Chloropicrin Soil Fumigation Reduces <Emphasis Type="Italic">Spongospora subterranea</Emphasis> Soil Inoculum Levels but Does Not Control Powdery Scab Disease on Roots and Tubers of Potato

The effect of chloropicrin fumigation on the soil populations of Spongospora subterranea and the development of powdery scab, formation of root galls and tuber yield was investigated in seven field trials conducted in Minnesota and North Dakota. Sixteen potato cultivars, with different levels of susceptibility to disease on roots and tubers, were planted in plots treated with chloropicrin at rates ranging from zero to 201.8 kg a.i. ha^?1. The amount of S. subterranea DNA in soil was determined using qPCR. Bioassays were conducted to further assess the effect of chloropicrin fumigation on root colonization by S. subterranea in two potato cultivars with contrasting disease susceptibility. In the field, chloropicrin applied at rates between 70.1 to 201.8 kg a.i. ha^?1 significantly decreased S. subterranea initial inoculum in soil but increased the amount of disease observed on roots and tubers of susceptible cultivars. The effect of increasing disease was confirmed in controlled conditions experiments. Although the amount of S. subterranea DNA in roots of bioassay plants increased with increasing chloropicrin rates, it remained similar among potato cultivars. Chloropicrin fumigation significantly increased tuber yield which in cultivars such as Shepody and Umatilla Russet were associated with the amount root galls (r = 0.30; P < 0.03). Results of these studies contradict earlier reports on the use of chloropicrin fumigation for the control of powdery scab. Factors other than inoculum level, such as environmental conditions that affect inoculum efficiency and host susceptibility, may be significant contributors to the development of powdery scab and root gall formation.     

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.     

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.     

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.     

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.     

Potato early dying—A serious threat to the potato industry

Potato early dying (PED) is characterized by a loss of plant vigor during mid to late summer followed by senescence and death of the crop a few weeks prior to normal maturity. This disease is of serious importance in areas of long-term or intensive potato production and is largely uncontrolled. Symptoms of PED, which are difficult to distinguish from normal senescence, especially in early-maturing cultivars, are uneven chlorosis and necrosis of vines and tan discoloration of vascular tissues. The basic cause of PED is the soil fungusVerticillium. Two species are involved—V. alboatrum predominating in cooler areas andV. dahliae in warmer areas. Both fungi commonly occur in cultivated soils and persist as melanized hyphae or microsclerotia, respectively. Infection occurs through roots followed by colonization of the vascular system. Contamination of uninfested fields can occur by wind or mechanical movement of soil-borne propagules or introduction of infested seed stock. AlthoughVerticillium is the primary pathogen in potato early dying, other soil organisms are involved, resulting in a “disease complex.” Research is under way in many areas to further our understanding of these pathogenic interactions and to exploit this knowledge for use in new systems of prediction and control.     

Verticillium wilt of potato in Israel

Potato early dying, incited byVerticillium dahliae, was found to be the major factor in reducing the yield potential of potato grown under our conditions. Immediate attempts at control involved the screening of commercial cultivars and lines bred in both the USA and Europe, and various soil fumigation treatments. This work led to the selection of cultivars with reasonably high tolerance and the elucidation of the role that the nematodePratylenchus thornei plays in the disease syndrome. Elimination of the nematode by sprinkler applied metham-sodium in conjunction with some of the above cultivars now produces yields of 40–60 metric T/ha, depending on the cultivar and season. More recently, our research has been directed towards the use of serological methods to detect the pathogen in locally produced seed tubers and the utilization of an extracellular toxin produced by the pathogen in determining resistance to the fungus. With regard to the former, we have been able to produce an antibody that reacts solely toV. dahliae and can be used in several serological tests to determine the presence of the pathogen. Our work has also shown that utilization of the toxin offers a highly reliable technique in determining at least certain forms of resistance.     

Row fumigation with Vorlex for control of a meadow nematode (Pratlylenchus penetrans) in potatoes

Row fumigation with Vorlex at 3 and 7 gallons per acre was applied for meadow nematode (Pratylenchus penetrans) control in potatoes. The material was applied 3 weeks before planting using one chisel per row at a depth of 8″, and ridged. The treatments resulted in a reduction in population ofP. penetrans in the soil and in the potato roots. On a soil with a relatively high population of meadow nematodes, row application of Vorlex at 3 and 7 gal per acre resulted in a yield increase of 10 and 14% respectively, and a lower incidence of tubers with vascular discoloration. By early October as many nematodes were present in treated rows as in untreated rows. Row fumigation is of value for only one potato crop.     

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.     

Effect of Soil Temperature,Injection Depth,and Rate of Metam Sodium Efficacy in Fine-Textured Soils with High Organic Matter on the Management of Verticillium Wilt of Potato

Metam sodium is a widely used soil fumigant for Verticillium wilt management in most potato production regions of the United States. Environmental concerns regarding volatilization losses have led to restrictions on the use of metam sodium. The potato industry adapted to these restrictions by replacing metam sodium applications through sprinkler irrigation with shank injection applications of the fumigant. Previous research established parameters for effective shank application of metam sodium based on soil temperature, injection depth and rate of fumigation. However, these recommendations were based on research conducted under coarse-textured soil conditions with a low organic matter (OM) content (<1.3%). However, many potato production soils in North Dakota and Minnesota have a finer silt loam texture and OM contents of >2.5%. Therefore, it is important to know whether metam sodium fumigation recommendations for coarse-textured soils can be adapted to fine-textured soils. Two field trials were conducted using a split strip-block design for studying metam sodium efficacy in managing wilt. In both years, metam sodium injection depth and soil temperature at the time of injection did not result in significant differences in any study variable evaluated. All metam sodium fumigation rates significantly (P?<?0.05), lowered Verticillium microsclerotia, reduced wilt severity, and improved tuber yield compared to non-treated plots. However, significant differences among fumigation rates were not observed across any variable evaluated. A relatively low rate of 373 l/ha is as effective as higher metam sodium rates for effective control of Verticillium wilt. Results presented here suggest that current metam sodium recommendations for shank injection applications in coarse-textured soils can be implemented in field soils with a fine texture and higher OM content.     

Effects of soil fumigation with Telone-C on the field inoculum potential of tobacco rattle virus and potato yields

Two Julesburg sandy-loam fields in Weld County, Colorado, with a history of Corky Ringspot (CRS) caused by tobacco rattle virus (TRV) were injected with Telone-C as fall or spring applications and planted to potatoes. Deep soil sample cores taken from the experimental fields were planted toNicotiana tabacum L. var. Samsun serving as TRV bait and indicator plants. Systemic infection of bait plants and assay of the bait plant roots for TRV indicate that potential TRV inoculum was present which escaped the effects of fumigation treatments. TRV was found to be widely but erratically distributed throughout the experimental fields. The low incidence of CRS in tubers harvested from the treated fields suggests that fumigation may have short term benefits but potential inoculum exists which could lead to infection after the effects of fumigation have dissipated. The total yield of tubers was increased in the two cultivars planted (Norgold and Norland) in the fumigated fields receiving spring or fall applications. The yield of U.S. No 1 Grade potatoes in the Norland cultivar was increased significantly by both fall and spring fumigation. These yield increases were directly related to the reduced incidence ofVerticillium wilt observed in the fumigated soils and reported as a personal communication from unpublished data.     

Challenges to Organic Potato Farming: Disease and Nutrient Management

For organic potato producers the two main challenges are disease and nutrient management. Both factors are limited by regulations that on the one hand prohibit the use of chemical fertilisers, especially nitrogen and, on the other hand, most synthetic pesticides. Late blight, caused by Phytophthora infestans is commonly thought to be the factor most limiting yield. However, because there is no really effective fungicide available to control late blight, there are virtually no yield loss data available for organic farming conditions. In this paper the state of the art of organic potato management with respect to disease and nutrient management is summarised. In a second part, the interactive effects of N-availability in the soil, climatic conditions and late blight were studied in the presence and absence of copper fungicides from 2002–2004 for the mid-early main-crop potato cv. Nicola. From the experimental work it became clear that copper fungicides in most cases do slow down epidemics adding an average of 3 days to the growth duration. However, only 30% of the variation in yield could be attributed to disease reduction. A model including disease reduction, growth duration and temperature sum from planting until 60% disease severity was reached, and soil mineral N contents at 10 days after emergence could explain 75% of the observed variation in yield. However, the model failed when N-supply was extremely high. The implications of the results on the management of organic potatoes with respect to cultivar choice, nutrient and disease management are discussed. In conclusion, several points emerge from the results: In organic farming, yields are foremost limited by nutrient availability in spring and early summer. The effects of late blight on yields may often be overestimated and cannot be deducted from results in conventional farming because of the strong interaction with nutrient status. Resistance clearly remains the most important strategy against late blight in organic potato production. However, as important or even more important than resistance is the early development and bulking behaviour and the ability of a cultivar to make use of organic nutrients efficiently. In the absence of efficient organic pesticides it is possible to reduce blight pressure to a certain extent by arranging the crop in small narrow fields perpendicular to the main wind direction neighboured either by non-hosts or completely resistant potatoes.     

Effects of soil fumigation with Telone C on soil and plant nitrogen,verticillium wilt and yield of Norland and Norgold potatoes

Two fields in Weld County, Colorado were fumigated in the fall or spring with Telone C in an attempt to restore their productivity. The soils were Julesburg sandy loams and had been cropped to either corn or potatoes for many years. Observations were made over three years on the following: soil ammonium and nitrate nitrogen carry over, seasonal changes in these two forms of nitrogen, petiole nitrate concentration,Verticillium population, verticillium wilt expression,Verticillium stem infection and potato yields of two varieties. No significant differences occurred between control and fumigated blocks in the carry over of soil ammonium or nitrate nitrogen. During the growing season of the first year, no significant differences in soil nitrate concentrations occurred, while during the second and third year at certain times, all fumigated blocks contained significantly less nitrate nitrogen than the control block. Significant seasonal differences measured between blocks in soil ammonium concentration were not related to fumigation. Petiole nitrate concentrations while similar during the first year, were significantly higher in most fumigated treatments the second year and generally lower than the controls in the third year. Verticillium populations were not consistently affected by fumigation. Fluctuations not associated with fumigation prevented drawing definite conclusions. Both spring and fall fumigation reduced wilt severity in the two varieties. Effects of spring fumigation carried through two seasons while fall fumigation did not. Plant infection, determined by culturing stem sections, corresponded with the wilt severity ratings. Fall fumigation increased market yields of Norland potatoes but the increase was not statistically significant. Usually, both fall and spring fumigation significantly increased market yield of Norgold potatoes and over three years, spring fumigation was significantly better. Yield increases resulting from fumigation were considered greater than could be explained from alteration in the soil ammonium to nitrate ratios and/or verticillium wilt control as measured in this study.     

Approaches to control of potato early dying caused byVerticillium dahliae

Potato early dying disease, caused byVerticillium dahliae, may be either suppressed or controlled by a variety of procedures. These methods include the growing of resistant cultivars, attention to cultural management, utilization of pesticides, and solar heating of soils. The greatest successes in control of Verticillium wilt have been achieved with the use of either soil fumigants or clonal resistance, but these approaches have been restricted. Widespread use of fumigants has been limited by cost, and the lack of commercial acceptance has often restricted the utilization of resistant potato cultivars. The Katahdin and Targhee cultivars are currently available with resistance and/or tolerance to this disease. The Russet Burbank potato also possesses a limited degree of resistance to the fungus, and with this resistance, the disease appears to be suppressed by optimal growing conditions. In potato fields that have been cropped for several years, disease suppression in Russet Burbank was found to be related to methods of irrigation and increased nitrogen availability. It is suggested that improved utilization of resistant germplasm and effective use of biological control strategies may ultimately lead to economical control of this disease.     

Controlling parasitic nematodes and soil-borne diseases of potatoes with soil fumigation

The fumigation of soil infested with a moderate population of parasitic nematodes and fungi very effectively improved the growth, quality and yield of potatoes. The beneficial effects of fumigation with DD and Vorlex were mainly attributed to control of nematodes, particularly the meadow nematode. However, the fumigants did effectively reduce the population ofRhizoctonia solani on potato roots and “scurf” on tubers caused byR. solani. Also less vascular browning of tubers developed in fumigated soil. Thus the effects of soil fumigation were complex, affecting many microorganisms and consequently producing numerous manifestations in the growth of potatoes.     

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.     

Effects of differential rates of nitrogen and phosphorus on early blight in potatoes

The Kennebec variety of potato was exposed toAlternana solani inoculum during the 1970 and 1971 seasons to determine the effects of differential rates of nitrogen and phosphorus on the incidence of early blight. Both high nitrogen and low phosphorus treatments significantly reduced the incidence of early blight and the combination of high nitrogen and low phosphorus consistently gave the lowest incidence of the disease during both years. The data suggest that this combination of nutrients may be related to early blight resistance in the plant by extending the period of meristematic activity permitting the plant to wall off infection. Yield data indicated that there was too great a yield difference between fertilization for optimum early blight control and fertilization for optimum yield. Therefore, in Maine, potatoes should be fertilized for optimum yield with a reasonable specific gravity, and early blight should be controlled by the application of fungicides and sanitation measures.     

Developing equations to estimate potato yield loss caused by early blight in Minnesota

Regression analyses were performed on field data from 1981 and 1982 to determine models for estimating yield loss caused by early blight in potato cvs. Norland and Russet Burbank. Percent yield loss (total tubers and US# 1 tubers) caused by early blight could not be satisfactorily explained by any regression model using only a single disease assessment as the input variable. Several multiple point models, which explained more than 70% of the variation in yield loss due to early blight, were determined for each cultivar. The models for cv. Norland used either early blight severity assessments on days 56, 66, 76 and 86 after sprout emergence, or blight severity increments for 10-day periods among days 46–86, as the input variables. The models for cv. Russet Burbank used the same input variables as for cv. Norland and additionally, blight severity on day 96. Yield loss in US#1 tubers was a more stable variable than loss in total tubers for estimating early blight effect.     

The influence of inoculum density and environment on the development of potato blackleg

Significant increases in pre-emergence seedpiece decay and preemergence shoot infection were observed asErwinia carotovora var.atroseptica inoculum density increased. As inoculum density increased, a point was reached where virtually 100 per cent of the disease was expressed as preemergence decay and no typical postemergence blackleg occurred. Soil temperature and soil moisture markedly affected blackleg incidence, especially at low inoculum densities. Increasing soil temperature (from O°C to 30°C) and soil moisture (from 65 to 100% of field capacity) usually resulted in an increased total blackleg incidence. The effect of soil temperature and soil moisture appeared to be masked at high inoculum densities. Conditions such as low inoculum density and/or cool soil temperatures and low soil moistures which apparently slowed the rate of seedpiece decay usually increased the amount of postemergence blackleg, but also reduced the total amount of disease which occurred. It is suggested that conditions which are not optimal for the pathogen result in increased amounts of typical easily diagnosed postemergence blackleg expression, whereas conditions which favor the pathogen may reduce this phase of the disease but actually increase the total loss by increasing the amount of seedpiece decay and preemergence death of potato shoots.     

Effects of moldboard plowing,chisel plowing and rotation crops on the Rhizoctonia disease of white potato

Two tillage practices, chisel plowing (30 cm) and deep moldboard plowing (22 cm), and five rotation crops (oats, lupine, buckwheat, broccoli and peas) were studied for their effects on the soil population ofRhizoctonia solani AG-3 and on Rhizoctonia disease on potato. All rotation crops were harvested except buckwheat, which was treated as a green manure crop. Chisel plowing significantly reduced (p = 0.05) the incidence and severity of stem lesions on potato caused byR. solani AG-3. In 1990, oats after moldboard plowing significantly increased disease when compared to other crops and broccoli after chisel plowing decreased disease severity. Soil populations ofR. solani AG-3 were significantly lower with chisel plowing. No interactions between tillage and rotation crops were observed. Rhizoctonia solani Kühn is a soil inhabiting plant pathogen found worldwide that affects many plant species including white potato (Solanum tuberosum L.).R. solani attacks potato at one or more stages in development resulting in distinct disease symptoms (25) often termed the Rhizoctonia disease complex of potato. In Maine, only strains AG-3 and AG-5 ofR. solani (4, 5) have been identified as attacking potato and causing four distinct types of symptoms: 1) black scurf (sclerotia) on tubers, 2) stem cankers, 3) aerial tubers and top rosetting, and 4) killing of sprouts. Crop rotation has been reported to reduce the incidence and severity ofR. solani on potato, but no single rotation method controls completely or to a high degree of reliability (11, 23, 24, 26). Deep moldboard plowing has been shown to reduce diseases caused byR. solani andSclerotium rolfsii Sacc. in crops other than potatoes (2, 7, 15, 19, 20). However, Gudmestadet al. (6) reported that deep moldboard plowing increased the severity ofR. solani on stems and stolons of potatoes. The reduction of diseases caused byR. solani andS. rolfsii by moldboard plowing is attributed to low inoculum densities in the upper soil layer by the burial of sclerotia to depths where germination and infection were prevented (14, 15, 19, 20). However, disking to a depth of 5–7 cm did not affect disease as the inoculum remained in the root zone (14, 15). Gurkin (7) states that the rationale for deep moldboard plowing is to promote decay of organic matter, remove organic matter from the infection court and to bury the sclerotia below the infection court. Cultural control techniques are largely preventive and are designed to reduce the quantity or the activity of inoculum by means of crop rotation, tillage practices, green manure crops, etc. (22). This study was conducted to determine individual effects and possible interactions of deep moldboard plowing versus chisel plowing in various rotation crops on the presence ofR. solani AG-3 in the soil and on the incidence of Rhizoctonia disease complex of potato.