Mineral Oil Inhibits Movement of Potato Virus Y in Potato Plants in an Age-Dependent Manner

Potato Virus Y (PVY) is one of the most devastating pathogens threatening potato production worldwide. It is a RNA virus that is disseminated by aphids in a non-persistent manner. Regular application of mineral oil on potato fields is known to reduce the number of PVY-positive tubers in post-harvest testing. The mechanism of this phenomenon is not well understood, but it is hypothesized to influence the virus-vector-plant relationships. Here, we present data from greenhouse and field trials that shed light on the effect of mineral oil on local and systemic accumulation of PVY^O in susceptible Shepody and Russet Burbank. The data suggests that mineral oil did not influence PVY^O levels in mechanically-inoculated leaves nor tubers of plants with a secondary infection. However, a reduction in systemic PVY^O levels was observed in mineral oil-treated older plants but not in younger plants, suggesting that mineral oil inhibits PVY^O movement in an age-dependent manner.     

Efficacy of Mineral Oil-Insecticide Mixtures for Protection of Potato Tubers Against PVY and PVM

The impact of a dozen mixtures of the most commonly applied aphicides: Mospilan 20 SP (acetamiprid), Pirimor 500 (pirimicarb) and Karate Zeon 050 CS (lambda-cyhalothrin), combined with the mineral oil Sunspray 850 EC, was researched in field conditions to assess their effectiveness in limiting potato tuber PVY, PVM and PLRV infection. In spite of the greatest reduction in the number of aphids occurring following application of Mospilan 20 SP, this treatment was not as effective in limiting PVY infection as, for example, applying Sunspray 850 EC mineral oil. Mineral oil, when used on its own or in a mixture with Pirimor 500 WG, was found to be the most effective measure for limiting PVY infection (the incidence of tubers infested with PVY was reduced by 64 % relative to control, i.e. no protection). A slightly weaker effect was observed in the case of a combination of the mineral oil with full doses of Karate Zeon 050 CS with a half of a dose of Mospilan 20 SP insecticide, however only for protection against PVY. A similar trend was observed for PVM even though a significant difference was only observed for Sunspray 850EC?+?Pirimor 500WG. In conclusion, the application of insecticide mixtures with mineral oil in protecting against PVY infection is not always as effective as the application of the oil itself only. Addition of the insecticide may sometimes improve the efficacy of protection, however, due to the extra costs involved, not always does it have to be economical.     

The Use of Mineral Oil in Potato Protection: Dynamics in the Plant and Effect on Potato Virus Y Spread

The focus of this study was to evaluate two mineral oils (Superior 70 and Vazyl-Y) in reducing the seasonal spread of Potato Virus Y (PVY). Three concentrations of oil (0, 5, and 10 L ha^?1 of Superior 70; 0, 7.5, and 15 L ha^?1 of Vazyl-Y) and three spray regimes for both oils (every 3–4, 7, and 10–11 days) were tested. Two weeks after top-kill, two tubers from each of 49 plants free of virus at emergence were harvested from treatment plots, sprouted, and tested for PVY with enzyme-linked immunosorbent assay (ELISA). Results revealed that in the case of Superior 70, PVY spread in mineral oil treated plots ranged from 2.1 to 12.2 %, while in the control plots it ranged from 20.4 to 37.7 % across three cultivars. In the case of Vazyl-Y, PVY spread in mineral oil treated plots ranged from 2.1 to 26.5 %, while in the control plots it ranged from 49.9 to 85.7 % across three cultivars. These data show that there was a significant reduction in PVY due to spray of mineral oils. In addition, mineral oil was quantified in plants from the Superior 70 treated and the control plots to understand the dynamics of mineral oil during the season. While there was little to no oil measured in the leaves at the early stages of plant growth, a considerable amount of mineral oil was detected close to plant maturity. A basic model of the concentration of oil in the treated foliage was formulated to confirm our understanding of the factors at play. The model could explain from 50 to 90 % of the variation in oil content observed in the field. Plant growth and size are important factors affecting oil content in mineral oil treated foliage.     

Comparison of Mineral Oil and Rapeseed Oil Used for the Protection of Seed Potatoes against PVY and PVM Infections

The aim of this research was to compare the effectiveness of mineral oil with rapeseed oil in the protection of potato seeds against Potato virus Y (PVY) and Potato virus M (PVM) infection. The research was carried out under field conditions in the north of Poland, in the Department of Potato Protection and Seed Science of the Plant Breeding and Acclimatization Institute??National Research Institute at Bonin. The effects of oil protection on potato seed infection by viruses, yield, and its structure and phytotoxity were assessed. Two rapeseed oils and one mineral oil were used: Olejan 85 EC (85% of natural rapeseed oil), alimentary oil Marlibo (100% of natural rapeseed oil) and Sunspray 850 EC (98.8% mineral oil+1.2% emulsifier). The effectiveness of oils in protection against PVY and PVM tuber infection was tested on two cultivars (Clarissa and Rosalind), which have a different level of resistance to the two viruses. The assessment of phytotoxicity was carried out on 10 potato cultivars from different earliness groups (Augusta, Bryza, Cekin, Clarissa, Impala, Krasa, Rosalind, Satina, Velox). Two oil concentrations, 2% and 4% were applied. During the growing period, eight to nine oil treatments were applied at 7-day intervals. The effectiveness of rapeseed oils against PVY and PVM was much weaker than that of the mineral oil and for the susceptible cultivar the percentage infected tubers did not differ significantly from untreated crops. However, Olejan 85 EC deserves attention as it significantly reduced PVY infection across years in the more resistant cultivar, especially in the case of a lower concentration (2% vs. 4%). When there are no other means of protection to use on organic seed plantations, this oil can be used as an alternative, especially in the case of cultivars which are moderately resistant to PVY. A disadvantage of its application may be the fact that in some years symptoms of phytotoxicity on potato plants were recorded and a lowering of tuber yield.     

Potato Cultivar and Seed Type Affect the Development of Systemic <Emphasis Type="Italic">Potato virus Y</Emphasis> (PVY<Superscript>N-Wi</Superscript>) Infection

Potato virus Y (PVY) infection is one of the greatest challenges to seed potato production in the United States. To determine how cultivar and seed type affect the development of systemic PVY infection, Russet Burbank and Russet Norkotah Colorado 3 cultivars were grown from two types of pre-nuclear seed (i.e., plantlets and minitubers) and Generation 3 (G3) tubers and challenged with PVY strain Wilga (PVY^N-Wi). Systemic PVY infection was measured by assaying spread of virus from the inoculation site to upper non-inoculated leaves. The Burbank cultivar had a lower incidence of systemic PVY infection compared to the incidence of systemic PVY that developed in the Colorado 3 cultivar. Furthermore, Burbank plants grown from G3 tubers had a lower incidence of systemic PVY infection, as compared to Burbank plants grown from plantlets. Together our results indicate that both cultivar and seed type affect the development of systemic PVY^N-Wi infections post-inoculation.     

Effectiveness of Combined Use of Mineral Oil and Insecticide Spray in Reducing <Emphasis Type="Italic">Potato Virus Y</Emphasis> (PVY) Spread under Field Conditions in New Brunswick,Canada

In the 2014 and 2015 crop seasons, the efficacies of different types, rates and combinations of mineral oil and insecticide foliar sprays for reducing Potato virus Y (PVY) spread were tested in controlled field trials in New Brunswick (NB), Canada. Experimental plots were planted with certified PVY-free Goldrush, supplemented with known virus-infected seed to raise PVY inoculum to 2.3% and 3% at the beginning of the 2014 and 2015 seasons, respectively. Treatments consisted of mineral oil-only sprays at different application rates, insecticide-only sprays of differing numbers, and several combined mineral oil and insecticide spray regimes, all compared to a no-spray control treatment. PVY spread to 18% (2014) and 22% (2015) of initially virus-free plants in no-spray control plots, with significant reductions observed in PVY spread in several treatments. Greatest PVY reductions, as low as 4% (2014) and 12% (2015), were in combined mineral oil and insecticide spray treatments, followed by oil-only sprays; while insecticide-only sprays did not significantly reduce PVY spread. As well as measuring PVY spread to marked test plants and randomly collected post-harvest tuber sample from the plots, exhibited similar treatment pattern for PVY incidence. Multiple logistic regression modeling confirmed the relative efficacy of combined oil and insecticide sprays for reducing PVY spread, while accounting for variable inoculum and aphid factors. Modeling also highlighted the importance of planting low-PVY seed initially, and of early application of foliar sprays. Local best management practice recommendations for reduction of in-field PVY spread were discussed.     

Payette Russet: a Dual-Purpose Potato Cultivar with Cold-Sweetening Resistance,Low Acrylamide Formation,and Resistance to Late Blight and Potato Virus Y

Payette Russet is a full season, russet-skinned potato cultivar notable for its cold-sweetening resistance and associated low acrylamide formation, making it ideally suited for processing into French fries and other potato products. Low asparagine and reducing sugar concentrations in Payette Russet tubers contribute to an 81 % reduction in acrylamide content in French fries relative to cultivars Ranger Russet and Russet Burbank following eight months storage at 9 °C. In three years of evaluations in the Western Regional Potato Variety Trials, average yield of Payette Russet was intermediate between Ranger Russet and Russet Burbank, but Payette Russet had the highest U.S. No. 1 yield when averaged across all eight trial locations. Acceptably low tuber glucose concentrations (<0.10 % glucose FWB) were maintained in Payette Russet following up to nine months storage at temperatures as low as 5.6 °C with consistently acceptable French fry color scores obtained (USDA value ≤2.0). Reducing sugars are also maintained uniformly throughout Payette Russet tubers, resulting in a low incidence of sugar ends and reduced mottling in French fries relative to standard processing cultivars. Long tuber dormancy also benefits long-term storage for processing. With its russet skin, Payette Russet could also be used for fresh-pack, and its assemblage of disease resistances makes it especially suitable for organic production, or for use by growers and companies seeking greater sustainability in their production. Payette Russet is resistant to foliar and tuber late blight, common scab, and has extreme resistance to PVY conferred by the presence of the Rysto resistance gene. Payette Russet also has a moderate level of resistance to Verticillium wilt, early blight, and corky ringspot. It is susceptible to Fusarium dry rot (F. sambucinum), therefore production and storage management guidelines are provided to minimize tuber infection. Payette Russet displays a low incidence of second growth and growth cracks, especially relative to Russet Burbank, and is intermediate between Ranger Russet and Russet Burbank for incidence of hollow heart/brown center. Blackspot bruise expression for Payette Russet is similar to Russet Burbank and reduced relative to Ranger Russet. Payette Russet was more susceptible to shatter bruise, internal brown spot, and tuber weight loss in storage relative to the industry standard cultivars. Payette Russet was released in 2015 by the USDA-ARS and the Agricultural Experiment Stations of Idaho, Oregon, and Washington, and is a product of the Northwest (Tri-State) Potato Variety Development Program.     

Attempts to extend seed potato production in Israel to new areas at high altitudes

Summary Attempts were made to grow seed potatoes in the Gilboa mountains and central Golan Heights both at ca 500 m altitude. The activity of winged aphids was low between May and September, the period suitable for growing seed potatoes. Potato virus X (PVX) and potato virus Y (PVY) were rare. The highest incidence of potato leaf roll (PLRV) was 17% at Gilboa and 5% at Nov (central Golan). In both areas, spindle sprout was a severe problem in progeny tubers and it was associated with a purple top syndrome, probably caused by a mycoplasma, that appeared during growth. Activity of the probable vector, the leafhopperHyalesthes obsoletus, ceases in the Golan after June. Delaying planting until the end of May or later reduced the incidence of purple top and spindle sprout to nil. Both were reduced by Temik treatment and by covering the plots with netting but not by treatments with Rogor and Croneton. All three chemicals reduced PLRV incidence. Therefore, delayed plantings and insecticides may reduce purple top, spindle sprout and PLRV. This work was partly supported by funds of BARD 339-80.     

Genetic Resistances to Potato Leafroll Virus, Potato Virus Y, and Green Peach Aphid in Progeny ofSolanum etuberosum

Increasing prevalence of potato leafroll virus (PLRV) and potato virus Y (PVY) has been reported in seed and commercial potato production, resulting in the rejection of potatoes for certification and processing. Host plant resistance to PLRV and PVY and their primary vector, green peach aphid,Myzus persicae, could limit the spread of these viruses. Host plant resistance to PLRV, PVY, and green peach aphid has been identified in non-tuber-bearingSolanum etuberosum (PI 245939) and in its backcross 2 (BC₂) progeny. Resistance to green peach aphid involved a reduction in fecundity and adult aphid size. In addition, one BC₂ individual was identified as possessing a genetic factor that was detrimental to nymph survival. PVY resistance was identified in all five BC₂ progenies evaluated in a field screening under intense virus pressure. PLRV resistance was identified in two of the five BC₂ progeny. This resistance was stable in field and cage evaluations with large populations of viruliferous aphids. Based on the segregation of virus resistances in the BC₂ , PVY and PLRV resistances appear to result from the action of independent genetic mechanisms that reduce the levels of primary and secondary virus infection. Two BC₂ individuals, Etb 6-21-3 and Etb 6-21-5 were identified as having multiple resistances to PLRV, PVY, and green peach aphid derived fromS. etuberosum. This germplasm could prove useful to potato breeders in the development of virus-resistant cultivars.     

Effects of Mulch and Potato Hilling on Development of Foliar Blight (<Emphasis Type="Italic">Phytophthora infestans</Emphasis>) and the Control of Tuber Blight Infection

Foliar and tuber blight caused by Phytophthora infestans accounts for significant losses in potatoes in field and storage. Nevertheless, limited research has been published on the effects of cultural practices on late blight control. Field experiments were conducted in two years on Howard gravely loam soil in New York State to evaluate the effectiveness of mulching using oat straw and hilling in preventing tuber blight infection for cvs Allegany and Katahdin. Potato hilling and mulching had little effect on foliar blight development. The cultivar affected the disease development in the foliage, with cv. Allegany showing lower foliar late blight than cv. Katahdin. Tuber blight incidence averaged 25% for cv. Allegany and 3% for cv. Katahdin in hilled plots, while in the mulched plots the incidence of tuber blight averaged 33% for cv. Allegany and 10% for cv. Katahdin. The straw hay mulch was ineffective in tuber blight control. Tubers set at a soil depth of more than 7 cm had lower tuber blight incidence than shallow tubers set at a depth of less than 7 cm. In both years, hilling provided partial protection of tubers but its effectiveness was limited in the presence of favourable conditions for late blight development. Even though large hills had proportionally a lower tuber blight incidence than medium-sized hills, the difference between the different hill sizes was not significant. These studies suggest that the use of cultivars with foliage resistance to late blight in combination with cultural practices may partially reduce the incidence of tuber blight. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation for endorsement by Cornell University or the US Department of Agriculture.     

Potato growth with and without plastic mulch in two typical regions of Northern China

Plastic film mulching is an important agricultural practice to save water and improve crop productivity in Northern China. Three field experiments were conducted to examine the effect of plastic mulch on soil temperature, potato (Solanum tuberosum L.) growth and evapotranspiration under drip irrigation in two typical regions of Northern China in 2001 and 2006. Results suggest that daily mean soil temperature under mulch was 2–9 °C higher than without mulch, especially during the early growth. Potato growth was restrained under mulching conditions in the North China Plain mainly due to the higher air temperature in this region and thus the higher soil temperature. The negative effects of mulching included a lower emergence and fewer marketable tubers per plant. Evapotranspiration and potato tuber yield were both reduced by mulch, especially in the North China Plain. In northwest China, mulch favorably increased the weight of jumbo tubers (W ≥ 300 g) per plant. Mulching duration had little effect on potato evapotranspiration in northwest China. However, both tuber yield and water use efficiency (WUE) decreased with increases in mulch duration, which suggests the plastic mulch should be removed early.     

Management of silver scurf (Helminthosporium solani) with fungicide seed treatments and storage practices

Thiabendazole insensitive strains ofHelminthosporium solani, the causal agent of silver scurf, make controlling the disease with seed treatment difficult. Potato tuber seed treatments and environmental storage management practices were investigated as means to minimize silver scurf. Fungicide seed treatments were evaluated for control ofH. solani; disease was evaluated during the growing season, at harvest, and after 5 months of storage. Silver scurf was observed on progeny tubers eleven weeks after planting. Fungicides that reduced silver scurf incidence and severity on the seed resulted in reduced incidence and severity of the disease in the progeny tubers at harvest and significantly lower disease ratings after storage. Only small increases in disease incidence (0-8%) were seen after storage. Thiophanate-methyl with mancozeb, Captan with mancozeb, and fludioxonil were among the most effective in reducing the incidence and severity of silver scurf on seed and in progeny tubers (Incidence on progeny tubers at harvest for these three treatments were 3%, 9%, and 8% respectively). Thiophanatemethyl alone was not effective for control of silver scurf (48% incidence compared to 43% incidence for the untreated control). Environmental conditions in storage affected disease development. Reduced humidity (85%) during the curing period (0–3 weeks after harvest) significantly reduced (11%) the surface area of tubers infected with silver scurf. Free moisture on the tuber surfaces during storage significantly increased (15%) tuber surface area infection.H. solani was shown to survive in soil and on some potato storage building materials for up to 9 months. The silver scurf disease of potatoes can be suppressed using effective seed treatment and storage management.     

How Long can Rogued Potato Plants Left in the Field Be a Source of <Emphasis Type="Italic">Potato Virus Y</Emphasis> for Aphids?

Removal of diseased plants (roguing) is commonly practiced in seed potato production. Diseased plants left to desiccate in fields could possibly serve as sources of Potato virus Y (PVY). PVY acquisition by three aphid species (Myzus persicae, Rhopalosiphum padi, Aphis fabae) was evaluated with leaflets from rogued plants for seven days. Results showed greater PVY acquisition rates in non-colonizing aphids species compared to colonizing ones. The proportion of aphids leaving leaflets increased with time (i.e. days after plants were uprooted) and some aphids were carrying PVY in their stylets on each of the seven days of the experiment, suggesting that aphids were able to probe and acquire PVY even when plants wilted. These results confirmed that diseased plants left in fields can serve as a source of PVY for aphids even after they wilted and emphasises that proper actions must be taken to efficiently remove diseased plants from fields.     

Identification of triplex (YYYy) Potato Virus Y (PVY) immune progenitors derived fromSolanum tuberosum ssp.andigena

As part of the International Potato Center’s (CIP) virus resistance breeding strategy, a group of 182 selected clones from intercrosses among duplex Potato Virus Y (PVY) immune progenitors derived fromSolanum tuberosum ssp.andigena (i.e., YYyy × YYyy) was sampled. These clones were test-crossed to the PVY susceptible tester 377964.5 (yyyy) to search for triplex (YYYy) and quadruplex (YYYY) PVY immune potato progenitors. Seedlings of each test-crossed progeny were screened for PVY immunity. Genetic analysis of observed ratios for immunity-susceptibility showed that two of the 182 clones segregated as triplex under the assumption of a random chromatid segregation model. The double reduction coefficient, α, and its standard error were estimated from the experimental data, and had values of 0.1566 and 0.0186 respectively. These results indicated that crossovers take place during meiosis between the locus and the centromere, permitting sister chromatids to migrate to the same pole producing a random chromatid segregation. In the population evaluated, no clone was identified as a quadruplex. The two triplex PVY immune genotypes will produce about 96% of progenies with PVY immunity when mated to PVY susceptible advanced clones or varieties. These progenitors have the potential to provide a durable PVY genetic control and diminish the present impact of this virus on the potato crop. Also, immunity to PVY will significantly simplify the seed production process.     

Potato Plants Grown from Minitubers are Delayed in Maturity and Lower in Yield,but are not at a Higher Risk of <Emphasis Type="Italic">Potato virus Y</Emphasis> Infection than Plants Grown from Conventional Seed

Potato virus Y (PVY) is the most important virus in North American seed potato (Solanum tuberosum L.) production. Planting virus-free minitubers in place of field-grown seed, which usually has a low PVY incidence, reduces initial PVY inoculum in the field. However, plants grown from minitubers are smaller and emerge later than those grown from conventional seed, which could make them more likely to become infected with PVY. We tested the effects of seed type of three potato cultivars (Dark Red Norland, Goldrush, and Red La Soda) on PVY incidence, tuber yield, and flowering time. The incidence of PVY in plants grown from minitubers did not differ from that of plants grown from conventional seed. Minituber-grown plants produced lower tuber yields than plants grown from conventional seed. Plants from minitubers also emerged and flowered later, but this did not increase their incidence of PVY. Cultivar-specific differences were observed in tuber yield and flowering times, suggesting that this variation may influence PVY incidence more than seed type.     

Occurrence and Distribution of Potato Pests and Diseases in Kenya

Potato plays an important role in food security in Kenya but yields are low (<10 t/ha), and this is partly attributed to the lack of healthy planting material. This study is the first wide-scale survey to determine the occurrence and distribution of common potato pests and diseases in Kenyan seed (certified and quality declared) and ware crops. Potato crops growing on 101 farms in 21 districts were examined. Approximately 36% of plants in farmers’ fields sampled both during the long rains (main potato-growing season) and short rains seasons displayed virus-like disease symptoms. Six viruses (potato leafroll virus (PLRV), Potato virus A (PVA), potato virus M (PVM), potato virus S (PVS), potato virus X (PVX), and potato virus Y (PVY)) were detected using double antibody sandwich enzyme-linked immunosorbent assay in potato samples. Sequencing of polymerase chain reaction products from PVY-infected plants revealed the presence of recombinant strains of PVY (NTN and Wilga). Four aphid species, Macrosiphum euphorbiae, Aphis gossypii, Myzus persicae, and Aphis fabae, colonized potato in all districts, occurring in greater numbers west of the Great Rift Valley than to the east. There was a positive correlation between virus incidence and aphid numbers in the long rains (main) potato-growing season. PLRV, PVM, PVS, PVX, and PVY were detected in solanaceous weeds. Ralstonia solanacearum was detected in soils from 13 farms in 8 of the 18 districts surveyed. Approximately 38% of soil samples were infested with Meloidogyne spp. Phytophthora infestans isolates belonging to the US 1 and 2_A1 genotypes were identified. Although many economically important diseases are present in Kenya, the lower aphid incidence in districts east of the Great Rift Valley may indicate that these districts are more suitable for seed potato production.     

Soybean aphid,Aphis glycines Matsumura, a new vector ofPotato virus Y in potato

Soybean aphid (Aphis glycines Matsumura), an exotic species first discovered in the North Central region of the United States in 2000, is a competent vector of severalPotyviridae. Soybean aphid has high fecundity and produces alatae (winged morphs) readily, characteristics typical of proficient virus vectors. When soybean aphids were exposed toPotato virus Y (PVY)-infected potato plants and then clip-caged on healthy potato plants in groups of five or as single aphids, PVY transmission ranged from 14% to 75% across all experiments. PVY^o, PVYⁿ, and PVP^ntn strains were transmitted by soybean aphid.     

Occurrence of Viruses Affecting Potato Crops in Khartoum State-Sudan

The incidence of Alfalfa mosaic virus (AMV), Potato leafroll virus (PLRV), and Potato virus Y (PVY) in potato crops derived from various types of seed potatoes was assessed visually and confirmed by direct tissue blot immunoassay, over two winter growing seasons (1999/2000, 2000/2001) at three locations, Elnaiya, Elshehinab, and Shambat in Khartoum State, the main potato growing region in Sudan. Virus infection was most prevalent in 2000/2001. In general, crops grown directly from imported certified seed potatoes and from “improved seed”, produced in Sudan from imported basic seed, showed the lowest levels of PLRV and PVY compared with crops grown from Sudanese farm saved seed. For AMV, however, only crops grown directly from imported certified seed potatoes had low levels of AMV. Crop location also affected virus incidence, although this varied with year. For AMV, levels were similar at all locations in 1999/2000, but were greatest at Elnaiya in 2000/2001. For PVY, levels were greatest at Elnaiya in 1999/2000 and Shambat in 2000/2001. For PLRV, no symptoms were observed in 1999/2000 and virus levels were similar for all locations in 2000/2001. This study reports for the first time the occurrence of AMV in potatoes grown in Sudan.     

Evidence of <Emphasis Type="Italic">Potato virus Y</Emphasis> Spread through Post-Emergence Management Practices in Commercial Potato Fields

Potato virus Y (PVY) transmission was studied in six commercial potato fields in 2015 and 2016 in New Brunswick, Canada. Plants emerged from PVY-positive tubers, or PVY-free plants that were artificially inoculated with PVY shortly after emergence, were used as PVY inoculum plants in the study. In all trials, equal numbers of PVY inoculum plants from each of three strains common in the region, PVY^O, PVY^N:O and PVY^NTN, were used. PVY inoculated into tractor-traffic rows showed 2 to 7 times as much PVY spread to previously virus-free plants by the end of the season (up to 48.5% in one tractor row, compared to a maximum of 16.3% in a distant control row unaffected by tractor traffic). Evidence supporting a hypothesis that tractor traffic enhances PVY transmission through aphid disturbance was observed by PVY spread in both directions along the rows, not biased in the direction of tractor travel, and that the ratio of spread of the three strains was nearly indentical in control and tractor rows. However, the lack of spread to immediately adjacent rows, and statistically significant spatial pattern matching the circumference of tractor wheels specific to each field support the hypothesis that direct mechanical transmission of wounded plant sap could also be a factor in the enhanced PVY transmission.