Optimizing the Integration of a Biopesticide (Bacillus subtilis QST 713) with a Single-Site Fungicide (Benzovindiflupyr) to Reduce Reliance on Synthetic Multisite Fungicides (Captan and Mancozeb) for Management of Apple Scab.

Apple scab is one of the most economically important diseases of apple in temperate production regions. In the absence of durable host resistance in commercially preferred cultivars, considerable applications of fungicides are needed to manage this disease. With the sequential development of resistance to nearly all classes of single-site fungicides in the apple scab pathogen Venturia inaequalis, synthetic multisite fungicides, such as mancozeb and captan, often comprise the core of chemical management programs for apple scab. Although these fungicides have demonstrable benefits for both disease and fungicide resistance management, the sustainability movement within agriculture aims to reduce reliance on such fungicides because of their broader environmental impacts. In this study, we establish a framework to enhance the feasibility of chemical management programs that do not rely on use of synthetic multisite protectant fungicides to manage apple scab. Specifically, we wish to evaluate chemical programs that integrate the biopesticide Bacillus subtilis QST 713 (Serenade Opti) in rotation with benzovindiflupyr (Aprovia), a single-site fungicide belonging to the class of succinate dehydrogenase inhibitors (SDHI), to circumvent the need for applications of synthetic multisite fungicides. During implementation of these programs, disease incidence data were taken at biweekly intervals. Regardless of the seasonal challenges presented in the 2 years of this study, when Bacillus subtilis QST 713 was used in place of captan and mancozeb mixtures, we did not observe any significant differences (P > 0.05) in development of apple scab symptoms between any of the management programs for the vertical axis or super spindle orchards in either year. This potential for substituting synthetic multisite fungicides with biopesticides is best realized when the programs are used with a decision support system in a super spindle planting system, where trees have reduced canopy densities. This 2-year study shows the potential to achieve adequate disease control using the integration of SDHI fungicides and biological controls without the use of synthetic multisite fungicides.

Effects of Consecutive Streptomycin and Kasugamycin Applications on Epiphytic Bacteria in the Apple Phyllosphere.

Antibiotic applications are essential for fire blight management in the eastern United States. Recently, streptomycin-resistant Erwinia amylovora strains were found in New York. There are growing concerns that streptomycin resistance may develop from postbloom streptomycin applications in local orchards. Our goal was to investigate the impacts of increasing streptomycin and kasugamycin applications on bacterial epiphyte community composition and antibiotic resistance in the phyllosphere of 'Idared' apple plantings in 2014 and 2015. Rinsate samples from leaves treated with 0, 3, 5, and 10 applications of streptomycin and kasugamycin were collected to isolate, enumerate, and identify epiphytic bacterial species. The majority of isolated epiphytic bacteria were identified as Pantoea agglomerans and fluorescent Pseudomonas spp., whereas E. amylovora was rarely found. Overall, postbloom streptomycin use did not result in an increased recovery of streptomycin-resistant E. amylovora. However, other streptomycin-resistant epiphytes (P. agglomerans and Pseudomonas spp.) did increase with increasing streptomycin applications. Increasing kasugamycin applications reduced the overall number and percentage of streptomycin-resistant epiphytes in the phyllosphere, which has important implications regarding the use of kasugamycin in orchards where streptomycin resistance is a concern.

Exploring Diversity and Origins of Streptomycin-Resistant Erwinia amylovora Isolates in New York Through CRISPR Spacer Arrays.

Streptomycin is the most effective and widely used chemical control in the eastern United States for blossom blight of apple caused by Erwinia amylovora; however, resistance to this antibiotic has been a concern in New York since 2002. From 2011 to 2014, statewide collections of E. amylovora were conducted resulting in the isolation of streptomycin-resistant (SmR) E. amylovora from several commercial orchards. Further genetic analysis of isolates was necessary to understand the origins and the diversity of these bacteria. Clustered regularly interspaced short palindromic repeat (CRISPR) spacer sequencing was employed to explore the diversity and possible origins of New York SmR E. amylovora isolates. The spacer array CR1, CR2, and CR3 regions of 27 SmR E. amylovora isolates and 76 streptomycin-sensitive (SmS) E. amylovora isolates were amplified and subsequently sequenced, revealing 19 distinct CRISPR spacer profiles for New York isolates. The majority of SmR E. amylovora isolates had the same CRISPR profile as SmR E. amylovora isolates discovered in 2002. This may infer that eradication efforts in 2002 failed and the bacterial populations continued to spread throughout the state. Several CRISPR profiles for SmR E. amylovora were identical to SmS E. amylovora collected from the same orchards, leading to the hypothesis that resistance may be developing within New York. Profiles not unique to New York were identical to many isolates from the Midwestern, eastern, and western United States, implying that streptomycin resistance may be due to the introduction of SmR E. amylovora from other regions of the United States. The increased understanding as to how SmR E. amylovora isolates are introduced, evolve, or have become established afforded by CRISPR profiling has been useful for disease management and restricting the movement of streptomycin resistance in New York.

Prevalence of Streptomycin-Resistant Erwinia amylovora in New York Apple Orchards.

Resistance to streptomycin in Erwinia amylovora was first observed in the United States in the 1970s but was not found in New York until 2002, when streptomycin-resistant (SmR) E. amylovora was isolated from orchards in Wayne County. From 2011 to 2014, in total, 591 fire blight samples representing shoot blight, blossom blight, and rootstock blight were collected from 80 apple orchards in New York. From these samples, 1,280 isolates of E. amylovora were obtained and assessed for streptomycin resistance. In all, 34 SmR E. amylovora isolates were obtained from 19 individual commercial orchards. The majority of the resistant isolates were collected from orchards in Wayne County, and the remaining were from other counties in western New York. Of the 34 resistant isolates, 32 contained the streptomycin resistance gene pair strA/strB in the transposon Tn5393 on the nonconjugative plasmid pEA29. This determinant of streptomycin resistance has only been found in SmR E. amylovora isolates from Michigan and the SmR E. amylovora isolates discovered in Wayne County, NY in 2002. Currently, our data indicate that SmR E. amylovora is restricted to counties in western New York and is concentrated in the county with the original outbreak. Because the resistance is primarily present on the nonconjugative plasmid, it is possible that SmR has been present in Wayne County since the introduction in 2002, and has spread within and out of Wayne County to additional commercial growers over the past decade. However, research is still needed to provide in-depth understanding of the origin and spread of the newly discovered SmR E. amylovora to reduce the spread of streptomycin resistance into other apple-growing regions, and address the sustainability of streptomycin use for fire blight management in New York.

First Report of White Pine Blister Rust Caused by Cronartium ribicola on Immune Black Currant Ribes nigrum Cv. Titania in Preston, Connecticut.

Since the relaxation of federal sanctions on the planting of Ribes crops because of the development of white pine blister rust (WPBR) immune Ribes cultivars (3), a small industry for the production of Ribes fruit for fresh and processing markets was established in New York and surrounding New England states. The majority of Ribes acreage in the region is planted to a WPBR immune black currant R. nigrum cv. Titania. From 2008 to June 2011, symptoms resembling those caused by WPBR pathogen Cronartium ribicola were observed at a large (>12 ha) R. nigrum cv. Titania planting in Preston, CT. In 2008, infection was restricted to a single field (100% incidence), but in 2009, all fields suffered from premature defoliation by late July. In 2010 and 2011, there was considerable incidence (>85%), but premature defoliation was kept in check by chemical management. Symptoms began as chlorotic lesions (0.5 to 4.3 mm in diameter) on both sides of the leaf. These chlorotic lesions had margins delineated by leaf veins and many developed necrotic (0.3 to 0.9 mm in diameter) centers on the upper surface of leaves within 2 to 3 weeks. The undersides of lesions developed blisters containing orange uredinia (0.1 to 0.33 mm in diameter) with smooth peridia that broke with the production of yellow-orange urediniospores (30 × 15 to 25 µm). Symptoms and signs were consistent with published descriptions of C. ribicola (1) and with WPBR infections on highly susceptible R. nigrum cv. Ben Alder planted at the New York State Agricultural Experiment Station in Geneva. Additional confirmation was provided by sequencing the two internal transcribed spacer (ITS) regions and the 5.8S gene (GenBank Accession No. JN587805; 98% identity with No. DQ533975) in the nuclear ribosomal repeat using primers ITS1-F and ITS4 as described previously (2,4). Furthermore, an attempt was made to confirm pathogenicity in the greenhouse by inoculating shoots of potted nursery stock of R. nigrum cv. Titania. Shoots were inoculated by rubbing leaves with either an uninfected currant leaf or a currant leaf from Preston, CT sporulating with urediniospores. Plants were subsequently misted with dH20 and covered with plastic bags for 24 h. Plants were watered biweekly and kept in a greenhouse with 39 to 65% relative humidity at 21 to 26°C. Shoots were monitored for symptom development on a weekly basis. After 3 weeks, 2 of the 10 plants inoculated with infected leaves developed chlorotic lesions and uredinia identical to those on leaves from Preston, CT, while all others remained healthy. Although not easily reproducible in a greenhouse, the breakdown of immunity in R. nigrum cv. Titania was observed for the last 4 years in Connecticut. Given that WPBR immunity was one of the conditions for legalized planting of Ribes, the breakdown of immunity has potentially deleterious implications particularly for nearby states like Massachusetts and New York, in which the Ribes industries are expanding. Moreover, Ribes growers may need to rely on chemical management programs to manage WPBR in the future. References: (1) G. B. Cummins. Illustrated Genera of Rust Fungi. Burgess Publishing Company, Minneapolis, MN, 1959. (2) M. Gardes and T. D. Bruns. Mol. Ecol. 2:113, 1993. (3) S. McKay. Hortic. Technol. 10:562, 2000. (4) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, Inc., San Diego, CA, 1990.

First Reports of Brown Fruit Rot on Sweet Cherry (Prunus avium) and Plum (P. domestica) and Shoot Blight on Apricot (P. armeniaca), Kwanzan Cherry (P. serrulata), and Sweet Cherry (P. avium) Caused by Monilinia laxa in New York, Rhode Island, and Massachusetts.

In the eastern United States, Monilinia laxa (Aderh. & Ruhl.) Honey has only been reported on tart cherry in New York (NY) (1). As a result of considerable rain in May of 2009 and 2011, an ornamental planting of Kwanzan cherries in Middletown, Rhode Island (RI), a planting of sweet cherry cvs. Ulster, Hedelfingen, Sam, and Lapins in Lanesboro, Massachusetts (MA), and plantings of apricot cvs. Harcot and Hargrande in Albion, Aurora, and Geneva, NY, and Harogem in Lanesboro, MA developed severe shoot blight (>15 to 100% of first-year shoots). Blighted shoots were wilted with the blight encompassing the distal end and often extending into second-year tissue with a distinct sunken margin. Leaves on symptomatic shoots had flushed, but were blighted and light brown. Blossom spurs were often blighted and gummosis was frequently observed at the base. In these same years, sweet cherry cv. Black Gold in Walworth, NY and plum cv. Stanley in Olcott, NY developed severe fruit rot (35 to 70% incidence). Plantings suffering from fruit rot had fruit lesions that began as pale brown, soft lesions with indiscriminant margins that covered 15 to 85% of the fruit surface area. Many blighted spurs, shoot tissues, and infected fruit were sporulating with tan-to-buff colored conidia produced in chains. From each planting with shoot blight, shoot tips were removed for pathogen isolation. Sections of symptomatic shoots (5 cm long) were surface sterilized in 0.6% NaOCl for 1 min and rinsed in sterile dH20. From plantings displaying blighted spurs or fruit rot, isolation was attempted directly from sporulating tissue. Cross sections of sterilized shoot tissue (3 mm thick) or tufts of sporulation from fruit and spurs were placed on potato dextrose agar amended with 50 µg/ml of streptomycin sulfate. After incubation at 24°C for 5 days, colonies with lobed margins, commonly described for M. laxa (4), were obtained. Several colonies resembling M. fructicola were isolated from all locations, but the majority of isolates from spurs and shoots resembled M. laxa. Conidia from both colony morphotypes were lemon shaped, but as expected, those from putative M. laxa isolates were smaller (10.75 × 12.0 µm) compared with those from putative M. fructicola isolates (15.75 × 18.25 µm) (4). Confirmation of M. laxa was further achieved by PCR amplification of the ß-tubulin gene using M. laxa-specific primers as previously described (3). Pathogenicity of M. laxa isolates was proven by inoculating fruit of the stone fruit crop from which they were isolated as previously described (2). Fruit inoculated with M. laxa developed brown, soft sporulating lesions identical to the original observations, while those inoculated with water remained healthy. M. laxa was reisolated from symptomatic shoots and spurs, but not from water-inoculated tissues. The presence of M. laxa has been reported on tart cherries in NY (1), but to our knowledge, this is the first instance of economically devastating shoot blight on apricot in NY and MA, ornamental cherry in RI, and sweet cherry in MA and fruit rot on sweet cherry and plum in NY caused by M. laxa. In wet seasons, stone fruit growers may need to revise their chemical management programs to better prepare for M. laxa epidemics on several stone fruit species. References: (1) K. D. Cox and S. M. Villani. Plant Dis. 94:783, 2010. (2) K. D. Cox and S. M. Villani. Plant Dis. 95:828, 2011. (3) Z. Ma et al. Pest Manag. Sci. 61:449, 2005. J.M. (4) G. C. M. van Leeuwen and H. A. van Kesteren. Can. J. Bot. 76:2042, 1998.

Confirmation of European Brown Rot Caused by Monilinia laxa on Tart Cherry, Prunus cerasus, in Western New York.

Monilinia fructicola (G. Wint.) Honey and M. laxa (Aderh. & Ruhl.) Honey are two pathogens causing brown rot in the United States. While the presence of M. fructicola has been confirmed in all major stone-fruit-production regions in the United States, M. laxa has yet to be detected in much of the eastern production regions. In July 2008, a planting of tart cherries cv. Surefire in Appleton, NY developed severe shoot blight. Blighted shoots (>15% of first-year shoots) were wilted and light brown with the blight encompassing the distal end and often extending into second-year tissue with a distinct sunken margin. Leaves on symptomatic shoots had flushed, but were blighted. Blossom spurs were either blighted at bloom or bore fruit, which were subsequently blighted. Gummosis was commonly observed from cankers at the base of spurs. Both mature and immature mummified fruit in addition to spurs and shoot tissue were sporulating in a manner characteristic of Monilinia (2). Eleven branches displaying symptoms were removed for isolation. Sections of symptomatic shoots (5 cm long) were surface sterilized in 0.6% NaOCl for 1 min and rinsed in sterile dH2O. Cross sections of shoot tissue (3 mm thick), in addition to spores from fruit and spurs, were placed on potato dextrose agar amended with 50 µg/ml of streptomycin sulfate. Following incubation at 24°C for 5 days, 24 colonies exhibiting morphology consistent with that of M. fructicola (uniform colony margin) were obtained, along with nine colonies exhibiting lobed colony margins, commonly associated with M. laxa (3). All colonies resembling M. fructicola were isolated from fruit, whereas those resembling M. laxa were isolated from spurs and shoots. Conidia from both colony morphotypes were lemon-shaped, but those from putative M. laxa isolates were smaller on average (10.75 × 12.0 µm) compared with those from putative M. fructicola isolates (15.75 × 18.25 µm). Confirmation of M. laxa was also accomplished by inoculation of mature green pear (2). Pears inoculated with 104 putative M. laxa conidia per ml produced a region of white-buff colored mycelium but no spores within the inoculated area, while M. fructicola-inoculated pears sporulated abundantly. Identity was further confirmed by PCR amplification of the ß-tubulin gene using M. laxa specific primers as previously described (1). Pathogenicity was proven by inoculating flowering shoots of tart cherry trees (cv. Montmorency) in spring 2009. Twenty shoots were spray inoculated with either 104 M. laxa conidia per ml or sterile dH2O and covered with plastic bags for 24 h. Shoots were monitored for symptom development on a weekly basis. Shoots inoculated with M. laxa developed characteristic shoot blight symptoms, while those inoculated with water remained healthy. M. laxa was reisolated from symptomatic shoots and spurs, but not water-inoculated tissues. The presence of M. laxa is reported for the Great Lakes region, which includes New York, but to our knowledge, this report is the first confirmed instance of economically devastating brown rot caused by M. laxa in New York. In the coming seasons, tart cherry growers must consider revising chemical management programs to protect against European brown rot infection during bloom. References: (1) Z. Ma et al. Pest Manag. Sci. 61:449, 2005. (2) J. M. Ogawa et al. Compendium of Stone Fruit Diseases. The American Phytopathological Society. St. Paul, MN, 1995. (3) G. C. M. van Leeuwen and H. A. van Kesteren. Can. J. Bot. 76:2042, 1998.

Genetic Diversity of Prunus necrotic ringspot virus Isolates Within a Cherry Orchard in New York.

A survey for Prunus necrotic ringspot virus (PNRSV) in an orchard of Prunus cerasus cv. Montmorency and Prunus avium cv. Hedelfingen in New York by enzyme-linked immunosorbent assay indicated an eightfold higher infection rate in sour cherry (33%, 32 of 96) than in sweet cherry (4%, 6 of 136) trees. The presence of PNRSV was confirmed by reverse transcription-polymerase chain reaction and amplification of the coat protein (CP) gene in total RNA from infected leaf tissue. Latent infection was prevalent in the majority of trees infected (87%, 33 of 38), while a few of them exhibited shock symptoms or had severely reduced growth (13%, 5 of 38). Asymptomatic PNRSV-infected trees clustered in spatial proximity to symptomatic trees. Sequence analysis of the CP gene (675 bp) indicated a population structure consisting of one predominant molecular variant for 10 isolates and six minor molecular variants for seven isolates. A high sequence identity was found between the CP gene of PNRSV isolates from cherry trees and other isolates from diverse hosts and various geographic origins at the nucleotide and amino acid levels (88 to 100%). Phylogenetic analyses showed a clustering of PNRSV isolates from cherry trees in New York in the predominant group PV-96.

Instability of Propiconazole Resistance and Fitness in Monilinia fructicola.

ABSTRACT The fitness and the dynamics of demethylation inhibitor fungicide (DMI) sensitivity in isolates of Monilinia fructicola sensitive (no growth at 0.3 mg/liter propiconazole) and resistant (>/=50% relative growth at 0.3 mg/liter propiconazole) to propiconazole were investigated. Overall, there was no considerable compromise in the fitness of resistant isolates compared to sensitive isolates of M. fructicola at the time of collection. Resistant and sensitive isolates differed in their sensitivity to propiconazole (P < 0.001) and incubation period (P = 0.044), but not in latent period, growth rate, spore production, and spore germination frequency (P > 0.05). Consecutive transferring on potato dextrose agar had an impact on conidia production, conidial germination, and growth rate (P < 0.0001). Consecutive transferring also had an impact on propiconazole sensitivity in resistant isolates. In the resistant isolates, sensitivity to propiconazole increased (R(2) = 0.960, P = 0.0034) within the first eight transfers. Similarly, sensitivity to propiconazole increased by 273% over the course of 34 months in cold storage in propiconazole-resistant isolates. Our results show that propiconazole resistance is unstable in vitro and that standard subculturing and cold storage procedures impact propiconazole sensitivity of resistant isolates. The instability of propiconazole resistance in M. fructicola may have important implications for disease management in that a reversion to propiconazole sensitivity could potentially occur in the absence of DMI fungicide pressure in the field.

Gastrodia anti-fungal protein from the orchid Gastrodia elata confers disease resistance to root pathogens in transgenic tobacco.

Diseases of agricultural crops are caused by pathogens from several higher-order phylogenetic lineages including fungi, straminipila, eubacteria, and metazoa. These pathogens are commonly managed with pesticides due to the lack of broad-spectrum host resistance. Gastrodia anti-fungal protein (GAFP; gastrodianin) may provide a level of broad-spectrum resistance due to its documented anti-fungal activity in vitro and structural similarity to insecticidal lectins. We transformed tobacco (Nicotiana tabacum cv. Wisconsin 38) with GAFP-1 and challenged transformants with agriculturally important plant pathogens from several higher-order lineages including Rhizoctonia solani (fungus), Phytophthora nicotianae (straminipile), Ralstonia solanacearum (eubacterium), and Meloidogyne incognita (metazoan). Quantitative real-time PCR and western blotting analysis indicated that GAFP-1 was transcribed and translated in transgenic lines. When challenged by R. solani and P. nicotianae, GAFP-1 expressing lines had reduced symptom development and improved plant vigor compared to non-transformed and empty vector control lines. These lines also exhibited reduced root galling when challenged by M. incognita. Against R. solanacearum expression of GAFP-1 neither conferred resistance, nor exacerbated disease development. These results indicate that heterologous expression of GAFP-1 can confer enhanced resistance to a diverse set of plant pathogens and may be a good candidate gene for the development of transgenic, root-disease-resistant crops.

Characterization of Armillaria spp. from peach orchards in the southeastern United States using fatty acid methyl ester profiling.

Limited information is available regarding the composition of cellular fatty acids in Armillaria and the extent to which fatty acid profiles can be used to characterize species in this genus. Fatty acid methyl ester (FAME) profiles generated from cultures of A. tabescens, A. mellea, and A. gallica consisted of 16-18 fatty acids ranging from 12-24 carbons in length, although some of these were present only in trace amounts. Across the three species, 9-cis,12-cis-octadecadienoic acid (9,12-C18:2), hexadecanoic acid (16:0), heneicosanoic acid (21:0), 9-cis-octadecenoic acid (9-C18:1), and 2-hydroxy-docosanoic acid (OH-22:0) were the most abundant fatty acids. FAME profiles from different thallus morphologies (mycelium, sclerotial crust, or rhizomorphs) displayed by cultures of A. gallica showed that thallus type had no significant effect on cellular fatty acid composition (P > 0.05), suggesting that FAME profiling is sufficiently robust for species differentiation despite potential differences in thallus morphology within and among species. The three Armillaria species included in this study could be distinguished from other lignicolous basidiomycete species commonly occurring on peach (Schizophyllum commune, Ganoderma lucidum, Stereum hirsutum, and Trametes versicolor) on the basis of FAME profiles using stepwise discriminant analysis (average squared canonical correlation = 0.953), whereby 9-C18:1, 9,12-C18:2, and 10-cis-hexadecenoic acid (10-C16:1) were the three strongest contributors. In a separate stepwise discriminant analysis, A. tabescens, A. mellea, and A. gallica were separated from one another based on their fatty acid profiles (average squared canonical correlation = 0.924), with 11-cis-octadecenoic acid (11-C18:1), 9-C18:1, and 2-hydroxy-hexadecanoic acid (OH-16:0) being most important for species separation. When fatty acids were extracted directly from mycelium dissected from naturally infected host tissue, the FAME-based discriminant functions developed in the preceding experiments classified all samples (n = 16) as A. tabescens; when applied to cultures derived from the same naturally infected samples, all unknowns were similarly classified as A. tabescens. Thus, FAME species classification of Armillaria unknowns directly from infected tissues may be feasible. Species designation of unknown Armillaria cultures by FAME analysis was identical to that indicated by IGS-RFLP classification with AluI.

Effect of Desiccants and Herbicides on Germination of Pseudosclerotia and Development of Apothecia of Monilinia vaccinii-corymbosi.

Pseudosclerotia (infected, mummified fruit) are the only source of primary inoculum of Monilinia vaccinii-corymbosi, the causal agent of mummy berry disease of blueberry. Laboratory applications of potential inhibitors of carpogenic germination were made to pseudosclerotia at three distinct developmental stages, i.e., ungerminated pseudosclerotia, pseudosclerotia with emerging stipes, and those with mature apothecia. Potential inhibitors evaluated included soybean oil and ammonium thiosulfate (two desiccants used experimentally as bloom thinners in fruit crops) and diuron and simazine (two commonly used herbicides), each applied in an aqueous suspension with 3% Latron B-1956 surfactant. Various aspects of carpogenic germination including the percentage of pseudosclerotia that produced stipes or apothecia, the number of stipes or apothecia per pseudosclerotium, the percentage of stipes that developed into apothecia, longevity of stipes and apothecia, and ascospore numbers were assessed. Compared with water, application of ammonium thiosulfate (2%) and diuron (2%) reduced stipe and apothecium production when sprayed on ungerminated pseudosclerotia, but these reductions were generally not significantly different from those achieved with Latron B applied alone (P > 0.05). The two compounds, however, completely inhibited the development of stipes into apothecia when applied to pseudosclerotia with stipes and caused a >3-fold reduction in apothecium longevity when applied to pseudosclerotia with mature apothecia. Application of simazine (2%) before germination or at stipe emergence resulted in the development of malformed apothecia from which no ascospores were recovered; stipe and apothecium longevity were also reduced. Soybean oil (15%) and Latron B applied alone had weak or inconsistent effects on most aspects of carpogenic germination of pseudosclerotia, although both compounds, when applied at stipe emergence, significantly reduced ascospore numbers in subsequently formed apothecia. The results suggest that diuron and simazine applied for weed control in commercial blueberry plantings may have beneficial side effects in reducing carpogenic germination of pseudosclerotia. The strong inhibitory effect of ammonium thiosulfate on all aspects of carpogenic germination, along with its value as a nitrogen fertilizer and ancillary herbicide, warrants further evaluation of this compound's performance and economics in the field.

Gradients of Primary and Secondary Infection by Monilinia vaccinii-corymbosi from Point Sources of Ascospores and Conidia.

Spread of mummy berry disease of blueberry, caused by Monilinia vaccinii-corymbosi, occurs in two discrete monocycles; primary infection by ascospores results in shoot blight, while secondary infection of open flowers by conidia leads to fruit mummification. Gradients of primary and secondary infection from point sources of ascospores and conidia placed in separate plant rows were recorded in each of 2 years at two sites with no history of the disease. Primary infection gradients were longer downwind than upwind, with 95% of blighted shoots occurring within 30 m of the ascospore point source. This observation, along with a positive correlation (r = 0.852, P = 0.0072, n = 8) between the distance over which shoot blight occurred and wind speed parallel to the row, supports the role of wind as a key factor in ascospore dispersal. By contrast, secondary infection gradients were shorter downwind and longer upwind, with 95% of infected fruit occurring within 20 m of the conidial point source. The shorter downwind spread of secondary infection, along with a nonsignificant correlation (r = -0.649, P = 0.0812, n = 8) between the distance over which infected fruit occurred and wind speed, suggests that factors other than wind are important in the transfer of conidia to open flowers; this could include conidial dispersal by bee pollinators, which have been shown previously to forage primarily upwind. Exponential and Pareto cumulative distribution functions were fitted to cumulative counts of blighted shoots and infected fruit to model spread of primary and secondary infection. The Pareto model, which is characterized by a longer tail and predicts more infection farther from the inoculum source, better fits the observed disease gradients in most cases.

Oversummer Survival of Monilinia vaccinii-corymbosi in Relation to Pseudosclerotial Maturity and Soil Surface Environment.

Pseudosclerotia (infected, mummified fruit) on the orchard floor act as oversummering and overwintering structures and the sole source of primary inoculum of Monilinia vaccinii-corymbosi, the causal agent of mummy berry disease of blueberry. Survival of pseudosclerotia may be affected by their maturity (degree of stromatization), which can vary considerably at the time of fruit abscission in early summer, and by variations in the soil surface environment. From July through October in 2 years, survival of pseudosclerotia of varying initial maturity (expressed as the proportion of fruit containing mature, melanized entostromata; immature, nonmelanized entostromata; or undifferentiated mycelia) was investigated in the laboratory relative to soil surface temperature and soil moisture content and in the field in relation to shading (full sun versus 50% shade) and ground cover (bare soil versus grass). In the laboratory, oversummer survival, expressed as the percentage of intact pseudosclerotia at the end of the experiment, was higher for cool soil temperatures (approximately 15°C), soils drier than field capacity, and pseudosclerotia containing mature entostromata. In the field, survival was related solely to initial maturity of pseudosclerotia and was highest for pseudosclerotia containing mature entostromata. Shading or grass ground cover did not significantly (P > 0.05) affect oversummer survival, presumably because they did not greatly modify soil temperature or soil moisture. When individual, intact pseudosclerotia were tested for viability using fluorescein diacetate staining, a linear relationship (r = 0.982, P < 0.0001, n = 90) between viable and intact pseudosclerotia was observed, supporting the use of the percentage of intact pseudosclerotia as a measure of oversummer survival.