Verticillium Wilt Symptoms Research Articles

A Soilless Verticillium Wilt Assay Using an Early Flowering Lettuce Line.

A soilless growth chamber assay was evaluated for rapid assessment of Verticillium wilt on lettuce (Lactuca sativa). Seedlings of the early flowering Plant Introduction (PI) 251246 were inoculated in tubes with conidial suspensions of isolates of Verticillium dahliae from lettuce or cauliflower. PI 251246 developed significant leaf and root symptoms by 21 days following inoculation with isolates of race 1 and race 2 from lettuce, but not following inoculation with an isolate from cauliflower. In contrast, leaf symptoms on the cultivar La Brillante and root symptoms on the cultivar Salinas were not easily differentiated from the symptoms observed on the noninoculated control treatments in the soilless assay, even at the highest inoculum concentration of 1 × 107 conidia/ml. Comparison of the soilless growth chamber assay and a soil-based greenhouse assay revealed a significantly higher proportion of PI 251246 with root vascular discoloration in the soilless assay compared with the soil-based greenhouse assay (χ2, 1 df = 8.9; P = 0.003). There is thus an advantage to using the soilless growth chamber assay with PI 251246. Compared with the soil-based greenhouse assay, the soilless assay reduced the time required for evaluation of Verticillium wilt symptoms on lettuce from about 90 days to 42 days.

Mode of action of a non-pathogenic Fusarium oxysporum strain against Verticillium dahliae using Real Time QPCR analysis and biomarker transformation

Verticillium wilt is a devastating disease of a wide range of herbaceous and woody plant hosts. It is incited by the soilborne fungus Verticillium dahliae. Management strategies are mainly focused on preventive measures. In a previous study, the efficacy of a non-pathogenic Fusarium oxysporum strain, designated as F2, isolated from a suppressive compost amendment, has been reported to reduce Verticillium wilt symptom development in eggplants under greenhouse and field conditions; in addition, antibiosis or parasitism were ruled out by using a dual culture test. In the present study, we investigated the mode of action of F2 against V. dahliae. For this purpose, the F2 and V. dahliae strains were transformed with the EGFP and DsRed2 reporter genes respectively, so as to visualize their presence on the root surface of eggplants. In addition, the ramification of both fungi into the plant vascular system was monitored by Real Time QPCR analysis. It was shown that F2 colonizes the root surface along the intercellular junctions excluding V. dahliae from the same ecological niche. In parallel, QPCR analysis showed that application of F2 reduces the levels of V. dahliae vascular colonization along with the disease severity. In a split root experiment it was demonstrated that F2 does not trigger the defense mechanisms of eggplants against V. dahliae. Therefore, it seems that competition for space or nutrients on the root surface are the main mechanism of action of F2 against V. dahliae.

The potential for early generation selection to identify potato clones with resistance to Verticillium wilt

Verticillium wilt (VW) of potato, caused primarily by the fungus Verticillium dahliae, results in yield loss and is therefore an important soil-borne disease. Resistance to VW exists in potato germplasm and is used by breeders during cultivar development. Breeders could make more rapid progress toward the development of VW resistant clones if they had an effective early generation selection strategy. The purpose of this study was to determine whether selection for VW resistance could be carried out in the first tuber generation on single hills. One hundred and fifty-two clones from 19 families were planted as single hills on a V. dahliae-infested field. Each plant was scored for vine maturity, VW symptom expression, yield, stem colonization (colony forming units (cfu), in dried basal stem segments) and incidence (percent infected stems). In the second clonal generation, which consisted of replicated four-hill plots, stem colonization scores and incidence values were used to identify clones which were more resistant than a moderately resistant cultivar and others which were more susceptible than a susceptible cultivar. The efficiency and reliability of the single-hill selection strategy, based on symptoms and yield, was then determined by comparison to the four-hill results. We determined that the best single-hill selection strategy was negative selection (discard clones with the lowest performance) with low stringency, based on yield.

Characterization of Verticillium dahliae and V. tricorpus Isolates from Lettuce and Artichoke.

Verticillium isolates collected from lettuce and artichoke were characterized for morphology, growth and pathogenicity. Several isolates were identified as Verticillium tricorpus based on morphological and cultural characteristics, including the production of dark resting mycelia, chlamydospores, microsclerotia, and yellow to orange pigmentation in culture. Compared with isolates of V. dahliae, these isolates also produced microsclerotia and conidia that were significantly larger and exhibited a distinct growth pattern at varying temperatures. Using database sequence information, primers were developed from the internal transcribed spacer region to produce a diagnostic 337-bp product specific to V. tricorpus and used to confirm the identification of isolates. Pathogenicity tests indicated that isolates of V. tricorpus were weak pathogens, causing a median disease severity (DS) of <1 (0-to-5 scale) on lettuce and artichoke. In contrast, isolates of V. dahliae consistently caused severe wilt with a median DS of >3.5 on lettuce and 5.0 on artichoke. Although lettuce and artichoke inoculated with isolates of V. tricorpus exhibited reduced height and fresh foliar and root weight, the reductions were not statistically significant, unlike in plants inoculated with isolates of V. dahliae. Lettuce co-inoculated with isolates of V. tricorpus and V. dahliae exhibited reduced symptoms of Verticillium wilt and improved growth relative to those inoculated with V. dahliae alone. The early introduction of V. tricorpus in soil-drench inoculations appeared to provide better relief from subsequent V. dahliae inoculation than when the two species were co-inoculated simultaneously using the root-dip method, suggesting competitive exclusion as a plausible mechanism. A spore-polymerase chain reaction assay developed using cultured spores directly as template and primers specific to V. tricorpus confirmed the presence of V. tricorpus on inoculated roots. This work demonstrates the potential use of V. tricorpus to directly reduce the effect of V. dahliae on lettuce and artichoke and, to our knowledge, is the first reported characterization of V. tricorpus isolates collected from lettuce and artichoke.

Alternaria alternata and Plectosporium tabacinum on Snap Beans: Pathogenicity, Cultivar Reaction, and Fungicide Efficacy

Verticillium wilt of sugar beet has not previously been reported in the Red River Valley of Minnesota and North Dakota where both Verticillium dahliae Kleb and V. albo-atrum Reinke &amp; Berthier cause wilt on potato. In August 2003, sugar beet in four fields from eastern North Dakota (sown to potato in 2002) exhibited typical symptoms of Verticillium wilt. Our objectives were to: (i) identify isolates from infected sugar beet and (ii) evaluate them for pathogenicity on sugar beet and potato. Accepted for publication 22 September 2008. Published 12 December 2008.

Development of a Real-Time Polymerase Chain Reaction Assay for Quantifying Verticillium albo-atrum DNA in Resistant and Susceptible Alfalfa

ABSTRACT A precise real-time polymerase chain reaction (PCR) assay was developed for quantifying Verticillium albo-atrum DNA. The assay was used in a repeated experiment to examine the relationship between the quantity of pathogen DNA detected in infected leaves and shoots and the severity of Verticillium wilt symptoms in several alfalfa cultivars expressing a range of disease symptoms. Plants were visually inspected for symptoms and rated using a disease severity index ranging from 1 to 5, and the quantity of pathogen DNA present in leaves and stems was determined with real-time PCR. No significant differences in pathogen DNA quantity or disease severity index were observed for experiments or for cultivar-experiment interactions. Significant differences were observed between cultivars for the quantity of pathogen DNA detected with real-time PCR and also for disease severity index ratings. In both experiments, the highly resistant check cultivar Oneida VR had significantly less pathogen DNA, and significantly lower disease severity index ratings than the resistant cultivar Samauri, the moderately resistant cultivar Vernema, and the susceptible check cultivar Saranac. In both experiments, the Spearman rank correlation between the amount of V. albo-atrum DNA detected in leaves and stems with real-time PCR and disease severity index ratings based on visual examination of symptoms was positive (>0.52) and significant (P < 0.0001). These results suggest that resistance to Verticillium wilt in alfalfa is characterized by a reduced colonization of resistant genotypes by the fungus.

Investigation of the Effect of Heating, Vesicular Arbuscular Mycorrhiza and Thermophilic Fungus on Cotton Wilt Disease

In this study, combinations of spores of a thermophilic fungus (Talaromyces flavus), Vesicular Arbuscular Mycorrhiza (VAM) and microsclerotia of Verticillium dahliae under various temperature treatment (31-38 degrees C) in triplicate trial, were investigated and results were compared with those of non-treated controls. Five cotton seeds were planted in each pot containing 3 kg of pasteurized soil. In each pot, combinations of 4x10(3) VAM spores, 2.5x10(9) spores of thermophilic fungus (T. flavus) and 5x10(5) microsclerotia of V. dahliae were added. Symptoms of Verticillium wilt were observed after 45 days. Index of disease severity was measured. Results indicated that pre-heating of microsclerotia at 31 and 35 degrees C for 10 and 14 h, respectively, caused a 15% reduction in leaf infection index. Presence of VAM and thermophilic fungus (T. flavus) spores caused 23 and 50% reductions in the disease development, respectively. Concurrent presence of mycorrhiza and thermophilic fungus spores caused a 10-20% reduction in disease development. These findings provide a promising approach to the control of Verticillium wilt of cotton. However, heat treatment of soil may prove difficult. Further studies in this regard are required and useful agricultural practices such as seasonal heating may be applied in the cotton fields.

Response of strawberry genotypes to inoculation with isolates of Verticillium dahliae differing in host origin

Eight strawberry genotypes known to differ in susceptibility to verticillium wilt were inoculated with eight isolates of Verticillium dahliae originally obtained from six different host crops: strawberry, potato, watermelon, mint, eggplant (aubergine) and cauliflower. Inoculation experiments were conducted in replicated field trials during two successive years. Known susceptible genotypes developed typical symptoms of verticillium wilt in both years. Although isolates manifested a wide range of aggressiveness, differences were significant only on the most susceptible strawberry genotype. Two isolates originating from strawberry were among the most aggressive of the eight tested, whereas the least aggressive isolate was obtained from cauliflower. Six strawberry genotypes that were regarded as resistant to verticillium wilt based on previous tests were also resistant in the present study, regardless of the isolate used. Overall, strawberry genotypes represented the largest source of variation in these experiments, with variance components approximately 10‐fold greater than those associated with either isolate or the isolate × genotype interaction. The results suggest it should be possible to develop resistance to verticillium wilt in strawberry that is broadly effective against isolates of diverse host origin.

Reduction of Verticillium dahliae microsclerotia viability in soil by dried plant residues

Experiments were conducted to assess the eradicative effectiveness of a number of organic amendments from various botanical species on microsclerotia (MS) of defoliating (D) and non-defoliating (ND) Verticillium dahliae isolates in infested soil. All the substrates studied significantly diminished the viability of V. dahliae MS when applied as amendments to infested soil. The efficacy was found to depend on the particular species, incubation time, pathogen isolate and whether the soil was sterilized. The effect generally increased with increasing incubation time. The ND isolate of the pathogen was more markedly affected than the D isolate. Organic tissue debris from Diplotaxis virgata, Lavandula stoechas and Thymus mastichina in the non-sterilized soil proved highly effective in reducing the viability of MS of both isolates over time. The D. virgata debris provided the most consistent reduction of MS viability among all types of organic matter and no viable inoculum of either isolate was detected after 6 weeks incubation (wai) in sterilized and non-sterilized soil. The L. stoechas amendment killed the ND V. dahliae isolate propagules in non-sterilized soil. Although the viability of D isolate MS was reduced only 58.6% 3 wai, no viable propagules were detected 6 or 9 wai. Similarly, the control rate with L. stoechas was lower in sterilized than in non-sterilized soil for both isolates. No viable MS were detected after 3 weeks incubation in the non-sterilized soil amended with T. mastichina. Plant debris from C. albidus and C. salvifolius had an intermediate effect on MS viability, but affected the viability of MS of both isolates more markedly in sterilized than in non-sterilized soil. The eradicative effect of organic amendments on V. dahliae MS was confirmed by examining the development of disease incidence and severity of symptoms of Verticillium wilt in cotton plants growing in infested, amended soil. All organic materials reduced the disease incidence and severity in soils infested with both isolates. Thus, no diseased cotton plants were detected in soils that were infested with the ND isolate and amended with any of the organic substrates. Similarly, the C. albidus and L. stoechas treatments completely prevented development of the disease in soil infested with the D isolate. The other organic treatments applied to this isolate strongly diminished the final disease incidence levels, which fell below 16.7%.

Spatial Pattern of Verticillium Wilt in Commercial Mint Fields.

Spatial patterns of mint plants with symptoms of Verticillium wilt caused by Verticillium dahliae were characterized in 10 commercial mint fields in Washington using several spatial analysis methods. The disease was assessed in 0.76-by-0.76-m quadrats (width of mint rows was 0.76 m) in randomly selected study sections varying in size from 5 to 76 m wide by 57 to 396 m long. The variance-to-mean ratio identified aggregation of diseased stems within quadrats even though probability distributions associated with cluster data did not fit the data well. Generally, there was more clustering within than across rows according to both doublets and runs analyses. Total number of wilt foci ranged from 5 to 170 per field, and mean size of foci ranged from 1 to 2.7 quadrats. In one field observed repeatedly, total foci increased from 24 to 104, and the mean size of foci increased from 1.0 to 1.3 quadrats in the same section of the field from one year to the next. Size of foci increased to 2.7 quadrats in a third year of sampling the same field. Mean focus size was larger within than across crop rows in 10 of 13 field-sampling occasions. The proximity index ranged from 0.88 to 1.00, indicating highly compacted disease foci. The statistical methods employed were useful in describing, quantifying, and visualizing spatial patterns of infected mint in commercial fields. Verticillium wilt spread during the life of the perennial mint crop. Inoculum for much of the secondary increase likely did not directly originate from micro-sclerotia present in soil before the crop was planted or from infected rhizomes that originally were planted.

Verticillium Wilt of Xanthium italicum Caused by Verticillium dahliae in Italy.

During the late summer of 2003, a wilt disease of the weed Italian cockleburr (Xanthium italicum Mor.) was observed in the Basilicata Region of southern Italy. Diseased plants were growing near an apricot orchard in which some trees were severely affected by Verticillium wilt. The most characteristic symptoms of the wilt disease affecting Italian cockleburr were yellowing, stunting, and gradual wilting. Also, diagonal and cross sections of stems revealed brown discoloration of their vascular tissues. To elucidate the etiology of the disease, we attempted detection and identification of the causal agents using traditional and polymerase chain reaction (PCR)-based methods. Small pieces of petiole and stem tissues from diseased and asymptomatic plants were surface disinfested in NaOCl solution, rinsed in sterile distilled water, blotted dry, and plated onto water agar (WA) medium. Following incubation at 22°C, the emerging colonies were transferred to potato dextrose agar (PDA). Verticillium dahliae (one isolate) was consistently identified on the basis of its morphological features according to the description of Smith (2). Using PCR assays with the primer pair ITS5/ITS4 (3), which are directed to fungal nuclear ribosomal DNA (rDNA) repeat sequences, an amplification product of approximately 560 bp was obtained by using total DNA extracted from wilt-affected Italian cockleburr plant tissues (five plants examined) as well as fresh mycelium from the V. dahliae-infected Italian cockleburr pure culture-maintained isolate mentioned above. No visible PCR products were obtained with total DNA from asymptomatic Italian cockleburr plants. Sequence analysis of the ITS5/ITS4 amplicons revealed no differences in their nucleotide positions. The obtained sequence of the V. dahliae-infected Italian cockleburr isolate (GenBank Accession No. AJ865691) was then used as query sequences in a BLAST 2.0 search (1). Sequence of the southern Italian isolate proved to be identical to that of the Greek strain "76 Greece" of V. dahliae (GenBank Accession no. AF104926). To prove Koch's postulates, 10 healthy Italian cockleburr seedlings were experimentally inoculated by dipping trimmed roots in a single-conidial suspension (1.5 × 106 CFU/ml) obtained from 10-day-old colonies of the V. dahliae-infected Italian cockleburr pure culture-maintained isolate. After 4 weeks, all inoculated Italian cockleburr plants showed symptoms identical to those of naturally infected field-grown plants. V. dahliae was consistently reisolated from inoculated plants. Additional inoculation experiments revealed that pepper and eggplant were also susceptible to the V. dahliae-infected Italian cockleburr isolate showing typical Verticillium wilt symptoms. To our knowledge, this is the first report of the occurrence of Verticillium wilt of X. italicum.

Rootstock Effects on Pistachio Trees Grown in Verticillium dahliae-Infested Soil

ABSTRACT In a field trial in soil infested with Verticillium dahliae, we compared the yield, growth, incidence of symptoms of Verticillium wilt, and mortality of two interspecific hybrid pistachio tree rootstocks (UCBI and PGII) with the standard rootstocks: the V. dahliae-resistant and susceptible Pistacia integerrima and P. atlantica, respectively. After 10 years, the trees were destructively sampled for V. dahliae in the xylem at the graft union. The results indicate that trees on the (P. atlantica 'KAC' x P. integerrima) hybrid UCBI rootstock grew and yielded as well as those on P. integerrima. Trees on the hybrid PGII yielded the least. Analysis of variance and log-linear models indicate that in soil infested with V. dahliae, three associations significantly affect pistachio nut yield. Rootstock affects scion vigor and extent of infection. Third, the extent of infection and scion vigor are inversely associated. Although trees on the P. integerrima rootstock had the highest ratings in a visual assessment of vigor, 65% were infected with V. dahliae in the trunk in the graft region compared with 73% in P. atlantica and 25% in UCBI. Thus, P. integerrima and UCBI have at least one different mechanism for resistance to V. dahliae.

Differential Infection of Mother and Runner Plant Generations by Verticillium dahliae in a High Elevation Strawberry Nursery

Strawberry cultivars Selva and Camarosa (Fragaria ×ananassa Duch.) were grown at a high elevation nursery in soil that was either naturally infested with Verticillium dahliae or was rendered pathogen-free through preplant fumigation with 2 methyl bromide: 1 chloropicrin (wt/wt) at 392 kg·ha-1. Plants grown in fumigated soil were inoculated with a conidial suspension of V. dahliae, prior to establishment. Just prior to harvest, plants were rated for disease based on symptoms of Verticillium wilt. At the same time, petiole samples were taken from mother plants and each of three generations of runner plants, along with the stolons subtending each of the sampled runner plants. Petioles and stolons were cultured to assay for the presence of V. dahliae, and scored as either infected or not infected. The experiment was conducted in each of two successive years, and the following conclusions were supported by results obtained in both years. First, symptoms of Verticillium wilt on mother plants of both cultivars were highly correlated with recovery of V. dahliae from petioles, but runner plants were consistently free of symptoms even though they were often infected. Second, runner plants sustained lower infection frequencies than mother plants, with the differences being significant in most cases. Lastly, infection of runner plants was due, at least in part, to transfer of inoculum from infected mother plants; in some cases this appears to have been the exclusive mode of infection.

Weed hosts ofVerticillium dahliae in crete: Susceptibility, symptomatology and significance

A survey of common and uncommon weed species usually showing Verticillium wilt symptoms was carried out during 1992–2000 in Crete, Greece.Verticillium dahliae was isolated in 48 out of 182 sampled fields, in which several weed species were grown, from several locations in Oropedio, Lasithi. Altogether, 124 isolates ofV. dahliae were recovered from the vascular stem-tissue of 19 weed species, belonging to ten botanical families. Pathogenicity trials with 13 out of 19 weed species that have never been reported as hosts of the fungus, using for inoculation isolates which originated from the same weed species, resulted in infection of all of them, showing various disease symptoms. Seven weed species (Anthemis melanolepis, Cardaria draba, Convolvulus arvensis, Erodium sp.,Euphorbia helioscopia, Helminthotheca echioides andSinapis alba) are new hosts worldwide, and six additional species (Euphorbia sp.,Lactuca serriola, Raphanus raphanistrum, Sinapis arvensis, Sonchus oleraceus andTrifolium sp.) are new hosts for Greece. The most susceptible (isolation frequency: 27.9–52.8%, moderate disease severity) species were:Capsella bursa-pastoris, C. draba, Chenopodium album, Senecio vulgaris andSolanum nigrum. Less susceptible (isolation frequency: 4.8–17.8%, slight disease severity) were:Amaranthus sp.,A. melanolepis, C. arvensis, Erodium sp.,Euphorbia sp.,E. helioscopia, H. echioides, L. serriola, Malva sylvestris, R. raphanistrum, S. alba, S. arvensis, S. oleraceus andTrifolium sp. Some species —C. draba, C. album, L. serriola andS. nigrum L. — that usually showed external and vascular wilt symptoms, occasionally exhibited only reduced growth. Visible symptoms under natural field conditions in all 13 weed species that had never been reported as hosts ofV. dahliae were similar to those observed after their artificial inoculation. The fungus was not isolated fromFoeniculum vulgare ssp.piperitum, Oxalis corniculata andStellaria media, among other species.

Pathogenicity and virulence of the two Dutch VCGs of Verticillium dahliae to woody ornamentals

Two experiments were performed in two consecutive years to test whether isolates of different vegetative compatibility groups (VCGs) differ in their ability to cause disease in woody ornamentals, to study the host specificity of the isolates and to get an insight into disease development in woody hosts. A range of woody ornamental plant species, including Acer campestre, Acer platanoides, Acer pseudoplatanus, Catalpa bignonioides, Cotinus coggygria, Robinia pseudoacacia, Rosa canina, Syringa vulgaris and Tilia cordata, were root-dip inoculated with six isolates of Verticillium dahliae, belonging to the two VCGs that occur in the Netherlands (VCG NL-1 and VCG NL-2). Isolates belonging to each VCG caused severe symptoms of verticillium wilt in most plant species tested. Disease progress differed between plant species, but was generally the same for the two VCGs. No overall differences in virulence were observed between the two VCGs for external wilt symptoms, number of dead plants, or shoot length. No significant VCG × plant species interactions were present for these characteristics. However, isolates of VCG NL-1 caused more vascular discolouration than did isolates of VCG NL-2. Isolates within VCGs often differed considerably in their virulence to certain hosts, as shown by highly significant isolate × plant species interactions. Isolates were more virulent on their original host. These findings imply that VCG identification does not contribute to disease prediction for a range of woody hosts.

Dual Role for Ethylene in Susceptibility of Tomato to Verticillium Wilt

AbstractEthylene has been observed to both inhibit and promote the symptoms of Verticillium wilt (caused by Verticillium dahliae) in tomato. To test the hypothesis that ethylene has different effects at different stages in the infection process, ethylene levels were manipulated in V. dahliae‐infected tomato plants by the application of an ethylene synthesis inhibitor aminoethoxyvinylglycine (AVG) and/or ethylene's biosynthetic precursor 1‐aminocyclopropane‐1‐carboxylate (ACC) and the effects on disease severity were examined. Statistically significant reductions in disease severity were consistently obtained for AVG‐treated plants that had ACC added at the time of inoculation. A model is therefore proposed in which post‐infection ethylene enhances Verticillium wilt development in tomato whereas its presence at the time of infection inhibits disease development.

Reduced symptoms of Verticillium wilt in transgenic tomato expressing a bacterial ACC deaminase

Summary Ethylene evolved during compatible or susceptible disease interactions may hasten and/or worsen disease symptom development; if so, the prevention of disease-response ethylene should reduce disease symptoms. We have examined the effects of reduced ethylene synthesis on Verticillium wilt (causal organism, Verticillium dahliae) of tomato by transforming tomato with ACC deaminase, which cleaves ACC, the immediate biosynthetic precursor of ethylene in plants. Three promoters were used to express ACC deaminase in the plant: (i) CaMV 35S (constitutive expression); (ii) rolD (limits expression specifically to the site of Verticillium infection, i.e. the roots); and (iii) prb-1b (limits expression to certain environmental cues, e.g. disease infection). Significant reductions in the symptoms of Verticillium wilt were obtained for rolD- and prb-1b-, but not for 35S-transformants. The pathogen was detected in stem sections of plants with reduced symptoms, suggesting that reduced ethylene synthesis results in increased disease tolerance. The effective control of formerly recalcitrant diseases such as Verticillium wilt may thus be obtained by preventing disease-related ethylene production via the tissue-specific expression of ACC deaminase.

Detection methods forVerticillium species in naturally infested and inoculated soils

The detection and identification of threeVerticillium species in field soils with a polymerase chain reaction (PCR)-based assay was compared to the traditional plating assay method. The two methods were both able to detect the commonVerticillium species in soils although the PCR method detectedV. tricorpus in three soil samples that the traditional method did not. In addition, the PCR assay was rapid, efficient, and required only 1 to 2 days for positive identification whereas the traditional methods required 4 to 8 weeks. The traditional method provided a quantitative measure of pathogen propagules in the soil with population levels ranging from 0 to 21, 625 colony-forming units per gram of soil. However, it was not able to differentiate between the weakly pathogenicV. albo-atrum strain 2 and the more aggressiveV. albo-atrum strain 1, but these two were distinguished with the PCR assay. Results from this study demonstrate that when symptoms of verticillium wilt are observed in potato plants in the field, the major verticillium wilt pathogens present in field soils can be rapidly and reliably detected by the PCR assay.

Intraspecific competition and fungal diseases as sources of variation in sunflower yield

We propose that sunflower yield is negatively associated with responsiveness to intraspecific competition but that this relationship can be affected by fungal diseases. This hypothesis was tested in environments where Sclerotinia head rot ( Sclerotinia sclerotiorum (Lib.) de Bary) and Verticillium wilt ( Verticillium dahliae Klebahn) were the prevalent diseases. Comparisons included cultivars released to the market in the last four decades (Experiments 1 and 2), and current hybrids of varying yield potential (Experiments 3 and 4). In each experiment, yield of undisturbed controls was compared with that achieved by reducing inter-row competition. Yield increase in response to reduced competition ranged from 0 to 84%, highlighting the variation among cultivars. In Experiments 1 and 3, the percentage of leaves with symptoms of Verticillium wilt accounted for 28–32% of the variation in control plot yield, while response to competition accounted for 60–75% of the residual variation; a negative association between control plot yield and response to competition was shown. In Experiments 2 and 4, incidence of head rot accounted for most of the variation in yield among cultivars. The lack of association between yield and response to competition in crops with head rot, and the negative relationship between yield and response to competition in crops with Verticillium wilt was consistent with our working hypothesis.

Evaluation of potato clones for severity of Verticillium wilt, yield and specific gravity in Maine

In 1994 and 1995, the effect of Verticillium wilt, caused byVerticillium dahliae andV. albo-atrum, on tuber yields, number and weight of U.S. No. 1 and B size tubers, and specific gravity was studied in northern Maine, an area with a short growing season. Seven clones (four resistant and three susceptible) were evaluated in a split-plot design with three replications. Clones were the whole-plot factor, and seed pieces in sub-plots were either uninoculated or inoculated with 50 ml of 4 × 104 cfu/mlVerticillium spp. at planting. Individual plants were scored for Verticillium wilt symptoms before harvest on a 1= 97% wilt. Differences among clones for wilting and specific gravity were significant. The inoculation treatment had no effect on any of the tuber traits measured. However, there were significant clone x inoculation interactions for most tuber traits. Reductions in yield, weight and number of U.S. No. 1 potatoes, and specific gravity were greater in the Verticillium wilt susceptible clones than in the resistant clones. These results suggest that breeding clones with resistance toVerticillium spp. will reduce yield losses, while maintaining tuber size and specific gravity under disease pressure.