Mechanism of broccoli-mediated verticillium wilt reduction in cauliflower

ABSTRACT Broccoli is resistant to Verticillium dahliae infection and does not express wilt symptoms. Incorporation of broccoli residues reduces soil populations of V. dahliae. The effects of broccoli residue were tested on the colonization of roots by V. dahliae, plant growth response, and disease incidence of both broccoli and cauliflower in soils with different levels of V. dahliae inoculum and with or without fresh broccoli residue amendments. The three soils included a low-Verticillium soil, a high-Verticillium soil, and a broccoli-rotation soil (soil from a field after two broccoli crops) with an average of 13, 38, and below-detectable levels of microsclerotia per g of soil, respectively. Cauliflower plants in broccoli-amended high-Verticillium soil had significantly (P 相似文献   

Effects of Irrigation and Verticillium dahliae on Cauliflower Root and Shoot Growth Dynamics

ABSTRACT Cauliflower root and plant growth and Verticillium wilt development were evaluated under different moisture regimes in the presence or absence of V. dahliae. Treatments included two main plots (V. dahliae-infested and fumigated), two subplots (furrow and subsurface drip irrigation), and three sub-subplots (deficit, moderate, and excessive regimes) that were arranged in a split-split-plot design in the field. Soil cores with roots were periodically sampled at 5 and 25 cm distance from plants. Total roots in each soil core were extracted with a hydropneumatic root elutriator, and root length from each sample was determined with a digital image analysis system. Incidence and severity of Verticillium wilt, plant height, number of leaves, and dry weights of leaves and roots were determined on 10 plants sampled at 7- to 10-day intervals 1 month after cauliflower transplanting and continued until harvest. To evaluate the effects of Verticillium wilt-induced stress on cauliflower plants, stomatal resistance was measured in upper healthy and lower (or diseased) leaves. Root length density at 5 and 25 cm from plant was significantly (P < 0.05) higher in subsurface drip than in furrow irrigation. Root length density was significantly higher in excessive irrigation regime than in the other regimes. Concomitantly, there was higher wilt incidence and severity in excessive and moderate regimes than deficit regime regardless of the irrigation method. Plant height was affected by irrigation methods and deficit regime. Neither the method of irrigation nor the quantity of water affected the other variables. Stomatal resistance in lower diseased leaves was significantly higher in infested than in fumigated plots but it was not in the upper healthy leaves. In this study, cauliflower yield was not affected by V. dahliae and irrigation method, but the deficit irrigation regime resulted in reduced yield even though it suppressed wilt in cauliflower. Thus, higher moisture levels resulted in higher root length density in V. dahliae-infested plots that in turn lead to greater incidence of Verticillium wilt and severity. The pathogen also affected physiological processes such as hydraulic conductance of cauliflower leaves, but not shoot growth or yield under these experimental conditions.     

Effects of Crop Rotation and Irrigation on Verticillium dahliae Microsclerotia in Soil and Wilt in Cauliflower

ABSTRACT Experiments were conducted in field plots to evaluate the effects of broccoli residue on population dynamics of Verticillium dahliae in soil and on Verticillium wilt development on cauliflower under furrow and subsurface-drip irrigation and three irrigation regimes in 1994 and 1995. Treatments were a factorial combination of three main plots (broccoli crop grown, harvested, and residue incorporated in V.dahliae-infested plots; no broccoli crop or residue in infested plots; and fumigated control plots), two subplots (furrow and subsurface-drip irrigation), and three sub-subplots (deficit, moderate, and excessive irrigation regimes) arranged in a split-split-plot design with three replications. Soil samples collected at various times were assayed for V. dahliae propagules using the modified Anderson sampler technique. Incidence and severity of Verticillium wilt on cauliflower were assessed at 7- to 10-day intervals beginning a month after cauliflower transplanting and continuing until harvest. Number of propagules in all broccoli plots declined significantly (P < 0.05) after residue incorporation and continued to decline throughout the cauliflower season. The overall reduction in the number of propagules after two broccoli crops was approximately 94%, in contrast to the fivefold increase in the number of propagules in infested main plots without broccoli after two cauliflower crops. Disease incidence and severity were both reduced approximately 50% (P < 0.05) in broccoli treatments compared with no broccoli treatments. Differences between furrow and subsurface-drip irrigation were not significant, but incidence and severity were significantly (P < 0.05) lower in the deficit irrigation regime compared with the other two regimes. Abundance of microsclerotia of V. dahliae on cauliflower roots about 8 weeks after cauliflower harvest was significantly (P < 0.05) lower in treatments with broccoli compared with treatments without broccoli. Rotating broccoli with cauliflower and incorporating broccoli residues into the soils is a novel means of managing Verticillium wilt on cauliflower and perhaps on other susceptible crops. This practice would be successful regardless of the irrigation methods or regimes followed on the susceptible crops.     

Spatial Patterns of Microsclerotia of Verticillium dahliae in Soil and Verticillium Wilt of Cauliflower

ABSTRACT The spatial patterns of microsclerotia of Verticillium dahliae in soil and wilt symptoms on cauliflower were determined at three sites in each of two fields in 1994 and 1995. Each site was an 8 x 8 grid divided into 64 contiguous quadrats (2 by 2 m each). Soil samples were collected to a depth of 15 cm with a probe (2.5 cm in diameter), and samples from four sites in each quadrat were bulked. Plants in each quadrat were cut transversely, and the number of plants with vascular discoloration and the number without discoloration were recorded. The soil was assayed for microsclerotia by the modified Anderson sampler technique. Lloyd's index of patchiness (LIP) was used as an indicator to evaluate the aggregation of microsclerotia in the field. Spatial autocorrelation and geostatistical analyses were also used to assess the autocorrelation of microsclerotia among quadrats. The LIP for microsclerotia was greater than 1, indicating aggregation of propagules; however, the degree of aggregation at most sites was not high. Significant autocorrelation within or across rows was detected in some spatial autocorrelograms of propagules, and anisotropic patterns were also detected in some oriented semivariograms from geostatistical analyses for microsclerotia, indicating the influence of bed preparation in the fields on pathogen distribution. The parameter estimates p and theta in the beta-binomial distribution and the index of dispersion (D) associated with the distribution were used to assess the aggregation of diseased plants at each site. A random pattern of wilt incidence was detected at 7 of 12 sites, and an aggregated pattern was detected at 5 of 12 sites. The degree of aggregation was not high. A regular pattern of wilt severity was detected at all sites. The high disease incidence (77 to 98%) observed at 11 of the 12 sites could be explained by high inoculum density.     

土壤大丽轮枝菌微菌核的快速定量检测

 微菌核是大丽轮枝菌在土壤中的主要存活结构和黄萎病的初侵染来源。对土壤中大丽轮枝菌微菌核进行定量是黄萎病监测和预警的基础。本研究以大丽轮枝菌Internal Transcribed Spacer (ITS)区特异性引物对P1/P2扩增产物的重组质粒为标准品,构建SYBR Green I实时荧光定量PCR反应的标准曲线,结合土样水筛法建立了土壤大丽轮枝菌微菌核定量检测体系。同时,建立了土壤中微菌核数量与棉花黄萎病发病率的关系模型。结果表明,实时定量PCR检测灵敏度比常规PCR高10倍,检测下限为1个微菌核/克土,在5.54×10²～5.54×10⁷copies范围内,DNA拷贝数的对数值与Ct值具有良好的线性关系。建立的土壤中微菌核个数n与Ct值之间的关系为n=e^7.3-Ct/3.905。温室人工接种微菌核数量与棉花黄萎病发病率间的线性关系为y=2.710n+0.251。     

北疆棉花黄萎病发病率调查及土壤中黄萎病菌微菌核数量与种群类型分析

为了解北疆棉花黄萎病的发生情况及发病与土壤中黄萎病菌微菌核数量的关系和病原菌种群类型, 2021年对北疆石河子?奎屯?博乐等8市(县)棉田棉花黄萎病发病率?土壤中黄萎病菌的微菌核数量?菌株种群类型进行了抽样调查?结果表明, 北疆未发生棉花黄萎病的棉田占49.2%, 0%<发病率<5.0%的棉田占32.7%, 发病率≥5.0%的棉田占18%?与2013年?2015年相比, 2021年无病田率分别增加17.7和12.7百分点, 发病率≥5%的棉田分别减少15.7和21.6百分点?从棉田黄萎病发病率与土壤中微菌核数量的关系来看, 整体上北疆棉田棉花黄萎病发病率与微菌核数量相关性不显著(r=0.119 1); 分区域看, 石河子-沙湾片区?奎屯-乌苏片区?精河-博乐片区棉田黄萎病发病率与微菌核数量的相关系数分别为0.033 2?0.007 6?0.062 3, 均无显著相关性; 而呼图壁-玛纳斯片区棉田黄萎病发病率与微菌核数量的相关系数为0.635 7, 呈中度正相关?土壤中的黄萎病菌菌株以菌核型为主, 占57.9%, 菌丝型占23.2%, 中间型占18.9%?用特异性引物进行PCR检测表明, 土壤中黄萎病菌落叶型菌株占97.6%, 占绝对优势?本研究将为北疆棉区棉花黄萎病的综合防控提供理论依据?     

一种新的棉花黄萎病快速接种技术及其在病原菌致病力和寄主抗病性鉴定上的应用

 棉花品种抗黄萎病鉴定一般在田间病圃中进行,其结果受病圃中病原菌分布均匀程度、气象等因素影响极大,往往导致鉴定结果不准确。为了使鉴定方法简单、科学、可靠,我们在温室条件下比较了3种苗期接种棉花黄萎病的方法,即切根蘸孢子法(接种浓度为10⁶分生孢子/mL);菌培养物土壤接种法(0.5%、1%、2%,w/w);微菌核土壤接种法(10³个微菌核/g土)。结果表明,切根蘸孢子法导致棉苗发病均匀、严重、迅速,播种35~45 d后即可得到均匀一致的发病结果。而其它2种接种方法在播种75 d后才得到相对稳定的发病结果。同时,研究还表明接种浓度为10⁴分生孢子/mL所导致的黄萎病显著比10⁵或10⁶的轻。利用切根蘸孢子法在室内鉴定12个棉花品种或品系的抗黄萎病能力,证明该方法是抗黄萎病快速鉴定的有效方法。此外,该鉴定方法还可快速鉴定黄萎病菌不同菌株的致病性,并可应用于作物对其它土传病害的抗病性鉴定上。     

Long-term effect of soil solarization in controlling verticillium wilt of globe artichokes in Greece

Soil solarization, either singly or in combination with a reduced dosage (34 g/m²) of methyl bromide, was effective in controlling verticillium wilt of globe artichokes for three successive cropping seasons. Both treatments resulted in greatly reduced natural populations of Verticillium dahliae microsclerotia, which were positively correlated with significant reductions in diseased plants as well as with the early formation of primary artichoke heads and increased yield. Propagules of Talaromyces flavus increased and survived better in solarized than in solarized and fumigated soils and could be partially involved in the effectiveness of solarization. Propagules of Aspergillus terreus invariably increased in treated plots but their involvement in the longevity of the treatment is questionable.     

Development of an assay for rapid detection and quantification of Verticillium dahliae in soil

ABSTRACT Verticillium dahliae is responsible for Verticillium wilt on a wide range of hosts, including strawberry, on which low soil inoculum densities can cause significant crop loss. Determination of inoculum density is currently done by soil plating but this can take 6 to 8 weeks to complete and delay the grower's ability to make planting decisions. To provide a faster means for estimating pathogen populations in the soil, a multiplexed TaqMan real-time polymerase chain reaction (PCR) assay based on the ribosomal DNA (rDNA) intergenic spacer (IGS) was developed for V. dahliae. The assay was specific for V. dahliae and included an internal control for evaluation of inhibition due to the presence of PCR inhibitors in DNA extracted from soil samples. An excellent correlation was observed in regression analysis (R(2) = 0.96) between real-time PCR results and inoculum densities determined by soil plating in a range of field soils with pathogen densities as low as 1 to 2 microsclerotia/g of soil. Variation in copy number of the rDNA was also evaluated among isolates by SYBR Green real-time PCR amplification of the V. dahliae-specific amplicon compared with amplification of several single-copy genes and was estimated to range from ≈24 to 73 copies per haploid genome, which translated into possible differences in results among isolates of ≈1.8 cycle thresholds. Analysis of the variation in results of V. dahliae quantification among extractions of the same soil sample indicated that assaying four replicate DNA extractions for each field sample would provide accurate results. A TaqMan assay also was developed to help identify colonies of V. tricorpus on soil plates.     

Theoretical approach to the dynamics of the inoculum density of Verticillium dahliae in the soil: first test of a simple model

A mathematical equation was developed that describes the inoculum densities of microsclerotia of Verticillium dahliae in the soil over a long time span. The equation was based on measurable parameters and ecologically meaningful principles. In the model, the number of systemic infections of plant roots during crop growth was related to soil inoculum density. In turn, formation of microsclerotia in debris and reduction of the amount of crop growth were related to the number of systemic infections. Finally, a gradual release and mortality of microsclerotia in the soil were included to calculate subsequent inoculum densities in the soil.
Fitting the function to experimental data of potato cvs Element, Ostara, Mirka and Astarte, flax, pea, barley, sugar beet, onion and faba bean gave a very high correlation between observed and predicted soil inoculum densities. The clear differences in inoculum production among potato cultivars and other crops were expressed in quantitative terms. The highest inoculum density after incorporation of the debris of a susceptible crop was estimated to occur at 2.3 thermal time units of 3600 degree days (base 0°C). Ten per cent of the initial input of inoculum was still present after 4.5 thermal time units. The model was used to predict the dynamics of soil inoculum densities for V. dahliae under various cropping frequency schemes and performed satisfactorily.     

Disease Progress Based on Effects of Verticillium dahliae and Pratylenchus penetrans on Gas Exchange in Russet Burbank Potato

ABSTRACT The interactive effects of concomitant infection by the nematode Pratylenchus penetrans and the fungus Verticillium dahliae on symptom expression in Russet Burbank potato was studied in growth chamber experiments. Treatments were P. penetrans at three initial densities, V. dahliae at one inoculum density, the combination of the nematode at these three densities and the fungus, and a noninfested control. Gas exchange was measured nondestructively in leaf cohorts of different ages, one to three times weekly, with a LI-COR portable photosynthesis system. The single-pathogen treatments had no effect on assimilation or transpiration rates, but joint infection had a significant impact. In concomitant infection, photosynthesis was impaired more than transpiration, so estimates of leaf health were based on carbon assimilation rates only. Reductions in assimilation rate were apparent before the onset of visual symptoms. Assimilation rates decreased as much as 44% in the top, and newest, leaves of concomitantly infected plants, compared to rates in control plants. Even so, the health of newly produced leaves did not become progressively worse through time. With light use efficiency less than 0.20 mol of CO(2) fixed per mol of photosynthetically active radiation used as the criterion for disease incidence, disease progressed acropetally from the oldest to the youngest leaves. In plants infected with P. penetrans (0.8 nematodes per cm(3) of soil) in combination with V. dahliae, all leaves in cohorts 1 and 2 were symptomatic by 45 days after planting, and leaves in cohorts 3 to 6 became symptomatic at weekly intervals thereafter. For the control and single-pathogen treatments, the first time that light use efficiency fell below 0.20 in all leaves in cohort 1 was 71 days after planting. Concomitant infection reduced leaf life span by about 3 weeks. Both visual and physiological symptom expression were invariant to differences in initial nematode inoculum densities ranging from 0.8 to 2.5 nematodes per cm(3) of soil in one experiment and from 1.3 to 4.1 nematodes per cm(3) of soil in a second experiment.     

A real-time PCR assay for detection and quantification of Verticillium dahliae in spinach seed

Verticillium dahliae is a soilborne fungus that causes Verticillium wilt on multiple crops in central coastal California. Although spinach crops grown in this region for fresh and processing commercial production do not display Verticillium wilt symptoms, spinach seeds produced in the United States or Europe are commonly infected with V. dahliae. Planting of the infected seed increases the soil inoculum density and may introduce exotic strains that contribute to Verticillium wilt epidemics on lettuce and other crops grown in rotation with spinach. A sensitive, rapid, and reliable method for quantification of V. dahliae in spinach seed may help identify highly infected lots, curtail their planting, and minimize the spread of exotic strains via spinach seed. In this study, a quantitative real-time polymerase chain reaction (qPCR) assay was optimized and employed for detection and quantification of V. dahliae in spinach germplasm and 15 commercial spinach seed lots. The assay used a previously reported V. dahliae-specific primer pair (VertBt-F and VertBt-R) and an analytical mill for grinding tough spinach seed for DNA extraction. The assay enabled reliable quantification of V. dahliae in spinach seed, with a sensitivity limit of ≈1 infected seed per 100 (1.3% infection in a seed lot). The quantification was highly reproducible between replicate samples of a seed lot and in different real-time PCR instruments. When tested on commercial seed lots, a pathogen DNA content corresponding to a quantification cycle value of ≥31 corresponded with a percent seed infection of ≤1.3%. The assay is useful in qualitatively assessing seed lots for V. dahliae infection levels, and the results of the assay can be helpful to guide decisions on whether to apply seed treatments.     

Incidence of Verticillium wilt of artichoke in eastern Spain and role of inoculum sources on crop infection

Surveys of 94 artichoke fields throughout the artichoke production areas of Comunidad Valenciana (eastern Spain) were conducted from 1999 to 2002 to determine the incidence and distribution of Verticillium wilt.Verticillium dahliae was isolated from 80.9% of the sampled fields, and detected in all artichoke-growing areas, with a mean disease incidence of 53.8% infected plants. The disease was found to cause severe damage to cv. ‘Blanca de Tudela’, which is the most important artichoke cultivar grown in Spain, and was also observed on the seed-propagated cv. ‘Imperial Star’. In field trials to study the role of infected planting material and soil inoculum on infection of artichoke plants during the cropping season,V. dahliae was transmitted from infected stumps to the plants, confirming that the use of infected stumps could have greatly contributed to the dissemination of the pathogen. Inoculum density ofV. dahliae in soil had an effect on crop infection, in that a higher number of microsclerotia per gram of soil resulted in a higher percentage of infected plants. In addition, yield of cv. Blanca de Tudela was significantly affected byV. dahliae infection, showing that a higher percentage of infection corresponded with lower yield. http://www.phytoparasitica.org posting July 21, 2005.     

Verticillium longisporum and Fusarium solani: two new species in the complex of internal discoloration of horseradish roots

This study was conducted to determine the causal agent(s) of internal discoloration of horseradish roots. In 1999, 133 roots from 31 fields, and in 2000, 108 roots from nine fields, were assayed to determine the incidence and severity of internal discoloration of horseradish roots as well as the pathogen(s) associated with discoloured tissue. Verticillium dahliae , Verticillium longisporum and Fusarium solani were isolated from 14, 16 and 23% of roots in 1999, and from 24, 20 and 19% of roots in 2000, respectively. Verticillium longisporum on horseradish was identified for the first time. Pathogenicity tests of isolated microorganisms were conducted on horseradish in the glasshouse. In one experiment on the susceptible cultivar 1573, roots (sets) were inoculated by dipping the sets in a suspension of either V. dahliae microsclerotia, V. longisporum microsclerotia, or F. solani conidia and then grown in a soil mix over 5 months. Plants inoculated with any of the three species developed root discoloration similar to that observed in commercial fields. Internal root discoloration symptoms developed over a period of 5 months. For all three pathogens, severity of root discoloration was significantly higher after 5 months compared with 2 months from inoculation. In another experiment on cultivar 1590, tissue culture-generated seedlings and sets were planted in an infested soil mix with V. dahliae or V. longisporum and grown in the glasshouse. Plants developed root discoloration, as observed in the field. The pathogens were reisolated from inoculated plants in both experiments. No pathogen was isolated from the control plants in the experiments. The results of this study suggest that internal discoloration of horseradish roots is a disease complex caused by at least three fungal species.     

大丽轮枝菌在茄田土壤中的分布及抽样方法

对茄子黄萎病在田间的空间分布及取样方法进行了研究,并探讨了田间土壤带菌量的准确测定方法。研究结果表明:用分布型指数法测定茄田土壤中微菌核的空间分布型,当菌核数量在8-63 ～39-93 个/g 干土范围内,为聚集分布。当茄田发病率小于75 % 时,病株在田间均属聚集分布;发病率大于75 % 时呈均匀分布。土壤中微菌核的调查方法以棋盘式为最适宜。     

大丽轮枝菌在茄田土壤中的分布及抽样方法

对茄子黄萎病在田间的空间分布及取样方法进行了研究,并探讨了田间土壤带菌量的准确测定方法。研究结果表明:用分布型指数法测定茄田土壤中微菌核的空间分布型,当菌核数量在8-63 ～39-93 个/g 干土范围内,为聚集分布。当茄田发病率小于75 % 时,病株在田间均属聚集分布;发病率大于75 % 时呈均匀分布。土壤中微菌核的调查方法以棋盘式为最适宜。     

The relationship between soil inoculum density and plant infection as a basis for a quantitative bioassay of Verticillium dahliae

Using potato, eggplant and thorn apple as test plants, the relationship between soil inoculum density and plant infection was studied as a basis for the development of a quantitative bioassay of Verticillium dahliae. A linear relationship was demonstrated (P < 0.05) between soil inoculum density and population density on roots for all three test plants and for soil inoculum density and population density in sap extracted from stems for eggplant. Correlation coefficients were higher with densities on or in roots (R² varying from 0.45 to 0.99) than with densities in stems (R² varying from 0.04 to 0.26). With eggplant, population densities on/in root and in sap extracted from stems were significantly correlated at 20 and 25°C with Pearson's correlation coefficients of 0.41 and 0.53, respectively. For potato, root colonization was higher at 15 than at 20°C, whereas the reverse applied to eggplant. Stems of potato were less colonized than stems of eggplant. The pathozone sensu Gilligan (1985) was calculated to be <300 µm, indicating that infection was caused by microsclerotia which were located close to the roots. To assess the density of V. dahliae in plant tissue pipetting infested plant sap on solidified ethanol agar medium without salts yielded higher densities than using pectate medium or mixing sap with molten agar. A bioassay for determining effects of (a)biotic factors on development of V. dahliae in the plant is recommended with eggplants as a test plant, grown in soil infested with 300 single, viable microsclerotia g^-1 soil at a matric potential of –6.2 kPa, and incubated at 20°C for 8 weeks.     

Distribution of Verticillium dahliae through watering systems in widely irrigated olive growing areas in Andalucia (southern Spain)

Verticillium dahliae Kleb. causes Verticillium wilts in many herbaceous and woody species. Many hosts of the pathogen are commonly cultivated in Andalucía (southern Spain), particularly major crops such as cotton, vegetables, almond, peach and, particularly, olive, in which the fungus causes Verticillium wilt of olive. Infective structures of the pathogen (microsclerotia), produced in the late phases of the infection cycle in senescent tissues of the infected plants, can be spread over short or long distances by a number of dispersal methods. Irrigation water is one of the factors implicated in this spread of V. dahliae. Indeed, increasing irrigation dosages in crops or an inadequate irrigation schedule have been identified as cultural practices favouring Verticillium wilt onset and severity in olive and other hosts. Most of the cultivated areas in the Guadalquivir Valley of Andalucía are irrigated by pumping stations using modern infrastructures that supply water to thousands of hectares of farm land, which are usually associated with irrigation communities. This study demonstrates that the pathogen survives in the sediment and particles suspended in water used for irrigation in different facilities of an irrigation community, that were involved in distributing water (main canal and reception tank of a investigated pumping station, irrigation pools and sand from filters). Thus microsclerotia moves from the pumping station to individual plots (olive and cotton cultivated farm) as viable microsclerotia, free or embedded in soil particles and plant debris, suspended in the irrigation water, or deposited in the sludge in piping systems or water storage ponds. We have detected amounts of inoculum in the solid pellet samples in these facilities that ranged from 2.7 to 6.7 microsclerotia per gram. Besides this, water from drippers in cultivated plots released into the soil a variable amount of infective propagules of the pathogen over time that accounted for 3.75 microsclerotia/m³ in some of the recording times. Therefore, irrigation water becomes an important source of inoculum that is very effectively involved in medium and long-distance spread of the pathogen.     

Response of Six Potato Cultivars to Amount of Applied Water and Verticillium dahliae

ABSTRACT Six potato cultivars were grown with or without the addition of Verticillium dahliae inoculum and were watered at 50, 75, or 100% estimated consumptive use. The applied water x cultivar interaction was significant (P = 0.009 and P = 0.001 for 1996 and 1997, respectively) for the relative area under the senescence progress curve (RAUSPC). With a decrease in water, there was an increase in RAUSPC. A significant interaction of inoculum density x cultivar also was found, based on RAUSPC (P = 0.0194 and P = 0.0033 for 1996 and 1997, respectively). In V. dahliae-infested plots, 'Katahdin' and 'Ranger Russet' were resistant to Verticillium wilt. Population size of V. dahliae in stem apices was significantly lower in 'Katahdin' in both 1996 and 1997 (P = 0.0001) and in 'Ranger Russet' in 1997 (P = 0.0001) than in the other cultivars. 'Russet Burbank' and 'Shepody' had large apical stem populations of V. dahliae and higher RAUSPC values associated with both V. dahliae inoculum and decreased amount of applied water. Marketable tuber yield was unaffected by V. dahliae in both years. Cultivar resistance to Verticillium wilt was related to cultivar tolerance to moisture deficit stress. Results suggest that moisture deficit stress response has the potential to be a useful tool in protocols for screening potato for Verticillium resistance.     

Effect of method of inoculation, inoculum density and seedling age at inoculation on the expression of resistance of cocoa (Theobroma cacao L.) to Verticillium dahliae Kleb.

Soil drench and stem puncture inoculation were compared as methods for selecting cocoa cultivars with resistance to Verticillium dahliae. Disease progress was more rapid and induced symptoms were more severe following stem puncture and, under glasshouse conditions, differences between cultivars were detected 15 days after inoculation. Moreover, using stem puncture, inoculum densities of 10⁴ conidia/ml were sufficient to differentiate resistant and susceptible cultivars, whereas with the soil drench method, inoculum densities of 10⁷ conidia/ml were necessary. Although a substantially higher proportion of plants were affected by stem puncture inoculation, the resistance of cultivar Pound-7 remained effective at high inoculum densities of 10⁸ conidia/ml. With either method, older seedlings were more susceptible to V. dahliae than younger ones. However, with stem puncture, 15-day-old seedlings were sufficiently susceptible for a valid disease assessment. In contrast, with soil inoculation, 60-day-old plants were required. In a nursery trial with 15-day-old seedlings, seven cocoa genotypes previously selected as resistant, moderately resistant or susceptible to Verticillium dahliae , on the basis of root inoculation, were ranked in the same order when stem punctured. Stem puncture inoculation of young seedlings is cost-effective in terms of time and space, and is therefore recommended for screening of cocoa for wilt resistance, especially in large-scale breeding programmes.