Progress in grafting watermelon to manage Verticillium wilt

Vegetable grafting for disease management was first used successfully when watermelon grafted onto a Cucurbita moschata rootstock overcame Fusarium wilt. Interspecific grafting has since been used effectively to mitigate several soilborne pathogens in a variety of solanaceous and cucurbitaceous cropping systems. Verticillium wilt caused by Verticillium dahliae is a significant disease in watermelon crops and is difficult to manage. Current management practices, including crop rotation, soil fumigation, and host resistance, are insufficient due to the ability of microsclerotia to persist in absence of a host, lack of efficacy of soil fumigants, and limited availability of resistant cultivars. Watermelon grafted onto commercial cucurbit rootstocks have increased tolerance to Verticillium wilt, although no cucurbit rootstocks are known to be completely resistant. Verticillium wilt incidence decreased on grafted plants grown in artificially and naturally infested soils, while scion health and growth as well as rootstock root mass and vigour increased. Commonly used rootstocks are Lagenaria siceraria, C. moschata, and C. maxima × C. moschata; of these, only C. maxima × C. moschata ‘Tetsukabuto’ reduced severity of Verticillium wilt across several scion cultivars, locations, years, and soil densities of V. dahliae. Although studies on Verticillium wilt resistance of grafted watermelon are few, their combined results suggest the threshold of V. dahliae soil density for watermelon may be around 5–12 cfu/g. This review summarizes available information on Verticillium wilt of watermelon and effects of different rootstock × scion combinations, assisting growers and breeding programmes in decisions to adopt watermelon grafting for management of Verticillium wilt.     

Highly infested soils undermine the use of resistant olive rootstocks as a control method of verticillium wilt

Verticillium wilt of olive (VWO) is probably the most devastating fungal disease for olive trees worldwide, and currently the main cultivars are susceptible or moderately susceptible to this disease. The evaluation of resistant cultivars as rootstocks to control the disease has scarcely been explored, and mainly in short-term studies under controlled conditions, which usually do not correspond with field evaluations. The main objective of this study was to assess the responses to VWO of different scion × rootstock combinations of the olive cultivars Picual, Arbequina, Changlot Real, and Frantoio in a long-term field experiment with a soil highly infested with the defoliating pathotype of Verticillium dahliae. The results showed that grafting the susceptible cultivar Picual onto resistant rootstocks delayed the onset of the disease symptoms; however, after 4 years, it was observed that all combinations that contain Picual (a) were extensively colonized by V. dahliae; (b) developed severe symptoms of the disease; and (c) had plant mortality similar to Picual growing on its own roots. This result highlights the importance of long-term field experiments to evaluate VWO and shows that grafting susceptible olive cultivars onto resistant ones does not provide a durable control of VWO under high inoculum potential, as V. dahliae is able to progress through the resistant rootstock and then extensively colonize and kill the susceptible scion. However, the high inoculum potential observed in this study does not allow us to consider the evaluated resistant cultivars as completely ineffective under lower inoculum densities.     

Prospects and strategies in controlling verticillium wilt of olive1

Control of verticillium wilt of olive currently depends on preventive measures. Since systemic fungicides are unable to prevent or control the disease, its control should primarily be based on cultural methods, including irrigation systems which restrict dissemination of Verticillium dahliae propagules by irrigation water and avoidance of intercropping with hosts susceptible to V. dahliae. Since leaves from affected olive trees contribute, through formation of microsclerotia, to the inoculum in the soil, pruning should be practised prior to branch defoliation. As for resistant olive rootstocks or cultivars, promising verticillium-wilt resistance has been found in two rootstocks selected in California (US). However, these have to be tested under local conditions before they can be released to Mediterranean growers, while further search for other resistant rootstocks is needed. Soil solarization applied to individual diseased trees in established olive groves could substantially contribute to recovery or long-lasting symptom remission in the treated trees. This effect is mainly attributed to the decrease or eradication of V. dahliae microsclerotia in the treated soil but also to heat-tolerant fungal antagonists of the pathogen. Using herbicides to control weeds, and limiting soil rotovation, can restrict symptom development. Biological control can also be considered as a promising trend in controlling the disease by searching, testing and exploiting potential fungal or bacterial antagonists.     

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.     

Sources of Verticillium dahliae Affecting Lettuce

ABSTRACT Since 1995, lettuce in coastal California, where more than half of the crop in North America is grown, has consistently suffered from severe outbreaks of Verticillium wilt. The disease is confined to this region, although the pathogen (Verticillium dahliae) and the host are present in other crop production regions in California. Migration of the pathogen with infested spinach seed was previously documented, but the geographic sources of the pathogen, as well as the impact of lettuce seed sparsely infested with V. dahliae produced outside coastal California on the pathogen population in coastal California remain unclear. Population analyses of V. dahliae were completed using 16 microsatellite markers on isolates from lettuce plants in coastal California, infested lettuce seed produced in the neighboring Santa Clara Valley of California, and spinach seed produced in four major spinach seed production regions: Chile, Denmark, the Netherlands, and the United States (Washington State). California produces 80% of spinach in the United States and all seed planted with the majority infested by V. dahliae comes from the above four sources. Three globally distributed genetic populations were identified, indicating sustained migration among these distinct geographic regions with multiple spinach crops produced each year and repeated every year in coastal California. The population structure of V. dahliae from coastal California lettuce plants was heavily influenced by migration from spinach seed imported from Denmark and Washington. Conversely, the sparsely infested lettuce seed had limited or no contribution to the Verticillium wilt epidemic in coastal California. The global trade in plant and seed material is likely contributing to sustained shifts in the population structure of V. dahliae, affecting the equilibrium of native populations, and likely affecting disease epidemiology.     

Establishment of apricot and almond trees using soil mulching with transparent (solarization) and black polyethylene film: effects on Verticillium wilt and tree health

Apricot ( Prunus armeniaca ) and almond ( P. dulcis ) trees at the first leaf stage were planted in soil infested with Verticillium dahliae and mulched with transparent or black polyethylene film, or not mulched, in the San Joaquin Valley of California, March-August 1990. During the 19-week mulching treatment, summer soil temperatures reached as high as 46, 41, and 33°C at 18 cm depth; and 41, 37, and 32°C at 30 cm depth under clear film, black film, and no film, respectively. Trees mulched from the time of planting with transparent polyethylene (solarization) did not survive or grow as well as those mulched with black film or not mulched. Incidence of foliar symptoms due to Verticillium wilt was reduced by 86–100% in both apricot and almond trees by black, as well as transparent film mulch the following season. Incidence of vascular discoloration symptoms of trunks and primary scaffolds due to Verticillium wilt was similarly reduced by both mulches. Mulching with black polyethylene film gave better overall results than solarization with transparent film. The intermediate soil temperatures produced did not chronically harm trees, as judged by tree survival and annual growth of trunk diameter, yet the prolonged period of soil heating provided control of Verticillium wilt equivalent to that of solarization with transparent polyethylene. These studies provided further evidence that in-season mulching can be used to conserve water during establishment of new orchards or replant trees in warm, arid climates.     

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。     

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.     

Sources of verticillium wilt resistance in wild olive germplasm from the Mediterranean region

Fifty-seven wild olive accessions collected from the Mediterranean basin were screened under greenhouse conditions for their resistance to verticillium wilt. Plants were root-dip inoculated. One defoliating and one non-defoliating isolate of Verticillium dahliae , both obtained from diseased plants in southern Italy, were used. Plants of the highly susceptible cv. Cima di Mola, frequently used as a rootstock in Apulia, were also included in this test. Disease reactions were evaluated on the basis of external symptoms, vascular browning and by calculating areas under disease progress curves ( audpc ). On the basis of audpc values and external symptom severity, accessions were grouped into four phenotypic groups: highly resistant, moderately resistant, susceptible and highly susceptible. Three accessions showed high resistance to both V. dahliae pathotypes. Forty resistant plants were selected from accessions that had shown the highest levels of resistance. Clones were obtained from each of these plants by in vitro micropropagation for further testing. Ten clones showed the resistance characteristics of their original mother plants, while others suffered greater levels of disease severity than their mother plants. Several new olive rootstocks were identified that were highly resistant to verticillium wilt and could be included in breeding programmes for resistance of olive to V. dahliae .     

Epidemiology of Verticillium dahliae on olive (cv. Picual) and its effect on yield under saline conditions

The epidemiology of Verticillium dahliae and its effect on yield was studied for 3 years in three plots of olive cv. Picual, planted in soil previously cropped with highly susceptible V. dahliae host plants and irrigated with saline water. Disease incidence increased 2·2-, 2·6- and 1·5-fold in plots 3A, 9A and 9C, respectively, within 39, 25 and 15 months of the first record taken. The highest severities were recorded in spring 2001 (4·0, 5·3 and 5·4 on a scale of 2–10, respectively). Disease incidence and severity increased during winter–spring and decreased during summer. Seasonal changes were also observed in the isolation of V. dahliae ; the highest isolation rates in diseased trees were in winter (34%) and spring (45%), and the lowest were in autumn and summer (19–20%). Verticillium dahliae was isolated on an average of 27, 28 and 19% from the bottom, middle and top of the tree canopy, respectively, and was isolated from trees with and without symptoms. The fruit yield from diseased trees was reduced by an average of 75% in comparison with symptomless trees in plot 3A each year. A similar yield reduction (89%) was recorded in plot 9A in 2000. The severe expression of the disease and its effect on yield in the present study could be due to the fact that the orchard was planted in infested soil, and that saline irrigation probably exacerbates the problem.     

Spatiotemporal analysis of spread of infections by Verticillium dahliae pathotypes within a high tree density olive orchard in southern Spain

The development of Verticillium wilt epidemics in olive cv. Arbequina was studied from November 1999 to May 2003 in a drip-irrigated, nontillage orchard established in a soil without a history of the disease at Córdoba, southern Spain. Disease incidence measured at 1-month-intervals increased from 0.2 to 7.8% during this period. Verticillium dahliae infecting the trees was characterized as defoliating (D) or nondefoliating (ND) pathotypes by a specific, multiplex-polymerase chain reaction (PCR) assay. Of the symptomatic trees, 87.2 and 12.8% were infected by the D or ND pathotypes, respectively. Dynamics of disease incidence were described by a generalized logistic model with a multiple sigmoid pattern. In the fitted model, the infection rate was highest in the winter to spring period and decreased to minimum values in the summer to fall period. Binary data of disease incidence was analyzed for point pattern and spatial correlation, either directly or after parsing them in contiguous quadrats. Overall, ordinary runs analysis indicated a departure from randomness of disease within rows. The binomial index of dispersion, interclass correlation, and Taylor's power law for various quadrat sizes suggested aggregation of diseased trees within the quadrat sizes tested. Spatial analysis by distance indices showed a nonrandom arrangement of quadrats containing infected trees. Spatial pattern was characterized by the occurrence of several clusters of infected trees. Increasing clustering over time was generally suggested by stronger values of clustering index over time and by the increase in the size of patch clusters. Significant spatial association was found in the clustering of diseased trees over time across cropping seasons; however, clustering was significant only for infections by D V. dahliae, indicating that infections by the D pathotype were aggregated around initial infections. The number and size of clusters of D V. dahliae-infected trees increased over time. Microsatellite-primed PCR assays of a representative number of V. dahliae isolates from diseased trees indicated that the majority of infecting D isolates shared the fingerprinting profile with D V. dahliae isolated from soil of a naturally infested cotton field in close proximity to the orchard, suggesting that short distance dispersal of the pathogen from this soil to the olive orchard may have occurred.     

Quantitative nested real-time PCR detection of <Emphasis Type="Italic">Verticillium longisporum</Emphasis> and <Emphasis Type="Italic">V. dahliae</Emphasis> in the soil of cabbage fields

Verticillium longisporum and V. dahliae, causal agents of Verticillium wilt, are spreading through the cabbage fields of Gunma Prefecture. Using the V. longisporum-specific intron within the 18S rDNA and differences between ITS 5.8S rDNA sequences in Japanese isolates of V. longisporum and V. dahliae, we developed three quantitative nested real-time (QNRT) PCR assays. The QNRT-PCR quantification of V. longisporum or V. dahliae in cabbage field soil was consistent with the severity of Verticillium wilt disease in those fields. In field trials of resistant cultivar YR Ranpo grown for three seasons in soil infested with the pathogen, disease severity and pathogen density in the soil were significantly reduced in a field moderately contaminated by V. dahliae, but only slightly reduced in a highly contaminated field. These results suggest that continuous cultivation of a resistant cultivar is an effective way to reduce the pathogen population. QNRT-PCR assays provide a powerful analytical tool to evaluate the soil population dynamics of V. longisporum and V. dahliae for disease management.     

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.     

Effects of crop rotation and removal of crop debris on the soil population of two isolates of Verticillium dahliae

Microsclerotia of Verticillium dahliae are produced in large numbers on senescing parts of host plants and remain viable in the soil for many years. Changes in the population density, i.e. density of microsclerotia, in the soil were measured in micro-plots using two isolates of V. dahtiae , specific to either field bean or potato, several crop sequences comprising potato, field beans and barley, and either the removal of aerial debris of the crops or incorporation into soil.
Potato was more susceptible to the potato isolate and field bean more susceptible to the field bean isolate. Removal of debris of potato and field bean reduced numbers of microsclerotia in the soil in the subsequent years, but removal of barley straw had no effect. Initially non-infested control micro-plots became infested, probably by the growth of potato roots into the naturally infested subsoil. The rate of increase of the microsclerotial population in the non-infested control micro-plots was larger than in the initially infested treatments, because more colonized debris was produced. It is concluded that removal of aerial debris of host crops is important to reduce the soil population of V. dahtiae.     

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。     

检疫性轮枝菌及其近似种的鉴定

 大丽轮枝菌(Verticillium dahliae)和黑白轮枝菌（V. albo-atrum）在世界范围内引起多种作物的黄萎病,属于我国重要进境植物检疫对象。本研究对采自我国部分地区和CBS保存的多种植物病原性轮枝菌,包括黑白轮枝菌、大丽轮枝菌及其变种大丽轮枝菌长孢变种(V. dahliae var. longisporum)、三体轮枝菌(V. tricorpus)、变黑轮枝菌(V. nigrescens)和云状轮枝菌(V. nubilum),采用生物学特性观察,结合rDNA-ITS序列分析的方法,进行了比较和分析。结果表明：不同种类轮枝菌在休眠结构形态上具有一定差异,部分菌株不产生任何休眠结构。各供试菌株在15～25℃范围内均可生长,但黑白轮枝菌在30℃下生长受到强烈抑制,而其他菌株受影响较小。对供试菌株rDNA-ITS序列分析结果表明植物病原性轮枝菌可聚为9个分支,包括三体轮枝菌、变黑轮枝菌、云状轮枝菌、V. theobromae、大丽轮枝菌、大丽轮枝菌长孢变种和3个不同的黑白轮枝菌分支,黑白轮枝菌、大丽轮枝菌及其长孢变种亲缘关系较近。采用生物学性状结合rDNA-ITS序列分析能够更加有效地将两种检疫性轮枝菌从其他植物病原性轮枝菌中区分出来。     

Phylogenetic Analyses of Phytopathogenic Isolates of Verticillium spp

ABSTRACT To better understand the genetic relationships between Verticillium dahliae isolates from lettuce and other phytopathogenic Verticillium spp. isolates from various hosts and geographic locations, the complete intergenic spacer (IGS) region of the nuclear ribosomal RNA gene (rDNA) and the beta-tubulin gene were amplified and sequenced. The sequences of the complete IGS region and the beta-tubulin gene were used alone and in combination to infer genetic relationships among different isolates of Verticillium with the maximum-likelihood distance method. Phylogenetic analyses set sequences into four distinct groups comprising isolates of V. albo-atrum, V. tricorpus, and V. dahliae from cruciferous and noncruciferous hosts. Within the four Verticillium groups, isolates of V. dahliae from cruciferous hosts displayed the closest affinity to V. dahliae from noncruciferous hosts. Isolates of V. dahliae from noncruciferous hosts could be further divided into four subgroups based on sequence similarities within the IGS region. Cross-pathogenicity tests demonstrated that most Verticillium isolates were as virulent on other hosts as on their hosts of origin. A phenogram based on the cross pathogenicity of individual isolates resembled those derived from the IGS and beta-tubulin sequence comparisons. On the basis of the data presented, the potential origin of some isolates of V. dahliae pathogenic on lettuce is proposed.     

Search for resistance to phytophthora root rot in West Indian avocado rootstocks in the Canary Islands1

Avocado root rot caused by Phytophthora cinnamoni is a major limitation for avocado expansion in the Canary Islands, as it is in other producing areas of the world. To date, no germplasm with resistance potential has been found among West Indian avocados, which would convert this type into an ideal rootstock for use in infested areas such as the Canary Islands. Our aim was a thorough screening of the local West Indian population, descendants of seedlings brought to the Canaries from the Caribbean in the early 1800s. Our selection methods were based on those used by Zentmyer in California (US) since 1960 (nutritivesolution tank, with controlled aeration, pH, and temperature, to which inoculum is added), which gave rise at that time to several resistant/tolerant Guatemalan and Mexican rootstocks. We also screened using pots filled with naturally infested soil. The plant material tested was: (a) West Indian seeds from the original, local population; (b) seeds of the Guatemalan x West Indian hybrid Gema, taken from 20-year-old local trees growing in highly infected groves; (c) seeds of the salt-resistant Israeli West Indian cv. Maoz. Also tested for comparison purposes were seeds of Mexican and Guatemalan avocados. Inoculations were done using the P. cinnamomi A2 strain, taken from proved diseased trees found in the Canaries. At present, one West Indian plant has survived the selection in a first trial, while 38 have survived in a second trial. One Mexican plant (cv. Topa-Topa) has survived in a third trial. Resistant material is now being reproduced both by tissue culture and by the etiolation technique.     

Variation in virulence of Greek isolates ofPhytophthora citrophthora as measured by their ability to cause crown rot on three peach rootstocks

The virulence ofPhytophthora citrophthora isolated from various host-plants on three peach rootstocks (GF677, PR204, KID I) was examined. There was no significant difference among the rootstocks with respect to their susceptibility to testedP. citrophthora isolates. The most virulent isolate originated from sycamore (Acer pseudoplatanus); isolates from pistachio trees (Pistacia vera) also showed high virulence but were significantly less virulent than the sycamore isolate. Isolates originating from plum (Prunus domestica), almond (Prunus amygdalus) and lemon (Citrus limon) trees were moderately virulent on peach rootstocks; those from cyclamen (Cyclamen persicum) showed the lowest virulence of those tested. There was thus great variation in virulence among the testedP. citrophthora isolates. It is possible that the isolates ofP. citrophthora from sycamore, pistachio, plum, almond and lemon trees are a threat to peach trees, whereas the low virulence of the isolates from cyclamen hosts suggests that these pathogens are not a serious threat to peach trees. http://www.phytoparasitica.org posting Jan. 3, 2002.