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.     

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.     

Evidence for the influence of weeds on corky ringspot persistence in alfalfa and Scotch spearmint rotations

Corky ringspot disease (CRS) of potato is caused by tobacco rattle virus (TRV). The virus is transmitted by the stubby root nematode (Paratrichodorus allius) in the Pacific Northwest potato-producing regions. Alfalfa (Medicago sativa L.) and Scotch spearmint (Mentha cardiaca Baker) rarely serve as hosts for TRV. Therefore,P. allius reared on these plants for 1 to 3 months are cleansed of TRV in greenhouse trials. However, weeds in alfalfa and Scotch spearmint rotation crops may serve as hosts for the virus. In greenhouse trials, hairy nightshade (Solanum sarrachoides), prickly lettuce (Lactuca serriola), henbit (Lamium amplexicaule) and, green foxtail (Setaria viridis) grown alone were found to be suitable hosts ofP. allius, whereas Powell amaranth (Amaranthus powellii) was not. ViruliferousP. allius added to hairy nightshade, prickly lettuce, henbit, green foxtail, or Powell amaranth in mixtures with alfalfa and/or Scotch spearmint occasionally remained viruliferous over a 3-to 4-month period, whereasP. allius maintained on weed-free alfalfa or Scotch spearmint became virus-free after 1 to 2 months. Potato grown in soil containingP. allius that had been maintained on weed-alfalfa or weed-Scotch spearmint mixtures for 3 to 4 months exhibited slight to severe CRS symptoms on new tubers, whereas potato following weed-free Scotch spearmint or alfalfa were free from CRS symptoms. Severe CRS symptoms on potato tubers were only observed when potatoes were grown in soil containingP. allius that were maintained on hairy nightshade or hairy nightshade mixtures with alfalfa or Scotch spearmint. These preliminary data suggest that the presence of weeds that serve as hosts of both TRV and P.allius may nullify the positive effects of growing alfalfa or Scotch spearmint for CRS control. Targeted control efforts of known weed hosts may be required to successfully eliminate CRS from fields using alfalfa and Scotch spearmint rotational crops.     

Management Effects of Disease-Suppressive Rotation Crops on Potato Yield and Soilborne Disease and Their Economic Implications in Potato Production

Soilborne potato diseases are persistent problems in potato production. Use of disease-suppressive rotation crops, such as Brassica spp. (mustards, rapeseed) and sudangrass, has shown potential for management of soilborne diseases and enhanced yield in various crop production systems. However, how to best implement these crops into productive potato cropping systems has not yet been determined. In this research, potential disease-suppressive crops were evaluated under four different types of production management (as a cover crop, green manure, harvested crop-residue incorporated, and harvested crop-residue not incorporated) in potato rotation field trials, and their effects on disease, yield, and economic viability determined. Mustard blend, sudangrass, and rapeseed rotations reduced the tuber disease black scurf (by 16–27 %) and increased yield (by 6–11 %) relative to a barley rotation control, but only mustard blend consistently reduced common scab (by 11 %). All rotation crops managed as green manures produced lower disease (by 15–26 %) and higher yields (by 6–13 %) than other management practices. Overall, the combination of mustard blend managed as a green manure was most effective, reducing scurf by 54 % and increasing yield by 25 % relative to a soybean cover crop. The use of mustard or rapeseed as a harvested crop with incorporation provided the best economic return, increasing net income by more than $860/ha relative to the standard barley rotation, but mustard blend grown as a green manure or non-incorporated harvest crop also substantially increased net income ($600 to $780/ha).     

Cropping sequence and tillage system influences annual crop production and water use in semiarid Montana,USA

Available water is typically the biggest constraint to spring wheat production in the northern Great Plains of the USA. The most common rotation for spring wheat is with summer fallow, which is used to accrue additional soil moisture. Tillage during fallow periods controls weeds, which otherwise would use substantial amounts of water, decreasing the efficiency of fallow. Chemical fallow and zero tillage systems improve soil water conservation, allowing for increased cropping intensity. We conducted a field trial from 1998 through 2003 comparing productivity and water use of crops in nine rotations under two tillage systems, conventional and no-till. All rotations included spring wheat, two rotations included field pea, while lentil, chickpea, yellow mustard, sunflower, and safflower were present in single rotations with wheat. Growing season precipitation was below average most years, resulting in substantial drought stress to crops not following fallow. Preplant soil water, water use, and spring wheat yields were generally greater following summer fallow than wheat recropped after wheat or alternate crops. Water use and yield of wheat following summer fallow was greater than for chickpea or yellow mustard, the only other crops in the trial that followed summer fallow. Field pea performed best of all alternate crops, providing yields comparable to those of recropped spring wheat. Chickpea, lentil, yellow mustard, safflower, and sunflower did not perform well and were not adapted to this region, at least during periods of below average precipitation. Following summer fallow, and despite drought conditions, zero tillage often provided greater amounts of soil water at planting compared to conventional tillage.     

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.     

Inoculum potential of Verticillium species in crop and weed residues overwintering in infested soil

Viable propagules ofVerticillium albo-atrum were recovered from roots and stems of naturally infected weeds and sunflower cultivars before or after burial in the field and from stems and roots of certain non-infested crops after burial in the field for seven months.Verticillium dahliae was recovered less frequently, then only from the roots ofSolanum nigrum (black nightshade). Survival of these pathogens after burial for 7 months varied considerably, ranging from 38 to 58% in sunflower stems and from 19 to 235% in roots of weeds. In stems of weeds and crops which did not contain propagules ofV. albo-atrum before burial, the fungus was recovered consistently after burial, indicating that saphophytic colonization probably occurred during interment.     

An economic analysis of potential rotation crops for Maine potato cropping systems

Potato cropping systems in Maine include both continuous potatoes and short-term potato rotations with small grains. Producers recognize the benefits of increased rotations, but the economics of producing a high-valued crop such as potatoes (Solanm tuberosum L.) create incentives for continuous potato production. Research at the USDA-ARS research site in Newport, ME, is evaluating the agronomic and economic impacts of five crops in two-year rotations on potato production and whole-farm profitability. The rotation crops are barley (Hordeum vulgare L.), sweet corn (Zea mays L.) green bean (Phaseolus vulgares L.), soybean (Glycine max L., Mer.), and canola (Brassica napus L.). Enterprise budgets for the five crops were developed. The budgets and historical prices and yields were used as inputs to a Monte Carlo simulation. The simulation was conducted to determine the impact of rotation crops on whole-farm profitability and income risk, as measured by income variability. The net incomes of the five rotation sequences were compared against continuous potatoes. Two rotation crops, sweet corn and green beans, resulted in an increase in net income relative to continuous potatoes. AU of the rotation crops were found to greatly reduce income risk and chance of economic losses. In the case of green beans and sweet corn, the analysis was rerun using data from the research trials on the following potato crop yields. Depending on whether the rotation effect was negative or positive, net income either fell or rose when compared to fist analysis. However, even when the rotation crop led to decreased yields in the following potato crop, income variability and likelihood of economic loss was still superior to the continuous potato rotation. These findings provide support for including rotation crops as a method to improve potato production and sustainability, increase wholefarm profitability, and reduce income risk.     

Diverse no-till irrigated crop rotations instead of burning and plowing continuous wheat

Field burning of residue is a traditional management tool for irrigated wheat (Triticum aestivum L.) production in the Inland Pacific Northwest of the United States (PNW) that can result in reduced air quality. A 6-year no-till field experiment to evaluate two complete cycles of a 3-year irrigated crop rotation of winter wheat–spring barley (Hordeum vulgare L.)–winter canola (Brassica napus L.) was sown (i) directly into standing residue of the previous crop, (ii) after mechanical removal of residue and, (iii) after burning of residue. The traditional practice of continuous annual winter wheat sown after burning residue and inverting the topsoil with a moldboard plow was included as a check treatment. Over-winter precipitation storage efficiency (PSE) was markedly improved when residue was not burned or burned and plowed after grain harvest. Grain yield of winter wheat trended higher in all no-till residue management treatments compared to the check treatment. Average grain yields of spring barley and canola were not significantly different among the no-till residue management treatments. Winter canola failed in 5 of 6 years due to a combination of a newly identified Rhizoctonia damping-off disease caused by Rhizoctonia solani AG-2-1 and cold temperatures that necessitated replanting to spring canola. Six-year average net returns over total costs were statistically equal over all four systems. All systems lost from $358 to $396 ha^?1. Soil organic carbon (SOC) increased linearly each year with no-till at the 0–5 cm depth and accumulated at a slower rate at the 5–10 cm depth. Take-all of wheat caused by Gaeumannomyces graminis var. tritici was most severe in continuous annual winter wheat. The incidence and severity of Rhizoctonia on roots of wheat and inoculum of R. solani AG-8, was highest in the no-till treatments, but there was no grain yield loss due to this disease in any treatment. Residue management method had no consistent effect on Rhizoctonia root rot on barley. The annual winter grass downy brome (Bromus tectorum L.) was problematic for winter wheat in the standing and mechanically removed residue treatments, but was controlled in the no-till residue burned and the burn and plow check. Another winter annual grass weed, rattail fescue (Vulpia myuros L.), infested all no-till treatments. This was the first comprehensive and multidisciplinary no-till irrigated crop rotation study conducted in the Pacific Northwest.     

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.     

Effects of broad-leaf crops and their sowing time on subsequent wheat production

Lupin, field pea, lentil, chickpea, canola, linseed, and barley were sown at different times (late April-early July) to study their effects on subsequent wheat production on a red earth at Wagga Wagga, New South Wales. The cultivars of field pea (Pisum sativum) included Dunn, Derrimut, Maitland and Dinkum; narrow-leaf lupin (Lupinus angustifolius) cultivars were Danja, Geebung and Gungurru, and either the L. angustifolius line 75A/330 (1989–1990) or the broad-leaf lupin L. albus cv. Ultra (1991–1992). Only one cultivar of the other crops was grown in any year and after 1989 lentil (Lens culinaris cv. Aldinga) was replaced by chickpea (Cicer arietinum cv. Amethyst). The canola (cv. Shiralee (1989–1991), cv. Barossa (1992)) and linseed (cv. Glenelg) rotations received annual applications of 40–50 kg N/ha as urea.Compared to barley and the oilseeds, grain legumes increased soil mineral N supply to the following wheat crop. Over 4 years the mean wheat grain yield response to a broad-leaf crop, relative to barley, was 115% for lupin, 84% for field pea, 88% for linseed and 86% for canola. However, the effects of the various crops on subsequent wheat grain yields and grain protein varied markedly with season. The main advantage of lupin over field pea occurred in years when disease reduced growth of field pea crops. In high rainfall years, wheat yields following lentil and chickpea were lower than those following lupin. The narrow-leaf cultivars of lupin promoted greater wheat yields than either the reduced branching line 75A/330 or the broad-leaf albus cultivar Ultra. Delayed sowing of lupin reduced yield and grain protein of wheat, except when low rainfall curtailed growth of either crop in the rotation. There was little effect of field pea sowing date on wheat grain yield but sowing in late June combined with a dry spring, reduced mineral N supply and grain protein. Late sowing of oilseeds had no consistent effect on wheat grain yield but increased grain protein in most years. Late sowing of barley typically increased wheat grain yield but had little effect on grain protein. The effects of sowing time were mainly attributed to effects on soil N supply and for barley on disease incidence in the subsequent wheat.     

Screening and Future Exploitation of Allelopathic Plants as Alternative Herbicides with Special Reference to Hairy Vetch

Summary

Allelopathic cover crops are one of the most promising application of allelopathy for weed control and reduce the synthetic herbicides input. Fifty three cover crop plant species (26 leguminous, 19 graminaceous and 8 others) were assessed for the allelopathic activity using Plant Box Method. It was found that leguminous cover crops such as hairy vetch (Vicia villosa) and velvetbean (Mucuna pruriens), graminaceous cover crops, such as oat (Avena sativa) and rye (Secale cereale), certain cultivars of wheat (Triticum aestivum) and barley (Hordeum vulgare) were promising. By bioassay screening, 23 species were selected for field tests. Fall-sown cover crops such as hairy vetch, rye, wheat, oat, grass pea, and mustard are more effective when compared to spring-sown cover crops. Hairy vetch was most promising for the weed control in abandoned fields because of its ability to die off during summer season to make a thick straw-like mulch.     

Effects of Soil Water Level,Black Dot (Colletotrichum coccodes) Infested Soil and Nutrient Depletion on Potato in a Controlled Environment

The effects of soil water level and soil infested or not infested with Colletotrichum coccodes were quantified and compared on Umatilla Russet potato in repeated greenhouse trials. Nitrogen levels in leaflets and tuber yield differed significantly for effect of water level but there was no effect for soil infestation in both trials. More leaflet N as measured by chlorophyll and less tuber yield occurred in the low than the medium and high soil water treatments. Number of progeny tubers was not affected by C. coccodes but numbers were significantly less for the low water level than the high water level in one trial. Root weight was significantly reduced by C. coccodes in both trials and was significantly less in the high than the low and medium soil water levels in one trial. Incidence of infected progeny tubers was significantly reduced in infested soils for the low soil water compared to the medium or high soil water levels in one trial. The effect of increasing levels of water in infested soils had large and significant increases for percentage of stem area with sclerotia in both trials. Managing soil water by not overwatering in irrigated potato fields in the presence of C. coccodes may reduce black dot severity and quantity of sclerotia that potentially can overwinter and serve as sources of infection for subsequent crops. Analyses demonstrated a potential for significant associations between plant and disease variables not evidence for cause and effect.     

Evaluating Incidence of Helminthosporium solani and Colletotrichum coccodes on Asymptomatic Organic Potatoes and Screening Potato Lines for Resistance to Silver Scurf

Silver scurf, caused by Helminthosporium solani, and black dot, caused by Colletotrichum coccodes, cause tuber blemishes on potato (Solanum tuberosum L.) which affect processing and fresh market trade. Tubers from ten cultivars were collected at harvest from three organic farms in Wisconsin and categorized as symptomatic or asymptomatic based on visual symptoms of silver scurf and black dot and/or signs of H. solani and C. coccodes. Tuber incubation and PCR assays were performed on asymptomatic tubers to detect H. solani and C. coccodes. Tuber incubation and PCR assays were in slight to fair agreement (kappa coefficient <0.4) for detecting both pathogens. Most asymptomatic tubers tested were positive by one or both assays for H. solani (75 %) or C. coccodes (94 %). Minituber inoculation assays were also performed to screen potato lines for resistance to silver scurf. Of the 14 lines tested, a diploid interspecific hybrid, C287, had consistently low sporulation, suggesting it has partial resistance to silver scurf. Since the majority of tubers harvested are already infected with one or both pathogens further research should focus on organically acceptable management practices that may inhibit disease development in field and in storage.     

Weed hosts ofParatrichodorus allius and tobacco rattle virus in the Pacific Northwest

The ability of several weed species to serve as hosts for tobacco rattle virus (TKV), the causal agent of corky ringspot disease of potato (CRS), and its nematode vector,Paratrichodorus allius, was investigated in greenhouse studies. ViruliferousP. allius multiplied on 24 out of 37 weed species tested, indicating they were suitable hosts of the vector. However, only 11 of these weeds were infected with TRV, as determined by ELISA. The nonhost status of a given weed species was not changed whether the viruliferous vector population originated from CRS problem fields in WA, OR, or ID. Several weeds served as hosts for the vector and virus including kochia, prickly lettuce, henbit, nightshade species (black, hairy, and cutleaf), common chickweed, and annual sowthistle. Virus-freeP. allius acquired TRV from the three nightshade species, volunteer potato grown from TRV-infected tubers, and prickly lettuce, and subsequently transmitted the virus to ‘Samsun NN’ tobacco indicator plants. Thus, some weeds may play a role in the epidemiology of CRS by perpetuating TRV and its vector in a problem field.     

The Potential Effect of Climate Change and Elevated Air Carbon Dioxide on Agricultural Crop Production in Central and Southeastern Europe

SUMMARY

The vulnerability and adaptation of major agricultural crops to different soils in Austria and Bulgaria under a changing climate and elevated air CO₂ were investigated. Several incremental and transient GCM climate change scenarios were created and applied. Warming will decrease the crop-growing duration of the selected crops in the regions of interest. All GCM scenarios, including the climate change effect only, projected reductions in grain yield of winter wheat and spring barley, caused by a shorter crop-growing period. However, when the direct effect of an increased CO₂ level was assumed, most GCM climate change scenarios projected an increase in wheat and barley yield and especially in soybean yield. An increased level of CO₂ alone had no significant impact on the simulated maize yield reductions under climate change.     

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.     

Clover cover crops under-sown in winter wheat increase yield of subsequent spring barley—Effect of N dose and companion grass

Four two-year field trials, arranged in randomised split-plots, were carried out in southern Sweden with the aim of determining whether reduced N fertiliser dose in winter wheat production with spring under-sown clover cover crops, with or without perennial ryegrass in the seed mixture, would increase the clover biomass and hence the benefits of the cover crops in terms of the effect on the wheat crop, on a subsequent barley crop and on the risk of N leaching. Four doses of nitrogen (0, 60, 120 or 180 kg N ha⁻¹) constituted the main plots and six cover crop treatments the sub-plots. The cover crop treatments were red clover (Trifolium pratense L.), white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) in pure stands and in mixtures. The winter wheat (Triticum aestivum L.) was harvested in August and the cover crops were ploughed under in November. The risk of N leaching was assessed in November by measuring the content of mineral N in the soil profile (0–30, 30–90 cm). In the following year, the residual effects of the cover crops were investigated in spring barley (Hordeum distichon L.) without additional N. Under-sowing of cover crops did not influence wheat yield, while reduced N fertiliser dose decreased yield and increased the clover content of the cover crops. When N was applied, the mixed cover crops were as effective in depleting soil mineral nitrogen as a pure ryegrass cover crop, while pure clover was less efficient. The clover content at wheat harvest as well as the amount of N incorporated with the cover crops had a positive correlation with barley yield. Spring barley in the unfertilised treatments yielded, on average, 1.9–2.4 Mg DM ha⁻¹ more in treatments with clover cover crops than in the treatment without cover crops. However, this positive effect decreased as the N dose to the preceding wheat crop increased, particularly when the clover was mixed with grass.     

Response ofColletotrichum coccodes to fungicidesin vitro

Black dot is an important disease of potato that affects all plant parts. The causal agent,Colletotrichum coccodes, is commonly found on potato tubers used for seed. Our objective was to determine thein vitro sensitivity ofC. coccodes to the fungicides maneb, thiabendazole, imazalil and CGA 173506, a phenylpyrrole. These fungicides are currently used, or are being tested for use, as potato seed treatments in the United States. All four fungicides reduced radial growth, sclerotial germination and spore germination ofC. coccodes at 10 μg/ml and higher. The fungicide concentrations that inhibited radial growth by 50% were 2 μg/ml, 5 μg/ml, 35 μg/ml, and 40 μg/ml for imazalil, CGA 173506, maneb and thiabendazole, respectively. TBZ was less inhibitory than the other three fungicides and stimulated sclerotial productionin vitro. Isolates ofC. coccodes differed in their sensitivity to maneb, thiabendazole and imazalil in radial growth, but not spore and sclerotial germination, assays. On CGA 173506, radial growth of fourC. coccodes isolates was inhibited, but these isolates produced sectors which grew normally. Germination of most sclerotia from these isolates was inhibited, but some germinated and produced normal colonies in the presence of CGA 173506. OneC. coccodes isolate was resistant to this fungicide in all assays.