Asparagus Fields Research Articles

Evaluation of the TOM-CAST Forecasting Model in Asparagus for Management of Stemphylium Leaf Spot in Ontario, Canada

In the last 5 years, asparagus acreage in Canada has increased by over 25%. Stemphylium leaf spot, caused by Stemphylium vesicarium, has emerged as the predominant foliar pathogen of asparagus. Typically, contact fungicides are applied every 14 days; however, regardless of the number of applications, growers are not achieving adequate control of the disease. The TOM-CAST forecasting model is used widely in Michigan asparagus fields, but it has never been assessed for suitability in Ontario or in the popular cultivar, Guelph Millennium. Six field trials were conducted in 2012 and 2013 to evaluate the TOM-CAST forecasting model in two asparagus cultivars. The fungicides chlorothalonil or azoxystrobin/difenoconazole were applied according to the forecasting model or on a 14-day interval. The effectiveness of the forecasting model differed between sites and cultivars. Even though TOM-CAST is used in all cultivars in Michigan, TOM-CAST was not effective on Guelph Millennium. In the cultivar Jersey Giant, however, TOM-CAST with a 20 disease severity value spray interval improved control of Stemphylium leaf spot without increasing the number of sprays, compared with a 14-day treatment. The results in Guelph Millennium differed between sites. At one site, TOM-CAST maintained similar levels of Stemphylium leaf spot, but increased the number of applications, compared with a 14-day application interval. Of more concern, none of the fungicide treatments differed greatly from the untreated control at the other site. Our results show that forecasting models need to be validated locally in asparagus cultivars relevant to production today.

Assessing annual and perennial flowering plants for biological control in asparagus

Despite the progress made to date to incorporate habitat diversification in farms, implementing it effectively for biological control programs in agriculture still faces many challenges. Our goal was to screen annual and perennial flowering plant species for their attractiveness to natural enemies and herbivores, and to select promising species to be used as flower enhancements in asparagus agroecosystems. Tested perennials were generally more attractive to predators than to parasitoid wasps, which were found in low numbers during our study. Among the annual plants, sweet alyssum (Lobularia maritima L. Desv.) was highly attractive to parasitoid wasps in common garden experiments and was selected for use in flowering borders, but it was not attractive to parasitoid wasps in asparagus field trials. We did not detect any effects of sweet alyssum on asparagus miner damage or on generalist herbivore abundance in asparagus fields. Thrips abundance was reduced in asparagus adjacent to sweet alyssum, but only at the field edge. Emergence rates from excised asparagus miner pupae were low with 18% (94) surviving, of which 77 were asparagus miners and 17 were parasitoids. In summary, although flower enhancements can often provide benefit for biological control of herbivores, our current results did not support this finding, but we suggest that continued effort is made to test the potential of perennial plants as flower enhancements in this agroecosystem as they require less maintenance than annuals.

Identification of purple spot disease on asparagus crops across spatial and spectral scales

Purple spot caused by the fungus Stemphylium vesicarium is a disease that causes heavy damage in asparagus crops. It infects the leaves and stem and results in degradation of the quality in the harvested asparagus shoots, lowering their commercial value. Precautionary spraying with fungicides is practiced to prevent its appearance, posing a threat to consumers and the environment. The aim of this work was to evaluate the ability of visible and near-infrared field spectroscopy (UNISPEC-DC) to identify the appearance of the purple spot symptoms at the level of asparagus plants and the ability of very high resolution (Pleiades-1A) and wide area (Landsat 8) satellite images to map fungal distribution within asparagus fields and across the river plain, respectively. Field measurements were used as reference to select the optimum infection identification models. Results indicate that field spectroscopy and wide area remote sensing can be used to create sufficiently accurate quantification models of disease severity in asparagus plants (R2 = 0.83 and RMSE = 9.68%) or across asparagus fields (R2 = 0.85 and RMSE = 8.26%). The accuracy drops when using satellite data to quantify infection within field (R2 = 0.34 and RMSE = 13.13%). The contributing wavelengths, bands and indices can be utilized to provide timely information to farmers and agronomists in order to support precision plant protection applications.

Biocontrol on the edge: Field margin habitats in asparagus fields influence natural enemy-pest interactions

Abstract We evaluated pest and predator spatial distributions in relation to asparagus field margins, developed molecular gut content analysis methods for two key asparagus pests, and determined trophic links between the two pests and arthropod predators. Our results indicated that the abundance of natural enemies is higher outside asparagus fields than inside, and fields bordered by forests had higher numbers of predators compared to other types of field margins. We screened 3646 field-collected predators from 10 commercial asparagus fields using molecular gut content analysis in 2014 and 2015, and found that 29 arthropod families feed on the two key pests. Significantly more predators positive for the two key pests’ DNA were found in field margins in both years than inside the asparagus field. We highlight the potential significance of unmanaged field margins, particularly forested ones, in providing biocontrol services in agricultural fields.

First report of stem blight of asparagus caused by Phomopsis asparagi in Myanmar

Recently, asparagus (Asparagus officinalis) growing has become popular in Myanmar and it has been widely cultivated in the central lowlands and eastern mountainous regions of the country. In 2015 the crop was grown on approximately 900 ha, more than twice the area grown in 2010. In 2016, approximately 65 to 80% of plants in asparagus fields near Pyinmana were damaged by a plant disease which had symptoms similar to those of asparagus stem blight (Fig 1). Initially the individual lesions were oval shaped and 1.0 - 4.3 cm in length with concentric rings of pycnidia on the stem of the plant (Fig. 2). Later the lesions coalesced and the stem collapsed when weather conditions were favourable for development of the disease. Diseased samples were collected and the fungal pathogen was isolated by culturing on 2% water agar media. The plates were incubated at 26°C for one day, and the emerged mycelium tip was transferred onto potato dextrose agar (PDA). The fungus produced black globose conidiomata (Fig. 3) containing alpha-conidia (Fig. 4). The mean length and width of alpha-conidia were 8.4 ±1.1 μm and 3.3 ±1.0 μm respectively. Based on the morphological characteristics, the fungus was initially identified as Phomopsis asparagi (Uecker & Johnson 2). Molecular characterisation of two fungal isolates was based on rDNA-ITS sequencing with the primers ITS1 and ITS4, and the sequence of each (GenBank Accession Nos. LC203583 and LC203585) had 99 and 100% identity with a reference sequence of P. asparagi (KJ801806) causing stem blight of asparagus in China. Confirmation of the causal agent was completed by a pathogenicity test. A disc of mycelium (5 mm) was cut from a fourteen-day-old PDA culture, put on a stem of asparagus and covered with wet cotton, paraffin film and a plastic sheet to conserve moisture. Inoculation with a PDA disc was used as control. Fourteen days after inoculation, typical stem blight symptoms were observed on the inoculated stem with numerous black pycnidia. Koch's postulates were fulfilled by re-isolation of P. asparagi from the inoculated plants. Phomopsis stem blight of Asparagus caused by P. asparagi has been recorded in most asparagus-growing countries in Africa, Asia, Australasia, Europe and North America (Elena, 1). This is the first report of stem blight of asparagus caused by P. asparagi in Myanmar. As asparagus growing develops in Myanmar, there is an increasing risk of an outbreak of stem blight. Further research on disease management is needed in Myanmar.

Effect of Leaf Mold Mulch, Biochar, and Earthworms on Mycorrhizal Colonization and Yield of Asparagus Affected by Fusarium Crown and Root Rot.

Asparagus can suffer from a crown and root rot caused by Fusarium oxysporum f. sp. asparagi and F. proliferatum. The disease is exacerbated when allelopathic toxins from old, rotting asparagus crowns are present in the soil. To minimize the damage from the replant problem, three strategies were examined: (i) biochar, (ii) application of earthworms (Lumbricus terrestris), and (iii) leaf mold to serve as a compost mulch and food source for earthworms. In a greenhouse, asparagus transplants were grown in soil amended with pathogen-infested asparagus residues or in nonamended soil, then both types of soil were augmented with biochar, earthworms, the combination of biochar and earthworms, or no treatment. Biochar increased arbuscular mycorrhizae (AM) colonization by 170% and reduced the incidence of root lesions by 57%; however, plant weight was not affected by any of the soil treatments and there were no significant interactions among the main effects. In the absence of infested asparagus residues, biochar reduced plant growth by 32%. Field plots that had severe crown and root rot, along with two other fields that had never been planted to asparagus, were planted with asparagus crowns and treated with leaf mold mulch, earthworms plus leaf mold mulch, biochar, or biochar plus earthworms plus leaf mold mulch. Untreated plots served as the control treatment. One year later, asparagus roots sampled from plots in the two new fields had a threefold increase in AM colonization when treated with biochar compared with control plots. Biochar did not increase yield over the duration of the 2012 to 2014 harvests when compared with that of the control plots. No soil treatment affected root colonization by AM in the field where Fusarium crown and root rot was severe. Compared with the untreated control plots, the leaf mold mulch treatment applied alone increased the marketable yields in each year of harvest. Combining leaf mold with earthworms provided no added benefit. Soil amendment with leaf mulch alone may hold promise for improving asparagus production in newly planted asparagus fields.

Differentiation of Stemphylium vesicarium from Stemphylium botryosum as causal agent of the purple spot disease on asparagus in Germany

The purple spot disease of asparagus is the most important disease in German asparagus growing regions. Two different Stemphylium species, S. vesicarium and the closely related species S. botryosum, are described as causal pathogens. Because of the strong phenotypical similarities the morphological differentiation is very difficult. Therefore the development of a suitable alternative to distinguish these species is an important need. The aim of this study was to develop a molecular and genetic based differentiation method for S. vesicarium and S. botryosum, and to analyze asparagus samples from Germany with this method to identify the prevalent causal agent of the purple spot disease in Germany. The sequences of three different DNA-markers were compared to get the most appropriate basis. Additionally to the commonly used ITS regions, parts of the protein-coding genes gapdh (glyceraldehyde-3-phosphate-dehydrogenase) and cytochrome b were analysed. The most significant difference between the two species was a 3 kb intron present in the S. botryosum cytochrome b region but not in S. vesicarium. This difference showed to be suitable for the distinction of these two Stemphylium species by a simple PCR-reaction. In addition to the qualitative analysis, the frequencies of these species were detected directly from asparagus field samples with the help of qPCR. In all german samples collected in 2010, 2011, 2013 and 2014 only S. vesicarium could be identified.

Effect of Trap Orientation and Interval Distance on Captures of Isoceras sibirica Alpheraky (Lepidoptera: Cossidae)

Studies were conducted in an asparagus field in taigu (37°18'N, 112°29'E, 824 m above sea level), Shanxi province, China, May to June, 2009 to 2011, to evaluate the influence of interval distance and orientation on catches of the carpenterworm, Isoceras sibirica Alpheraky (Lepidoptera: Cossidae) in pheromone-baited traps. The results showed that catches of male I. sibirica moths in upwind were higher than in other traps for any intertrap distances. When intertrap distances were shorter than 30 m, interference between traps occurred. These results reveal the effective trap orientation and interval distance for I. sibirica and thus provide guidelines for improving the effectiveness of traps in monitoring and controlling I. sibirica in fields.

The Parasitoids of the Asparagus Miner (Diptera: Agromyzidae): Field Parasitism and the Influence of Food Resources on Life History

The goals of this study were to identify pupal parasitoids of the asparagus miner, Ophiomyia simplex Loew (Diptera: Agromyzidae), and examine the effect of different diets and floral resources on the lifespan of adult asparagus miners and their parasitoids. We also measured the effect of parasitism on stem damage caused by the asparagus miner. The identity and abundance of the parasitoids of the asparagus miner were determined in asparagus fields in Michigan from weekly asparagus miner pupal collections during the 2010-2013 seasons. Twelve species of hymenopterous parasitoids were reared from asparagus miner pupae, including Chorebus rondanii (Giard) (Ichneumonoidea: Braconidae), 10 species in three families of Chalcidoidea, and one species of Bethylidae (Chrysidoidea), that represent new host records for the asparagus miner. C. rondanii and Thinodytes cephalon (Walker) (Pteromalidae) were the most common parasitoids. The effects of different diets and flowers on the lifespan of the pest and parasitoid adults were also evaluated. Buckwheat resulted in the shortest life span for the asparagus miner, whereas Riddell's goldenrod significantly increased its lifespan relative to the control. Parasitoid lifespan was doubled when individuals were fed sugar-rich diets. In the field, parasitoids preferred stems that contained more pupae and damage. The two most commonly reared parasitoids should be considered as targets for future conservation biological control efforts of the asparagus miner.

Characterization of Fusarium Isolates from Asparagus Fields in Southwestern Ontario and Influence of Soil Organic Amendments on Fusarium Crown and Root Rot

Fusarium crown and root rot (FCRR) of asparagus has a complex etiology with several soilborne Fusarium spp. as causal agents. Ninety-three Fusarium isolates, obtained from plant and soil samples collected from commercial asparagus fields in southwestern Ontario with a history of FCRR, were identified as Fusarium oxysporum (65.5%), F. proliferatum (18.3%), F. solani (6.4%), F. acuminatum (6.4%), and F. redolens (3.2%) based on morphological or cultural characteristics and polymerase chain reaction (PCR) analysis with species-specific primers. The intersimple-sequence repeat PCR analysis of the field isolates revealed considerable variability among the isolates belonging to different Fusarium spp. In the in vitro pathogenicity screening tests, 50% of the field isolates were pathogenic to asparagus, and 22% of the isolates caused the most severe symptoms on asparagus. The management of FCRR with soil organic amendments of pelleted poultry manure (PPM), olive residue compost, and fish emulsion was evaluated in a greenhouse using three asparagus cultivars of different susceptibility in soils infested with two of the pathogenic isolates (F. oxysporum Fo-1.5 and F. solani Fs-1.12). Lower FCRR symptom severity and higher plant weights were observed for most treatments on 'Jersey Giant' and 'Grande' but not on 'Mary Washington'. On all three cultivars, 1% PPM consistently reduced FCRR severity by 42 to 96% and increased plant weights by 77 to 152% compared with the Fusarium control treatment. Populations of Fusarium and total bacteria were enumerated after 1, 3, 7, and 14 days of soil amendment. In amended soils, the population of Fusarium spp. gradually decreased while the population of total culturable bacteria increased. These results indicate that soil organic amendments, especially PPM, can decrease disease severity and promote plant growth, possibly by decreasing pathogen population and enhancing bacterial activity in the soil.

Patterns of Spatial and Temporal Distribution of the Asparagus Miner (Diptera: Agromyzidae): Implications for Management

The asparagus miner is an obligatory feeder on asparagus and a putative vector for pathogenic fungi implicated in the early decline of asparagus fields. To date, the distribution of the asparagus miner over space and time is poorly understood. Our study evaluated the spatial and temporal pattern of adult asparagus miners in commercial asparagus fields in Michigan in 2011 and 2012. We sampled adults and damage weekly during the growing season using yellow sticky traps outside, at the edge, and inside commercial fields. Yellow sticky traps at each trapping location were placed at the canopy and ground level to determine vertical distribution of adults. During the first generation, adults were more evenly distributed throughout the field. In the second generation, adults were more commonly found on the edge of the field. Overall, there was a greater percent of mining damage near the edge of the field. Additionally, three times as many asparagus miners were found in the canopy compared with ground-level traps. There were 12 times as many asparagus miner adults on edges bordered by another asparagus field than on ones bordered by forest. Taken together, our results indicate that while asparagus miner management in the beginning of the growing season should focus on the entire field, in the latter half of the season, growers could save money and resources by targeting miner adults at the edges of fields. Finally, conserving the remaining naturally forested landscape and planting borders of trees may help ameliorate pest pressure in asparagus fields.

RELATIONSHIPS BETWEEN AIRBORNE PLEOSPORA HERBARUM AND ALTERNARIA SP. SPORES IN NO-TILL ASPARAGUS FIELDS

Pleospora herbarum (anamorph Stemphylium vesicarium) is the causal agent of purple spot of asparagus. Airborne concentrations of P. herbarum ascospores and conidia and Alternaria sp. conidia were monitored using a Burkard volumetric spore sampler for three growing seasons in two Michigan no-till asparagus fields. While P. herbarum ascospores and conidia and Alternaria sp. conidia were all detected early in the sampling season, most P. herbarum (greater than 78%) conidia were sampled following the last spear harvest in each field each year. This suggests that conidia were primarily responsible for disease on the fern. Diurnal periodicities were observed for atmospheric concentrations of P. herbarum and Alternaria sp. conidia with peak concentrations from 700 to 1300 h and 900 to 1700 h, respectively. P. herbarum ascosporal concentrations did not follow a diurnal pattern. The concentration of airborne Alternaria sp. conidia was positively correlated with airborne P. herbarum conidia, but not with P. herbarum ascospores. The results of this study suggest that an Alternaria disease forecaster such as FAST or TOM-CAST could be useful to predict disease on asparagus ferns since P. herbarum conidial concentrations were positively correlated with Alternaria sp. conidial concentrations. TOM-CAST has been adopted by most growers in Michigan and has been validated as a useful tool to prompt chlorothalonil applications to 'Jersey' cultivars of asparagus. As the asparagus industry transitions from 'Jersey' to 'Milennium' cultivars, new fungicides become available, and weather parameters not included in TOM-CAST are identified as key for disease development, modification of TOM-CAST may be necessary for satisfactory disease control.

EVOLUTION OF SOIL CHEMICAL CHARACTERISTICS AND YIELDS AFTER THE INSTALLATION OF A COMMERCIAL ASPARAGUS FIELD IN ICA

The rapid increase of the area cultivated with asparagus in Ica generated a pressure for opening new lands to agriculture. The following research was conducted to evaluate the effect of long-term asparagus management on properties of a desert soil. Soil chemical properties and crop biometrical parameters were measured in alternate years in a commercial asparagus field in Ica. Asparagus fertilization affected soil pH and levels of available P and K. Little effect was observed on soil organic matter. Drip irrigation increased soil salinity after 10 years. Plant size and foliar dry matter decreased slightly but the number of stems increased progressively. The nutrient content was adequate through the evaluation period. Intensive asparagus production can be performed while in the meantime increasing soil quality.

EFFECT OF SOIL AMENDMENTS ON ALLELOPATHY AND TOLERANCE TO VIOLET ROOT ROT IN MYCORRHIZAL ASPARAGUS PLANTS

Allelopathy tolerance of arbuscular mycorrhizal (AM) fungi [Glomus intraradices (Gi) and Gigaspora margarita (Gm)] - inoculated asparagus seedlings (Asparagus officinalis L., ‘Welcome’) were investigated in the presence of different soil amendments: activated charcoal (AC) and coffee residue (CR). The growthinhibitory effect of allelopathy in asparagus decline field soil was confirmed by bioassay. Ten weeks after AM fungal inoculation growth of control asparagus seedlings was enhanced in CR and AC-added decline soil plots compared with healthy and non-added decline soil. Gi colonization level in the root system was comparatively higher than Gm. Moreover, maximum dry weight of shoots and roots were observed, in addition of CR with Gi-inoculated commercial soil plot, which was also more effective than AC with Gi-inoculated decline and commercial soil plots. These results reveal that allelopathy tolerance appeared in AM fungal inoculated asparagus plants by addition of coffee residue or activated charcoal, though the effect differed between the soil amendments. In violet root rot study, symptom of violet root rot appeared in all the treatments 12 weeks after H. mompa inoculation and the severity of the incidence was less in AM fungal inoculated asparagus plants than non-inoculated plants. Cultivar difference (‘Welcome’ and ‘Mary Washington 500W’) was also observed and the disease indices revealed that the incidence was remarkably low in Gi-inoculated ‘Welcome’ than ‘Mary Washington 500W’. These results suggest that tolerance to violet root rot appeared in mycorrhizal plants, though the tolerance level differed with the cultivars.

Influence of Environment on Airborne Spore Concentrations and Severity of Asparagus Purple Spot.

Environmental conditions, airborne concentrations of Pleospora herbarum ascospores and conidia, and purple spot disease severity on spears and fern were monitored for 3 years in two no-till asparagus fields in Michigan. Purple spot lesion development on spears was correlated with low temperature and vapor pressure deficit and high rainfall. Low vapor pressure deficit and high rainfall were also correlated with airborne ascospore concentrations at both sites. Lesion development on the fern was favored by longer periods of leaf wetness and low vapor pressure deficit and rainfall. Daily airborne conidia concentrations were positively correlated with average temperature and the number of hours of leaf wetness per day and negatively correlated with vapor pressure deficit. Airborne conidia concentrations displayed a diurnal periodicity with greater concentrations between 700 and 1300 h. The results of this study indicate that a model to predict purple spot on asparagus spears should include temperature, vapor pressure deficit, and rainfall; a model for disease on the fern should include temperature, leaf wetness, vapor pressure deficit, and rainfall.

Sphaerodes mycoparasitica sp. nov., a new biotrophic mycoparasite on Fusarium avenaceum, F. graminearum and F. oxysporum

A new species, Sphaerodes mycoparasitica (Ascomycetes, Melanosporales), was isolated from isolates of Fusarium avenaceum and Fusarium graminearum originating from wheat fields in Saskatchewan, and from Fusarium oxysporum originating from asparagus fields in Quebec, Canada. The species is characterized by a unique combination of ascospore size, shape (fusiform and triangular) and wall ornamentation (reticulate and smooth). Also, conidia are produced from simple phialides on the surface of ascoma peridial wall, on ascoma surrounding hyphae, and on irregularly branched conidiophores arising from hyphae. The closest relation of S. mycoparasitica is Sphaerodes quadrangularis, which has no detected anamorphic stage. The description of S. mycoparasitica, its phylogenetic position—based on DNA sequences of large subunit ribosomal RNA gene (LSU)—as well as a key for all known Sphaerodes species are provided.

Influence of Environmental Factors on Green Asparagus Flavonoids

We have previously reported that there is high correlation between antioxidant capacity and total phenol content in green asparagus (RODRÍGUEZ et al. 2005), which suggests that phenols could be mainly responsible for that activity as happens for other plant-derived products (VINSON et al. 1998; KAMAT et al. 2000; NINDO et al. 2003; PELLEGRINI et al. 2003). It has also been established that flavonoids are the most abundant phenolics in green asparagus and that their profile is significantly different among genotypes (FUENTES-ALVENTOSA et al. 2007, 2008). The composition of flavonoids in plants is influenced by both genetic factors and environmental conditions. It has been previously reported that the former seem to be a determinant factor because significant differences were found between green (triguero) native spears and commercial hybrids of green asparagus both cultivated in the same area (FUENTES-ALVENTOSA et al. 2008). The main objective of the present work is to determine the influence of cultivation area and harvest date on the content and composition of flavonoid glycosides from several green asparagus genotypes. The variability of flavonoid composition among spears of the same genotype collected from different plants has also been investigated. Quantitative evaluation of flavonoid content was carried out by HPLC-DAD-MS, as described by FUENTES-ALVENTOSA et al. (2007). Flavonoid glycosides and flavonols were identified by using their retention times and both spectroscopic and mass spectrometric data. Quantification of individual flavonoids was directly performed by HPLC-DAD using an eight-point regression curve in the range of 0-250 μg on the basis of standards. Where standards were not available, quantification was based on an average value for that class of compound, since responses were essentially similar within classes. Results were calculated from the mean of three replicates. From the results of this study it can be concluded that flavonoid composition of green asparagus was influenced by both cultivation area and harvest date, whereas no significant differences were found among spears of the same genotype from different plants within an asparagus field.

The effect of plant growth-promoting rhizobacteria on asparagus seedlings and germinating seeds subjected to water stress under greenhouse conditions

Plant growth-promoting rhizobacteria (PGPR) can have positive effects on vigour and productivity, especially under stress conditions. In asparagus (Asparagus officinalis L.) field culture, seeds are planted in high-density nurseries, and 1-year-old crowns are transplanted to production fields. Performance can be negatively affected by water stress, transplant shock, and disease pressure on wounded roots. PGPR inoculation has the potential to alleviate some of the stresses incurred in the production system. In this study, the effects of PGPR (Pseudomonas spp.) treatment were determined on 3-week-old greenhouse-grown seedlings and germinating seeds of 2 asparagus cultivars. The pots were irrigated to a predetermined level that resulted in optimum growth or the plants were subjected to drought or flooding stress for 8 weeks. The cultivars responded differently to PGPR: single inoculations of seedlings enhanced growth of 'Guelph Millennium' under optimum conditions and 'Jersey Giant' seedlings under drought stress. Seed inoculations with PGPR resulted in a positive response only for 'Guelph Millennium', for which both single or multiple inoculations enhanced plant growth under drought stress.

Heterorhabditis sonorensis n. sp. (Nematoda: Heterorhabditidae), a natural pathogen of the seasonal cicada Diceroprocta ornea (Walker) (Homoptera: Cicadidae) in the Sonoran desert

A new Heterorhabditis species was isolated from nymphal stages of the seasonal cicada Diceroprocta ornea (Walker) in an asparagus field in the state of Sonora, Mexico. Concomitantly, another isolate of the same nematode species was also collected from an oak woodland habitat in the Chiricahua mountain range in southeastern Arizona. Morphological and molecular studies together with cross-hybridization tests indicate these two isolates are conspecific and represent a new undescribed Heterorhabditis sp. This new species is distinguished from other species in this genus by a combination of several qualitative and quantitative morphological traits. Key diagnostic features include: presence of a pronounced post-anal swelling in the hermaphrodite; male with nine pairs of bursal rays, with pairs 4 and 7 bent outwards and one pair of papillae placed on the cloacal opening, value of D% (average: 79); infective juveniles with a well developed cuticular tooth, long tail (average: 105μm) and values of D% (average: 90) and E% (average: 99). In addition to these diagnostic characters, cross-hybridization tests between the new species with H. bacteriophora and H. mexicana yielded no fertile progeny. Comparison of ITS rDNA sequences with other available sequences of described species depicted the two isolates as a new species. Phylogenetic analysis of these sequence data placed H. sonorensis n. sp. as a member of the indica-group.

THE POTENTIAL OF BIOLOGICAL SOIL DISINFESTATION TO MANAGE FUSARIUM FOOT AND ROOT ROT IN ASPARAGUS

In a field experiment on an abandoned asparagus field we studied the effect of Biological Soil Disinfestation (BSD) on survival of buried inoculum samples of three test pathogens (Fusarium redolens f.sp. asparagi (FRA), Rhizoctonia tuliparum (RT) and Verticillium dahliae (VD)) and on the Fusarium infestation level. The BSD treatments involved incorporation of grass into moist soil and covering the soil with airtight plastic. The amount of grass incorporated was varied (42, 62 or 102 tons of grass/ha) as well as the depth of incorporation (40 or 80 cm). It was found that BSD greatly reduced all three pathogens in buried soil samples and that incorporation of 62 or 102 tons of grass per ha to 80 cm soil depth resulted in a significant decrease in soil infestation in the upper 40 cm; in the deeper layer the decrease was lower. Asparagus plants grown from seed in the field for one year showed a strong decrease in Fusarium root rot severity with all BSD treatments. The results clearly show the potential of BSD to decrease soil infestation levels of Fusarium pathogens and to contribute to an enhanced life span of replanted asparagus crops