PM-101
Carrot cavity spot incidence and severity as affected by pigmentation and weather
M.R. McDonald1, K. Vander Kooi1, M.T. Tesfaendrias1 and P.W. Simon2
1 Dept. of Plant Agriculture, University of Guelph, Guelph, Ontario, Canada N1G 2W1
2 USDA, ARS and University of Wisconsin, Madison WI 53706 USA
Cavity spot of carrot, caused by several species of Pythium, is an important soilborne disease of carrots grown in organic and mineral soils in Canada. Field trials were conducted in the Holland/Bradford Marsh region of Ontario, from 2002-2007, to determine the effect of carrot pigmentation and weather on cavity spot incidence and severity. White, yellow, red and purple carrot breeding lines from the USDA breeding program at the University of Wisconsin were seeded in organic soil in late May, harvested in late October and assessed for disease in early December, each year from 2002-2007. Commercially available cultivars with orange pigmentation were included for comparisons. Disease incidence and severity were highest in carrot cultivars with red pigmentation in five of six years, while purple pigmented carrots had the lowest incidence and severity in all years. Results indicated the influence of rainfall and temperature was greatest two and three months after seeding. Disease incidence in all the lines increased with increasing rainfall in July (r = 0.79), which ranged from 29 to 102 mm over the 6 years. High mean maximum air temperatures in August were correlated with low incidence (r = -0.87) and severity of cavity spot (r = -0.90), over a range of 24 to 27ºC. Further confirmation of the importance of rainfall in contributing to cavity spot development, and high temperatures in reducing cavity spot, could allow a forecast of disease incidence and severity at harvest. Red pigmented carrot cultivars were very susceptible and would be useful as a susceptible check in cavity spot research.
PM-102
Integration of nitrogen and fungicides for management of carrot leaf blight in Ontario, Canada
C. Saude1, K. Vander Kooi1, S. Westerveld2 and M. R. McDonald1
1 Dept. of Plant Agriculture, University of Guelph, Guelph, Ontario, Canada NIG 2W1
2 Ontario Ministry of Agriculture, Food and Rural Affairs Simcoe Resource Centre, Simcoe, Ontario, Canada N3Y 4N5
Carrot leaf blight (CLB) in Ontario, Canada is caused by both Alternaria dauci (Kühn) Groves and Skolko) and Cercospora carotae (Pass.) Solheim). The disease can weaken carrot tops, which can break during mechanical harvesting, reducing harvested yields. Fungicides are commonly used to manage the disease, but there is potential to reduce fungicide applications through nitrogen (N) management. Trials were conducted on mineral soils from 2006 to 2008, to determine the relative importance of applied N and fungicide applications to control carrot leaf blight. Three rates of N (0, 110, and 220 kg/ha) and 0, 3 or 5 (2006 and 2007) or 6 (2008) fungicide applications were applied. Carrot leaf blight severity was assessed biweekly throughout the season and at harvest. Alternaria leaf blight severity tended to be highest in carrots that received no fungicide and was lowest in carrots that received nitrogen plus three or more fungicide applications. In 2007 and 2008 the application of N fertilizer resulted in lower leaf blight severity, but there was no effect of N in 2006. High rates of N (220kg/ha) were associated with low plant stands in 2007 and 2008. Total and marketable yield increased with increasing number of fungicide sprays in 2006 and 2007, but were not influenced by N application. Results suggest that carrot leaf blight severity can be minimized through a combination of N and fungicide applications.
PM-103
Comparison of resistance response of carrot accessions (Daucus carota L.) to Alternaria dauci and A. radicina
R. Krämer, T. Nothnagel and F. Rabenstein1;
Institute for Breeding Research on Horticultural and Fruit Crops, 1Institute for Epidemiology and Pathogen Diagnostics, Julius Kühn-Institute, Federal Research Centre for Cultivated Plants, Erwin-Baur-Strasse 27, 06484 Quedlinburg, Germany
Leaf blight and black rot belong to the most important diseases of carrots. Successful controlling by breeding for resistance to these diseases needs sensitive, practicable and reproducible screening methods. A research project aimed to develop effective screening methods for evaluation of new resources of Alternaria resistance in carrots will be presented.
Different bioassays with detached leaf segments (A. dauci) as well as petioles and root slices (A. radicina) respectively were established. Development of disease symptoms could be quantified with a digital image analysis system (DIAS). This method allowed the accurate assessment of individual plant reaction to A. dauci and A. radicina at the same time.
A set of ten carrot accessions was screened threefold with these bioassays to compare i) resistance response of the accessions, ii) the reproducibility of the three tests as well as iii) correlation between both pathogens. Results and possible application in breeding research will be discussed.
In connection with resistance research a plate trapped antigen-immunosorbent assay (PTA-ELISA) was established for detection the disease severity and also to investigate spreading of the fungi in plant material. Furthermore effective methods for inoculation of whole carrots in the greenhouse were developed. The resistance response of whole carrots to both fungal pathogens was compared to those which were obtained in the bioassays. Selected results will be presented.
PM-104
Potential effect of powdery mildew on yield of carrot seed crops in the Columbia Basin of Washington
Lindsey du Toit and Mike Derie
Washington State University Mount Vernon NWREC, Mount Vernon, WA 98273
Carrot seed crops in the Pacific Northwest (PNW) region of the US produce up to 50% of the US and 20% of the world supply of carrot seed. Powdery mildew of carrot, caused by Erysiphe heraclei, has become endemic in this semi-arid region. Previous research suggested powdery mildew usually does not affect carrot seed yield or quality (germination). However, recent empirical observations suggest fungicide applications can significantly reduce losses to powdery mildew on susceptible cultivars, particularly in drip or furrow irrigated crops. The objective of this project was to determine whether crop maturity at the time of development of powdery mildew can affect carrot seed yield and quality. A field trial was set up in the Columbia Basin of Washington in each of 2006, 2007, and 2008 in a grower’s direct-seeded, furrow-irrigated seed crop of an open-pollinated cultivar highly susceptible to powdery mildew. A randomized complete block design was used with five replications per treatment. Powdery mildew-infested carrot plants were transplanted into the appropriate plots in April, May, June, July, or August. Plots not yet inoculated were treated with fungicides (boscalid + pyraclostrobin and/or chlorothalonil) twice a month to protect against powdery mildew. Non-inoculated control plots sprayed with fungicides through the season were also included. The treatments successfully generated different times of infection by E. heraclei each year. The greatest impact of powdery mildew on seed yield was detected in 2007. In all three years, severity of powdery mildew within a month of harvest did not differ significantly among plots inoculated in April, May, or June (70 to 81% mean severity in 2007), but was significantly lower for plots inoculated in July (70% severity in 2007), and lowest for plots inoculated in August or not inoculated (<10% severity in 2007). In 2007, seed yield did not differ significantly among plots inoculated in April, May, or June (1,480 to 1,519 lb seed/acre), but increased 20% and 44% for plots protected against powdery mildew through July (1,784 lb/acre) and August (2,143 lb/acre), respectively. Germination of seed after cleaning and sizing was not affected by powdery mildew. The increase in seed yield detected in 2007 in plots protected with fungicides until July and August, increased gross income by approximately $456 and $1,014/acre ($1,140 to $2,535/ha), respectively; and net income by $200 and $700/acre ($500 to $1,750/ha), respectively. Final results will also be presented for the 2008 trial. The dense canopy of carrot seed crops in the two to three months before harvest, particularly open-pollinated crops, necessitates thorough coverage of the canopy with efficacious fungicide(s) in order to reduce losses to powdery mildew.
PM-105
Development of a quantitative molecular carrot seed assay that differentiates DNA from viable vs. non-viable cells of Xanthomonas hortorum pv. carotae
Lindsey du Toit1, Todd Temple2, Mike Derie1, and Ken Johnson2
1 Washington State University Mount Vernon NWREC
2Oregon State University, Corvallis
Xanthomonas hortorum pv. carotae (Xhc) is a seedborne pathogen of carrot prevalent in most areas of carrot seed production. Carrot seed lots are assayed routinely to determine the population of seedborne Xhc. Traditional Xhc seed assays entail dilution plating a seed wash onto semi-selective agar media. The assays require about a week to determine the pathogen population size associated with the seed. A relatively new DNA-based polymerase chain reaction (PCR) assay for Xhc can detect the pathogen within 24 hours at a similar level of sensitivity to seed-wash dilution plating assays. However, traditional PCR does not differentiate DNA from viable (live) vs. nonviable (dead) cells, and does not quantify the amount of bacterial cells in a seed lot. Therefore, the PCR assay is less useful for testing seed treated with hot water because the effectiveness of the heat treatment cannot be determined. We evaluated the use of a DNA-intercalacting dye, propidium monoazide (PMA), for selective detection of DNA from live cells of Xhc. The dye was combined with a real-time PCR assay to quantify the population of live Xhc in carrot seed lots, particularly for testing the efficacy of hot water seed treatments at eradicating or lowering the level of seed contamination by Xhc. In addition, an alternative DNA assay to real-time PCR was developed for detecting Xhc: loop-mediated isothermal amplification (LAMP). The LAMP assay for Xhc has quantitative capacities similar to real-time PCR, but can be completed at a much lower expense, more rapidly, and with less sophisticated equipment than real-time PCR. The LAMP protocol proved sensitive and robust on carrot seed lots naturally infested with Xhc. Combining PMA treatment with the LAMP protocol for detecting seedborne Xhc significantly reduces the time and costs associated with testing carrot seed lots quantitatively for live Xhc following hot water treatment.
PM-106
Impact of commercial foliage trimming on disease suppression and yield of processing carrots in Nova Scotia, Canada
G. McIsaac1*, K.R. Sanderson2, R.D. Peters2, D. Garbary1 and A. Ells3.
1 Aquatic Resources/Biology, St. Francis Xavier University, P.O. Box 5000, Antigonish, NS B2G 2W5, Canada
2 Agriculture and Agri-Food Canada, 440 University Ave., Charlottetown, PE C1A 4N6, Canada
3 Bragg Lumber Company Ltd., P.O. Box 60, Collingwood, NS B0M 1E0, Canada
A mechanical carrot foliage trimmer was developed in Prince Edward Island, Canada to trim a portion of the canopy between adjacent rows. The foliage trimmer has been tested in experimental plots in Prince Edward Island since 2006 and has provided substantial suppression of Sclerotinia rot of carrot (SRC), without the need for chemical fungicides. Trimming opens the carrot canopy, allowing increased sunlight penetration and airflow, which prevents moisture buildup and creates unfavorable conditions for SRC development. Several commercial carrot producers throughout North America have adopted Agriculture and Agri-Food Canada’s foliage trimmer prototype and carrot foliage trimming is becoming standard practice in the carrot industry. In 2008, the impact of commercial foliage trimming on SRC (caused by Sclerotinia sclerotiorum), as well as several other carrot diseases was assessed in Nova Scotia, Canada. Sixteen fields throughout Colchester and Kings Counties in Nova Scotia representing various carrot cultivars and seeding densities were surveyed for the incidence and severity of SRC, Alternaria leaf blight, Cercospora leaf spot, and bacterial soft rot. Both trimmed and untrimmed portions of the selected fields were assessed for disease to determine the effect of trimming on disease incidence and severity. In addition, sample yields were recorded to determine if foliage trimming caused biological yield loss. This is the first research of the effects of commercial foliage trimming on carrot diseases in Nova Scotia, a province with 800 ha devoted to carrot production.
Oral Presentation: Grace McIsaac
Phone: 902-393-3423
E-mail: x2005ezn@stfx.ca.petersr@agr.gc.ca.punja@sfu.ca)
PM-107
Post-harvest application of fludioxonil for control of Sclerotinia rot of carrots in storage
P. Monaghan1, R.D. Peters2*, K.R. Sanderson2 and L.R. Hale1
1 Department of Biology, University of Prince Edward Island, 550 University Ave., Charlottetown, PE C1A 4P3, Canada
2 Agriculture and Agri-Food Canada, 440 University Ave., Charlottetown, PE C1A 4N6, Canada
Sclerotinia rot of carrot (SRC), caused by Sclerotinia sclerotiorum, is one of the most economically important diseases of carrots. Post-harvest outbreaks are particularly devastating, with storage losses of some commercial growers reported as high as 50% in Canada. Studies were undertaken in Prince Edward Island, Canada to assess various fungicides for their in vitro activity against S. sclerotiorum and for their ability to suppress SRC in stored carrots. Local isolates of S. sclerotiorum were found to be extremely sensitive to fludioxonil (EC50 < 0.001 mg/L) in amended agar assays. Similarly, carrots treated with fludioxonil prior to inoculation with S. sclerotiorum and subsequent storage were found to be virtually free of disease, even after several months in storage. A significant reduction, compared to untreated controls, in the spread of disease was achieved when fludioxonil-treated carrots were placed adjacent to severely infected carrots prior to storage. Post-harvest application of fludioxonil to carrots going into storage provides substantial suppression of SRC and studies are currently underway to provide data to support minor use of this product on carrots in Canada.
Oral Presentation: Rick D. Peters
Phone: (902) 566-6846
Fax: (902) 566-6821
E-mail: petersr@agr.gc.ca
PM-108
Enhanced resistance to foliar fungal pathogens in carrot by application of elicitors
Jayaraman Jayaraj and Zamir K. Punja*
Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada. (*Presenter and author for correspondence. E-mail:
Treatment of greenhouse-grown carrot plants with salicylic acid (SA) (100 μM), chitosan (0.02%) and the nutrient-chelate product Alexin (1%), followed 10 h later by inoculation with the necrotrophic fungal pathogens Alternaria radicina and Botrytis cinerea, significantly reduced disease development 10 days after inoculation compared to control plants sprayed with water. The most effective treatment was chitosan, followed by Alexin and SA. Additional sprays of elicitors resulted in significantly lower disease development 25 days after inoculation. Treated plants had elevated transcript levels of PR1 (pathogenesis-related protein 1), chitinase, LTP (lipid-transfer protein), chalcone synthase, NPR-1 (non-expressor of PR1) and PR5 (pathogenesis-related protein 5) genes compared to control plants when assayed 10-70 hr after treatment. The activity of peroxidase, polyphenoloxidase, phenylalanine ammonia lyase, chitinase, b-1,3- glucanase and lipoxygenase was significantly increased in elicitor- treated plants compared to control plants 12-72 hr after treatment.
Microscopic examination of treated leaves revealed reduced fungal growth and colonization, 48 hr after treatment, accompanied by fewer lesions at 120 hr, compared to the control. Protein extracts from elicitor-treated plants reduced spore germination and germ tube elongation of the pathogens in vitro by 30-45 %. Elicitor-treated plants accumulated higher amounts of total phenolics, 6-methoxymellin and hydrogen peroxide compared to the control. Both chitosan and Alexin induced responses similar to that of SA, suggesting that these elicitors may activate the salicylate pathway, leading to induction of defense genes, enzymes, phytoalexin and phenolics, which collectively reduced fungal colonization of carrot leaves.
PM-109
Management of insect pests in Danish carrots: the importance of monitoring and forecasting in organic and pesticide-low productions
Peter Esbjerg, Professor
Department of Ecology, Faculty of Life Sciences, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
In the Scandinavian countries a critical attitude to pesticide use is deeply rooted, and Denmark has a notorious limitation of permitted pesticides. Carrot was early in focus for pesticide reduction as this fresh market crop was earlier intensively treated on a schedule basis against the two main pests, Carrot fly (Chamaepsila rosae) and Turnip moth (Agrotis segetum). Growers claimed that production without certain insecticides and organic production in particular would be impossible but today the major production is registered organic or Integrated (IP/IPM). For the carrot specialised Carrot fly crop rotation, and forecasting based on monitoring with yellow sticky traps are key tactics in facing the difficulties of having no larvicidal agent. For the strongly fluctuating, non-specialised Turnip moth monitoring with pheromone traps is highly important. With the catches as entry point and information from an array of experimental and observational studies the time periods of tiny larvae (cutworms) and their survival/mortality rates linked to soil moisture and temperature can be centrally estimated for forecasting at farm level. This enables either precisely timed irrigation against the very young cutworms in organic production or a later but also rather precise insecticidal treatment. Both ways are widely used by growers have significantly lowered the use of insecticides and improved the economic output.
PM-110
Detection of Pythium sulcatum and P. violae in Soil
Sam Livingston1, Mike Davis1, Jianjun Hao2
1 Dept. of Plant Pathology, University of California, Davis, CA, 95616
2 Dept. of Plant Pathology, Michigan State University, East Lansing, MI 48824
Cavity spot caused by Pythium spp. is serious disease in carrot (Dacus carota L.) growing regions of San Joaquin Valley of California. Detection of specific pathogenic species has been difficult using traditional methods. We developed a Nested-PCR to detect Pythium species in soil prior to sowing and to study Pythium population dynamics from carrot tissues. Samples of carrots tissues and soil from severely infected fields were used to compare direct isolation of the pathogens, to detection by PCR-methods. Direct isolation on PDA or PARP yielded Pythium only from 8% of the carrots. Freeze-dried root tissue from infected carrots and soil DNA were used as templates for PCR and Nested-PCR. In freeze-dried tissue, direct PCR detected Pythium in 12 % of the samples whereas Pythium species were detected in 28% of samples using Nested-PCR. Although P. sulcatum and P. violae were not detected in soil by direct PCR, they were detected in 26 and 94 % of the samples respectively by Nested-PCR. To our knowledge this is a first report of the detection of P. sulcatum and P. violae in naturally infested soils. Nested-PCR proved to be a highly sensitive method for detecting low titers of Pythium in soil and host tissue.
PM-111
Seedcorn maggot and mite control trials in carrots in the Pacific
Tim Waters, Washington State University Extension, Benton Franklin County, 1016 N. 4th Ave. Pasco, WA
Doug Walsh, Washington State University, Dept. of Entomology, IAREC, 24106 N. Bunn Rd. Prosser, WA
Springtime seedcorn maggot (Delia platura) infestations can reduce germination and stand establishment of winter- and spring-planted Columbia Basin vegetables. Crops impacted include carrots, onions, beans, peas, and corn. These crops have traditionally been treated post-planting with organophosphate, carbamate, or pyrethroid insecticides, with varying degrees of effectiveness. For the past several years seed treatments of cloronicotinyl insecticides including imidacloprid, thiamethoxam, and clothianidin have provided effective control of maggots, but to date there were no effective treatments available for maggot control for organic carrot producers. Trials conducted in 2007 and 2008 show stand establishment of several vegetable crops treated with spinosad was significantly greater (p<0.05) then stand establishment in plots planted with untreated seed. Additionally, spinosad seed treatment was equivalent to conventional post-planting insecticide treatments for stand establishment in carrots.
Two-spotted spider mite Tetranychus urticae is an indirect pest with economic impacts on many crops, including carrots. Leaves of carrot plants infested with spider mites show a distinct spotted effect called stippling. In heavy infestations the leaves become bronzed, silvery, or yellow in appearance. Eventually the plants lose vigor and the leaves begin to defoliate causing a negative impact on carrot quality and yield. Mite populations typically outbreak during hot dry weather in later July and early August for fresh market carrots or in late August and September for processing carrots. These timeframes are when carrots are in the critical growth phase referred to as “bulking.” In 2005 and 2006, Washington State carrot growers experienced economic injury from late-summer spider mite outbreaks. While several insecticides are registered for use against mites on carrots, they are not effective and are inappropriately broad in spectrum. At present no miticides are registered for spider mite control on carrots in Washington State. A substantial number of candidate miticides were tested and found to control mites on carrots.
PM-112
Evaluation of alternative nematicides for the control of root-knot nematodes in a commercial carrot field
Wendy Berry, University of Nebraska and Joe Nunez, University of California Cooperative Extension
Nematodes are likely the number one pest of carrots grown in California, particularly root knot nematodes. Currently the preferred method of nematode control for carrots is with the use of pre-plant soil fumigants, which negatively impact the environment. Alternative methods of nematode control need to be studied to quickly identify other possible control strategies. The aim of this study is to evaluate the nematicidal properties of several botanical and biological products. One product, Neemcake is the residue after neem seeds are crushed to extract neem oil. It is sold as a soil supplement but reports show that it is an effective amendment for nematode control. Nemagard is another botanical nematicide composed of ground sesame stalks. Previous research by others has shown sesame plants to have nematicidal effects. Dragonfire-CPP is sesame oil that is also registered for nematode control. The breakdown of chitin from shrimp and crab shell meal is reported to stimulate chitinase production from various fungi and bacteria in the soil. The egg cases of nematodes are composed of chitin, so in the presence of the chitinase enzyme nematode egg cases are broken down thus killing the nematode egg. Ne-Plex is another product that creates an unfavorable environment for nematodes. Mustard meal is the residue after crushing mustard seed for oil. It is high in isothiocyanates that can have a biofumigation effect. Movento is registered as an insecticide with the active ingredient Lipid Biosynthesis Inhibitor. Preliminary testing suggests that this product is effective as a nematicide.
PM-113
Delivery of nematicides via seed-coating: a new carrot plant protection tool?
J. Ole Becker, Department of Nematology, University of California, Riverside, CA 92521, obecker@ucr.edu
Protection of young seedlings against root-knot nematodes (Meloidogyne spp.) is a major concern in California carrot production. While pre-plant soil fumigants typically provide good efficacy against soilborne pathogens, the utilization of soil pesticides is likely to decline because of regulatory pressure to limit potential air quality and non-target exposure problems. These developments might further foster efforts to develop pesticide-based seed coatings for plant protection. The low application rates associated with seed coatings result in reduced risk for the user and environment as well as increased production efficacy. Recent greenhouse trials have indicated that carrot seed coatings with the nematicide abamectin provides useful protection against the early attack of these pathogens. With application rates equivalent to 16 g a.i./acre, the abamectin treatments increased stand in M. incognita infested sandy loam and typically reduced galling by two rating classes after 8 weeks. While such treatment will not be comparable to fumigant efficacy, it might be useful in combination with other nematode management tactics, in fields with spotty or light nematode infestations and in fumigant-free buffer zones. In combination with a potent fungicide seed coating, it might provide an environmentally safe, effective and economically attractive plant protection tool.
PM-114
Influence of gibberellic acid on carrot growth and severity of Alternaria blight
R.M. Davis, University of California, Davis and J.J. Nunez, University of California Cooperative Extension Kern County
Foliar applications of gibberellic acid (GA) consistently reduced the percentage of leaf area affected by Alternaria dauci compared to nontreated plants. The degree of blight reduction by two applications of GA was similar to that achieved by four applications of the fungicide iprodione. At rates examined (2.5 to 250 mg/l GA), foliage dry weights were generally increased by GA. Although root weight was significantly reduced by 250 mg/l GA, applications of lower rates (40 mg/l or less) reduced blight severity without affecting root quality. Applications of GA usually resulted in plants with longer leaves, wider petioles, and a more upright growth habit. In one trial, leaf length and petiole diameter increased linearly with increasing rates (20, 30, and 40 mg/l GA). When applied twice at 30 mg/l, GA did not affect cuticle, epidermal, or leaf thickness. In general, the initial timing of two applications of 20 to 40 mg/l GA (4, 6, or 8 weeks after plant emergence) did not influence the effects of GA. However, in one trial there was a greater incidence of core separation from the root cortex when 40 mg/l GA was applied initially at 4 weeks. GA at 30 mg/l slightly but significantly decreased inner root color in one of two trials.
PM-115
Control of cavity spot in a disease nursery
Jim Farrar, Department of Plant Science, California State University, Fresno, Fresno, California, USA
Cavity spot of carrot is one of the most important diseases of fresh market carrots. Although the cavity spot blemished are superficial, they decrease the cosmetic appeal of the carrot. Disease losses range from a low incidence of blemished carrots that must be sorted out during packaging to abandonment of fields due to high incidence of disease. In California, cavity spot is caused by Pythium violae, P. ultimum, P. sulcatum, and P. irregulare. A cavity spot nursery was developed at California State University, Fresno in order to conduct replicated trials for disease control. A number of disease control studies have been conducted in the nursery. Reason (fenamidone), Ranman (cyazofamid), and Quadris (azoxystrobin) applications result in significantly fewer diseased carrots than the untreated control and are equivalent to the industry standard Ridomil Gold (mefanoxam) applications. In addition, fungicide programs which alternate applications of Ridomil Gold with either Reason or Ranman result in significantly fewer diseased carrots that the untreated control. Previcur (propamocarb), Presidio (fluopicolide), Aliette (fosetyl-Al), Phostrol (phosphorous acid salts), Prophyt (potassium phosphate) and phosphorous acid-based fertilizers applications do not result in significantly fewer diseased carrots. The biological control agent, Plant Helper PC, and systemic acquired resistance products, Kendal and Resist, also do not provide significant levels of cavity spot control. Compost, gypsum, and fish emulsion do not significantly reduce cavity spot incidence. Commercial Imperator-type varieties and USDA breeding lines display a range of susceptibilities to cavity spot but none are completely resistant.
Key words: cavity spot, pythium, disease control, fungicides, pesticides
PM-116
Root-knot nematode resistance expression and selection in carrots
Philip A. Roberts, Department of Nematology, University of California, Riverside, CA 92521
Philipp W. Simon, USDA-ARS, Vegetable Crops Unit, Dept. of Horticulture, University of Wisconsin-Madison,1575 Linden Drive, Madison, WI 53706
William C. Matthews, Jr., Department of Nematology, University of California, Riverside, CA 92521
Joe Nunez, Vegetable/Plant Pathology Advisor, UC Cooperative Extension, 1031 South Mount Vernon Avenue, Bakersfield, CA 93307
Two species of root-knot nematodes, Meloidogyne javanica and M. incognita, are major parasites of carrots in California fresh carrot production fields and many warm climate areas worldwide. Relatively low soil population densities of these species at planting cause significant forking and galling disfiguration of taproots and economic loss of marketable yield. Excellent sources of resistance to both species have been identified, including the Mj-1 gene now in advanced breeding materials, and novel genes in the early stages of introgression into elite backgrounds. The resistance prevents nematode feeding and development, thereby suppressing infection symptoms and yield loss. We have assessed the behavior of the Mj-1 resistance efficacy under high temperature, in different genetic backgrounds, and with diverse nematode populations. The resistance shows allelic dosage response and varies in the strength of resistance to M. incognita populations in some breeding line backgrounds indicating epistasis. These effects are being assessed in homozygous lines and segregating populations using combined greenhouse and field-based phenotypic assays, and markers for the Mj-1 locus. Our goal is to maximize the effectiveness of the Mj-1 based resistance in fresh carrot cultivars, singly and in combination with novel resistance sources.
PM-117
Cruciferous Crops for Root-knot Nematode Control
Antoon Ploeg* and Scott Edwards
Department of Nematology, University of California Riverside
Riverside, CA 92521, USA
Cultivation of cruciferous cover crops, followed by incorporating green biomass into the soil (biofumigation) has been shown to reduce soil-borne pathogens and pests. Using this method to manage root-knot nematodes poses a risk, as many cruciferous crops are moderately good hosts for these nematodes. To avoid nematode build-up during cover crop cultivation, it is important to select poor–or non-host cover crop varieties. Previously, we evaluated the host range of 32 cruciferous varieties for an M. incognita race3 population in a pot-trial. The results indicated that there is a large variation in host-status between the cruciferous varieties. These results were confirmed in micro-plot trials, and also showed that selecting poor- or non-host varieties significantly reduced galling and root-knot nematode infestation in a following nematode-susceptible tomato crop, and resulted in tomato yields not different from a nematicide treatment (ca. 3x yield after fallow control). Recent results with the same 32 cruciferous varieties, but with a population of M. javanica show that the host status of the varieties is generally very similar for both root-knot nematode species (M. incognita, M. javanica). We will use selected varieties as a cover crop in a root-knot nematode infested field, to evaluate their potential to control nematode damage in a following carrot crop.