Etiology of potato early dying in Delaware

Greenhouse and field plot experiments along with surveys of commercial potato fields were conducted to determine the etiology of potato early dying in Delaware and its impact on yield ofSolanum tuberosum cultivar Superior.Verticillium dahliae was the primary pathogen causing early dying, and no increase in disease severity or yield loss was observed when plants were inoculated with bothV. dahliae andColletotrichum coccodes. The field survey showed thatV. dahliae was the predominant pathogen in symptomatic plants, and nematodes were not essential in association withVerticillium to cause early dying. Disease severity was the same in the presence of a low population or absence of nematodes.     

Plant-parasitic nematodes in the potato growing areas of Maine

A study was conducted from October 1987 to November 1988 to accumulate data on the distribution of plant-parasitic nematodes, to examine rotation schemes and time of sampling in relation to nematode populations, and to assess the occurrence ofVerticillium spp.-Pratylenchus spp. in potato growing areas of the state of Maine.Pratylenchus penetrans andP. crenatus were encountered most frequently, with detection being 19% higher in early season (May–June) than late season (September–November). All rotation schemes supportedPratylenchus spp.Meloidogyne hapla was encountered in 14% of the early samples and 19% of the late samples. Oats-potato, potato-potato, and clover-potato supported the highestM. hapla population levels. Other nematodes detected in fewer samples and in lower numbers than the two above genera wereParatylenchus spp.,Criconemella spp.,Helicotylenchus spp. andTylenchorhynchus sp. Two plants from each of 27 fields were examined for nematode-fungus associations. Samples from 7 fields were positive forV. dahliae andP. penetrans only,V. alboatrum andP. penetrans were found in 1 field,V. alboatrum alone was detected in 1 field andP. penetrans,V. dahliae andV. alboatrum were found together in 2 fields. The detection of these nematode-fungus associations suggests that the potato early dying complex defined elsewhere in the United States may be occurring in Maine.     

Pratylenchus spp. andVerticillium spp. in New Brunswick potato fields

A survey of New Brunswick potato fields with symptoms of early dying was conducted in September 1990 and 1991. Root lesion nematodes (Pratylenchus spp.) were detected in 43 of 46 fields in 1990 and in 37 of 43 fields in 1991. In 1990 and 1991 populations averaged 910 and 410 nematodes g-¹ dry root and 1030 and 720 nematodes kg¹ dry soil, respectively. Previous crops did not affect nematode populations, except in one comparison where the number of nematodes in potato roots in 1990 was higher when the previous crop was cereal than in 1991 when the previous crop was peas.Pratylenchus crenatus was more prevalent thanP. penetrans. Root-knot nematodes(Meloidogyne hapla) were detected at a few sites at low population levels only in 1991. In 1991,Verticillium albo-atrum was detected in all 37 potato fields where fungal analyses were conducted, and the average population was 350 propagules g¹ dry soil. Soil populations ofV. alboatrum did not differ in fields in which either Russet Burbank or Shepody were being grown, nor did previous crops have any effect.Verticillium dahliae was detected in 17 fields at very low populations, usually below 1 propagule g¹ dry soil. There were no significant correlations (P<0.05) between populations of P.crenatus orP. penetrans in soil or roots and populations ofV. albo-atrum orV. dahliae propagules in soil.     

Potato early dying disease in the Pacific Northwest caused byErwinia carotovora pv.carotovora andE. carotovora pv.atroseptica

Premature death of potato vines is widespread in irrigated potato fields of the Pacific Northwest.Verticillium dahliae is a major cause of early dying in fields with a history of potato production, but in fields new to potato production, or those that have been fumigated and have a lowV. dahliae population in the soil, premature death of vines can also occur.Erwinia carotovora pv.carotovora (Ecc) andE. carotovora pv.atroseptica (Eca) have been implicated in the early dying disease syndrome in these fields. Both bacterial pathogens are associated with symptoms similar to those caused byV. dahliae; i.e., a progressive chlorosis and necrosis of the foliage and vascular discoloration that may extend a short distance up the stem. Greenhouse pathogenicity studies on cv. Russet Burbank cuttings in which symptom development was identical to that produced byV. dahliae have verified thatEcc andEca can cause potato early dying.     

Seasonal variation and effects of wheat rotation on populations ofVerticillium dahliae Kleb. in Ohio potato field soils

Soil population levels ofVerticillium dahliae in Ohio were monitored from May–October, 1982–1985, in 15 fields in potato-wheat rotation and two fields in potato monoculture. Population levels in fields in rotation ranged from 0 to 86 microsclerotia/10 g of air-dried soil with average values during each 6-month sampling period of 9.7, 12.9, 9.6 and 19.6 microsclerotia/10 g of soil for 1982, 1983, 1984 and 1985, respectively. In at least 2/3 of the fields sampled each year, soil populations ofV. dahliae peaked in either July or August, regardless of whether fields were under wheat rotation or potato monoculture. Of the 15 fields under potato-wheat rotation, 13 showed a general pattern of increased populations ofV. dahliae in one or both years following cropping to potato. Among those 13, significantly higher populations (P < 0.05) were observed in one wheat field in 1983 and in eight fields in 1985. In the two fields in potato monoculture, one consistently had population levels ofV. dahliae 3–4 times higher than any other field sampled in this study. Factors that may contribute to periodic changes inV. dahliae populations, implications of these changes in interpreting soil population data, and usefulness of a potato-wheat rotation in managingV. dahliae populations in Ohio are discussed.     

Fertility and management practices to control verticillium wilt of the russet burbank potato

Management practices for the suppression of Verticillium wilt of Russet Burbank potato include sanitation, use of optimum sprinkler-irrigation practices, soil solarization, and an adequate soil fertility program. Among all cultural factors considered, nitrogen (N) deficiency in potato tissue was most commonly associated with the severity of Verticiilium wilt in Russet Burbank potato. Field studies have shown that increased N availability suppresses Verticiilium incidence on cv Russet Burbank while having no effect on cv Norgold Russet. Studies from both greenhouse and field show that the colonization ofV dahliae in potato stem tissue is suppressed in cv Russet Burbank when the availability of Phosphorous (P) is increased to the optimum. Following one season of cropping with Russet Burbank potato, the treatment providing the optimal N availability also suppressed the increases ofV dahliae populations in the soil during the following year of consecutive cropping. Similarly, after two seasons of continuous potato cropping, treatments with optimal P had lower soil populations ofV. dahliae in soil. Results show the suppression of Verticiilium wilt with optimal fertility. Verticiilium wilt [caused by eitherVerticiilium dahliae Kleb, (microsclerotial form) orVerticiilium albo- atrum Reinke and Berthold (dark mycelial form)] is one of the most severe diseases of potato in the United States. Potato yields, tuber size, and specific gravity may be substantially reduced by this disease, depending on severity, time of occurrence, and growing season. In Idaho and other arid growing regions of the United States, Verticiilium wilt is caused byVerticillium dahliae Kleb. Idaho field studies involving cropping practices, soil fumigation treatments, solarization, and Verticillium-resistant potato clones all support the importance ofV dahliae upon potato yield. Data from these studies show that yield losses due toV. dahliae commonly approximate 5 to 12 metric T/ha (5, 6). Table 1 illustrates the effect of several cropping practices upon relativeV dahliae populations in soil with the effects upon both disease severity and potato yield. With continuous cropping of Russet Burbank,V dahliae populations in soil increased, and yields were reduced by 18 to 19 metric Tlha when compared to locations that had been allowed to remain either weed-free and fallow or where corn had been previously cropped. It was estimated thatV dahliae was influencing yield by about 10 to 12 metric Tlha based on a regression analysis of yield as a function ofV dahliae populations in soil. Similarly, whenV dahliae was suppressed by fumigation treatments involving mixtures of dichloropropene and picfume, yields more significantly increased by 6.5 to 12 metric T/ha (5). Although many factors may influence yields, clones with higher Verticillium resistance than Russet Burbank generally out-yield Russet Burbank. Figure 1 demonstrates the effect of solarization (a technique involving the laying of clear plastic on soil for the purpose of elevating soil temperatures to killV dahliae). When Verticillium was controlled in this manner (lo), there was no significant yield difference between the Russet Burbank potato and the highly resistant A68113-4 clone. The A68113-4 clone grown in non-solarized soil out-yielded the Russet Burbank in non-solarized soil by 31 percent while the solarization treatment significantly increased yield for the Russet Burbank and A68113-4 clones by 46 and 18 percent, respectively. These yield responses were observed in a field where inoculum levels ofV dahliae were relatively low (2.10 cfulgrn of soil). With these losses, the need to develop control procedures is great and to achieve this control the need also exists to accurately evaluate the effect of treatments uponV dahliae. Our Idaho studies have routinely utilized such a procedure (9) in combination with comparisons of both disease severity and yield.     

The effect of netted scab (Streptomyces spp.) andVerticillium dahliae on growth and yield of potato

Summary The effect ofStreptomyces spp. (netted scab) on the growth of potato was investigated in three pot experiments, in two of which the effect ofVerticillium dahliae was also assessed. The netted scab organisms attacked all underground plant parts of susceptible potato cultivars early in the growing season; the roots were especially seriously attacked, markedly reducing tuber yield and number but prolonging the duration of the growing season. The nematicide oxamyl had little effect on the incidence of netted scab. Repeated growing of the susceptible cv. Bintje greatly increased soil contamination with the netted scab pathogens. V. dahliae reduced haulm growth before wilt symptoms were evident and it reduced tuber yield but not number. Oxamyl delayed infection byV. dahliae by controlling parasiticPratylenchus nematodes (mainlyP. thornei).     

Lesion nematode suppression with the butte potato and relationships to verticillium wilt

Field studies conducted over a 5 yr period to compare the effect of continuous cropping of several potato clones (Butte, Russet Burbank, Targhee, A66107-51, and A68113-4) showed Butte to be highly resistant toPratylenchus neglectus while being highly susceptible toVerticillium dahliae. This was further evidenced during the 6th yr when Russet Burbank was cropped over all plot-locations. Populations ofP. neglectus were shown to be significantly less (0.05 P) in both soil and roots when Russet Burbank was grown over sites previously cropped with the Butte potato than over site-locations cropped by either Russet Burbank, Targhee, A66107–51, or A68113–4. Additional field and greenhouse studies corroborated the high degree of resistance of Butte toPratylenchus spp. In addition toP. neglectus resistance, a greenhouse investigation also showed resistance to P. penetrans. A field study designed to compare the effects of Butte with nematicides showed that Butte reducedP. neglectus populations in both soil and roots when compared with Russet Burbank. This degree of reduction was equivalent to the effects of either of two systemic nematicides (aldicarb and isofenphos) at 3.4 to 4.0 kg a.i./ha, respectively. Results demonstrate biological control with the Butte potato clone forP. neglectus suppression. Although populations ofP. neglectus were reduced by either nematicides or Butte, wilt severity was higher with Butte than Russet Burbank. Results showed wilt suppression andP. neglectus reduction with treatments of either nematicide, while having no significant effect on the colonization of potato stems byV. dahliae.     

Fungal pathogens andPratylenchus penetrans associated with preceding crops of clovers,winter wheat,and annual ryegrass and their influence on succeeding potato crops on Prince Edward Island

The incidence of soilborne fungal pathogens of potatoes and root lesion nematodes were determined in preceding crops of clovers, annual ryegrass, and winter wheat, in the succeeding potato crops and soils and severity of associated potato diseases recorded in two, 2-yr field trials. Differences were not evident in yearly recovery ofRhizoctonia solani, Verticil-Hum dahliae, V. alboatrum, andColletotrichum coccodes from previous crops and soils. However, overall trends were present with a higher recovery ofV. dahliae from wheat and a greater recovery ofR. solani from clover than either wheat or ryegrass. A previous crop of ryegrass resulted in lower levels ofR. solani in succeeding potato soils in both trials and after clover in one trial at the pre-harvest sampling time. In trial 1 greater numbers ofPratylenchus penetrans were recovered from clover soils than from soils of ryegrass or wheat. One instance of lower nematode recovery in potato soils following ryegrass occurred but no differences in their recovery from potato roots were observed. Potato wilt incidence was lowest after clovers. Stem rot severity was positively correlated with recovery ofR. solani andV. dahliae from potato stems and roots and negatively correlated withV. alboatrum from potato stems only. Nematode populations were not related to potato disease symptom severity ratings. Results reported herein indicate that only a portion of soilborne diseases of potatoes on Prince Edward Island may be related to previous crops produced in the 3-yr rotation selected for this study.     

Infection of aerial parts of potato plants byColletotrichum coccodes and its effects on premature vine death and yield

Collectotrichum coccodes, the pathogen causing black dot disease of potato, was investigated to determine its potential for causing symptoms on the aerial parts of potato plant, premature vine death, and reduction in yield under greenhouse and field conditions. Spray inoculations with conidial suspensions (10⁶ colony forming units/ml) ofC. coccodes caused sunken, dark, necrotic lesions on stems, leaves and petioles, vein and leaf necrosis, dieback and premature vine death under growth chamber and greenhouse conditions. Field inoculations in 1986 and 1987 provided evidence for an increased incidence of premature vine death in seven potato genotypes (Russet Burbank, Norgold Russet, Superior, Butte, NDA 8694-3, A68113-4 and A66107-51). Late season application of metribuzin at 0.84 kg/ha significantly increased the incidence of stems with black dot symptoms in cv. Russet Burbank at one location. Field inoculations in 1987 at two locations resulted in yield reduction in the late-maturing cv. Russet Burbank (6.3 and 6.5 t/ha) and clone A68113-4 (5.0 t/ha), but not in the early maturing cv. Norgold Russet. Similar results were obtained in greenhouse studies following soil inoculations withC. coccodes: yield reduction was observed in cv. Russet Burbank but not in cvs. Norgold Russet, Superior and Katahdin. Colonization of stem ends of tubers byC. coccodes from spray inoculated field plots was higher than in tubers from uninoculated plots at one of the two locations. These results point to a much broader potential ofC. coccodes as a potato pathogen, than hitherto documented. Colletotrichum coccodes (Wallr.) Hughes (syn.C. atramentarium (Berk. & Br. Taub.) was described in 1825 (21) and has a wide host range (5). Potato and tomato are the more economically important hosts. This fungus has been variously referred to as an unspecialized, minor pathogen or secondary invader (18), weak pathogen (22), specialized parasite (5), or superficial colonizer and widespread epiphyte (19). The symptoms produced by this pathogen on potato have been described as black dot (10, 14) on stems, stolons, tubers and roots, sometimes associated with leaf scorch or wilt (24), rubbery tuber wilt (1), and tuber skin necrosis (17). The fungus is commonly associated with senescent, decaying root and stem tissue, especially at the end of the season. In Idaho, instances of premature death of potato without typical symptoms of Verticillium wilt, but accompanied by a high incidence of stems with black dot symptoms, have been observed in potato crops grown on fields fumigated with metam sodium. Additional observations by growers have also implicated the possible interaction of the herbicide metribuzin with the incidence of black dot. Although it was observed by some workers to accelerate leaf senescence (12, 22), Schmiedeknecht (21) concluded thatC. coccodes was unable to infect the green leaves of potato plant. Even thoughVerticillium dahliae is generally considered the major factor in potato early dying (20), several workers (7, 11, 13, 19, 22) have investigated the possible role ofC. coccodes in causing premature death of potato (8). However, these studies examinedC. coccodes only as a soilborne, root and tuber pathogen of potato; its effects on shoot portions (aerial parts) of the plant were not considered. In addition to causing anthracnose of fruits of tomato, pepper and squash (18), recent reports indicate thatC. coccodes causes foliage blights on eastern black nightshade (2) and velvetleaf (25). The objectives of this study were: (1) to examine the potential ofC. coccodes to cause symptoms on aerial parts of potato plant, (2) to investigate the effect of spray inoculation in the field withC. coccodes on incidence of premature vine death, tuber colonization and yield of potato, and (3) to document the effect of soil inoculation withC. coccodes alone or in combination withV. dahliae on potato varieties under greenhouse conditions.     

Inoculum potential of verticillium—infested potato cultivars

Viable propagules ofVerticillium albo-atrum andV. dahliae were recovered from potato stems before and after burial in the field during 1968–1971. Of 14 cultivars tested: Red Pontiac and Superior ranked highest in numbers of propagules ofV. albo-atrum. Russet Burbank, Kennebec, and Irish Cobbler ranked highest withV. dahliae, respectively. Survival ofV. albo-atrum in infected stems was evident (5 ? 34 × 10³ ppg) even after burial in field soil for 17 months. In general, survival ofV. albo-atrum (47–69%) was higher thanV. dahliae (23 – 61%) following field burial of infected stems for 7 months.     

Recovery ofVerticillium dahliae from North American certified seed potatoes and characterization of strains by vegetative compatibility and aggressiveness

Samples of tubers collected from commercial potato seed lots produced across North America were assayed forVerticillium dahliae. V. dahliae was successfully isolated from 65 of 224 seed lots tested, a successful isolation rate of nearly 30%. Vegetative compatibility of the isolates was assessed through complementation tests using nitrate non-utilizing mutants. AllV. dahliae isolates belonged to vegetative compatibility group (VCG) 4. Of the 162 isolates recovered, 64% belonged to VCG 4A, 33% to VCG 4B, and 3% to VCG 4AB. All 39 of the isolates tested in the greenhouse on potato cv. Superior were pathogenic to potato. Disease symptoms developed earlier, were more severe, and plants died earlier when inoculated with VCG 4A compared to 4B isolates. As a group, AUSPC values were significantly higher (p=0.05) for VCG 4A than for 4B isolates. These data suggest that (1) commercial certified seed tubers from diverse locations are commonly infected withV. dahliae and thus may serve as primary sources of the pathogen; (2) potato isolates ofV. dahliae in North America belong to VCG 4A and 4B and these strains are widely distributed via seed tubers; and (3) VCG 4A and 4B are distinct pathotypes ofV. dahliae that vary in their aggressiveness to potato.     

Potato production and incidence ofVerticillium dahliae following rotation to nonhost crops and soil fumigation in the state of Washington

For three years, potato yields, tuber quality, and incidence ofVerticillium dahliae were measured in field plots previously cropped for one year to the following nonhosts: sudan grass, green peas followed by sudan grass (same year), spring wheat, spring wheat followed by sudan grass (same year), sweet corn and field corn. One year rotation to nonhosts did not reduce the population ofV. dahliae propagules in the soil and in only one of the three years wereV. dahliae propagule numbers in potato stems significantly reduced in plots following nonhost crops, compared with plots following potatoes. Verticillium wilt symptoms in potatoes were not reduced by one year rotation to any of these crops and only in one year in three was yield significantly increased. In two of three years, percent U.S. No. 1 tubers was increased following one-year rotation with green peas plus sudan grass, and in one of the two years, specific gravity was increased by rotation. Rotational cropping to nonhosts for two years between potatoes significantly reduced preplant soil populations ofV. dahliae propagules in the soil and in potato stems the following fall. However, soil populations in these two year rotational plots the spring following potato were not reduced compared to plots previously cropped to potatoes two consecutive years. Cropping to nonhosts for two years had no consistent effect on incidence of Verticillium wilt in subsequent potato crops. Two years’ rotation to nonhosts increased plant height and yield compared to continuous cropping to potato but not percent U.S. No. 1 tubers. The various nonhost crops all had about the same non-significant effect on yield. In two out of three years’ trials, fumigation significantly reduced both the incidence of Verticillium wilt and number ofV. dahliae propagules in stems in plots compared to plots non-fumigated. In only one trial, fumigation significantly increased tuber yields and percent U.S. No. 1 tubers.     

Root lesion nematodes in potatoes

Pratylenchus penetrans andP. crenatus were the dominant plantparasitic nematodes in potato roots and soil in Prince Edward Island. No nematodes were recovered from potato tubers. Forage legumes and grasses grown in rotation with potatoes also harbored large populations of root lesion nematodes. The amount of water moving through the stems of potato plants infected withP. penetrans in the greenhouse generally was less than in nematode-free plants;P. crenatus did not produce this effect. Telone-II, Telone-C-17, and Temik reduced the number of nematodes substantially in field plots. Yields of Superiors, Kennebecs, Sebagos and Netted Gems in nematicide-treated plots were not significantly greater than in untreated plots. There was a small but significant reduction in the specific gravity of tubers in the Telone-treated plots.     

Effects of crop rotation and granular nematicides on the incidence ofVerticillium dahliae Kleb. andColletotrichum coccodes (Wallr.) Hughes,in potato

Summary The incidence ofVerticillium dahliae andColletotrichum coccodes was studied in a crop rotation experiment on sandy soil from 1983 to 1986. Early in the growing season the percentage of stems infected byV. dahliae increased with increasing cropping frequency of potato, depending on the cropping sequence, and decreased with the application of granular nematicides. However, later in the growing season the initial differences decreased.C. coccodes infections of stems were not affected by these factors. The percentage of plants infected byV. dahliae closely correlated with the percentage of plants with wilt symptoms but this relationship was absent forC. coccodes. Early in the growing season, the more vigorous the plants, the greater the level ofV. dahliae infection. Soil infestation withV. dahliae increased with increasing cropping frequency of potato but was not affected by annual applications of granular nematicides.     

Differential diagnosis ofVerticillium dahliae in potato with antisera to partially purified pathogen-produced extracellular antigens

Summary An extracellular protein-lipopolysaccaride antigen (PLP) was purified from culture fluids ofVerticillium dahliae. Antiserum produced in rabbits to the PLP detected the antigen in homogenates of tubers, stems and leaves ofV. dahliae-infected potato plants but not in extracts of healthy potato tissue or extracts of potato plants infected by other fungal pathogens. The antigen was not detectable in extracts of potato isolates ofV. tricorpus, V. nigrescens andV. nubilum or various other fungi. The antigen was shown to be different from cross-reactive antigens detected by antisera to mycelial antigens. When used as a tool for specific diagnosis ofV. dahliae infection in potato, antiserum to PLP was more reliable than that prepared against fungal body antigens. Publication of the Agricultural Research Organization No 245-E.     

Effect of Soil Temperature,Injection Depth,and Metam Sodium Rate on the Management of Verticillium Wilt of Potato

Verticillium wilt, caused by Verticillium dahliae Kleb., is a primary component of the early dying complex of potato (Solanum tuberosum L.) in the United States. Although genetic resistance to V. dahliae exists and has been incorporated into several potato cultivars, the commercial potato industry is still dominated by cultivars susceptible to the pathogen. As a result, soil fumigation with metam sodium remains an important means by which Verticillium wilt is controlled, despite its expense and potentially negative environmental impact. Recent restrictions on metam sodium use by the Environmental Protection Agency directed at reducing exposure to vapor emissions have increased the need to improve shank injection of the soil fumigant. In studies reported here, the application of metam sodium reduced the severity of Verticillium wilt, however, soil temperature at the time of injection, metam sodium injection depth, and application rate had little overall effect. In 2011, temperature at the time of metam sodium injection did not result in significant differences in any parameter evaluated. However, in 2012, soil populations of V. dahliae, wilt severity and host colonization were significantly reduced when metam sodium was applied at 4 °C compared to 13 or 15 °C. No significant differences were observed between a single or two metam sodium injection depths in any parameter evaluated across the 2 years the study was conducted. While all rates of metam sodium significantly reduced soil populations of V. dahliae compared to the non-treated control, significant differences across rates were rarely observed. Improved control of Verticillium wilt and increased yield can be achieved as a result of these studies. The effective control of Verticillium wilt can be obtained by using metam sodium at a comparatively low rate of 373 l/ha, particularly when applied at a relatively cold soil temperature of 4 °C using a single injection depth of 25 cm. The potential impact of these application modifications of metam sodium in reducing emissions and non-target exposure is discussed.     

Long term effects of preplant fumigation of potato fields

The application of nematicides resulted in increased potato yields, and populations ofPratylenchus penetrans were lower for 3 years after application; but longevity of benefits of fumigating potato fields varied with the kind and amount of nematicide, application methods and with growing conditions after fumigation. Fumigation of two fields in the fall of 1965 with Vorlex, Telone, D-D, and mixtures of chloropicrin with Telone or D-D resulted in 70% to 90% control of the meadow nematode,P. penetrans, after two crops in one field, and 50% to 70% control after three crops in another. Yield increases of Kennebec potatoes averaged 42% and 16% in successive crops in the first field and yield increases of Katahdin potatoes averaged 13%, 22% and 16% in three successive crops in the other field. Fumigation resulted in 30 to 70% less vascular browning in tubers in the first two crops but there was no effect in the third crop. There was no effect on black scurf in any crop. Root injury varied proportionally with populations ofP. penetrans in roots in 1967. Populations ofP. penetrans regained injurious levels in three other fields after one crop when abundant root growth in moist 1967 followed spring fumigation with Telone and D-D. Under poor conditions for sealing of the soil following fumigation in the spring of 1968, counts ofP. penetrans collected at different soil depths after fumigation showed excellent kill below 3 inches and poorer kill in the upper 2 inches of soil.     

Effect of annual sequence of removing or flaming potato vines and fumigating soil on verticillium wilt of potato

The effects of removing or flaming potato vines and soil fumigation on population density ofVerticillium dahliae in soil, severity of Verticillium wilt, and tuber yield were studied in a field near Alliston, Ontario, between 1993 and 1996. Vines were physically removed or flamed using a propane flamer in September just before harvest and soil was fumigated with metam-sodium (Vapam) at 550 L/ha in October after harvest. Vine removal had no effect on soil populations ofV. dahliae, area under the disease progress curve (AUDPC), or tuber yield. Flaming once (1993), twice (1993 and 1994), or three (1993,1994, and 1995) times reduced the soil population density ofV. dahliae, and flaming twice (1993 and 1994) reduced AUDPC compared to the nontreated control, but had no effect on tuber yield. Fumigation once (1993) or twice (1993 and 1994) reduced pathogen density in soil and AUDPC, but did not increase tuber yield. Fumigation once (1993) or twice (1993 and 1994), in combination with flaming twice (1993 and 1994), was equally and significantly effective in reducing both population density ofV. dahliae in soil and AUDPC values and in increasing tuber yield in 1995. Annual flaming of vines in combination with soil fumigation once (1993) or twice (1993 and 1994) in the fall improved the control of Verticillium wilt of potato and realised the greatest profits.