Influence of light-enhanced glycoalkaloids on resistance of potato tubers to Fusarium sulphureum and Fusarium solani var. coeruleum

Increases in resistance to Fusarium sulphureum and Fusartium solani var. coeruleum were determined in seed tubers of cultivars Desiree, Pentland Hawk and Kerr's Pink with light-enhanced glycoalkaloids. Glycoalkaloid concentration in the tubers ranged from 78.0 to 885.9 mg kg⁻¹ fresh weight. Exposure of tubers to light prior to inoculation decreased fungal invasion, the greater reductions generally reflecting longer light exposure periods. In most cases, no direct relationship between resistance and glycoalkaloid concentrations was recorded for F . sulphureum. Surface infection indices and reduced depth of rot caused by F. solani var. coeruleum were closely correlated with tuber glycoalkaloid content. No significant effects on spore length and width were recorded. Exposure to light of seed tubers that are not destined for human or animal consumption could provide a pretreatment to reduce fungal invasion by F. sulphureum and F. solani var. coeruleum .     

Resistance to thiabendazole in Fusarium species isolated from potato tubers affected by dry rot

Fusarium species obtained from stored potato tubers affected with dry rot were grown on agar containing thiabendazole. All 40 isolates of F. coeruleum and 60 isolates of F. avenaceum tested were sensitive to the fungicide, but 68% of the 85 isolates of F. sulphureum and one isolate of F. culmorum were classified as resistant. When isolates were made from dry rots on tubers that had been treated with thiabendazole during loading into store, all 81 isolates of F. sulphureum were resistant, whereas all the isolates of F. coeruleum (25), F. avenaceum (4) and Phoma foveata (10) were sensitive. Resistance was not found in five isolates of Cylindrocarpon destructans. All the Fusarium spp. were sensitive to imazalil and were pathogenic when inoculated into potato tubers. Resistant and sensitive isolates of F. sulphureum caused rots of similar size.     

Pathogen resistance in aerial tubers of potato cultivars

Increases in resistance of aerial tubers (cultivars Désirée, Epicure and King Edward) against nine pathogenic potato fungi were determined in vitro using freeze-dried powder derived from subterranean and aerial tubers as a culture medium, and in vivo by inoculating both tuber types with the storage pathogens Fusarium sulphureum, F . solani var. coeruleum, Phoma exigua var. foveata and Polyscytalum pustulans . Significantly reduced growth rates and spore and conidiophore numbers were recorded when pathogens were cultured on freeze-dried powder derived from aerial tubers. No effects on spore size of F . solani var. coeruleum were recorded but spore and conidiophore lengths of remaining pathogens were reduced and width increased, except for Colletotrichum coccodes , where the diameter of acervuli increased, and P . exigua var. foveata , where spore length and width were reduced. Reduced disease incidence, severity and spore numbers were recorded on aerial compared with subterranean tubers inoculated in vivo with the four storage pathogens. Spore length of P . pustulans was reduced and spore width increased, while spore length and width of P . exigua var. foveata were reduced on aerial compared with subterranean tubers inoculated with these pathogens. No effects on spore size of Fusarium spp. were recorded. Concentrations of glycoalkaloids, chlorogenic acid, calcium and magnesium were higher in aerial than in subterranean tuber tissue, which may have contributed to increased resistance and effects on size and morphology of fungal spores. The implications of using aerial tubers as an alternative technique for the production of seed potatoes and their importance to the potato industry are discussed.     

Use of quantitative molecular diagnostic assays to investigate fusarium dry rot in potato stocks and soil

ABSTRACT Specific and sensitive quantitative diagnostics, based on real-time (TaqMan) polymerase chain reaction (PCR) and PCR enzyme-linked immunosorbent assay, were developed to detect dry-rot-causing Fusarium spp. (F. avenaceum, F. coeruleum, F. culmorum, and F. sulphureum). Each assay detected Fusarium spp. on potato seed stocks with equal efficiency. Four potato stocks, sampled over two seed generations from Scottish stores, were contaminated with F. avenaceum, F. sulphureum, F. culmorum, F. coeruleum or a combination of species, and there was a general trend towards increased Fusarium spp. contamination in the second generation of seed sampled. F. sulphureum and F. coeruleum caused significantly (P < 0.05) more disease in storage than the other species when disease-free tubers of potato cvs. Spunta and Morene were inoculated at a range of inoculum concentrations (0, 10(4), 10(5), and 10(6) conidia/ml). Increased DNA levels were correlated with increased disease severity between 8 and 12 weeks of storage. The threshold inoculum levels resulting in significant disease development on both cultivars were estimated to be 10(4) conidia/ml for F. sulphureum and 10(5) conidia/ml for F. coeruleum. To study the effect of soil infestation and harvest date on disease incidence, seed tubers of cvs. Morene and Spunta were planted in a field plot artificially infested with the four Fusarium spp. F. culmorum and F. sulphureum were detected in soil taken from these plots at harvest, and F. sulphureum DNA levels increased significantly (P < 0.05) at the final harvest. All four Fusarium spp. were detected in progeny tubers. There was a trend toward higher levels of F. culmorum detected in progeny tubers at the earliest harvest date, and higher levels of F. sulphureum at the final harvest. The use of diagnostic assays to detect fungal storage rot pathogens and implications for disease control strategies are discussed.     

Characterization of Fusarium spp. responsible for causing dry rot of potato in Great Britain

A 3-year survey was undertaken to establish the relative frequency of different Fusarium spp. present as inoculum on potato tubers collected from four regions of Great Britain. A total of 219 samples (comprising 10 950 tubers) were collected from the 2000, 2001 and 2002 crops and processed to recover dry rot-producing isolates. In total, 228 isolates of Fusarium spp. were recovered. Most (94·7%) of these isolates were attributed to one of four Fusarium species: F. coeruleum, F. avenaceum, F. culmorum and F. sambucinum (formerly F. sulphureum) . The incidence of the combined Fusarium spp. increased the further south the crops had been grown. Fusarium coeruleum was the most commonly isolated species in each survey year, comprising 37 to 52% of the total Fusarium species. Selected isolates of each species were evaluated for their ability to produce rots in potato tubers. Fusarium sambucinum was a more aggressive pathogen than the other Fusarium species in eight out of 10 cultivars. Fusarium avenaceum and F. culmorum were relatively weaker pathogens. However, these species were aggressive on some cultivars, notably Hermes. The selected isolates were also assessed for their sensitivity to the fungicides thiabendazole and imazalil. Using in vitro tests, 65% of F. sambucinum isolates were resistant to thiabendazole and 7% of F. avenaceum isolates were resistant to imazalil. Tubers treated with imazalil yielded a higher proportion of isolates of F. avenaceum than those that were untreated. Similarly, a higher proportion of F. sambucinum isolates were recovered from tubers treated with thiabendazole than from those that were not treated.     

Endophytic bacterial communities in the periderm of potato tubers and their potential to improve resistance to soil-borne plant pathogens

To evaluate whether the location of bacterial endophyte communities contributes to disease resistance in potato tubers ( Solanum tuberosum ), the population density, biodiversity and antibiotic activity of endophytic bacteria was examined from the tuber peel (periderm plus top 3 mm of tissue) of four cultivars (Russet Burbank, Kennebec, Butte and Shepody). There were no significant differences for population density of bacteria among the layers of peel examined and no cultivar × peel layer interaction. Endophytic bacteria from several layers of peel were challenged in in vitro bioassays to the soil-borne plant pathogens Fusarium sambucinum , Fusarium avenaceum , Fusarium oxysporum and Phytophthora infestans (mating types A1 and A2). In general, antibiosis of bacterial endophytes against these pathogens was significantly higher ( P  = 0.01) in isolates recovered from the outermost layer of tuber peel and decreased progressively toward the centre of the tuber. Antibiosis against P. infestans was variable, with a progressive decrease in antibiotic activity from outer to inner layers of peel occurring in cvs Russet Burbank and Kennebec only. For antibiosis there were significant cultivar × peel, and cultivar × pathogen interactions ( P  = 0.01). In all cases the inhibitory activity of endophytic bacteria was significantly greater ( P  = 0.01) against the A1 than the A2 mating type of P. infestans . In four of seven cases, where the same species of bacteria were recovered from all three peel layers, antibiosis to pathogens decreased significantly ( P  = 0.01) with depth of recovery (from the periderm to inside the tuber), indicating that in certain communities of endophytic bacteria, defence against pathogens may be related to bacterial adaptation to location within a host and may be tissue-type and tissue-site specific.     

The effect of foliar application of phosphonate formulations on the susceptibility of potato tubers to late blight

Foliar sprays of potato plants with phosphonic acid (partially neutralised with potassium hydroxide to pH 6.4) substantially reduced infection of the tubers by Phytophthora infestans, the cause of late blight, in glasshouse and field experiments over a 4-year period. Healthy tubers of blight-susceptible cultivars removed from treated plants and artificially inoculated by spraying with sporangial/zoospore suspensions of P infestans did not develop disease symptoms, demonstrating that the phosphonate applications had directly reduced the susceptibility of tubers to infection, probably as a result of translocation into tuber tissue. In contrast, foliar application of fosetyl-aluminium did not significantly reduce tuber blight development following inoculation. Five to six sprays of partially neutralised phosphonic acid (2 kg ha-1) applied at 10-14 day intervals resulted in the least tuber infection, but such a treatment regime may not be economic. In trials where the effect of timing and rate of application of 2-4 kg phosphonic acid ha-1 was examined, a single treatment of 4 kg ha-1 applied mid- or late-season proved the most effective. A spray programme in which one or two applications of phosphonic acid are combined with use of a non-systemic or systemic fungicide to enhance foliar protection offers the possibility of controlling both foliage and tuber blight and could have a major impact in reducing overwinter survival of P infestans in tubers.     

BABA effects on the behaviour of potato cultivars infected by <Emphasis Type="Italic">Phytophthora infestans</Emphasis> and <Emphasis Type="Italic">Fusarium solani</Emphasis>

Since most plants possess resistance mechanisms which can be induced upon pre-treatment with a variety of chemical compounds, the use of β-aminobutyric acid (BABA) as a defence inducer without reported toxic effect on the environment was studied. The aim of this work was to analyse the effectiveness of BABA to induce resistance against Phytophthora infestans and Fusarium solani in potato cultivars differing in their level of resistance to late blight. The behaviour of some components of biochemical mechanisms by which BABA increases resistance against P. infestans, as well as the effect of BABA on the activity of a potential pathogenic factor of F. solani, were studied. Plants with four applications of BABA throughout the crop cycle produced tubers more resistant to P. infestans and F. solani than non-treated plants. In addition, tuber slices from treated plants, inoculated with P. infestans, showed an increase in phenol and phytoalexin content. The aspartyl protease StAP1 accumulation was also higher in tubers obtained from treated plants and inoculated with P. infestans. This result was observed only in the more resistant potato cv. Pampeana, early after infection. In the potato–F. solani interaction, infected tubers coming from BABA-treated plants showed minor fungal proteolytic activity than infected, non-treated ones. For potato cvs Pampeana and Bintje, the BABA treatment improved the yield of harvested tubers. The number of tubers per plant and total weight of harvested tubers was greater for those obtained from treated plants with two early or four applications of BABA. The results show that the BABA treatment increases the resistance of potatoes but the degree of increase depends on the original level of resistance present in each cultivar.     

Aggressiveness and host specificity of Brazilian isolates of Phytophthora infestans

The population of Phytophthora infestans in Brazil consists of two clonal lineages, US-1 associated with tomatoes and BR-1 associated with potatoes. To assess whether host specificity in these lineages resulted from differences in aggressiveness to potato and tomato, six aggressiveness-related epidemiological components – infection frequency (IF), incubation period (IP), latent period (LP), lesion area (LA), lesion expansion rate (LER) and sporulation at several lesion ages (SSLA) – were measured on detached leaflets of late blight-susceptible potato and tomato plants. Infection frequency of US-1 was similar on potato and tomato leaflets, but IF of BR-1 was somewhat reduced on tomato. Incubation period was longer on both hosts with US-1, although this apparent lineage affect was not significant. Overall there was no host effect on IP. On potato, BR-1 had a shorter LP (110·3 h) and a larger LA (6·5 cm²) than US-1 (LP = 162·0 h; LA = 2·8 cm²). The highest LER resulted when isolates of BR-1 (0·121 cm² h⁻¹) and US-1 (0·053 cm² h⁻¹) were inoculated on potato and tomato leaflets, respectively. The highest values of the area under the sporulation capacity curve (AUSC) were obtained for isolates of US-1 inoculated on tomato leaflets (6146) and for isolates of BR-1 on potato leaflets (3775). In general, higher values of LA, LER, SSLA and AUSC, and shorter values of LP were measured when isolates of a clonal lineage were inoculated on their original host than with the opposite combinations. There is evidence that there are quantitative differences in aggressiveness components between isolates of US-1 and BR-1 clonal lineages that probably contribute to host specificity of P. infestans populations in Brazil.     

Epidemiological importance of Solanum sisymbriifolium, S. nigrum and S. dulcamara as alternative hosts for Phytophthora infestans

Lesions of Phytophthora infestans were found on woody nightshade ( Solanum dulcamara ), black nightshade ( S. nigrum ) and S. sisymbriifolium during a nationwide late blight survey in the Netherlands in 1999 and 2000. Pathogenicity and spore production of P. infestans isolates collected from potato ( S. tuberosum ), S. nigrum , S. dulcamara and S. sisymbriifolium were determined on several host plant species, and oospore formation in naturally infected and inoculated foliage of hosts was quantified. The present population of P. infestans in the Netherlands is pathogenic on S. nigrum , S. dulcamara and S. sisymbriifolium . Oospores were produced in leaves of S. nigrum , S. dulcamara and S. sisymbriifolium following infection with A1 and A2 isolates. Therefore these plant species should be regarded as alternative hosts for the late blight pathogen. In the case of S. nigrum and S. dulcamara infection was a relatively rare event, suggesting that diseased plants do not significantly contribute to the overall late blight disease pressure present in potato-production areas. Oospore production in ageing S. nigrum and S. dulcamara plants in autumn, however, may generate a considerable source of (auto) infections in following years. Considerable numbers of sporangia and oospores were produced on S. sisymbriifolium following infection with P. infestans . Additional field infection data are needed to evaluate the epidemiological consequences of a commercial introduction of S. sisymbriifolium as a potato cyst nematode trap crop.     

Development of a species-specific and sensitive detection assay for Phytophthora infestans and its application for monitoring of inoculum in tubers and soil

A specific and sensitive PCR assay for the detection of Phytophthora infestans , the cause of late blight of potato, in soil and plant tissues was developed. A P. infestans -specific primer pair (INF FW2 and INF REV) was designed by comparing the aligned sequences of rDNA internal transcribed spacer regions of most of the known Phytophthora species. PCR amplification of P. infestans DNA with primers INF FW2 and INF REV generated a 613 bp product, and species specificity was demonstrated against DNA from nine other Phytophthora species and seven potato-blemish pathogens. In a single-round PCR assay, 0·5 pg pure P. infestans DNA was detectable. Sensitivity was increased to 5 fg DNA in a nested PCR assay using Peronsporales-specific-primers in the first round. As few as two sporangia or four zoospores of P. infestans could be detected using the nested assay. Procedures are described for detection of P. infestans in leaves, stem and seed potato tubers before expression of symptoms. A soil assay in which 10 oospores per 0·5 g soil were detectable was developed and validated using samples of field soil. The PCR assay was used to examine the long-term survival of sexual (oospores) and asexual (sporangia and mycelium) inoculum of P. infestans in leaf material buried in a replicated experiment under natural field conditions. Oospores were consistently detected using the PCR assay up to 24 months (total length of the study) after burial in soil, whereas the sporangial inoculum was detected for only 12 months after burial. Sporangial inoculum was shown to be nonviable using a baiting assay, whereas leaf material containing oospores remained viable up to 24 months after burial.     

Novel in vivo use of a polyvalent Streptomyces phage to disinfest Streptomyces scabies-infected seed potatoes

A highly virulent and polyvalent Streptomyces phage was isolated from a potato field near Albany, Western Australia. The efficacy of the isolated phage to disinfest seed potato tubers artificially inoculated with a common scab-causing streptomycete was evaluated. The phage suspension was prepared in a mini-bioreactor. Diseased potatoes were bathed in a phage suspension (1 × 10⁹ plaque-forming units per mL) for 24 h. The suspension was constantly circulated within a novel 25 L phage bath by means of an air-sparging pipe driven from an air compressor. Phage-treated scab-affected seed potatoes planted into free-draining polystyrene boxes containing steam-pasteurized field soil produced tuber progeny with significantly ( P  < 0·05) reduced levels of surface lesions of scab (1·2%) compared with tubers harvested from nonphage-treated tubers (23%). The number of scab lesions was also significantly reduced ( P  < 0·05) by phage treatment of mother tubers. No significant differences were recorded in weight, size or number of harvested tubers from phage-treated or nontreated mother tubers. This is the first in vivo study that has used Streptomyces phage to significantly disinfest seed potatoes of Streptomyces scabies and thereby reduce contamination of soil from seed-tuber-borne inoculum and reduce infection of daughter tubers.     

Metalaxyl sensitivity of isolates of Phytophthora erythroseptica in Prince Edward Island

Isolates of Phytophthora erythroseptica , causal agent of pink rot of potato, were obtained from diseased tubers collected in Prince Edward Island (PEI), Canada. In an in vitro assay of field and single-zoospore isolates, all isolates of the pathogen from PEI were sensitive to metalaxyl (EC₅₀ < 0·5  µ g mL⁻¹), unlike moderately or highly resistant reference isolates obtained from Maine, USA. Allozyme-banding patterns at the glucose-6-phosphate isomerase ( Gpi ) locus were identical (91/91) for all isolates of P. erythroseptica examined from PEI and Maine, but could be used to distinguish isolates of P. erythroseptica from local isolates of Phytophthora infestans . Inoculation of potato tubers from plants treated with foliar applications of metalaxyl in the field indicated that compounds inhibitory to metalaxyl-sensitive isolates of P. erythroseptica were present in the periderm, even after 4 months' storage. By contrast, moderately or highly resistant isolates of the pathogen caused significantly ( P  ≤ 0·05) more disease than sensitive isolates in tubers from plants treated with metalaxyl. The effectiveness of metalaxyl in controlling pink rot in a particular region will depend on resistance levels of local populations of P. erythroseptica . Preventing the development and spread of resistant strains of P. erythroseptica will be critical in maintaining metalaxyl as an effective chemical for control of this pathogen.     

Enhancement of natural disease resistance in potatoes by chemicals

The mechanism involved in systemic acquired resistance (SAR) can be non-specifically induced in susceptible plants. In response to pathogens, plants' natural defence mechanisms include the production of lignin and phytoalexins and the induction of plant enzymes. The aim of this research was to study the induction of SAR mediated by the chemical activator DL-3-aminobutyric acid (BABA) and the fungicide fosetyl-aluminium in potato cultivars with different levels of resistance against Phytophthora infestans (Mont) de Bary. To study the chemical induction of the resistance, the foliage of several potato cultivars was sprayed with BABA, fosetyl-aluminium or water (as a control treatment). After 3 days the foliage was inoculated with P. infestans. Seven days after inoculation, development of disease symptoms in the foliage was assessed. In postharvest tuber samples, evidence for enhancement of the defence response was evaluated by measuring the protein content of several hydrolytic enzymes as well as the phenol and phytoalexin content. The highest level of protection against late blight was observed when the chemicals were applied at early stages of crop development. An increase in resistance to late blight was also detected in tubers after harvest. There was also an increase in the protein level of beta-1,3-glucanase and aspartic protease as well as in the phenol and phytoalexin content of potato tuber discs obtained from postharvest tubers of treated plants. Thus the protective effect seemed to persist throughout the whole crop cycle. This treatment may offer the possibility of controlling both foliage and tuber blight and could have a major impact in reducing over-winter survival of P. infestans in tubers.     

Phytophthora infestans in a subtropical region: survival on tomato debris, temporal dynamics of airborne sporangia and alternative hosts

Little is known about inoculum dynamics of late blight caused by Phytophthora infestans in tropical/subtropical areas, particularly in Brazil. The objectives of the present study were to assess (i) the survival of the pathogen on stems, leaflets and tomato fruits, either buried or not in soil; (ii) the pathogenicity of P . infestans to mostly solanaceous plant species commonly found in Brazil that could act as inoculum reservoir; and (iii) the temporal dynamics of airborne sporangia. Phytophthora infestans survived in tomato plant parts for less than 36 days under greenhouse and field conditions. In greenhouse tests, pathogen structures were detected earlier on crop debris kept in dry than in wet soil conditions. Isolates of two clonal lineages of P. infestans , US-1 from tomato, and BR-1 from potato, were inoculated on 43 plant species. In addition to potato and tomato, Petunia  ×  hybrida and Nicotiana benthamiana were susceptible to the pathogen. Airborne inoculum was monitored with Rotorod and Burkard spore traps as well as with tomato and potato trap plants. Sporangia were sampled in most weeks throughout 2004 and in the first two weeks of 2005. Under tropical/subtropical conditions, airborne inoculum is abundant and is more important to late blight epidemics than inoculum from crop debris or alternative hosts.     

Genetic diversity and host differentiation among isolates of Phytophthora infestans from cultivated potato and wild solanaceous hosts in Peru

To test the hypothesis that isolates of Phytophthora infestans attacking wild Solanaceae exhibit specialization for particular host species, 115 isolates of P. infestans were collected from cultivated potatoes, nontuber-bearing Solanum spp. of the Basarthrum section and wild tomatoes from five departments in the northern and central highlands of Peru, and characterized using several neutral markers. All isolates belonged to one of four clonal lineages described previously in Peru: EC-1, US-1, PE-3 and PE-7. There was a strong association of three lineages with host species: PE-3 was only isolated from cultivated potato, while PE-7 and US-1 were only isolated from nontuber-bearing Solanum spp. ( Basarthrum section and wild tomatoes). EC-1 was isolated from all host groups sampled. A subset ( n  = 74) of the isolates was evaluated for metalaxyl resistance. High levels of resistance were found almost exclusively in EC-1 and PE-3, while US-1 and PE-7 isolates were generally sensitive. In a detached-leaf assay for lesion diameter using five EC-1 isolates from S. caripense and seven EC-1 isolates from cultivated potato, there was a significant interaction between isolate origin and inoculated host, caused by higher aggressiveness of EC-1 from cultivated potato on its host of origin. In a comparison of EC-1 (seven isolates from cultivated potato) and US-1 (three isolates from S. caripense ), each pathogen lineage was more aggressive on its original host species, causing a highly significant interaction between isolate origin and inoculated host. Wild tomatoes and nontuber-bearing Solanum spp. harbour several pathogen lineages in Peru and could serve as reservoirs of inoculum that might contribute to epidemics on potato or tomato. Potential risks associated with the use of wild Solanum hosts as sources of resistance to P. infestans are discussed .     

Effects of benzothiadiazole and acetylsalicylic acid on β-1,3-glucanase activity and disease resistance in potato

Benzothiadiazole (BTH), as Bion WG50, and acetylsalicylic acid (ASA) treatments of potato foliage of field- and glasshouse-grown potato plants were compared for control of two foliar diseases, early blight ( Alternaria solani ) and powdery mildew ( Erysiphe cichoracearum ). The effect of these treatments on harvested tubers wound-inoculated with the dry rot fungus ( Fusarium semitectum ) was also evaluated. BTH (50 mg a.i. L⁻¹) gave almost complete control of both foliar pathogens on inoculated glasshouse-grown plants and reduced the severity of leaf spotting diseases (mainly early blight) in the field. BTH (100 mg a.i. L⁻¹) and ASA (400 mg a.i. L⁻¹) reduced the severity of dry rot in field-grown tubers in some post-harvest wound-inoculated treatments but not others and a similar reduction occurred with tubers inoculated post-harvest from BTH-treated plants grown under glasshouse conditions. BTH treatment increased β-1,3-glucanase activity in leaves > stem > tubers > stolons but not in roots. Increased enzyme activity was recorded for up to 45 days post-treatment.     

Volatile metabolite profiling to detect and discriminate stem-end rot and anthracnose diseases of mango fruits

The volatile metabolites from the headspace gas of containerised mango ( Mangifera indica ) cv. Tommy Atkins fruits, surface wounded and inoculated with the two fungal anamorphic pathogens Colletotrichum gloeosporioides and Lasiodiplodia theobromae , or non-inoculated (controls), were profiled using a portable gas chromatograph/mass spectrometer to discriminate diseases of mango. Thirty-four compounds were detected relatively consistently among replicates. Several of these were disease/inoculation-discriminatory and were classified into three groups: (i) compounds unique to only one treatment; (ii) compounds common to two or more treatments, but not to all; and (iii) compounds common to all treatments, but varying in their abundance. Two compounds, 1-pentanol and ethyl boronate, were detected in L. theobromae- inoculated mangoes alone, while thujol was observed only in C. gloeosporioides- inoculated mangoes. Discriminant analysis models based on the abundance of significant mass ions and consistent compounds correctly classified diseases/inoculations in up to 100% of cases. The disease-discriminatory compounds and discriminant analysis models developed here have the potential to be used in the early detection of postharvest diseases of mango fruits after validation under commercial conditions.     

几种植物提取物诱导马铃薯对致病疫霉的抗性

 采用纤维素膜技术研究了大蒜、洋葱、芹菜、丁香、花椒、大黄等24种植物离体组织及其提取物(其中8种中草药为水煎剂,16种其它植物为水浸剂)对马铃薯晚疫病菌(Phytophthora infestans)静止胞萌发、附着胞形成及侵入丝形成的直接影响和提取物诱导块茎对晚疫病病菌侵入机构形成的影响,并在此基础上进一步就提取物免疫处理种薯、诱导成株期抗病性、增加块茎产量的影响做了初步探讨。在洋葱、葱、大蒜等12种植物组织表面,静止胞萌发率明显低于对照(擦镜纸),而且在6种植物组织表面未见附着胞形成,10种植物组织表面未见侵入丝形成;在大蒜、韭菜等13种植物组织提取液中,静止胞萌发率明显低于对照(擦镜纸),其中6种植物组织提取液中附着胞未见形成,7种植物组织提取液中侵入丝未见形成。经8种植物组织提取物诱导处理的块茎切片,不仅保鲜期长,而且表现出较强的诱导抗性;静止胞萌发率、附着胞形成率及侵入丝形成率均明显低于对照;经8种植物组织提取物诱导处理的种薯在出芽、早期生长、抗晚疫病及产量等各方面均优于对照,病害保护率达54%、增产率达31%。     

Phytophthora infestans Populations from Tomato and Potato in North Carolina Differ in Genetic Diversity and Structure

ABSTRACT Phytophthora infestans causes a destructive disease on tomato and potato. In North Carolina (NC) potatoes are mostly grown in the east, whereas tomatoes are grown in the mountainous areas in the western part of the state. Five genotypes of P. infestans were identified from 93 and 157 isolates collected from tomato and potato over a 5 year period between 1993 and 1998. All isolates collected from potato in eastern NC were the US-8 genotype, whereas only a single isolate was the US-1 genotype. Tuber blight was found on immature daughter tubers in a single field in 1997, however infection on mature tubers was not observed. Within potato fields, a range of sensitivity to metalaxyl was observed among isolates but all were either intermediate or highly resistant to the fungicide. In contrast, isolates from tomatoes included previously reported US-7 and US-8 genotypes and two new genotypes called US-18 and US-19 (A2 mating type, allozyme genotype Gpi 100/100 and Pep 92/100). These genotypes had unique restriction fragment length polymorphism banding patterns, were sensitive to metalaxyl, and have not been reported elsewhere. All genotypes, with the exception of the US-1, were the Ia mitochondrial haplotype. Thus, isolates of P. infestans from tomato were more genetically diverse over time in NC than those from potato and include two new genotypes that are sensitive to metalaxyl.