Stone Fruit Orchards Research Articles

Development of Synthetic Food-Related Attractant for Carpophilus davidsoni and its Effectiveness in the Stone Fruit Orchards in Southern Australia

An effective synthetic attractant, based on host-related volatile compounds, was developed for the nitidulid beetle, Carpophilus davidsoni, as part of a program for managing this pest in Australian peach orchards. Fermenting peach juice and whole peaches and nectarines served as chemical models for the attractant, and synthetic formulations were sought that matched the natural sources, both with respect to emission rates of key chemicals (in ng/min) and attractiveness to flying beetles. All volatile sources were kept in airstreams during laboratory chemical measurements, and these airstreams were subsequently directed to the wind-tunnel behavioral assay, allowing the determined emission rates of compounds to be associated with particular levels of attractiveness. Chemical sampling of airstreams was by solid-phase microextraction, and analysis was by gas chromatography and mass spectrometry. A volatile delivery system was coupled to the wind tunnel and allowed facile manipulation of natural and synthetic scents. The final blend, modeled after fermenting peach juice, contained ethanol as the main constituent and 2-methyl-1-propanol, 2-methyl-1-butanol, 3-methyl-1-butanol, acetaldehyde, and ethyl acetate as minor constituents, all in water solution. Blend components were synergistic, and use of a mixture was essential for optimal attractiveness. The synthetic blend was highly effective in the field and is intended to replace fermenting peach juice and overripe peaches, which were previously used as synergists of the Carpophilus spp. aggregation pheromones in attract-and-kill stations.

The incidence of New Zealand flower thrips in stonefruit orchards between flowering and harvest

New Zealand flower thrips Thrips obscuratus (Crawford) (NZFT) feeds on the nectar and pollen of stonefruit flowers and was thought to then disappear from orchards for 23 months until the fruit ripen Aerial populations of NZFT were sampled in Central Otago orchards using white sticky traps Samples were taken in a peach orchard for 6 months from flowering until after harvest and in two cherry orchards for six weeks during harvest In all three orchards populations of NZFT reached high numbers in December In the peach block NZFT numbers peaked in December several weeks before the fruit began to ripen It was concluded that NZFT can live in stonefruit orchards probably feeding on newly emerged leaves of peaches or cherries or in the vegetation of the irrigated orchard floor Trapped thrips could also have come from other blocks within the larger orchard areas or from outside sources

The Changing Needs with Time for Mineral Nutrition of an Organic Stone Fruit Orchard Under Mediterranean Conditions

Fruit growers apply fertilizer at high rates with soil organic matter (SOM) below 2.0%. As organically certified fertilizers are costly, our objective was to compare the effects of two modes of organic nitrogen nutrition to conventional control on plum tree yield and soil properties. The orchard is located on a Vertisol soil, and planted to plums in 1998. The tested treatments were: A) conventional control, receiving an average of 350 kg of N/ha per year; B) fertilization using cattle manure compost (15 tons/ha per year) + feather meal (1 ton/ha per year); and C) a combination of the same amount of compost + 500 kg of feather meal/ha per year + leguminous cover crop (alfalfa, Medicago sativa cv. Gilboa). By 2003, the SOM of treatment A remained stable and that of treatments B and C increased by 36% and 91%, respectively. As a result, soil bulk density of treatments B and C declined with no change in A. During the first year, levels of soil nutrients were lower in treatments B and C than those in A, but they became higher after 2 to 3 years. Nitrification capacity of the soils of treatments B and C was higher than that of treatment A. This enabled a drastic reduction of the application rates of organic amendments without resulting a decline in the soil's nutrient content in the organic treatments over the next 3 years, due to continued mineralization of the SOM pool. Various soil microbial characteristics (microbial counts, total hydrolytic activity, functional richness, and diversity) were determined. In all these parameters the organic treatments showed higher levels than treatment A. Stem circumferences and yields were similar for all treatments. It can be concluded that soil productivity is affected by SOM, so that after achieving a threshold level of SOM, fertilization needs are reduced considerably.

Possibilities of brown rot management in organic stone fruit production in Hungary

In this study, possibilities of environmentally-friendly plant protection against two brown rot species was summarized for organic stone fruit orchards. Symtomps of the two most important brown rot species (Monilinia fructigena (Aderh. & Ruhl.) Honey and Monilinia laxa (Aderh. & Ruhl.) Honey) were described and then cultivar susceptibility to brown rot was discussed. Several sustainable plant protection methods were selected and discussed in details such as mechanical, agrotehcnical, biological, and other control possibilities (elemental sulphur, lime sulphur and copper).

Geostatistical characterization of the spatial distribution of Grapholita molesta and Anarsia lineatella males in an agricultural landscape

Abstract: The results obtained in spatial analysis of pheromone trap catches of oriental fruit moth, Grapholita molesta (Busck) and Peach twig borer, Anarsia lineatella (Zeller), are reported. The studies were performed in the Molise region, central Italy, during 2002 and 2003. Local distributions of oriental fruit moth and peach twig borer males were considered in a heterogeneous landscape. The aim of the study was to determine the temporal and spatial variation in distribution and abundance of the insect pests inside small plots of fruit orchards (apple, kiwi, peach, pear and plum fruits) and outside (in field crops, irrigation channels, hedgerows and a river), and to evaluate the importance of the host plants in relation to the adult distributions. Results showed that the main ‘hot spot’ for both lepidopterous pests was in a stone fruit orchard in the northern zone of the study area; other infested areas were in stone orchards and, in the case of A. lineatella, also in plum orchards. The river seems to act as a barrier rather than an ecological corridor. The observed spatio‐temporal distributions of G. molesta and A. lineatella differed and were determined mainly by the location of the most important host plants and by the tendency of males to move in the environment, i.e. inside and outside the breeding and mating sites. Adequate knowledge of these spatial processes should be considered as an essential prerequisite for integrated pest management programmes in a precision farming approach.

An attract‐and‐kill system to control Carpophilus spp. in Australian stone fruit orchards

AbstractDuring two growing seasons, the use of an attract‐and‐kill system for control of Carpophilus spp. (Coleoptera: Nitidulidae) and the effective range or drawing power of the attract‐and‐kill stations were examined in stone fruit orchards in the Goulburn Valley, northern Victoria, Australia. Three attract‐and‐kill stations, baited with synthetic aggregation pheromone plus co‐attractant, were placed about 50 m apart in the upwind corner of each treated block 5–6 weeks before the fruit began to ripen. Large numbers of Carpophilus spp. were caught in the attract‐and‐kill stations immediately after placement. By the time fruit had ripened, the number of Carpophilus spp. caught had decreased greatly. Fruit damage caused by Carpophilus spp. in treated blocks, especially in 2000–2001 season, was almost zero (0.1% and 0.6%) in trees and on the ground, respectively, whereas the damage levels in control blocks were 5.2% and 19.9% in trees and on the ground, respectively. This study indicates that excellent protection of ripening stone fruit may be achieved by using attract‐and kill‐stations.

Diversity of Plum pox virus isolates in Bosnia and Herzegovina

Sixteen Plum pox virus (PPV) isolates from several stone fruit cultivars, host species, orchards and geographical areas of Bosnia and Herzegovina were selected for typing, using serotype‐specific monoclonal antibodies (MAbs) and PCR–RFLP, targeting the 3′ terminal region of the coat protein (CP) and P3‐6K1 with restriction enzymes RsaI and DdeI. Four PPV isolates were identified as PPV‐M by serology and PCR; eight isolates were identified as PPV‐D based on PCR–RFLP on both genomic regions, but were not recognized by the D‐specific MAb4DG5. Four isolates from plum were identified as natural D/M recombinants (PPV‐Rec), based on conflicting results of CP and P3‐6K1 typing. To investigate the genetic diversity of Bosnian PPV isolates in more detail, five isolates (three PPV‐Rec, one PPV‐M and one PPV‐D) were partially sequenced in the region spanning the 3′ terminal part of the NIb gene and the 5′‐terminal part of the CP gene, corresponding to nucleotides 8056–8884. Nucleotide sequence alignment of recombinant isolates showed that they were closely related at the molecular level to previously characterized recombinants from other European countries, and shared the same recombination break point in the 3′ terminal part of the NIb gene. This is the first report of naturally infected Prunus trees with PPV‐M, PPV‐D and PPV‐Rec in Bosnia and Herzegovina. The high variability of the Bosnian PPV isolates fits with the presence of this virus in the country over a long period.

Aphid (Hemiptera: Aphididae) Species Composition and Potential Aphid Vectors of Plum Pox Virus in Pennsylvania Peach Orchards

Plum pox, an invasive disease recently identified in Pennsylvania stone fruit orchards, is caused by the aphid-transmitted Plum pox virus (genus Potyvirus, family Potyviridae, PPV). To identify potential vectors, we described the aphid species communities and the seasonal dynamics of the dominant aphid species within Pennsylvania peach orchards. Aphids were trapped weekly in 2002 and 2003 from mid-April through mid-November within two central Pennsylvania orchards by using yellow and green water pan traps. In total, 42 aphid species were identified from both orchards over 2 yr. Within orchards, actual species richness ranged from 24 to 30 species. The Abundance Based Coverage Estimator predicted species richness to range from 30 to 36 species, indicating that trap catches were identifying most aphid species expected to occur in the orchard. Three species, Rhopalosiphum maidis (Fitch), Aphis spiraecola Patch, and Myzus persicae (Sulzer), were consistently dominant across locations and years. Orchard-trapped populations of these three species peaked in a similar chronological sequence each year. As expected, trap color influenced the total number and distribution of the predominate species collected. However, the same dominant species occurred in both yellow and green traps. Based on the seasonal population dynamics reported here and on published vector efficacy studies, the most probable significant PPV vector was identified as A. spiraecola. If the PPV pathogen escapes current quarantine or if subsequent reintroductions of PPV occur, these data will be useful for developing plum pox management strategies.

Aphid (Hemiptera: Aphididae) Species Composition and Potential Aphid Vectors of Plum Pox Virus in Pennsylvania Peach Orchards

Plum pox, an invasive disease recently identified in Pennsylvania stone fruit orchards, is caused by the aphid-transmitted Plum pox virus (genus Potyvirus, family Potyviridae, PPV). To identify potential vectors, we described the aphid species communities and the seasonal dynamics of the dominant aphid species within Pennsylvania peach orchards. Aphids were trapped weekly in 2002 and 2003 from mid-April through mid-November within two central Pennsylvania orchards by using yellow and green water pan traps. In total, 42 aphid species were identified from both orchards over 2 yr. Within orchards, actual species richness ranged from 24 to 30 species. The Abundance Based Coverage Estimator predicted species richness to range from 30 to 36 species, indicating that trap catches were identifying most aphid species expected to occur in the orchard. Three species, Rhopalosiphum maidis (Fitch), Aphis spiraecola Patch, and Myzus persicae (Sulzer), were consistently dominant across locations and years. Orchard-trapped populations of these three species peaked in a similar chronological sequence each year. As expected, trap color influenced the total number and distribution of the predominate species collected. However, the same dominant species occurred in both yellow and green traps. Based on the seasonal population dynamics reported here and on published vector efficacy studies, the most probable significant PPV vector was identified as A. spiraecola. If the PPV pathogen escapes current quarantine or if subsequent reintroductions of PPV occur, these data will be useful for developing plum pox management strategies.

Environmentally-benign plant protection possibilities against domestic Monilinia spp. in organic apple and stone fruit orchards

In this study, possibilities of environmental-friendly plant protection against domestical brwon rot species were summarized for oecological pome and stone fruit orchards. Symtomps of the two most important brown rot species (Monilinia fructigena (Aderh. & Ruhl.) Honey and Monilinia laxa (Aderh. & Ruhl.) Honey) were described and then cultivar susceptibility to brown rot was discussed. Furthermore, mechanical, agrotecnical, biological, and other control possibilities (stone powders, plant extracts and restricted chemical materials) were shown.

Copper and streptomycin resistance in bacterial strains isolated from stone fruit orchards in New Zealand

Stone fruit orchards in New Zealand are sprayed with copperbased compounds and streptomycin to control bacterial diseases such as bacterial blast (Pseudomonas syringae pv syringae) and bacterial spot (Xanthomonas arboricola pv pruni) About 50 of the bacteria isolated from nectarines from the orchard at Clyde Research Centre Central Otago in September 2003 were found to be resistant to copper The percentage of strains of P syringae pv syringae isolated in 2004 from nectarines from the same orchard that were resistant to 500 mg/litre of copper was 58 This percentage rose to 92 seven days after treating the trees with copper In 35 of the 48 samples from commercial stone fruit orchards tested more than half of the bacteria isolated were resistant to 500 mg/litre of copper Only five strains of Xanthomonas sp out of the 306 strains tested were resistant to copper; none were resistant to streptomycin

Evaluation of the predatory wasp, Ancistrocerus gazella, for biological control of leafrollers in Otago fruit crops. II. Wasp phenology and seasonal changes in prey composition

The phenology of the predatory wasp, Ancistrocerus gazella (Panzer), colonising artificial nests was studied over 3 years in seven Study Areas in or adjacent to mixed pome and stonefruit orchards in Otago, New Zealand. Each Study Area had three to four nest sites comprising groups of wooden blocks drilled with 6 mm diameter nest tubes 75–150 mm in length. The timing of nest tube colonisation was recorded by observing the presence of nest closures (mud plugs) and their subsequent opening during adult emergence. The nest tubes from some entire nest sites were dissected to determine the stages of development of the wasps and their subsequent emergence, and samples of nest tubes were dissected weekly from a range of nest sites to determine changes in prey composition over the season. A. gazella was found to be primarily bivoltine, but part of each generation entered diapause and a small number of second generation wasps also emerged before winter in warm sites. The adult foraging of each generation was well synchronised with late larval instars of the two generations of three pest leafroller species. However, very low numbers of leafroller larvae were collected because A. gazella was highly polyphagous, predominantly exploiting a sequence of other lepidopterous species. It was concluded that management of A. gazella for leafroller control would be uneconomic but it provides a useful component of the natural enemy complex of pest leafroller species.

Ground-penetrating Radar to Detect and Quantify Residual Root Fragments Following Peach Orchard Clearing

Consecutive replanting of peach (Prunus persica) trees on the same orchard site can result in various replant problems and diseases, including armillaria root disease (Armillaria spp.), which develops upon contact between the roots of newly planted trees and infested residual root pieces in the soil. There is little information regarding the quantity of roots remaining in stone fruit orchards following tree removal and land clearing. We investigated the utility of ground-penetrating radar (GPR) to characterize reflector signals from peach root fragments in a controlled burial experiment and to quantify the amount of residual roots remaining after typical commercial orchard clearing. In the former experiment, roots ranging from 2.5 to 8.2 cm in diameter and buried at depths of 11 to 114 cm produced characteristic parabolic reflector signals in radar profiles. Image analysis of high-amplitude reflector area indicated significant linear relationships between signal strength (mean pixel intensity) and root diameter (r = -0.517; P = 0.0097; n = 24) or the combined effects of root diameter and burial depth, expressed though a depth × diameter term (r = -0.630; P = 0.0010; n = 24). In a peach orchard in which trees and roots had been removed following typical commercial practice (i.e., trees were pushed over, burned, and tree rows subsoiled), a GPR survey of six 4 × 8-m plots revealed that the majority of reflector signals indicative of root fragments were located in the upper 30 to 40 cm of soil. Based on ground-truth excavation of selected sites within plots, reflectors showing a strong parabolic curvature in the radar profiles corresponded to residual root fragments with 100% accuracy, whereas those displaying a high amplitude area represented roots in 86.1% of the cases. By contrast, reflectors with both poor curvature and low amplitude yielded roots for less than 10% of the excavated sites, whereas randomly selected sites lacking reflector signals were devoid of any roots or other subsurface objects. A high level of variability in the number of residual roots was inferred from the radar profiles of the six plots, indicating an aggregated distribution of root fragments throughout the field. The data further indicated that at least one residual root fragment would be present per cubic meter of soil, and that many of these fragments have diameters corresponding to good to excellent inoculum potential for armillaria root disease. Further GPR surveys involving different levels of land clearing, combined with long-term monitoring of armillaria root disease incidence in replanted trees, will be necessary to ascertain the disease threat posed by the levels of residual root biomass observed in this study.

POTENTIAL USE OF TRANSGENIC PLUMS RESISTANT TO PLUM POX VIRUS FIELD INFECTION

Research to date indicates that post-transcriptional gene silencing (PTGS) is an effective strategy for the development of stable high-level resistance to Plum pox virus in plum. Field tests in Poland, Romania, and Spain show that after 5–6 years of natural aphid vectored inoculation, trees of the PTGS clone C5 remain virus-free. The effectiveness of transgene-based PTGS for imparting potyvirus resistance and the relatively rapid incorporation of this trait into Prunus germplasm suggests continued exploration and employment of this strategy for producing virus resistant stone fruits and other woody perennial tree fruits. INTRODUCTION Plum pox virus (PPV), the causal agent of sharka disease, is one of the most serious viruses affecting stone fruits, a group of commercially-produced Prunus species that includes peach, plum, apricot, and cherry (Nemeth, 1994). PPV is spread by aphids and through the use of infected budwood. Symptoms of plum pox infection include leaf chlorosis, fruit chlorosis and deformation, premature fruit drop, and tree decline, particularly in the presence of other common Prunus viruses (Roy and Smith, 1994). Originally reported from Bulgaria in 1932, PPV has spread throughout Europe where it has caused the destruction of over 100 million stone fruit trees. The scarcity of natural resistance and the lack of efficient control measures have made it difficult to stop the spread of PPV. Quarantine, plant sanitation, and eradication are the only available control strategies. Serious consequences result when these measures fail. Recent outbreaks of PPV in Chile, U.S., and Canada illustrate the potential for spread of this disease and the devastating consequences to the fruit industry. For example, during the first 2.5 years following the appearance of PPV in a limited area of Pennsylvania over 550 ha of stone fruit orchards were destroyed in order to contain and ostensibly eradicate the virus. Important and useful work is underway on the development of resistant cultivars of stone fruits utilizing naturally occurring resistance genes (Hartmann, 2002; Kegler et al., 1998; Polak et al., 2002). Yet there are few highly resistant genotypes, and the number of genes affecting resistance, their inheritance, and their effectiveness against various strains of PPV are areas of inquiry that require further study. It is clear that a multifaceted approach to developing resistant plants is necessary. Using plum (Prunus domestica L.) as a model, we have shown that through genetic transformation, stone fruits highly resistant to PPV can be developed (Scorza et al., 1994; Ravelonandro et al., 1997). This resistance is based on post-transcriptional gene silencing (PTGS) (Turner and Schuch, 2000) induced by the PPV coat protein transgene that was inserted into plum Proc. XXVI IHC – Genetics and Breeding of Tree Fruits and Nuts

Assessing dormant season organophosphate use in California almonds

Organophosphate (OP) pesticides were recommended during the dormant season to control overwintering insects such as peach twig borer, San Jose scale, European red mite, and brown mite in California almond and stone fruit orchards. However, since 1990, dormant OP use had fallen under increased scrutiny due to surface water contamination concerns. Studies have shown positive correlation between OP use and residue load in surface water. The purpose of this study is to assess the trends and regional patterns of OP use in almond orchards, and to identify factors that may have influenced those trends, including weather, pest pressure, and use of alternatives to OP such as pyrethroid, dormant oils, and Bacillus thuringiensis (Bt) for the assessment of the impacts to surface water quality. Pesticide use data from the California Department of Pesticide Regulation were analyzed. Regression analyses were used to assess trends from 1992 to 2000, and a geographic information system (GIS) was used to visualize the spatial variation in pesticide use. Results from this study indicated that, statewide, dormant OP use decreased while the use of some alternatives, such as dormant pyrethroid, no dormant insecticides, and in-season pyrethroid, oil alone, and Bt, increased in the last 9 years. The significant decreasing trend of OP use was observed for the measures of kilogram per hectare crop planted, percentage of total planted hectare treated, and numbers of growers who applied dormant OP. The reduction of dormant OP use appeared in all major almond-growing counties. Correlation analyses revealed that more rain was associated with less dormant OP use. A higher percent of almond damage, or rejects, was related to higher OP use in the following dormant season and in-season periods. However, the effects of weather and percent of nut rejects can only explain small portion of the variations in dormant OP use. Therefore, in addition to weather and pest pressures, economic pressures and various outreach and extension programs may also have played a role in encouraging farmers to reduce their use of dormant OP.

Mycoviruses in Monilinia fructicola

DsRNAs were detected in 36 of 49 Monilinia fructicola isolates from stone fruit orchards in New Zealand. The dsRNA profiles were highly variable, even between isolates from a single tree. Comparison of pathogenicity on detached fruit, in vitro growth rate, and sporulation of 14 isolates showed no obvious correlation with presence of dsRNAs. Partially purified extracts from four isolates were examined for the presence of virus-like particles by transmission electron microscopy. One isolate contained 45 nm isometric particles similar in appearance to totiviruses and partitiviruses. A second isolate contained 200-250 x 25 nm rigid rods similar in appearance to the plant pathogenic tobraviruses and furoviruses. This is the first report of the presence of viral-like agents in the brown rot fungus Monilinia fructicola.

Mating disruption barriers in pome fruit for improved control of oriental fruit moth Grapholita molesta Busck (Lep., Tortricidae) in stone fruit under mating disruption

Abstract:Oriental fruit moth (OFM) Grapholita molesta Busck (Lep., Tortricidae) is a very important pest in commercial orchards in Victoria, Australia. Pheromone‐mediated mating disruption (MD) applied in stone fruit orchards successfully controlled OFM populations for many years, but damage to shoot tips and fruit at the edge of peach blocks located adjacent to pear blocks under insecticide treatments has become problematic. To improve protection of stone fruit against edge damage and outbreaks of OFM, all orchards were treated with sex pheromone dispensers for MD on an area‐wide basis. Area‐wide MD treatment, including all pome and stone fruit orchards in a discrete area, successfully controlled edge infestations of OFM, but was expensive. To reduce the cost of OFM control, sex pheromone dispensers for MD were applied as barrier treatments to 54–60 m of neighbouring pears adjacent to peaches under MD. Detailed monitoring of the OFM population, shoot tip and fruit damage assessments indicated that application of MD barriers on pears during two consecutive seasons provided sufficient control of OFM on peaches. This MD barrier treatment was able to reduce the number of OFM caught in all experimental peach blocks, with damage to shoot tips and fruit giving similar results to MD treatment of the whole neighbouring pear block. Extending the MD treatment area for 54–60 m into the neighbouring pear block significantly reduced the edge damage in MD‐treated peaches in the first season and almost eliminated OFM damage in the second season.

Distribution of the oriental fruit moth Grapholita molesta Busck (Lep., Tortricidae) infestation on newly planted peaches before and during 2 years of mating disruption

Abstract: Oriental fruit moth (OFM) Grapholita molesta Busck (Lep., Tortricidae) is one of the most important pests of commercial stone fruit orchards in the Goulburn‐Murray Valley region of Victoria, Australia. OFM populations have been successfully controlled by the use of the mating disruption (MD) technique for many years, but damage to shoot tips and fruit has now started to increase. The most severe damage under MD is found at the edge of peach blocks, adjacent to the pear blocks under insecticide treatment. In 1997–98, OFM infestation levels were examined in a newly planted peach block surrounded by older peaches, pears, apples and pasture. The infestation distribution was followed up for four consecutive years. No treatments were used against OFM for the first 2 years in the newly planted peaches, but in years 3 and 4 the whole block was treated with MD. At the end of year 2, shoot tip damage was randomly distributed throughout the newly planted peach block with no ‘edge effect’. After MD was applied in year 3, the damage was confined to the edges of the block adjacent to insecticide‐sprayed apples and pears. No ‘edge effect’ was detected along the border with an older peach block treated with MD or on the border with pasture. Extending the MD treated area for 25–30 m into the neighbouring apples and pears in year 4 reduced the ‘edge effect’.

Nested PCR Assays for Detection of Monilinia fructicola in Stone Fruit Orchards and Botryosphaeria dothidea from Pistachios in California

AbstractNested polymerase chain reaction (PCR) assays were developed based on microsatellite regions for detection of Monilinia fructicola, the causal agent of brown rot of stone fruits, and Botryosphaeria dothidea, the causal agent of panicle and shoot blight of pistachio. The nested PCR primers specific to M. fructicola were developed based upon the sequence of a species‐specific DNA fragment amplified by microsatellite primer M13. The external and internal primer pairs EMfF + EMfR and IMfF + IMfR amplified a 571‐ and a 468‐bp fragment, respectively, from M. fructicola, but not from any other fungal species present in stone fruit orchards. The nested PCR primer pairs specific to B. dothidea were developed based upon the sequence of a species‐specific 1330‐bp DNA fragment amplified by microsatellite primer T3B. The external and internal primer pairs EBdF + EBdR and IBdF + IBdR amplified a 701‐ and a 627‐bp fragment, respectively, from B. dothidea, but not from any other fungal species associated with pistachio. The nested PCR assays were sensitive enough to detect the specific fragments in 1 fg of M. fructicola or B. dothidea DNA or in the DNA from only two conidia of M. fructicola or B. dothidea. The nested PCR assays could detect small numbers of M. fructicola conidia caught on spore‐trap tapes and detect visible infections of B. dothidea in pistachio tissues. Microsatellite regions with high numbers of copies are widely dispersed in eukaryotic genomes. The results of this study indicate that microsatellite regions could be useful in developing highly sensitive PCR detection systems for phytopathogenic fungi.

The coincidence of thrips and dispersed pollen in PNRSV‐infected stonefruit orchards – a precondition for thrips‐mediated transmission via infected pollen

SummaryRecent laboratory studies have demonstrated that Prunus necrotic ringspot virus (PNRSV) (family Bromoviridae) can be readily transmitted when thrips and virus‐bearing pollen are placed together on to test plants. For this transmission mechanism to result in stonefruit tree infection in the field, PNRSVbearing pollen must be deposited onto surfaces of stonefruit trees on which thrips also occur. In a previous paper, we demonstrated that almost all pollen in a PNRSV‐infected Japanese plum orchard in southeastern Queensland was deposited onto flowers, whereas few grains occurred on leaves and none on stems. Here, we present results of our investigation of thrips species composition, distribution and abundance on stonefruit trees in the same study area as our previous pollen deposition study. We collected a total of 2010 adult thrips from 13 orchards during the 1989, 1991 and 1992 flowering seasons of which all but 14 were in the suborder Terebrantia. Most (97.4%) terebrantian thrips were of three species, Thrips imaginis, Thrips australis and Thrips tabaci. Thrips tabaci as well as species mixtures that included T. imaginis, T. australis and T. tabaci have been shown to transmit PNRSV via infected pollen in laboratory tests. Adult thrips were frequently collected from flowers but rarely from leaves and never from stems. Large and significant differences in numbers of T. imaginis, T. australis and T. tabaci adults in flowers occurred among orchards and between seasons. No factor was conclusively related to thrips numbers but flowers of late‐flowering stonefruit varieties tended to hold more thrips than those of early‐flowering varieties. Our results indicate that the common thrips species present on stonefruit trees in the Granite Belt are also ones previously shown to transmit PNRSV via infected pollen in the laboratory and that these thrips are concentrated in tonefruit flowers where most stonefruit pollen is deposited. These results contribute to mounting circumstantial evidence that stonefruit flowers may be inoculated with PNRSV via an interaction of thrips with virus‐bearing pollen and that this transmission mechanism may be an important cause of new tree infections in the field