Peter L. Sholberg

Fumigation of table grapes with acetic acid to prevent postharvest decay

Acetic acid (AA) fumigation controlled decay of Summerland Selection 494 and Selection 651 table grapes in repeated trials over a 3-year period when grapes were fumigated with 0.27% vol/vol AA and stored for 6 weeks at 2 or 5°C. Fumigation of Selection 651 at approximately 2-week intervals with AA or SO 2 controlled both Botrytis and Penicillium decay and reduced berry shatter equally in two separate years. No significant difference occurred between SO 2 and AA treatments in yield components (cluster and berry weight), fruit composition (°Brix, titratable acidity, pH, and color), and degree of rachis drying.

Fumigation of stonefruit with acetic acid to control postharvest decay

Abstract Acetic acid was an effective postharvest fumigant to destroy fungal spores on peaches, nectarines, apricots, and cherries. Decay by Monilinia fructicola and Rhizopus stolonifer on Harbrite peaches was prevented by as little as 1.4 or 2.7 mg l −1 acetic acid, respectively. Harbrite peaches fumigated with 2.7 mg l −1 acetic acid were slightly injured, the phytotoxicity indicated by light brown streaks. Higher concentrations of acetic acid increased injury; the streaks darkened and became much more pronounced. Glohaven peaches treated in the orchard with captan at 5% bloom, full bloom, ripening fruit, and 2 days before harvest then fumigated with 2.7 mg l −1 acetic acid after harvest had significantly less postharvest brown rot (12.5%) than fruit treated with captan alone (25.0%). Decay of Lambert cherries, primarily due to Alternaria spp. , was reduced from 38.9 to 10.0% by fumigation with 2.7 mg l −1 acetic acid. Unfortunately, small pits developed in the fruit surface during storage at 1 °C. Brown rot ( M. fructicola ) of Tilton apricots was reduced from 100 to 25% by fumigation with 2.0 mg l −1 acetic acid without signs of severe phytotoxicity.

Development of a DNA Macroarray for Detection and Monitoring of Economically Important Apple Diseases

Short DNA gene sequences (oligonucleotides) from the ribosomal spacer regions of bacterial and fungal pathogens were used to identify and monitor economically important apple diseases. The oligonucleotides or probes were attached to a nylon membrane by an amine modified linker arm and arranged in a precise pattern to form an array for detecting five pathogens corresponding to five apple diseases. Initially the specificity of the probes was determined by hybridizing pure cultures of the pathogens to the probes. The DNA array correctly identified Botrytis cinerea, Penicillium expansum, Podosphaera leucotricha, Venturia inaequalis, and Erwinia amylovora and eliminated closely related species. When the array was used to monitor V. inaequalis ascospores collected from spore traps located in orchards, it confirmed the presence of ascospores as predicted by the disease forecasting model. Preliminary tests to quantify P. leucotricha populations using grayscale values was effective to 20 conidia per leaf disk. The DNA array is a promising new detection system for accurate identification of several pathogens in a single test with the potential for being a new tool for epidemiological studies.

Isolation and characterization of eight bacteriophages infecting Erwinia amylovora and their potential as biological control agents in British Columbia, Canada

Abstract Nineteen active bacteriophages against Erwinia amylovora, the causal agent of fire blight, were collected from apple and pear orchards in the Okanagan and Fraser Valleys of British Columbia. Eight survived the isolation, purification and storage processes. Five bacteriophage isolates included in this study lysed more than 50% of the 20 E. amylovora strains tested from BC. Examination by transmission electron microscopy revealed that all eight phages belong to the order Caudovirales, the tailed phages, and included members of the families Myoviridae and Podoviridae. Bacteriophages were characterized by digestion of the phage DNA with four restriction endonucleases and two sets of PCR primers. Two novel groups, RFLP groups 7 and 8, were identified based on differences in restriction fragment patterns. Phages ΦEa1337-26 and ΦEa2345-6 reduced infection by 84% and 96%, respectively, when tested on detached pear blossoms using the epiphyte bacterium Pantoea agglomerans Eh21-5 as a carrier. In addition, bacteriophage ΦEa2345-6, applied in combination with Eh21-5, reduced infection of fire blight on apple flowers of potted apple trees by 56% and compared well with the antibiotic streptomycin.

First report of Phytophthora capsici on cucurbits and peppers in British Columbia.

Phytophthora capsici is a devastating pathogen of several commercial cucurbit and solanaceous crops. In August 2004, P. capsici was isolated and identified for the first time in British Columbia from an infected pumpkin (Cucurbita pepo var. medullosa) purchased from a market garden in Kelowna. Visual symptoms of fruit rot were also evident on several varieties of squash (Cucurbita spp.), pepper (Capsicum annuum), and eggplant (Solanum melongena var. esculentum) from the same area. Further characterization of the causal agent was needed to confirm its identification, pathogenicity, and ability to overwinter. Eight isolates obtained from infected pumpkin and from five different squash varieties grew on selective media for P. capsici. Single-spore isolates were produced from these cultures, and DNA was extracted from them. On the basis of sequence analysis, the eight isolates were P. capsici, differing by two or fewer nucleotides in the ribosomal spacer (internal transcribed spacer 2 (ITS2)) regions compared with previously identified strains. In pathogenicity studies, isolates 2256 and 2289, originally from butternut squash (Cucurbita moschata) and pumpkin, respectively, infected sweet pepper (Capsicum annuum var. frutescens) and butternut squash, killing pepper plants in 8 days. The isolates were less pathogenic on squash, according to the area under the disease progress curve, which was approximately six times higher in pepper than squash. Phytophthora capsici was recovered from both sweet pepper and butternut squash, as well as from infected zucchini (Cucurbita pepo var. melopepo) and watermelon (Citrullus lanatus) plants. Mating studies showed that isolates 2256 and 2289 were probably the A1 mating type because they produced oospores only if they were paired with the A2 tester strain obtained from the Centraalbureau voor Schimmelcultures culture collection. A hypothesis on how the disease developed and spread in British Columbia is presented.

Methodology for Determining Relationships Between Inoculum Concentration of Botrytis cinerea and Penicillium expansum and Stem End Decay of Pear Fruit

The objective of this research was to determine quantitative relationships between incidence of stem end decay of pear fruit and inoculum concentration of Botrytis cinerea and Penicillium expansum using dry conidia applied to pear fruit in a settling tower. Five concentrations of conidia were applied to pear fruit, fruit were stored at -1°C for 8 months, and stem end decay was evaluated. In addition, conidia were washed from the surface of inoculated fruit, and DNA was extracted and quantified with real-time polymerase chain reaction (PCR). The linear regression relationships between percent stem end gray mold and B. cinerea conidia per liter of air or per square centimeter of fruit surface were significant (P = 0.01). At the highest inoculum dose introduced into the settling tower, conidia per liter of air, conidia per square centimeter, and percent stem end gray mold at 8 months after inoculation were 12, 31, and 39, respectively for 2000 and 6, 33, and 67, respectively for 2001. Similarly, the linear regression relationships between percent stem end blue mold and P. expansum conidia per liter of air or per square centimeter of fruit surface were significant (P = 0.01 and 0.05, respectively). At the highest inoculum dose introduced into the settling tower, conidia per square centimeter and percent stem end blue mold at 8 months after inoculation were 39 and 26, respectively for 2000 and 66 and 23, respectively for 2003. Real-time PCR provided a rapid, quantitative measure of B. cinerea and P. expansum DNA on pear fruit surfaces. Because of possible year-to-year shifts in susceptibility of fruit to decay, disease incidence:inoculum dose relationships may be of most value compared within years rather than across years. This would facilitate comparison of decay risk among orchards in order to determine which fruit is most suitable for long-term storage.

Phage Biopesticides and Soil Bacteria: Multilayered and Complex Interactions

Soil ecosystems are impacted by the application of biological control agents to aerial portions of plants. The use of bacteriophages, “phages,” as agriculture biopesticides has been adopted and utilized in a number of host–pathogen systems. Phages may be applied as aerial sprays, preplant treatments and soil drenches in concentrations up to 108–10PFU/ml. The short- and long-term ecology of local soil microbial communities may be impacted by the use of phages since the soil ecosystem provides a suitable template for phage–bacterial interactions. The soil rhizosphere is modulated by exudates from plant roots that allow the development of bacterial populations and interaction with phages. Phages likely regulate host populations by lysis but may compensate by having longer lysogenic stages that prevent further infection and lysis. Phages appear to be large reservoirs of genes that encode proteins that may be novel to any bacterial strain. Studies with both virulent and lysogenic phage gene sequences show that phage–bacterial interactions that occur in soil permit genetic exchange from one host to another. The exchanges may involve the exchange of genetic material via lysogeny, lytic action, abortive infections, transduction, and pseudolysogeny.

Control of common bunt (Tilleta tritici and T. laevis) of wheat (Triticum aestivum cv. ‘Laura’) by fumigation with acetic acid vapour

Common bunt caused by Tilletia tritici and T. laevis remains an important disease of wheat, particularly in organic production where effective fungicides are not available. Acetic acid (AA), a potential organic seed fumigant, was evaluated for control of common bunt of wheat. The highly susceptible spring wheat cultivar Laura was inoculated with bunt spores and then fumigated with 2 and 4 g kg-1 AA vapour in 23 L chambers for 1 h at 20°C. Fumigation reduced field infection levels of common bunt in trials conducted at Lethbridge, AB during 2000, 2001, and 2003. The 4 g kg-1 rate was more effective than the 2 g kg-1 rate in reducing bunt infection, although both rates were as effective as Vitavax, the standard seed-treatment fungicide treatment. Some reduction in tiller numbers was associated with the AA treatments especially at the 4 g kg-1 rate. In vitro tests on artificial growth media showed that AA significantly decreased seed-borne mold contamination caused by several species of fungi. Acetic acid fum...

Use of PCR and DNA hybridization for identification of pear powdery mildew caused by Podosphaera leucotricha

Polymerase chain reaction (PCR) amplification of the ribosomal gene internal transcribed spacer (ITS) region has been used to classify Podosphaera leucotricha from apple but not pear. Molecular information is needed to show the relationship between apple and pear powdery mildew. DNA was successfully extracted from cleistothecia, conidia and mycelial fragments from leaves and shoots of apple, pear and six unrelated hosts with symptoms of powdery mildew. PCR amplification of the ITS regions of P. leucotricha with primers that amplify non-specific fungal DNA resulted in a 506 bp PCR product. Sequencing of the PCR products from apple and pear from different orchard blocks produced identical sequences for the ITS region that did not differ from P. leucotricha sequences in the GenBank database. The sequence data was also used to select species-specific oligonucleotides for P. leucotricha that could be used in a confirmatory hybridization assay. The hybridization reactions were positive for eight out of nine apple and pear powdery mildew samples. Six other hosts with powdery mildew symptoms were negative when tested with the assay and showed that this DNA test was specific for P. leucotricha. In conclusion the PCR and hybridization assays should prove to be useful tools in monitoring both apple and pear powdery mildew for epidemiological studies.

A novel technique for evaluation of apple (Malus × domestica Borkh.) cultivars for susceptibility to powdery mildew

A technique for the evaluation of apple cultivars for susceptibility to powdery mildew caused by Podosphaera leucotricha (Ell. & Ev.) E. S. Salmon (anamorph Oidium farinosum Cooke) was tested at the Pacific Agri-Food research Centre (PARC), Summerland, BC. Shoots of test cultivars [Braeburn, Creston, Elstar, Empire, Fuji strain BC2, Gala strain Royal, Jonagold, McIntosh, Delicious red strain Bisbee, Shamrock, Silken, Sunrise, and one breeding selection (11w-17-85)], approximately 6 cm in length, were grafted on to branches of mature Jonagold trees in the spring and evaluated in the summer of the same year and in subsequent years after growth on the host tree. Incidence and severity of powdery mildew were evaluated in 1994, 1995, 1997, and 1998. They were not evaluated in 1996 because approximately half the grafts from the previous year did not grow and had to be replaced. The highest incidence of powdery mildew averaged 84% for Gala, compared with only 15% for Bisbee Delicious with the lowest incidence. G...