolin and mefenoxam concentrations using HPLC-MS/MS on those calculated from bioassay standard curves indicated that the bioassays overestimated fungicide amounts in the extracts. The bioassay, however, can be considered an alternative option comparable with costly residue analyses in fungicide mobility studies in plants. Uptake of oxathiapiprolin at sufficient but low concentrations into plant roots provides an explanation for its long-lasting high activity in the management of Phytophthora root rot.