Cavity Spot Of Carrots Research Articles

Exploring the Effects of Trichoderma virens Biofungicide on Carrot Cavity Spot and Soil Fungal Community Dynamics

Carrot cavity spot (CCS) has conventionally been managed with fungicides. However, fungicide resistance, their potential risks to human health and the environment, and the increasing demand for organic produce necessitate the exploration of biofungicides as alternatives. In this study, we evaluated varying concentrations of SoilGard (Certis USA, Columbia, MD), a Trichoderma virens-based biofungicide, for efficacy against different CCS-causing Pythium species in vitro. Additionally, its effects on taxonomic and functional diversities of soil fungal communities were studied in vivo in the greenhouse. To our knowledge, this is the first study reporting SoilGard's effectiveness against CCS, with emphasis on its potential as an alternative for fungicide-resistant Pythium isolates. Our in vitro study revealed that SoilGard efficacy was significantly dose-dependent and isolate-specific, thus highlighting the importance of selecting its application rate and the target isolate. Analysis of soil fungal communities using Illumina MiSeq sequencing revealed that SoilGard exerted a significant, albeit temporary, effect on the fungal community structure. It negatively impacted co-occurrence network complexity and alpha diversity in carrot-cultivated soil, whereas bare soil communities remained largely unaffected, thus explaining why preplant applications may yield better results. Our study showed that carrot cultivation without SoilGard enhanced fungal diversity, which was more pronounced late in the season, possibly due to carrot root-associated exudates. Our study sheds light on how complex interactions within soil fungal communities can be impacted by the application of beneficial/pathogenic microbes.

First Report of Strains Within the Pythium spinosum Species Complex Causing Carrot Cavity Spot in California.

Carrots (Daucus carota L. subsp. sativus (Hoffm.)) with typical symptoms of cavity spot, i.e., sunken, round to elliptical lesions of 2-5 mm long (Hiltunen and White 2002), were collected from two locations in California, Bakersfield and Riverside, in January and July 2019, respectively. Carrots were rinsed in tap water, 4-mm2 lesion fragments were pressed into selective corn meal agar (CMA-PARP; Schrandt et al. 1994) and incubated at 23ºC in the dark for four days. Identification of pure cultures was performed via amplification and sequence analysis of two genomic regions, the Internal Transcribed Spacer 1-5.8S-ITS2 (ITS) region and the cytochrome C oxidase subunit 1 (COI) gene, using the universal primers UN-UP18S42/UN-LO28S576B (Schroeder et al. 2006) and OomCOXI-Levup/OomCOXI-Levlo (Robideau et al. 2011), respectively. Via BLAST, two isolates from organically grown carrots in Bakersfield (MCIF19) and Riverside (JSCS19), with identical ITS sequences (GenBank Acc. Nos. MZ799354 and MZ799355, respectively), showed 99.61% similarity (1021/1025 bp) to that of Pythium spinosum (AY598701.2). Yet, the COI of MCIF19 (MZ803207) showed 98.72% similarity (692/701 bp) to that of Pythium paroecandrum (GU071818.1), while the COI of JSCS19 (MZ803208) was identical (701/701 bp) to that of Pythium kunmingense (GU071820.1), a rarely isolated species considered within the species complex of P. spinosum (Robideau et al. 2011). According to these results, the isolates were identified as belonging to the P. spinosum species complex, part of Pythium Clade F (Lévesque and De Cock 2004; Robideau et al. 2011). Further research is needed to clarify the exact taxonomic status of both isolates. Koch's postulates were completed using two different assays. Each assay was done twice and with carrots of the cultivar Maverick. Surface-sterilized, freshly harvested, mature carrots, in a plastic box lined with moistened sterile paper towels, were inoculated each with four CMA plugs (5-mm diameter) with actively growing mycelium of each isolate. CMA plugs, non-inoculated or colonized by a known pathogenic P. violae strain, were used as the negative and positive control, respectively. Boxes were closed to maintain humidity and incubated at 23ºC in the dark. Lesions similar to the ones caused by P. violae were observed at day 3 for all plugs of both isolates. No symptoms were observed for the negative control, even after extending the incubation to 7 days. In a more natural assay, four non-treated carrot seeds were planted in tree seedling pots (25 x 6.5 cm) containing sterilized 50/50 peat moss/sand combined with 15-ml V8 broth (Schrandt et al. 1994) with densely grown mycelium. The same inoculation treatments were used as for the carrot disk assay. Plants (one plant/pot, four plants/treatment) were maintained at 23ºC under a 16 h photoperiod with daily watering (20 ml). At 14 weeks, the carrots inoculated with P. violae and the two test isolates showed cavity spot lesions while no symptoms were observed on carrots growing in non-inoculated medium. For both assays, pathogens were re-isolated from rinsed symptomatic tissue and their identity was confirmed using the molecular analysis described above. No oomycetes were recovered from the non-inoculated carrots. Although several Pythium species have been associated with cavity spot before, this is, to our knowledge, the first report of strains within the P. spinosum species complex causing carrot cavity spot in California and elsewhere. Funding: This research was made possible by the California Fresh Carrot Advisory Board (FRA-21).

Cropping history shapes fungal, oomycete and nematode communities in arable soils and affects cavity spot in carrot

Fungal, nematode and oomycete communities in arable soil serve diverse ecological functions. However, little is known about the dynamics among these three taxonomic groups. In order to identify the driving factors and networks shaping these communities, and the influence of communities on the development of carrot cavity spot, we collected soil samples from more than sixty fields in which carrot was planned as the next crop, and analyzed the microbial communities using metabarcoding. Crop grown before sampling was best in explaining variation in community composition, but also soil type was shaping fungal and nematode communities, but not oomycete communities. Network analysis revealed that correlations between taxa within taxonomic groups were mostly positive whereas negative correlations dominated between taxa across the different taxonomic groups. Pythium intermedium and P. irregulare from soil samples were found to be associated with cavity spot disease in carrot, and were also found in the lesions of cavity spot. Our study demonstrate that previous crop and soil type significantly affect microbial communities and the development of symptoms of cavity spot in carrot. We also found that differences in co-occurrences within and between taxonomic groups exist.

Development of a disease suppressive soil for managing cavity spot of carrots

Development of a disease suppressive soil for managing cavity spot of carrots

Incidence and Severity of Cavity Spot of Carrot as Affected by Pigmentation, Temperature, and Rainfall.

Field trials to determine the effect of carrot pigmentation and weather parameters on cavity spot (CS) of carrot were conducted in the Holland/ Bradford Marsh region of Ontario between 2002 and 2009. In all, 23 colored carrot cultivars from the United States Department of Agriculture (USDA) Agricultural Research Service breeding program at the University of Wisconsin (n = 5) and commercial seed companies (n = 18) were seeded in organic soil (pH 6 to 7, 45 to 75% organic matter) in late May to early June and harvested in late October or early November. Carrot roots were assessed for CS severity midseason and postharvest. Evaluations postharvest indicated that the purple pigmented carrot from breeding line 'USDA 106-3' and cultivars 'Purple Rain' and 'Purple Haze' consistently had low CS severity. The orange-pigmented 'USDA 101-23', 'Cellobunch', 'YaYa', and 'Envy' had moderate CS; and the red-pigmented carrot breeding line 'USDA 104-3' and cultivars 'Atomic Red', 'Proline Red', 'Dragon', and an unnamed line from India had high CS. Differences in CS severity in carrot cultivars between evaluations at midseason and postharvest suggest that some carrot cultivars are more susceptible to Pythium spp. inoculum in soil (alloinfection) and others to secondary infection (autoinfection) that can be attributed to the Pythium sp. involved in CS. CS severity was positively correlated with total rainfall 2 and 3 months after seeding, and was negatively correlated with number of days with air temperature ≥30°C 3 and 4 months after seeding. Soil temperature and total rainfall were the best predictors of CS incidence and severity. These results could allow a forecast of disease incidence and severity at harvest.

First Report of Pythium sulcatum Causing Cavity Spot in Processing Carrot Crops in the Columbia Basin of Washington State.

In October 2012, symptoms of cavity spot (1) were observed on roots of two 50 ha, Red Core Chantenay processing carrot (Daucus carota L. subsp. sativus (Hoffm.)) crops in the Columbia Basin of central Washington. Symptoms consisted of sunken, elliptical lesions (3 to 15 mm long) on the root surface. Approximately 6% of the roots in each crop were affected, which was sufficient to present sorting problems for the processor. Symptomatic roots were washed thoroughly in tap water, and then small sections of tissue from the lesion margins were removed aseptically and plated onto water agar (WA) without surface-sterilization. Isolates with morphological characteristics typical of Pythium sulcatum Pratt & Mitchell (2) were obtained consistently from the symptomatic tissue. The genus and species identity of seven isolates was confirmed by sequence analysis of the internal transcribed spacer (ITS) 1-5.8S-ITS2 region of ribosomal DNA (rDNA) using universal eukaryotic primers UN-UP18S42 and UN-LO28S576B with the PCR protocol described by Schroeder et al. (3). The ITS consensus sequences of the seven isolates (Accession Nos. KF509939 to KF509945) were 98 to 99% homologous to ITS sequences of P. sulcatum in GenBank. Pathogenicity of all seven isolates was confirmed by inoculating mature carrot roots of cv. Bolero. Each root was washed with tap water, sprayed to runoff with 70% isopropanol, and dried in a laminar flow hood on sterilized paper toweling. The roots were then placed in plastic bins lined with paper toweling moistened with sterilized, deionized water. Each root was inoculated by placing two 5 mm-diameter agar plugs, taken from the edge of an actively growing WA culture of the appropriate isolate, on the root surface approximately 3 cm apart. Non-colonized agar plugs were used for a non-inoculated control treatment. Four replicate roots were inoculated for each isolate and the control treatment. After inoculation, the roots were misted with sterilized, deionized water, a lid was placed on each bin, and the roots were incubated in the dark at 22°C. Roots were misted daily to maintain high relative humidity. Dark, sunken lesions were first observed 3 days post-inoculation on roots inoculated with the P. sulcatum isolates, and all inoculated roots displayed cavity spot lesions by 7 days. No symptoms were observed on the non-inoculated control roots. Colonies with morphology typical of P. sulcatum were re-isolated from the symptomatic tissue of roots inoculated with the P. sulcatum isolates, and the species identity of the re-isolates was confirmed by ITS rDNA sequence analysis, as described above. Although P. sulcatum is one of several Pythium species that can cause cavity spot of carrot (1), to our knowledge, this is the first report of P. sulcatum causing cavity spot in Washington State, which has the largest acreage of processing carrot crops in the United States (4).

Antifungal activity of aluminium‐containing salts against the development of carrot cavity spot and potato dry rot

AbstractAs an alternative to the use of synthetic chemical fungicides to control plant disease, aluminium‐containing salts were evaluated for their effects on the mycelial growth of various fungal or fungus‐like pathogens and their ability to control carrot cavity spot (Pythium sulcatum) and potato dry rot (Fusarium sambucinum). Results showed that various aluminium‐containing salts provided strong inhibition of all the tested pathogens (Alternaria solani, Botrytis cinerea, F. sambucinum, P. sulcatum and Rhizopus stolonifer) with minimal inhibitory concentration of 1–10 mM. Aluminium chloride and aluminium sulphate were generally the most effective, inhibiting mycelial growth of pathogens by as much as 47% and 100%, respectively, at a salt concentration of 1 mM. Applied at 5 mM, aluminium sulphate also provided 28% and 100% inhibition of dry rot and cavity spot, respectively. Aluminium chloride (5 mM) reduced dry rot by 25% whereas aluminium lactate (5 mM) decreased cavity spot lesions by 86%. These results indicate that various aluminium‐containing salts may provide an alternative to the use of synthetic fungicides to control these pathogens.

First Report of Pythium recalcitrans Causing Cavity Spot of Carrot in Michigan.

During a survey of carrot (Daucus carota L.) cavity spot in Michigan in September 2010, carrot roots with typical cavity spot symptoms were collected from production fields in Fremont Co. The lesions were excised from infected roots, surface-disinfested with 0.62% NaClO for 3 min, rinsed in sterilized, distilled water three times, cut into 0.5 cm long pieces, and then plated on water agar (WA) amended with carbendazim (10 μg/ml), ampicillin (50 μg/ml), rifampicin (50 μg/ml), and pentachloronitrobenzene (10 μg/ml) (cumulatively referred to as CARP). Plates were incubated at 22 ± 1°C in the dark for 3 days. Pure cultures of the isolates were obtained by transferring a single hyphal tip of each colony to potato dextrose agar (PDA) amended with CARP. Among the 33 isolates obtained, M2-05 was identified as a Pythium sp. that differed from the known cavity spot pathogens of carrot. The isolate has spherical hyphal swellings but no other distinguishing morphological characteristics. M2-05 was further classified by analyzing the partial sequences of four genes: the internal transcribed spacer (ITS) region of ribosomal DNA, beta-tubulin (β-tub), cytochrome c oxidase subunit 2 (cox 2), and NADH dehydrogenase subunit 1 (nadh 1) (1,3). A BLAST search of these sequences for M2-05 was conducted using the nucleotide database of GenBank, resulting in 100% similarity to all four sequenced genes of P. recalcitrans (2). The DNA sequences of M2-05 were deposited in GenBank (JQ734349, JQ734229, JQ734289, and JQ734409 for ITS, β-tub, cox 2, and nadh 1, respectively). Koch's postulates were conducted by inoculating mature carrot roots (cv. Nantindo) with mycelial plugs (4 mm in diameter) cut from the margin of actively growing colonies of M2-05 on PDA plates. Two mycelial plugs were placed on each carrot root at 3-cm intervals, with the mycelial side facing the root; and two non-colonized agar plugs were placed similarly for the non-inoculated control treatment. In comparison, carrot roots also were inoculated with an isolate of each of P. sulcatum and P. violae using the same method. There were four replicate carrot roots inoculated for each isolate and each of the control treatments. The inoculated roots were placed on a plastic grid (7 mm in height) in a closed plastic container, with moist paper towels underneath the grids. The container was incubated in the dark at 22 ± 1°C, and the roots were sprayed gently daily with sterilized, distilled water to maintain high humidity. Brown lesions were observed on all inoculated carrot roots 5 days after inoculation. The lesions measured 0.68 ± 0.48, 1.20 ± 0.71, and 0.56 ± 0.31 mm2 averaged over all eight lesions for the isolates of P. recalcitrans, P. sulcatum, and P. violae, respectively. Symptomatic tissues from the inoculated roots were excised and incubated on WA-CARP plates, and the culture from each lesion confirmed as the isolates inoculated using the same molecular methods described above. The carrot tissue under the control agar plugs remained symptomless, and no Pythium was recovered from the control roots. P. recalcitrans was described in 2008 as infecting roots of Beta vulgaris and Vitis vinifera (2). To our knowledge, this is the first published report of P. recalcitrans naturally infecting carrot roots, not only in Michigan, but anywhere in the world.

Fungicide Sensitivity of Pythium spp. Associated with Cavity Spot of Carrot in California and Michigan.

The identity of 172 isolates of Pythium spp. from cavity spot lesions on carrot produced in California and Michigan was determined, and their sensitivity to three fungicides was examined. Pythium violae accounted for 85% of California isolates, with P. irregulare, P. dissotocum (the first report as a carrot pathogen in the United States), P. ultimum, and P. sulcatum making the balance. P. sulcatum, P. sylvaticum, and P. intermedium were the most commonly recovered (85%) species in Michigan; others from Michigan included P. intermedium, P. irregulare, and an unclassified strain, M2-05. On fungicide-amended media, 93% of isolates were sensitive to mefenoxam (inhibition of mycelial growth was >60% at 10 μg active ingredient [a.i.]/ml); however, two of five isolates of P. irregulare from California were highly resistant (≤60% inhibition at 100 μg a.i./ml); about half of the isolates of P. intermedium and P. sylvaticum and a single isolate of P. violae were highly or intermediately resistant to mefenoxam (>60% inhibition at 100 μg a.i./ml, or ≤60% inhibition at 10 μg a.i./ml). P. dissotocum, P. irregulare, P. sulcatum, M2-05, and three of seven isolates of P. intermedium were insensitive to fluopicolide (effective concentrations for 50% growth inhibition [EC50] were >50 μg a.i./ml), while P. sylvaticum, P. ultimum, P. violae, and some isolates in P. intermedium were sensitive (EC50 < 1 μg a.i./ml). All isolates were sensitive to zoxamide (EC50 < 1 μg a.i./ml). Sensitivity baselines of P. violae to zoxamide and fluopicolide were established.

Antifungal activity of sulfur-containing salts against the development of carrot cavity spot and potato dry rot

In the search for alternatives to synthetic fungicides to control postharvest disease, sulfur-containing salts were evaluated for their effects on the mycelial growth of various fungal or fungus-like pathogens and their ability to control carrot cavity spot ( Pythium sulcatum) and potato dry rot ( Fusarium sambucinum). Results showed that metabisulfite-containing salts provided strong inhibition of all the tested fungi. Furthermore, some sulfate-containing salts were also directly inhibitory to P. sulcatum (calcium sulfate and ammonium sulfate) and to F. sambucinum (sodium sulfate). The metabisulfite salts also provided 100% inhibition of cavity spot and dry rot at concentrations of 50 and 200 mM, respectively. Calcium sulfate and sodium sulfate also significantly reduced carrot cavity spot lesions at 50 mM and ammonium sulfate, magnesium sulfate, potassium sulfate and sodium sulfate reduced potato dry rot lesions at 200 mM. These results indicate that various sulfate and metabisulfite salts could be used to control these postharvest microorganisms.

Antimicrobial efficacy of cinnamon, ginger, horseradish and nutmeg extracts against spoilage pathogens

In the search for alternatives to the use of synthetic fungicides, aqueous spice extracts were evaluated for their effects on the mycelial growth of various spoilage pathogens and their ability to control potato dry rot and carrot cavity spot in vivo. Results showed that cinnamon, ginger and nutmeg significantly inhibited the mycelial growth of Aspergillus niger (Ascomycota), Fusarium sambucinum (Ascomycota), Pythium sulcatum (Oomycota) or Rhizopus stolonifer (Zygomycota), whereas horseradish extract did not lead to the inhibition of any microorganism at the tested concentration. Among the most effective extracts, 0.05 g mL‑1 of cinnamon extract completely inhibited A. niger and P. sulcatum, and 0.10 g mL‑1 of cinnamon extract completely inhibited F. sambucinum. A concentration of 0.05 g mL‑1 of ginger extract also caused 100% inhibition of P. sulcatum. In vivo, cinnamon extract significantly reduced lesions of potato dry rot and carrot cavity spot, and ginger extract reduced lesions of carrot cavity spot. These results indicate that aqueous cinnamon and ginger extracts could provide an alternative to the use of synthetic fungicides to control these pathogens.

Lateral roots of carrot have a low impact on alloinfections in cavity spot epidemic caused by Pythium violae

Carrot cavity spot, caused by a complex of Pythium species, is characterized by sunken elliptical lesions on the taproot. Recent epidemiological studies of P. violae have demonstrated the occurrence of both primary and secondary infections, with two types of secondary infection, autoinfection and alloinfection. Investigating the mechanisms underlying alloinfection and the role of carrot lateral roots, we asked whether direct physical root contact plays a role in alloinfection and whether root exudates enhance mycelial growth in soil alone. A rhizobox system was designed to differentiate the effects of each mechanism: a buffer zone created by nylon mesh was used to test the first mechanism, and young carrots with a root system similar to lateral roots were used to test the second. Alloinfections were generated in rhizoboxes via diseased taproots transplanted close to healthy, mature carrots. The nylon mesh had no significant effect on disease intensity (reflecting alloinfection), providing evidence that mycelial growth in soil contributed more to disease spread than did physical contact among roots. Nor did young carrots significantly affect alloinfection; thus root exudates had little effect on mycelial growth.

Pathometric relationships reveal epidemiological processes involved in carrot cavity spot epidemics

Carrot cavity spot (CCS) is one of the most important soilborne diseases affecting the carrot crop. The few epidemiological studies that have investigated the temporal and spatial dynamics of the disease have been based solely on diagrammatic scales or semi-quantitative indices. To reveal epidemiological processes involved in the development of CCS epidemics, we investigated pathometric relationships. To this end, standardised measurements were defined (disease incidence i, lesion density d, conditional lesion density cd, lesion size ls, and total diseased area tda). The evolution of a cohort of CCS lesions according to their size suggested that lesions can expand over time. Two pathometric relationships were tested: a first one, between i and tda, is given by the equation i = 100(1−exp(−a(t)tda)), where t is thermal time, and a second one, between tda, d, and ls, is given by the equation tda = c(t)πd(ls/2)2. These relationships were validated for CCS epidemics in the case of field experiments, a survey in commercial fields, and a controlled-conditions experiment. The temporal linear decrease of the time-dependent parameter a(t) in the first relationship suggested that CCS epidemics followed classical epidemiological phases driven by successive processes: (1) the mobilisation of soil inoculum leading to primary infection, (2) the spread of disease to neighbouring taproots (alloinfection), and (3) the intensification of disease on the taproot (autoinfection). This is consistent with complementary experimental results which demonstrated that auto- and alloinfections occur in CCS epidemics.

PCR-based identification ofPythiumspp. causing cavity spot in carrots and sensitive detection in soil samples

On the basis of ITS sequences PCR primers were designed for the identification of the five Pythium species found to be most important for the development of carrot cavity spot in Norway: P. intermedium, P. sulcatum, P. sylvaticum, P. violae and P. ‘vipa’. The P. ‘vipa’ isolates had a unique ITS sequence, differed morphologically from all other Pythium isolates, and thus probably represent a new species. The PCR primers were species-specific with no cross-reaction to other Pythium species or to fungal isolates from carrot tested. The detection limits varied for the different primer pairs. The two most sensitive assays allowed detection of as little as 5 fg DNA. All five Pythium species could be detected in lesions from diseased carrots. Weak positive signals were obtained from some carrot samples without symptoms. PCR assays allowed detection of pathogens in soil. In samples of soil known to produce cavity spots on cropped carrots, strong signals were obtained. In several soil samples more than one of the five Pythium species could be detected. The utilization of this diagnostic PCR assay in analysis of field soil and carrot tissue might in the future be exploited to reduce the incidence of this serious carrot disease.

Modulation of primary and secondary infections in epidemics of carrot cavity spot through agronomic management practices

The relative importance of primary and secondary infections (auto‐ and alloinfections) in the development of a carrot cavity spot (CCS) epidemic caused by Pythium spp. were investigated. Three cropping factors: fungicide application, soil moisture and planting density, were selected as the key variables affecting the disease tetrahedron. Their effects on: (i) disease measurements at a specific time, (ii) the areas under the disease progress curves (AUDPCs) and (iii) a time‐dependent parameter in a pathometric incidence‐severity relationship, were studied. Mefenoxam applications 5 and 9 weeks after sowing reduced the intensity of a field CCS epidemic that involved both primary and secondary infections. In microcosm experiments, mefenoxam reduced secondary infections by Pythium violae obtained by transplanting infected carrot roots and slowed disease progress (1·6 lesions per root in treated versus 5·8 lesions in non‐treated microcosms). A deficit of soil moisture limited the movement of Pythium propagules to host tissue, and thus reduced primary infections in the field; it also promoted the healing of lesions, limiting lesion expansion and the potential for alloinfections (6·8–7·5 lesions per root in irrigated plots compared with 2·4 lesions in non‐irrigated plots). A negative relationship between the mean root‐to‐root distance and the rate of alloinfections was established in microcosms; a reduction in mean planting density was also effective in limiting CCS development (0·5, 1·6 and 2·0 lesions per root in microcosms containing 8, 16 and 31 roots, respectively). An integrated disease management system based on a combination of cultural methods, such as optimized fungicide application, date of harvest versus soil moisture content, and host density versus planting pattern, may make a useful contribute to the control of CCS.

Demonstration of secondary infection by Pythium violae in epidemics of carrot cavity spot using root transplantation as a method of soil infestation

Cavity spot of carrot (CCS), one of the most important soilborne diseases of this crop worldwide, is characterized by small sunken elliptical lesions on the taproot caused by a complex of pathogens belonging to the genus Pythium, notably P. violae. In most soilborne diseases the soil is the source of inoculum for primary infections, with diseased plants then providing inoculum for secondary infections (both auto‐ and alloinfection). Using fragments of CCS lesions to infest soil, it was demonstrated that CCS lesions on carrot residues can cause primary infection of healthy roots. Using a novel soil infestation method, in which an artificially infected carrot root (the donor plant) was placed close to healthy roots (receptor plants) the formation of typical CCS lesions were induced more efficiently than the use of classical soil inoculum and showed that CCS can spread from root to root by alloinfection from transplanted diseased roots. The method also demonstrated the polycyclic nature of a CCS epidemic caused by P. violae in controlled conditions. Secondary infections caused symptoms and reduced root weight as early as two weeks after transplantation of the diseased carrot. This reproducible method may be used for delayed inoculation and for studying the effect of cropping factors and the efficacy of treatments against primary and secondary cavity spot infections.

Modélisation de cinétiques de la maladie de la tache de la carotte provoquée par un complexe d'agents pathogènes du genre Pythium dominé par le Pythium violae

Carrot cavity spot is caused by a complex of pathogens belonging to the genus Pythium, including Pythium violae. The disease is characterized by the appearance of sunken elliptical lesions on the taproot. The objective of the present study is to identify, understand, and prioritize the processes that induce the spatio-temporal kinetics of epidemics of cavity spot. We favoured the hypothesis that considers primary infection (from soilborne inoculum) and secondary infection (root-to-root contaminations from existing lesions, i.e., auto- and allo-infections). Epidemics were obtained during a 3-year field experiment after artificial soil infestation of plots with P. violae at increasing inoculum doses. Different kinetics of cavity spot were described using standardized disease measurements. The analysis of the incidence curves showed the early appearance of a plateau, with differences in the disease level that are positively correlated with the inoculum doses. These differences disappeared at the end of the cropping season. Three simulation models, integrating or not the concomitance of the primary and secondary infections, were fitted to the disease incidence to test if progress curves were compatible with the hypothesis of the occurrence of both infection processes: the logistic model, the bilogistic model of Hau and Amorim, and the model of Brassett and Gilligan without temporal decline of the soil inoculum potential. The quality of the fitting was correct in the three cases, and the hypothesis of the occurrence of secondary infections was not refuted. This result is an important argument to demonstrate the polycyclic nature of the disease.

The ecology of a Pythium community in relation to the epidemiology of carrot cavity spot

Carrot cavity spot (CCS) is characterised by the appearance of small sunken elliptical lesions on the tap root. It is caused by a complex of Pythium species, but the species diversity and interactions within the complex have never been studied for modelling CCS epidemics. The diversity of a pathogenic Pythium community was assessed during 3 consecutive years in a field experiment after an initial artificial soil infestation with P. violae. 1241 lesions were examined, yielding 728 Pythium isolates. Conventional microbiological methods and restriction polymorphism of the internal transcribed spacer regions of the rDNA of 209 representative Pythium isolates allowed us to identify 655 isolates as belonging to six Pythium species, including P. violae and five indigenous species ( P. sulcatum, P. intermedium, P. sylvaticum/ irregulare, P. coloratum, and P. ultimum). Biological traits, such as pathogenicity, optimum temperature for mycelial growth and saprophytic survival of the inoculum, explained the fluctuations in the composition of the complex over 17 successive samplings during the 3-year period, most notably the prevalence of first P. violae and then P. sulcatum. P. violae and P. sulcatum were occasionally isolated in mixture from single lesions (10.4% and 9.6%, respectively). Other species were more frequently isolated in mixture: 30.8% for P. intermedium, 33.8% for P. sylvaticum/irregulare, 42.9% for P. ultimum, and 66.7% for P. coloratum. A contingency analysis allowed us to define ‘major’ and ‘minor’ species on both pathological and ecological criteria (frequency of occurrence in the complex, pathogenicity and ability to induce lesions by themselves), and demonstrated that infection by one ‘major’ pathogen species ( P. violae or P. sulcatum) is not positively correlated with the presence of a second Pythium species. The ratio between ‘observed’ and ‘expected’ mixed infection frequency under the assumption of independent infection ( mir) was less than 1 for P. violae, P. sulcatum, P. intermedium, and P. sylvaticum/irregulare ( P < 0.05). For all Pythium species, there was a negative linear relationship between mir and pathogenicity ( R 2 = 0.638): the less a Pythium species was pathogenic on carrot, the more often it was isolated from a CCS lesion in mixture with at least one other species. The non-significance of interactions between species during the infection phase suggests that CCS epidemics can be analysed as if they were caused by a single Pythium species.

(258) Differential Response of Colored Carrots to Cavity Spot

Cavity spot of carrot, caused by several species of Pythium, is endemic in many carrot production areas of the world, including the Holland/Bradford Marsh region of Ontario, Canada. Field trials were conducted from 2002–04 to determine if carrots with different pigments varied in susceptibility to the disease. Carrots from the USDA breeding program at the University of Wisconsin were seeded in muck soil (pH 6.4, 60% organic matter) on 28, 30, and 27 May, harvested 22, 22, and 23 Oct., and assessed for disease on 5, 8, and 10 Dec. 2002, 2003, and 2004, respectively. The carrots were white (W 105-7), yellow (W 102-1), dark orange (W 101-23), red (W 104-3), and purple (W 106-3). Cultivar `Cellobunch' was included in 2003 and 2004. Twenty-five carrots of each of four replicate plots were assessed in 2002 and 2003, and 50 carrots were assessed in 2004, for disease incidence and severity [disease severity index (DSI), based on the size of the largest lesion per carrot]. Disease incidence was moderate in 2002 and 2003 (34%, 33%), and high in 2004 (60%). Consistent differences in susceptibility to cavity spot were identified over the three years of trials. The purple carrot had the lowest incidence (12%) and severity (7 DSI) of cavity spot, followed by the dark orange carrot (39%, 22 DSI) as compared to the susceptible yellow carrot (58%, 41 DSI). There was no difference in disease reaction between the yellow and white carrots. `Cellobunch' had the same reaction as the dark orange carrot. Studies are needed to determine whether the pigments themselves cause differences in the disease response.

Evaluation of fungicides for the control of carrot cavity spot

Cavity spot is one of the most common and serious diseases of carrot (Daucus carota L). The disease, caused by different species of Pythium, including P. violae Chesters & Hickman, P. sulcatum Pratt & Mitchell and P. sylvaticum Campbell & Hendrix, leads to frequent high rejection rates during grading worldwide. In the area of the city of Québec, the disease is caused mainly by P. sulcatum, P. sylvaticum and P. macrosporum Vaartaja & van der Plaats-Niterink. Cavity spot can be controlled with metalaxyl, but reports are emerging that this treatment show little or no efficacy in many regions. This situation reinforces the need for alternative fungicides. The objectives of the present study were: (1) to determine the sensitivity of 14 pathogenic isolates of P. sulcatum, P. sylvaticum and P. macrosporum collected from carrots produced in the area of the city of Québec to different broad-spectrum and oomycete-specific fungicides (chlorothalonil, etridiazole, fludioxonil, fosetyl-Al, metalaxyl, zoxamide), (2) to evaluate the efficacy of the fungicides in controlling cavity spot, and (3) to evaluate the risk of resistance development of isolates with the best-performing fungicide(s). The determination of EC50 for the fungicides tested showed that most isolates were highly sensitive to both metalaxyl and zoxamide but insensitive to fludioxonil, fosetyl-Al and chlorothalonil. In greenhouse assays, only zoxamide provided significant and consistent disease control as measured by the number of cavity spot lesions caused by P. sulcatum. Investigations into the risk of resistance development to zoxamide showed that, for specific isolates, repeated exposure to the fungicide resulted in a loss of sensitivity.