Mature Plant Resistance Research Articles

Sequential breakdown of the Cf-9 leaf mould resistance locus in tomato by Fulvia fulva.

Leaf mould, caused by Fulvia fulva, is a devastating disease of tomato plants. In many commercial tomato cultivars, resistance to this disease is governed by the Cf-9 locus, which encodes five paralogous receptor-like proteins. Two of these proteins confer resistance: Cf-9C recognises the previously identified F. fulva effector Avr9 and provides resistance during all plant growth stages, while Cf-9B recognises the yet-unidentified F. fulva effector Avr9B and provides mature plant resistance only. In recent years, F. fulva strains have emerged that can overcome the Cf-9 locus, with Cf-9C circumvented through Avr9 deletion. To understand how Cf-9B is circumvented, we set out to identify Avr9B. Comparative genomics, transient expression assays and gene complementation experiments were used to identify Avr9B, while gene sequencing was used to assess Avr9B allelic variation across a world-wide strain collection. A strict correlation between Avr9 deletion and resistance-breaking mutations in Avr9B was observed in strains recently collected from Cf-9 cultivars, whereas Avr9 deletion but no mutations in Avr9B were observed in older strains. This research showcases how F. fulva has evolved to sequentially break down the Cf-9 locus and stresses the urgent need for commercial tomato cultivars that carry novel, stacked resistance genes active against this pathogen.

Phloem connectivity and transport are not involved in mature plant resistance (MPR) to Potato Virus Y in different potato cultivars, and MPR does not protect tubers from recombinant strains of the virus

The aims of this study were: i) to investigate mature plant resistance (MPR) against four strains of Potato virus Y (PVYO, PVYN, PVYNTN and PVYN−Wi) in potato cultivars that differ in maturity (e.g. early or maincrop) at different developmental stages, and ii) to determine whether phloem translocation of photoassimilates at different stages including the source-sink transition influences MPR. The data showed that MPR was functional by the flowering stage in all cultivars, and that the host-pathogen interaction is highly complex, with all three variables (potato cultivar, virus strain and developmental stage of infection) having a significant effect on the outcome. However, virus strain was the most important factor, and MPR was less effective in protecting tubers from recombinant virus strains (PVYNTN and PVYN−Wi). Development of MPR was unrelated to foliar phloem connectivity, which was observed at all developmental stages, but a switch from symplastic to apoplastic phloem unloading early in tuber development may be involved in the prevention of tuber infections with PVYO. Recombinant virus strains were more infectious than parental strains and PVYNTN has a more effective silencing suppressor than PVYO, another factor that may contribute to the efficiency of MPR. The resistance conferred by MPR against PVYO or PVYN may be associated with or enhanced by the presence of the corresponding strain-specific HR resistance gene in the cultivar.

Impact of the potato inoculation date on potato virus Y load and viral distribution in daughter tubers at harvest

AbstractAged plants are more difficult to infect than young plantlets. This modification of susceptibility is described as mature plant resistance (MPR). For potato virus Y (PVY), MPR is known to lead to low infection rates of plants inoculated at the postflowering stage and a decrease in the number of infected daughter tubers. However, the impact of inoculation date on the capacity of PVY to accumulate in daughter tubers has not been studied so far. Field and greenhouse experiments were carried out to better understand PVY epidemiology and to help potato growers to evaluate consequences of early/late infections on the quality of their crops. In field trials, potato plants (cv. Bintje) were covered by insectproof nets from planting to harvest except for a 14‐day period to expose plants to natural PVY infections. Under controlled conditions, potato plants were mechanically inoculated with PVY at different dates from preflowering stages (early inoculations) to postflowering stage (late inoculations). At harvest, daughter tubers were individually collected and analysed to define proportions and viral load of infected tubers according to the time between virus inoculation and harvest. Our results showed that although the age of plants at the time of inoculation can modify their susceptibility to PVY infection, in return, early and late PVY inoculations lead to similar rates of infected tubers at the plant scale and equivalent viral accumulation in infected tubers. All together, these data revealed that both early/late infections are high risks for the sanitary quality of potato tubers.

The effect of mature plant resistance in sugar beet (Beta vulgaris spp. vulgaris) on survival, fecundity and behaviour of green peach aphids (Myzus persicae).

Several studies have shown the negative effects of mature plant resistance (MPR) on aphids in sugar beet, which is correlated to the formation of black deposits in their stomach. However, the underlying mechanism of MPR still needs to be elucidated, by understanding the toxicity effects of MPR on aphids and the role of the plant phenological stage and the environment. Here, we report that MPR in sugar beet does not only affect Myzus persicae mortality rate and the formation of a black deposit in the aphid stomach, but also aphid fecundity and behaviour. In addition, experiments in climate-controlled and field settings showed quantitative variation in MPR to M. persicae between six genotypes of sugar beet. Our results indicate that environmental effects, such as temperature, play a major role in MPR and underscore the importance of proper climate-controlled experiments for investigating MPR. In climate-controlled experiments, 83.3% of aphids on old leaves developed a black deposit, in contrast to only 16.8% of aphids on young leaves. This shows that not only plant age, but also leaf age plays a major role in the intensity of MPR. Further research will be needed to identify the underlying mechanism, before MPR can be used as a viable and sustainable solution to aphid pests in sugar beet.

A first approach to develop a quantitative screening method to identify resistance to bacterial leaf spot disease caused by Acidovorax anthurii in anthurium

Resistance of anthurium cultivars to infection by Acidovorax anthurii, the causal agent of bacterial leaf spot disease was investigated using seven inoculation methods, four inoculum concentrations and two different plant ages towards developing a screening method for identification of disease resistance. Injection of 1 mL of 1.5 × 109 CFU mL−1 three-quarter way down the intact petiole from the intact leaf lamina of a stage-4 leaf consistently and repeatedly elicited 100% infection in susceptible anthurium cultivars, and was the best inoculation method. All 9 measures of resistance using the optimized inoculation method were able to differentiate among 14 anthurium cultivars. Of the three most discriminating measures, proportion of infected leaves per plant showed the largest correlations with field resistance measures obtained through a farm survey and a field trial conducted in a sick plot, and was the best measure of resistance. The study provided evidence for two independent mechanisms of resistance, one at the level of entry and infection establishment and the other at the level of severity of infection. Only two anthurium cultivars showed resistance at both levels. In addition to a cultivar dependent mechanism of resistance, the study showed that there is a general mechanism of mature plant resistance. The study provides a quantitative screening method that can be used to breed for resistance to A. anthurii in anthurium.

The movement of potato virus Y (PVY) in the vascular system of potato plants

Potato virus Y (PVY) is responsible for major viral diseases in most potato seed areas. It is transmitted by aphids in a non-persistent manner, and it is spread in potato fields by the winged aphids flying from an infected source plant to a healthy one. Six different PVY strains groups affect potato crops: PVYC, PVYN, PVYO, PVYN:O, PVYNTN, and PVYN-Wi. Nowadays, PVYNTN and PVYN-Wi are the predominant strains in Europe and the USA. After the infection of the leaf and accumulation of the virus, the virus is translocated to the progeny tubers. It is known that PVYN is better translocated than PVYO, but little is known about the translocation of the other PVY strains. The translocation of PVY occurs faster in young plants than in old plants; this mature plant resistance is generally explained by a restriction of the cell-to-cell movement of the virus in the leaves. The mother tuber may play an important role in explaining mature plant resistance. PVY is able to pass from one stem to the other stems of the same plant through the vascular system of the mother tuber, but it is unknown whether this vascular link between stems is permanent during the whole life of the plant. Two greenhouse trials were set up to study the spread of PVY in the vascular system of the potato plant. The PVY-susceptible cultivar Charlotte was used for both trials. It was demonstrated that all stems growing from a PVY-infected tuber will become infected sooner or later, and that PVYN-Wi translocates more efficiently to progeny tubers than PVYNTN. It was also demonstrated that the progressive decay of the mother tuber in the soil reduces the possibility for virus particles to infect healthy stems through the vascular system of the mother tuber. This new element contributes to a better understanding of the mechanism of mature plant resistance.

Screening Sorghum for Resistance to Striga Hermonthica (Del.) Benth

The root hemiparasitic weed Striga hermonthica is a serious constraint to grain production of economically important cereals in sub-Saharan Africa. Six cultivars of sorghum Abu sabien, were tested for resistance and tolerance to this root hemi-parasite by applying one of two levels of Striga infestation (2 and 4 mg/pot). Results indicated that Striga emergence infesting on roots of all cultivars, indicating that none exhibited a true resistance mechanism. However, cultivar, V6 sustained the lowest Striga emergence and no significant decrease in host biomass. Cultivar V3 exhibited the highest in height and biomass on infection indicating the operation of a tolerance mechanism. Under high infestation, one promising variety namely V6 constantly recorded low number of parasite plants and lowest Striga growth. Sorghum V3 sustained the highest plant growth although it has high Striga infestation. Mature plant resistance was also expressed by delay of parasite emergence and inhibition of its development, low reduction in sorghum growth and production (dry matter) in comparison with susceptible cultivars. Adoption of an integrated approach encompassing high yielding Striga resistant and/or tolerant cultivars may provide a cheap and easy to apply method for Striga control under low-input farming systems.

Seedling Resistance to Sclerotinia sclerotiorum as Expressed Across Diverse Cruciferous Species.

Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, is a serious disease of many cruciferous crops and frequently poses a threat to the sustainable and profitable production of these crops worldwide. Differences in seedling resistance to S. sclerotiorum across 46 diverse cruciferous genotypes from 12 different species were assessed by comparing the extent of pathogenesis on inoculated cotyledons under controlled conditions. Selections of Brassica carinata, B. incana, B. juncea, B. napus, and B. napus introgressed with B. carinata, B. nigra, B. oleracea, B. rapa var. rosularis, B. rapa var. chinensis, B. tournefortii, Raphanus raphanistrum, R. sativus, and Sinapis arvensis were tested. The average size of lesions on cotyledons 48 h post inoculation varied from 0.8 to 7.3 mm. The three most resistant genotypes with the smallest lesions were all from B. oleracea (viz., B. oleracea var. italica 'Prophet' and B. oleracea var. capitata 'Burton' and 'Beverly Hills'). Representatives of R. raphanistrum, S. arvensis, B. juncea, and B. carinata were the most susceptible to S. sclerotiorum, with the largest lesions. To our knowledge, this is the first report of high levels of resistance to S. sclerotiorum in B. oleracea at the cotyledon stage and also the first report of the host cotyledon reactions against S. sclerotiorum for all tested species except B. napus and B. juncea. The mean lesion size for B. napus introgressed with B. carinata was 5.6 mm, which is midway between the lesion size for the two parent species B. napus (5.1 mm) and B. carinata (5.8 mm). Separate genetic control for cotyledon versus mature plant resistance was demonstrated by the lack of correlation between lesion size from S. sclerotiorum on the cotyledon with the severity of disease initiated by stem inoculation or natural processes in a previous field test. On the most resistant genotypes, B. oleracea var. italica Prophet and var. capitata Burton, growth of S. sclerotiorum on the cotyledon surface prior to penetration was severely impeded, production of appressoria inhibited, and both cytoplasm shrinkage and protoplast extrusion in S. sclerotiorum hyphae prevalent. This is the first report of such resistant mechanisms in B. oleracea. Genotypes with cotyledon resistance identified in this study will be of great value not only in furthering our understanding of resistance mechanisms across different cruciferous species but also could be exploited for developing commercial crucifer cultivars with high-level resistance against S. sclerotiorum.

Bell and Banana Pepper Exhibit Mature-Plant Resistance to Tomato Spotted Wilt Tospovirus Transmitted by <I>Frankliniella fusca</I> (Thysanoptera: Thripidae)

Tomato spotted wilt virus (family Bunyaviridae, genus Tospovirus, TSWV) causes annual economic losses in pepper, Capsicum annuum L., across the southern United States and is transmitted by several species of thrips, including the tobacco thrips, Frankliniella fusca (Hinds). Reduced virus transmission and symptom severity as plant age increases is known as mature-plant resistance. TSWV transmission to pepper plants was examined in three and four age classes in field and greenhouse trials, respectively. In the field trial, 'Camelot' bell pepper plants were exposed to potentially viruliferous F. fusca 37, 51, or 65 d postsowing. Two greenhouse trials of Camelot bell and one trial each of 'Bounty' and 'Pageant' banana pepper plants were exposed to potentially viruliferous F. fusca, 43, 57, 71, or 85; 48, 62, 75, or 90; 42, 56, 70, or 84; and 43, 57, 71, or 85 d postsowing, respectively. Linear and hyperbolic regressions of percentage of infected plants per block on days postsowing indicated mature-plant resistance in all trials. All models were significant, but hyperbolic curves better fit the data than linear models. Hyperbolic models were used to calculate the number of days posttransplant at which a 50% decrease from the predicted percentage of infected plants at transplant age (42 d postsowing) was expected. This was referred to as days posttransplant-50 (DPT50). DPRT50 occurred within 9 days posttransplant age for all trials, indicating that early TSWV management in pepper is critical.

Response of Brassica oleracea var. capitata to Wound and Spray Inoculations with Xanthomonas campestris pv. campestris

Eighteen cabbage breeding lines and cultivars were evaluated for resistance to black rot caused by Xanthomonas campestris pv. campestris following wound and spray inoculations at the juvenile and mature stages. Plants were evaluated using four inoculation procedures (juvenile wound, juvenile spray, mature wound, and mature spray) in replicated greenhouse and field experiments. The breeding lines Badger #16, Cornell 101, Cornell 102, NY 4002 and accession PI 426606 exhibited high levels of resistance following all inoculation procedures. `Silver Dynasty' was the most resistant commercial cultivar based on the four tests, yet ranked 12th following the juvenile wound inoculation. The juvenile spray inoculation had a high correlation with both wound and spray inoculations in field experiments (0.89 and 0.86, respectively); however, the juvenile wound inoculation did not correlate well with mature wound and spray inoculations (0.58 and 0.51, respectively). The results indicate that the juvenile wound inoculation is not the most appropriate approach for determining field resistance in Brassica oleracea, and that resistant material could be selected against using this approach. A high correlation between juvenile spray inoculation disease severity ratings and mature plant resistance indicates that plants can be evaluated effectively at the juvenile stage for mature plant resistance to black rot.

Temporal expression of PR-1 and enhanced mature plant resistance to virus infection is controlled by a single dominant gene in a new Nicotiana hybrid.

A new variety of Nicotiana edwardsonii, designated N. edwardsonii cv. Columbia, expresses pathogenesis-related (PR) proteins in a temporal manner 45 to 49 days postplanting and also exhibits enhanced resistance to Tobacco mosaic virus, Tobacco necrosis virus, and Tomato bushy stunt virus. In contrast, PR proteins were not expressed in the original N. edwardsonii variety at comparable ages but were induced after onset of a hypersensitive response to viral infection. The temporal induction of PR proteins in 'Columbia' was correlated with increases in salicylic acid and glycosylated salicylic acid. Earlier studies noted that some Nicotiana hybrids derived from interspecific crosses constitutively express PR proteins, but the genetic basis of this phenomenon had not been investigated, likely because many interspecific Nicotiana crosses are sterile. However, the close genetic relationship between N. edwardsonii and 'Columbia' indicated that a hybrid between these two plants might be fertile, and this proved to be true. Genetic crosses between 'Columbia' and N. edwardsonii demonstrated that a single, dominant gene conditioned temporal expression of PR proteins and enhanced resistance. This gene was designated TPR1 (for temporal expression of PR proteins).

Temporal spread of wheat dwarf virus and mature plant resistance in winter wheat

Wheat dwarf is a potentially severe disease of winter wheat. The causal agent is wheat dwarf virus (WDV), a geminivirus transmitted by the leafhopper Psammotettix alienus. Field surveys of WDV incidence and vector occurrence were conducted for 4 years in winter wheat fields in central Sweden. The objective was to estimate the extent of primary spread of WDV in autumn, by adult leafhoppers migrating into the fields, and the secondary spread in the following early summer, by the progeny of the immigrants. We also related the rate of virus spread in early summer to the phenological development of the vector and the host plant. Furthermore, the development of mature plant resistance was assessed by inoculating winter wheat plants in various growth stages with infectious leafhoppers. Field survey data indicate that the rate of primary infections did not exceed 5%, and that infection rates increased to up to 50% during the summer. Infection rates ceased to increase as the crops reached the end of stem elongation, and virus transmission experiments showed that plants became resistant to infection at growth stage DC 31 (first node detectable).

Age‐related Resistance in Bell Pepper to Cucumber mosaic virus

SummaryWe demonstrated the occurrence of mature plant resistance in Capsicum annuum‘Early Calwonder’ to Cucumber mosaic virus (CMV) under greenhouse conditions. When Early Calwonder plants were sown at 10 day intervals and transplanted to 10‐cm square pots, three distinct plant sizes were identified that were designated small, medium and large. Trials conducted during each season showed that CMV accumulated in inoculated leaves of all plants of each size category. All small plants (with the exception of the winter trial) developed a systemic infection that included accumulation of CMV in uninoculated leaves and severe systemic symptoms. Medium plants had a range of responses that included no systemic infection to detection of CMV in uninoculated leaves with the systemically infected plants being either symptomless or expressing only mild symptoms. None of the large plants contained detectable amounts of CMV in uninoculated leaves or developed symptoms. When plants were challenged by inoculation of leaves positioned at different locations along the stem or different numbers of leaves were inoculated, large plants continued to accumulate CMV in inoculated leaves but no systemic infection was observed. When systemic infection of large plants did occur, e.g. when CMV‐infected pepper was used as a source of inoculum, virus accumulation in uninoculated leaves was relatively low and plants remained symptomless. A time‐course study of CMV accumulation in inoculated leaves revealed no difference between small and large plants. Analyses to examine movement of CMV into the petiole of inoculated leaves and throughout the stem showed a range in the extent of infection. While all large plants contained CMV in inoculated leaves, some had no detectable amounts of virus beyond the leaf blade, whereas others contained virus throughout the length of the stem but with limited accumulation relative to controls.

Some epidemiological approaches to the control of aphid-borne virus diseases in seed potato crops in northern Europe

An account is given of progress during the last 30 years in knowledge of the epidemiology of diseases caused by aphid-borne viruses in seed potato production areas of northern Europe. During this period, Potato leafroll virus (PLRV) and strains of Potato virus Y (PVY O, PVY N) were consistently the most prevalent and harmful. The main factors influencing spread involve the amount of initial virus inoculum in seed crops, agricultural practices in relation to seed potato production, the status of aphids as colonising or transient vectors in the crop, variation in their life cycles and behaviour, effects of environmental factors on aphid population dynamics, and the interaction between aphid phenology and mature plant resistance. Lessons have been drawn from comparisons of the causes of outbreaks of PLRV and PVY, and various comprehensive models have been designed to predict virus spread and optimise control. The accuracy and reliability of virus detection, and chemical control of aphids have been considerably improved, but use of the latter has been severely affected by the development in Myzus persicae of resistance to most types of insecticide. It is suggested that more attention should be given to breeding virus-resistant cultivars that would lead to decreased pesticide use and thus promote more integrated environment-friendly strategies for control.

Phenotypic Variation for Downy Mildew Resistance among Inbred Broccoli

Downy mildew, caused by Peronospora parasitica (Pers. Fr.) Fr., is one of the most economically important diseases of broccoli (Brassica oleracea L. Italica group). Previous studies have shown that resistance to downy mildew in broccoli is dependent on plant age with seedling resistance being independent of mature-plant resistance. The objectives of this study were to: 1) determine if valid evaluations for downy mildew resistance can be conducted at both the cotyledon and the three to four true-leaf stages on the same plants of a given broccoli entry; 2) determine if doubled-haploid (DH) lines derived from the resistant hybrid `Everest' also exhibit resistance to downy mildew and if so, characterize the resistance phenotype(s) in these lines; and 3) determine if identified resistant DH lines exhibit resistance to isolates of P. parasitica acquired from different geographic regions of the United States. Twenty-three DH broccoli inbreds and two commercial hybrids were evaluated for reaction at different developmental stages to infection by P. parasitica in a controlled environment. Results showed that broccoli plants can be evaluated for downy mildew resistance in a two-stage process. Inoculation at the cotyledon stage did not offer any cross-protection or otherwise influence the expression of reaction phenotype (RP) when the same plants were subsequently inoculated at the three to four true-leaf stage. Three different RPs to infection by P. parasitica were identified in DH inbreds. These were: 1) susceptibility at both the cotyledon stage and the true-leaf stage; 2) resistance at both the cotyledon and true-leaf stage; and 3) susceptibility at the cotyledon stage but resistance at the true-leaf stage. There was no effect of two pathogenic isolates from different geographic regions on RP of DH broccoli inbreds. Selection of plant resistance to downy mildew at the cotyledon stage will effectively identify plants with high levels of resistance at subsequent developmental stages.

Expression of mature plant resistance to Striga hermonthica in maize

Twelve maize genotypes with different degrees of resistance were evaluated in plots inoculated with seeds of the phytoparasite Striga hermonthica (Del.) Benth. and in Striga-free (control) plots for three seasons between 1991 and 1993. Resistant genotypes, although showing similar levels of underground infection as susceptible 9 weeks after maize planting (WAP), had significantly fewer emerged parasitic plants and sustained lower damage. Host damage was not determined by the severity of infection. Percent height and dry matter reductions increased from 3 to 6 WAP for resistant and susceptible genotypes. While percent height and dry matter reductions for resistant genotypes declined at 9 and 12 WAP, susceptible genotypes either maintained or increased their levels of damage. Correlation between the severity of height and dry matter reductions at the vegetative (3 and 6 WAP) and reproductive (9 and 12 WAP) stages of maize growth were not significant, indicating that damage at early stages of growth cannot be used to predict mature plant response. Maize damage score (1–9) at 8 WAP, a non destructive and composite shoot indicator of host performance under Striga infestation, was significantly correlated (r = 0.88, r = 0.82; p < 0.01) to the level of shoot reduction at mature plant stages. Emerged Striga count at 8 WAP was significantly correlated (r = 0.98, p < 0.01) to the count at 10 WAP, the time when parasite emergence was highest. Under high and uniform infestation, mature plant resistance can be detected at 8 WAP, the onset of flowering in maize.

033 Characterization of Downy Mildew Resistance in USDA Broccoli Inbreds

Downy mildew, caused by Peronospora parasitica (Pers. ex Fr.), is one of the most economically important diseases in broccoli (Brassica oleracea L. Italica group). Previous studies reported that resistance to downy mildew in broccoli depends on plant age and that seedling resistance appears to be independent of mature-plant resistance. The objectives of our studies were to evaluate resistance and susceptibility of USDA broccoli inbreds to downy mildew and to investigate the interaction between the host and pathogen at two plant stages with single or double inoculation. Multiple screening tests at both cotyledon and three-expanded leaf stages using 38 entries, including USDA inbreds and commercial hybrids, were conducted in randomized complete-block designs. In these tests, every leaf of each plant was thoroughly sprayed with P. parasitica isolate PP1 at a concentration of 10,000 sporangia per ml at both stages. Ratings for downy mildew reaction phenotype were made at 9 days postinoculation on a 0-9 scale of increasing disease severity. We found significant phenotypic variation to infection among broccoli entries. We observed three general phenotypes: 1) resistance at both stages; 2) susceptible at cotyledon stage combined with resistance at three-expanded leaf stage; and 3) susceptibility at both stages. Additionally, inoculation at the cotyledon stage had no effect on inoculation at the three-expanded leaf stage.

The effect of plant age and infection with virus yellows on the survival ofMyzus persicaeon sugar beet

Summary.Survival ofMyzus persicaeconfined in clip‐cages on mature leaves of sugar beet declined as the plants aged. Death of aphids was often preceded by the appearance of a black deposit in the aphids' stomachs, which may have been the cause of death. Both the rate of death and the proportion of aphids dying with black deposits was significantly less when plants were infected with beet yellows virus or beet mild yellowing virus, by comparison with healthy plants. The implication of these phenomena on the onset of mature plant resistance is discussed.

Yield losses in virus‐infected crops

Virus disease is not considered as a major cause of crop losses by some authorities on agricultural economy, although past and recent specialized literature has stressed the importance of several virus diseases in reducing crop productivity. This review considers data published from 1988 to 1995 on loss estimation in virus‐infected crops, and analyzes the major factors affecting such losses (type of cultivar, mature plant resistance, incidence of field infection, symptom severity, etc.). A short discussion on the physiological basis of crop losses is also presented. Analysis of the data clearly confirms the marked effect of virus diseases on crop yield, whose control brings heavy economic cost. Accurate analysis of losses caused by virus diseases and the profits arising from successful control of the infections should be performed for single crops in every region or country to estimate the potential gain.

Translocation and mechanical spread of a Minnesota isolate of potato virus S in potatoes

Potato cultivars Norland, Kennebec, and Russet Burbank were inoculated with a Minnesota isolate of potato virus S (PVS) in the field. The incidence of secondary infection, measured by enzyme-linked immunosorbent assay (ELISA), indicated that significantly more PVS infection occurred in Russet Burbank than Norland or Kennebec, and that Norland had significantly more infection than Kennebec (P≤ 0.01). Mature plant resistance was not observed. Rapid reinfection of PVS-free seedlots resulted from inoculum point-sources when routine cultivation practices were followed. Reinfection rates were significantly greater for seedlots grown at Grand Forks, ND, when compared with seedlots grown at Becker, MN, during both years of the study (P≤ 0.01). After two years in the field, reinfection rates for Norland (71.8%) and Russet Burbank (73.0%) did not differ significantly, but were significantly greater than the reinfection rate for Kennebec (29.5%) (P≤ 0.01). In greenhouse-grown plants, PVS moved out of rub-inoculated leaves within 24 hr, however, 13 and 20 days were required before PVS was detected with ELISA in foliage above and below the inoculated leaf, respectively. The frequency of PVS detection was significantly greater in foliage above the inoculated leaf compared to foliage below the inoculated leaf (P≤ 0.05). Translocation of PVS from inoculated leaves to daughter tubers occurred within 13 days for Russet Burbank and Norland and within 20 days for Kennebec.