Papers by Srđan G Aćimović
PhytoFrontiers, 2022
PATHMAP (Pathogen And Tree fruit Health Map) is a smart phone application (app) and interactive d... more PATHMAP (Pathogen And Tree fruit Health Map) is a smart phone application (app) and interactive dashboard developed specifically for support specialists, extension personnel, and university scientists supporting the tree fruit industry. The PATHMAP app collects detailed information about observed diseases, insect pests, and disorders and the option for attaching photos. The data is then visualized using a graphical interface dashboard displaying an interactive color-coded map. Prior to the development of PATHMAP, abundant tree fruit disorder data were collected each year, but a central interactive repository for archiving data and facilitating communication of field observations did not exist. PATHMAP has been betatested by university extension personnel, private consultants, and university scientists to ensure usability and functionality. PATHMAP will be used within the tree fruit industry for monitoring known pest patterns, occurrences, and outbreaks of emerging pathogens. It will augment existing extension diagnosis listservs which have value in visual diagnosis but are cumbersome and have no archiving capabilities. Data obtained through the tool can be used in epidemiological meta-analyses, to develop new predictive models, and can serve as a platform to track emerging pathogens, insects, and disorders for a variety of cropping systems.
Plant Disease, 2022
Apple orchards with minimal or reduced fungicide inputs in the Mid-Atlantic region of the United ... more Apple orchards with minimal or reduced fungicide inputs in the Mid-Atlantic region of the United States have experienced outbreaks of severe premature defoliation with symptoms that matched those of apple blotch disease (ABD) caused by Diplocarpon coronariae. Fungal isolates obtained from symptomatic apple leaves and fruit produced uniform slow-growing, dark-gray colonies on peptone potato dextrose agar and had conidia. Internal transcribed spacer DNA sequences matched with D. coronariae and Koch’s postulates were fulfilled when typical ABD symptoms occurred when reinoculated onto apple leaves and fruit. Spore dispersal in nonfungicide-treated orchards detected with quantitative PCR was low in early spring and dropped to undetectable levels in late May and early June before rising exponentially to highs in July and August, which coincided with symptom development. Only low spore numbers were detected in fungicide-treated orchards and nearby forests. In preliminary fungicide tests, fluxapyroxad, thiophanate methyl, and difenoconazole effectively inhibited mycelial growth of isolates in vitro. When apple cultivars Fuji and Honeycrisp were inoculated with D. coronariae, Honeycrisp showed delayed onset of symptoms and lower disease severity, and the transcription profile of seven host defense-related genes showed that PR-2, PR-8, LYK4, and CERK1 were highly induced in Honeycrisp at 2 and 5 days postinoculation. This is the first report of ABD in the Mid-Atlantic United States, which includes studies of seasonal D. coronariae spore dispersal patterns, preliminary fungicide efficacy, and host defense-related gene expression to assist development of best ABD management practices.
Frontiers in Microbiology, 2022
Erwinia amylovora causes fire blight, a disease responsible for enormous economic losses in the p... more Erwinia amylovora causes fire blight, a disease responsible for enormous economic losses in the pome fruit-producing areas where it is present. Despite the abundant research on fire blight, information about E. amylovora population dynamics and survival in fire blight cankers and the plant defense responses to this pathogen in the infected bark are limited. In our study, we obtained fire blight cankers in apple, pear, and Asian pear cultivars showing differing resistance to the disease by shoot inoculation with E. amylovora. We collected cankers from irrigated and non-irrigated trees every 3 months in two independent field experiments and analyzed samples by viability digital PCR. We also assessed the expression of pathogenicity-related (PR) genes in the bark of selected apple and Asian pear cultivars. A logistic regression analysis revealed the impact of environmental and host factors on E. amylovora detection rates in cankers. The chances of detecting live E. amylovora cells in cankers increased significantly in those collected from irrigated trees, in July, and/or during an experiment performed in a year with an expected average rainfall when compared to samples from non-irrigated trees, collected in January, and/or during an experiment performed under environmental conditions dominated by drought. We found a positive correlation between the pathogen detection rates in cankers and the host resistance to fire blight that might be explained by lower E. amylovora survival rates in more damaged tissues of susceptible hosts. The genes PR-1, PR-2, PR-5, and PR-8 were induced in the bark surrounding apple and Asian pear fire blight cankers. Our study, involving the analysis of more than 800 canker samples, provides new knowledge about the fire blight disease cycle and lays the foundation for improved fire blight management and eradication strategies in pome fruit orchards.
Plant Disease, 2022
Apple growers in the Mid-Atlantic region of the United States have reported increased losses to b... more Apple growers in the Mid-Atlantic region of the United States have reported increased losses to bitter rot of apple. We tested the hypothesis that this increase is because the Colletotrichum population has developed resistance to commonly used single-mode-of-action (single-MoA) fungicides. We screened 220 Colletotrichum isolates obtained from 38 apple orchards in the Mid-Atlantic region for resistance to 11 fungicides in FRAC (Fungicide Resistance Action Committee) groups 1, 7, 9, 11, 12, and 29. Eleven (5%) of these isolates were resistant to FRAC group 1 with confirmed beta-tubulin E198A mutations, and two (< 1%) were also resistant to FRAC group 11 with confirmed cytochrome-b G143A mutations. Such low frequencies of resistant isolates indicate that fungicide resistance is unlikely to be the cause of any regional increase in bitter rot. A subsample of isolates was subsequently tested in vitro for sensitivity to every single-MoA fungicide registered for apple in the Mid-Atlantic US (22 fungicides; FRAC groups 1, 3, 7, 9, 11, 12, and 29), and thirteen fungicides were tested in field trials. These fungicides varied widely in efficacy both within and between FRAC groups. Comparisons of results from our in vitro tests with results from our field trials and other field trials conducted across the Eastern US suggested that EC₂₅ values (concentrations that reduce growth by 25%) are better predictors of fungicide efficacy in normal field conditions than EC₅₀ values. We present these results as a guideline for choosing single-MoA fungicides for bitter rot control in the Mid-Atlantic US.
Plant Disease, 2021
To reduce the severity of shoot blight and prevent the resulting development of cankers on perenn... more To reduce the severity of shoot blight and prevent the resulting development of cankers on perennial apple wood, we evaluated eight fire blight post-infection spry programs of prohexadione-calcium (PCA) alone or with acibenzolar-S-methyl (ASM) over two years. On mature trees of cv. Royal Court, a single application of the high PCA rate (247 mg/L) at 2 to 3 days after inoculation resulted in 89.5 and 69.5% reduction of shoot blight severity after inoculation. Two applications of PCA 247mg/L, 12 or 14 days apart, with the first one applied 2 to 3 days after inoculation, resulted in 78.8 and 74.5% reduction of shoot blight severity in both years. A 100% control of canker incidence on perennial wood from infected shoots in both years was achieved with a single application of PCA (247mg/L) applied at 2 or 3 days after the inoculation, while three applications of PCA (125 mg/L) + ASM (25mg/L) 12 – 16 days apart, reduced canker incidence by 83.5 and 69% in the two years. The other programs with lower PCA rates and frequencies of application reduced shoot blight severity for 50.8 and 51.8% (PCA) and for 62.6 – 72% and 59.3% (PCA + ASM), over two years, respectively. Reduction of canker incidence on wood by other programs was 66.5% and 69 – 90.4% in the two years, respectively. As fire blight cankers lead to death of dwarf apple trees and serve as primary sources of inoculum, our effective PCA and PCA + ASM programs could serve as viable post-infection management options. These treatments can reduce or prevent canker development and thus significantly abate tree losses in high-density apple orchards after fire blight epidemics occur.
CABI Agriculture and Bioscience, 2021
Background: Apple blotch (AB) caused by Diplocarpon coronariae (Dc) has been established in Europ... more Background: Apple blotch (AB) caused by Diplocarpon coronariae (Dc) has been established in Europe since 2010. AB is a serious apple disease, mostly in low input orchards and in cider production areas in Northern Italy, Switzerland, Austria and Germany. However, the epidemiology and population genetic structure of this pathogen is unknown. Methods: We developed twelve Dc-specific microsatellite markers and screened DNA of both pure fungal isolates and infected apple leaves. The marker data of 313 European samples of Dc were compared to Dc isolates from Asia (n = 7) and the USA (n = 3). Results: We found 31 distinct multilocus genotypes (MLGs) in European samples, and seven additional MLGs in the Asian and USA samples. The European samples had the typical genetic signature of a recently introduced species including high clonality, a low number of private alleles and one dominant MLG across all the sampling sites. All European MLGs were genetically distant from those MLGs of Asian and USA origin. Based on the lack of linkage disequilibrium observed, there is evidence that Dc undergoes regular cycles of sexual recombination in the European population, although the sexual stage (apothecia) has not been observed in Europe. Conclusions: The twelve newly developed SSR markers reported here provide a useful tool to characterize the population genetic diversity and structure of Dc in Europe. Our study supports the hypothesis that Dc is a recently introduced pathogen in Europe, but of currently unknown origin. Dc has a large effective population size during field epidemics, so we believe that the pathogen has substantial evolutionary potential. Application of the SSR markers to large-scale and diverse Dc samples will help to better understand the epidemiology of AB, which has become a global apple disease, and will help guide effective mitigation strategies based on disease management and resistance breeding.
Insects, 2021
The samurai wasp, Trissolcus japonicus (Ashmead), has been proposed as a biocontrol agent against... more The samurai wasp, Trissolcus japonicus (Ashmead), has been proposed as a biocontrol agent against brown marmorated stink bugs (BMSB), due to its ability to parasitize and kill BMSB eggs. However, the wasps’ small size makes it challenging for those untrained in morphological identification to determine the wasps’ species. To circumvent this problem, a molecular method was created to identify T. japonicus. The method uses species-specific primers, designed in this study, which target the variable region of the mitochondrial Cytochrome Oxidase 1 (CO1) locus. After confirming successful DNA extraction from samples, the PCR amplification using our primers produced 227-bp PCR products for all T. japonicus specimens and no amplification in other microhymenoptera candidates. Additionally, DNA from BMSB-parasitized eggs gave positive PCR amplification, while the control BMSB samples showed no amplification. This indicates that PCR with our primers specifically and sensitively differentiates T. japonicus specimens from other similar wasp species and discriminates between T. japonicus-parasitized and non-parasitized BMSB eggs. Finally, an in silico analysis of CO1 sequences demonstrated that our primers match the sequences of four different haplotypes of T. japonicus, indicating that our diagnostic method could potentially be applied to analyze T. japonicus populations throughout North America, Europe, and parts of Asia.
Phytopathology, 2021
Apple growers in the Mid-Atlantic region of the United States have been reporting an increase in ... more Apple growers in the Mid-Atlantic region of the United States have been reporting an increase in losses to bitter rot of apple and are requesting up-to-date management recommendations. Management is complicated by variations in apple cultivar susceptibility, temperature and rainfall, and biology of the Colletotrichum species that cause bitter rot. Over 500 apples with bitter rot were obtained from 38 orchards across the Mid-Atlantic and the causal species identified as C. fioriniae and C. nymphaeae of the C. acutatum species complex and C. chrysophilum, C. noveboracense, C. siamense, C. fructicola, C. henanense, and C. gloeosporioides sensu stricto of the C. gloeosporioides species complex, the latter two being first reports. Species with faster in vitro growth rates at higher temperatures were more abundant in warmer regions of the Mid-Atlantic, while those with slower growth rates at higher temperatures were more abundant in cooler regions. Regional bloom dates are earlier and weather data shows a gradual warming trend that likely influenced, but was not necessarily the main cause of the recent increase in bitter rot in the region. A grower survey of apple cultivar susceptibility showed high variation, with the increase in acres planted to the highly susceptible cultivar ‘Honeycrisp’ broadly corresponding to the increase in reports of bitter rot. These results form a basis for future studies on the biology and ecology of the Colletotrichum species responsible, and suggest that integrated bitter rot management must begin with selection of less-susceptible apple cultivars.
Scientific Reports 10 (11043): 1-19, 2020
Apple bitter rot caused by Colletotrichum species is a growing problem worldwide. Colletotrichum ... more Apple bitter rot caused by Colletotrichum species is a growing problem worldwide. Colletotrichum spp. are economically important but taxonomically un-resolved. Identification of Colletotrichum spp. is critical due to potential species-level differences in pathogenicity-related characteristics. A 400-isolate collection from New York apple orchards were morphologically assorted to two groups, C. acutatum species complex (CASC) and C. gloeosporioides species complex (CGSC). A sub-sample of 44 representative isolates, spanning the geographical distribution and apple varieties, were assigned to species based on multi-locus phylogenetic analyses of nrITS, GAPDH and TUB2 for CASC, and ITS, GAPDH, CAL, ACT, TUB2, APN2, ApMat and GS genes for CGSC. The dominant species was C. fioriniae, followed by C. chrysophilum and a novel species, C. noveboracense, described in this study. This study represents the first report of C. chrysophilum and C. noveboracense as pathogens of apple. We assessed the enzyme activity and fungicide sensitivity for isolates identified in New York. All isolates showed amylolytic, cellulolytic and lipolytic, but not proteolytic activity. C. chrysophilum showed the highest cellulase and the lowest lipase activity, while C. noveboracense had the highest amylase activity. Fungicide assays showed that C. fioriniae was sensitive to benzovindiflupyr and thiabendazole, while C. chrysophilum and C. noveboracense were sensitive to fludioxonil, pyraclostrobin and difenoconazole. All species were pathogenic on apple fruit with varying lesion sizes. Our findings of differing pathogenicity-related characteristics among the three species demonstrate the importance of accurate species identification for any downstream investigations of Colletotrichum spp. in major apple growing regions.
Arboriculture & Urban Forestry, 46(1): 44-50, 2020
Diplodia tip blight (Diplodia pinea) affects mature pines, including Austrian (Pinus nigra) and P... more Diplodia tip blight (Diplodia pinea) affects mature pines, including Austrian (Pinus nigra) and Ponderosa (P. ponderosa) pines. Infections spread from needle fascicles to branch and, if unchecked, to the entire tree. Efficacy studies of fungicide injections in conifers are limited. Minute vascular tissues and resin exudate, a response to drilling, present impediments to injection. The efficacy of Propizol (14.3% propiconazole) for control of Diplodia tip blight in Austrian pines was evaluated. We evaluated (1) time of year, (2) injection spacing, and (3) fungicide dilution with respect to injection efficiency. Late fall injections expedited uptake, which is consistent with the reduced monoterpene emission rates in autumn and winter reported by Kim et al. 2005 and Lim et al. 2008. The time required for the dose to be administered was recorded for close and wide spacing of injection sites. Close spacing had the greatest impact on reducing the application time, irrespective of time of year. Low volume injections required less time to apply compared to high volume. Regardless of the application method, we observed a significant decrease in disease incidence in Propizol-treated trees. Injections applied in late fall resulted in a mean reduction in infections of new candles in the next growing season. Injections in the following spring, however, did not result in improvement in candle condition until a year later. We believe that these differences are based on whether the fungicide was applied prior to or after infection. Based on these findings, we recommend Propizol prior to infection for optimal results.
Scientific Reports, 9(11530): 1-17, 2019
Fire blight is a devastating disease of apple and pear caused by the bacterium Erwinia amylovora.... more Fire blight is a devastating disease of apple and pear caused by the bacterium Erwinia amylovora. One of its main symptoms is canker formation on perennial tissues which may lead to the death of limbs and/or the entire tree. E. amylovora overwinters in cankers which play an important role in initiating fire blight epidemics. However, knowledge of pathogen biology in cankers is scarce, in part due to limitations of classical microbiology methods and the inability of most molecular techniques to distinguish live from dead cells. In this work, a viability digital PCR (v-dPCR) protocol using propidium monoazide (PMA) was developed, allowing for the first time the selective detection and absolute quantification of E. amylovora live cells in apple and pear cankers collected in two time periods. Some key factors affecting the v-dPCR performance were the maceration buffer composition, the target DNA amplicon length, the thermal cycle number and the use of sodium dodecyl sulfate or PMA enhancer for Gram-negative bacteria to improve the effect of PMA. In the future, this methodology could shed light on E. amylovora population dynamics in cankers and provide clues on the effect of management practices, host cultivar, host water/nutritional status, etc., on bacterial survival.
Fruit Quarterly, Winter Issue, Vol 26 (4), pg. 21-27, 2018
reduce number of early season fungicide sprays, depending on the year. It seems that the RIMpro a... more reduce number of early season fungicide sprays, depending on the year. It seems that the RIMpro apple scab model incorporates valuable advances in describing V. inaequalis biology by increasing the accuracy for infection prediction early in the season and at the end of the season when ascospore inoculum is depleted from leaf litter. However, RIMpro will benefit only those growers willing to (1) learn how to use it, (2) check model outputs on a regular basis as they plan their fungicide spray applications, and (3) properly maintain their NEWA RainWise stations to ensure accurate weather data recording."
Plant Disease, Vol. 103 (5), pg. 1032, 2019
Bitter rot of pome fruit is caused by up to 18 fungal species within the genus Colletotrichum. It... more Bitter rot of pome fruit is caused by up to 18 fungal species within the genus Colletotrichum. It is mainly considered to affect apples (Malus pumila Mill.) but Asian pear (Pyrus pyrifolia (Burm.f) Nakai) and common pear (Pyrus communis L.) can also be severely affected (Ivic et al. 2013; Baroncelli et al. 2017). During summer of 2017 bitter rot symptoms occurred on Asian and common pear in different orchards on Long Island and in Hudson Valley, New York (NY). In general, disease incidence on Asian pear was higher (80%) than in common pear (2-10%). A total of 65 symptomatic Asian and common pear fruit (‘Olympic’ and ‘Harrow Sweet’, respectively) from two orchards were sampled. They exhibited brown, dry, flat to sunken lesions, 2 to 6 cm in diameter on the surface, sometimes with acervuli producing orange conidial masses in concentric rings. For fungal isolation, fruit were rinsed with water and surface-disinfected with 70% ethanol. Fruit flesh pieces 2-3 mm in diameter were taken from the lesion edge, placed on potato dextrose agar (PDA) and dark-incubated at 25°C for 7 days. Fungal isolates produced abundant white-to-light gray mycelium, becoming dark with age and turning pink to red on the underside. Conidia were cylindrical to fusiform, pointed at one or both ends. No perithecia were observed on PDA. Morphological and cultural characteristics coincided with those described for C. acutatum sensu lato (Damm et al. 2012). One isolate from Asian pear (10-3) and two from common pear (4-2 and 8-2) were characterized by molecular methods. According to multiplex PCR developed by Hu et al. (2015) all isolates belonged to the C. acutatum species complex. Results of a BLAST analysis using partial sequences of ACT, TUB2 and GAPDH genes (Accession Nos. MH819301-9) revealed 99-100% identities and E-values equal or below 0.0 with C. fioriniae strains already deposited on GenBank (LC381035.1, MH003627.1, LC406922.1, JQ949614.1, JQ949948.1, KY828145.1, KY995536.1, MH547629.1, KY828145.1). Finally, multilocus phylogenetic analyses of individual and concatenated sequences of the above-mentioned genes identified bitter rot isolates as C. fioriniae. Pathogenicity was examined on detached organic Asian and common pears (‘Olympic’ and ‘Bartlett’, respectively). Fruit were surface-disinfected, wound-inoculated by attaching a 5-mm-diameter PDA plug from a 7-day-old culture, or sterile PDA in negative controls, using a sterile pipette tip and dark-incubated for 10 days in a moist chamber at 25°C. Three fruit were inoculated per isolate. Inoculated pears showed similar symptoms to those observed in the orchard, while control fruit remained asymptomatic. Fungi were re-isolated from the infected fruit and confirmed as C. fioriniae as previously described. This is the first report of C. fioriniae causing bitter rot on Asian and common pear in NY. This report raises concern over the possible role of cultivated Pyrus spp. in NY (ca. 1,200 acres) acting as inoculum sources for infections of apple orchards (ca. 44,000 acres). Since Colletotrichum spp. differ in biology, ecology and fungicide resistance, knowledge on which species are causing bitter rot on pome fruit crops in the USA is an important tool for improving bitter rot management strategies.
Fruit Quarterly, Winter issue, Vol 25 (4), pg. 25-29, 2017
Fruit Quarterly, Fall issue, Vol 25 (3), pg. 15-20, 2017
Plant Disease, Vol. 102 (10): pg. 1950-1957, 2018
Coast redwood (Sequoia sempervirens) is among the most widely planted landscape trees in Californ... more Coast redwood (Sequoia sempervirens) is among the most widely planted landscape trees in California (CA) but is in decline outside its natural range due to factors including prolonged drought and plant pathogens. We investigated associations of Botryosphaeriaceae fungi with declining coast redwood trees throughout CA. More than 100 samples were collected from 11 coastal and inland locations in CA. Fifty-nine Botryosphaeria-like fungal strains were isolated and 18 were selected for further study. Phylogenetic analysis of ITS and EF-1α sequence data confirmed the presence of Botryosphaeria dothidea, Neofusicoccum australe, N. luteum, N. mediterraneum and N. parvum. Pathogenicity testing showed that although the Neofusicoccum species vary in virulence, all are more virulent that B. dothidea. Neofusicoccum australe caused the largest lesions, followed by N. luteum, N. parvum, and N. mediterraneum. Of the species recovered, only B. dothidea has been previously confirmed as a pathogen of coast redwood in CA. These results confirm that multiple Botryosphaeriaceae species are associated with branch decline and dieback on coast redwood in CA, which agrees with similar studies on woody agricultural crops. Accurate diagnosis of fungal pathogens of coast redwood is important for the development of disease management strategies and may help improve horticultural practices in maintenance of urban stands.
Plant Disease, Vol. 102 (9): pg. 1848, 2018
In May 2017, dead buds at pink and tight cluster growth stages were observed on young apple trees... more In May 2017, dead buds at pink and tight cluster growth stages were observed on young apple trees of cultivar SnapDragon at two orchard sites in Marlboro, NY. Disease symptoms included necrosis of lateral and terminal flower buds that were wilted and drooping. Bark below the dead buds on severely affected trees showed black streaking with or without interspersing white streaks. On the cross-section of branch wood, just below the buds, light brown xylem and glossy water-soaked sections resembling frost damage were observed. After bark shaving, dark xylem tissue was visible along the branch length, in contrast to the light or pale color of the healthy tissue. The incidence of the disease on trees was 3 to 5%, and 0.5% of trees died. There were five near-frost events recorded on 4, 8, 9, 10, and 11 May 2017, leading us to suspect that the observed symptoms were blossom blast caused by the ice-nucleating strains of Pseudomonas syringae. Isolation from the affected tree samples was performed on King’s B (KB) and nutrient sucrose agar (NSA) media. After 3 days at 25°C, most of bacterial isolates formed round, shiny, gray-white colonies producing green fluorescent pigment on KB medium and typical levan-positive colonies on NSA. Eight isolates, inducing a hypersensitive reaction on tobacco plants, were further characterized by conventional bacteriological and molecular tests (Braun-Kiewnick and Sands 2001). The analyzed bacterial isolates were Gram-negative, aerobic, hydrolyzed gelatin and aesculin, but not pectate, and were negative for oxidase and arginine-dihydrolase activity. All strains were ice-nucleation active and utilized sucrose, sorbitol, and inositol as carbon sources, but not tyrosine and tartaric acid. Results of levan, cytochrome c oxidase, potato soft rot, arginine dihydrolase, and tobacco hypersensitive reaction (LOPAT) tests indicated that the isolates belong to P. syringae group Ia, and gelatin hydrolysis, aesculin hydrolysis, tyrosinase activity, and tartrate utilization (GATTa) tests designated them as pathovar (pv.) syringae (Kaluzna et al. 2012). Polymerase chain reaction (PCR) with specific primers B1/B2 (Sorensen et al. 1998) and SyD1/SyD2 (Bultreys and Gheysen 1999) showed that isolates possessed genes responsible for synthesis (syrB) and secretion (syrD) of syringomycin, respectively. Partial sequencing of the 16S rDNA gene of three isolates (GenBank MG788015 to MG788017) showed 99 to 100% identity with sequences of P. syringae pv. syringae previously deposited in the NCBI GenBank database. To complete Koch’s postulates, pathogenicity of the four isolates was confirmed by inoculation of 1-month-old potted apple plantlets grown from seeds collected from mature apple fruit of cultivar SnapDragon. One plant per isolate was inoculated multiple times as follows: using a needleless syringe, two leaves per plant were spot-infiltrated with bacterial suspension (106 CFU/ml); on the same plants, stem was also inoculated by inserting a sterile toothpick previously dipped into a bacterial colony. Sterile distilled water was used as a negative control. Inoculated plantlets were sealed within plastic bags for 48 h and further incubated at 22°C. Inoculation test was repeated in two independent experiments. Necrosis of the leaf and stem tissue developed at the point of inoculation, and its progression was observed daily for 2 weeks. No symptoms developed on the control plants. Bacteria were reisolated from inoculated tissues and confirmed as P. syringae pv. syringae by PCR for syrB and syrD genes. This is the first report of P. syringae pv. syringae causing blossom blast symptoms on developing apple buds in commercial orchards in the United States. Even though it rarely occurs on apples, with most recent frosts significantly lowering apple yields in Michigan (2012) and New York (2016), blossom blast might become a more common problem in U.S. orchards. Previously, P. syringae pv. syringae was isolated from dormant apple buds without and with internal necrosis in the United States (Burr and Katz 1984).
Plant Disease, Vol. 101 (2): pg. 380, 2017
Over the last decade, Diplodia corticola (Dc) has emerged as an important canker pathogen of oaks... more Over the last decade, Diplodia corticola (Dc) has emerged as an important canker pathogen of oaks (Quercus spp.) in the U.S. (Aćimović et al. 2016, Dreaden et al. 2011, and Lynch et al. 2013) and Europe (Linaldeddu et al. 2013). In fall 2014, large overstory Q. rubra with premature leaf browning and drop, bleeding / sooty bark cankers, and associated mortality were reported from Seneca State Forest (SSF) in Greenbriar Co., WV. Surveys conducted in 2015-16 revealed crown dieback and mortality of additional Q. rubra throughout the SSF, with some showing prolonged radial growth decline visible in cross section. Disease symptoms included bleeding / sooty cankers, crown dieback, and bark cracking as described for Q. rubra (Aćimović et al. 2016). From the necrotic margin of bole cankers and vascular streaking up to 6 m above the cankers, wood plugs were excised, surface-disinfested in 5% sodium hypochlorite, and plated onto glucose yeast extract agar (GYE). After 3-5 days, a fungus with abundant gray mycelium characteristic of Diplodia (Alves et al. 2004) grew from canker and vascular tissue plugs from 5 of 6 symptomatic Q. rubra from the SSF. ITS1+5.8S+ITS2 of the ribosomal RNA gene repeat (rDNA), and β-tubulin gene (Bt) were PCR amplified and sequenced. GenBank BLASTn searches showed all rDNA sequences (deposited as KX766145 & KX766144 for WV isolates DC103 & DC121) and Bt sequences (KX766146 & KX766147) were all 99% & 97% identical to D. corticola (KF766156 or DQ458853, respectively). Isolates DC103 & DC121 along with isolates OTL181 & OTCT3-1 from Q. rubra in ME (Aćimović et al. 2016) and DC110 (KX766143) from Q. velutina in MA were used to confirm pathogenicity by placing 0.5-cm diam. GYE plugs from 10-day-old Dc cultures or sterile agar plates (control) into five scalpel-wounded 3-year-old Q. rubra saplings (mean diam. 9.4 mm) per treatment and parafilmed. Inoculated trees were maintained in a growth room at 21°C for 8 weeks. External symptoms were recorded weekly starting 3 weeks post-inoculation (WPI). By 8 WPI, all Dc-inoculated saplings had cankers on all stems while controls remained canker-free. Combined but not individual canker area means for WV Dc isolates were significantly larger (p = 0.04, 4.8 cm2) than the controls (0.2 cm2). Similarly, all sampled Dc-inoculated plants (n = 3 / treatment) showed vascular streaking and associated occlusion, visible in cross section, 8 WPI while controls remained asymptomatic. Combined means for WV Dc isolates as well as individually for DC103 showed significantly longer streaking (p < 0.04, 23.6 & 31.2 cm, respectively) than the controls (0.0 cm). Mean AUDPC (area under the disease progress curve) values were significantly higher (p < 0.001) for all individual Dc treatments (combined 26.4) than the control (12.0) with mortality (1 sapling / treatment) reported for 4 of 5 Dc treatments (DC103, OTL181, OTCT3-1, and DC110). Onset of external symptoms (e.g. wilt, epicormic sprouts) occurred 4 WPI or later. Epicormic sprouts occurred in DC121, DC110 and OTCT3-1. Isolations of Dc from 80% of cankers and 25% of symptomatic vascular tissues distal to cankered regions across all Dc treatments confirmed pathogenicity of Dc on healthy Q. rubra. To our knowledge, this is the first documented report of stem cankers caused by D. corticola on Q. rubra in WV.
Urban Forestry & Urban Greening, Vol. 19, pg. 151–157, Jul 12, 2016
Excessive tree wounding is a common concern regarding the use of trunk injection technology for t... more Excessive tree wounding is a common concern regarding the use of trunk injection technology for tree protection purposes in landscapes and urban greening. We investigated the rate of healing of injection ports (points) in apple trees by monitoring parameters such as port diameters, the size of bark cracking, and port depths. We compared drilled injection ports from 4.4 and 9.5 mm drill bits, with latter being sealed with plastic-silicone plug (Arborplug®) or not, and the lenticular port from a double-edged blade. Depending on port size and type, port closure ranged from one to more than two years. Bark cracking around injection ports was more pronounced longitudinally. On the sealed 9.5 mm port, bark cracking was largely similar to all drilled ports. The depth of port wounds decreased faster on the port from the 4.4 mm drill bit and on lenticular injection port versus the unsealed port from the 9.5 mm drill bit. Plastic-silicone plugs, which simulate removed bark, slowed the healing of 9.5 mm drill port with callus and increased the port depths over time due to callus formation over the top of the plug. From fastest-healing to slowest-healing, on average the injection ports were: lenticular port from blade (70.8%), the unsealed 9.5 mm drill port (44.4%), 4.4 mm drill port (43.9%), and 9.5 mm drill port sealed with plastic-silicone plug (20.4%).
Arboriculture & Urban Forestry, Vol. 42 (2): pg. 84–94, Mar 2016
Bark beetles carry a number of associated organisms that are transferred to the host tree upon at... more Bark beetles carry a number of associated organisms that are transferred to the host tree upon attack that are thought to play a role in tree decline. To assess the pathogenicity to western white pine (WWP; Pinus monticola) of fungi carried by the mountain pine beetle (MPB; Dendroctonus ponderosae), and to evaluate the potential for systemic prophylactic treatments for reducing fungal impacts, experiments were conducted with WWP seedlings to meet three objectives: 1) evaluate pathogenicity of two MPB-associated blue-stain fungi; 2) evaluate phytotoxicity of tree injection products; 3) evaluate the anti-fungal activity of tree injection products, in vitro and in vivo, toward the associated blue-staining fungi. To evaluate pathogenicity, seedlings were inoculated with Grosmannia clavigera or Leptographium longiclavatum, common fungal associates of MPB. Seedling mortality at four months after inoculation was 50% with L. longiclavatum and 90% with G. clavigera, both significantly higher than controls and thereby demonstrating pathogenicity. Phytotoxic effects of TREE-äge®, Alamo®, and Arbotect® were evaluated by stem injection; no phytotoxic effects were observed. Anti-fungal properties of the same three products were evaluated in vitro against G. clavigera, where Alamo was most active. Co-inoculation of G. clavigera and L. longiclavatum into seedlings after a stem injection of Alamo showed significantly less mortality and lesion formation than either species alone. Results support the hypothesis that MPB blue-stain associates, particularly G. clavigera, promote death of WWP when attacked by MPB. These findings suggest that the administration of a fungicide with insecticide for tree protection against bark beetles may be advantageous.
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Papers by Srđan G Aćimović