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.     

Synergistic interactions betweenRhizoctonia solani Kühn,Verticillium dahliae Kleb.,Meloidogyne spp. andPratylenchus neglectus (Rensch) Chitwood & Oteifa,in potato

Summary Samples of a sandy soil and a marine clay soil sterilized by steam were put in 55-1 containers insulated with polystyrene and placed outdoors on a brick pavement. Sandy soil was infested singly or in all possible combinations with root-knot nematodes (Meloidogyne spp.) and the fungiRhizoctonia solani andVerticillium dahliae, and the marine clay soil was infested with the root-lesion nematodePratylenchus neglectus and the same fungi to evaluate the effects of these organisms on the yield of potato. The experiments were carried out from 1983 to 1986. Tuber yield was reduced by single infestations of theMeloidogyne spp. andV. dahliae but not significantly byR. solani orP. neglectus. A three-factor interaction: nematode ×R. solani × V. dahliae was found in both experiments.R. solani andV. dahliae showed significant synergistic effects when soil was infested with theMeloidogyne spp. orP. neglectus.     

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.     

Effect ofUlocladium consortiale,a nonpathogen,on the severity of tuber blight caused byAlternar1a solani

Inoculation withAlternaria solani andUlocladium consortiale caused significantly more severe infection of tubers than inoculations ofA. solani alone.U. consortiale, when inoculated alone, caused no significant tuber infection. Infection resulting from a plug-type inoculation was significantly greater than that from a flap-type inoculation.     

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.     

Response of selected soilborne fungi and bacteria to herbicides utilized in potato crop management systems in Maine

The response of nine soilborne fungi associated with potato production:Rhizoctonia solani, Fusarium solani ‘Coerculeum,’F. roseum ‘Sambucinum,’F. avenaceum, Helminthosporium solani, Verticillium alboatrum, Alternaria solani, Trihcoderma viride, andLaetisaria arvalis; and five soilborne bacteriaAzotobacter chroococcum, Bacillus subtilis, Erwinia carotovora subsp. carotovora,Pseudomonas aeruqinosa andPseudomonas fluorescens to the herbicides linuron, metribuzin, dinoseb, paraquat, EPTC, and dalapon were determined by growth on amended media. None of the herbicides increased radial growth of the fungi, and only dinoseb significantly (P = 0.05) reduced radial growth of all fungi at concentrations less than 64 ppm. EPTC, dalapon, linuron, paraquat, and dinoseb severely inhibited bacterial survival (91, 82, 44%, respectively) when evaluated on trypticase soy and cimmons citrate agar media amended with herbicides at concentrations of 1, 10, and 100 ppm. Dinoseb, when applied to field soil in pots, significantly reduced total microorganism populations in the top 5 cm of soil. Results suggest that soil applications of herbicides can selectively inhibit soil microflora with possible effects on disease incidence and severity, and may also be a factor in determining effectiveness of biolgical control agents.     

Induction of soil suppressiveness to verticillium wilt of potato by successive croppings

Nine soils collected in New Jersey and Maryland were infested with microsclerotia ofVerticillium dahliae and planted with potato cv. Superior for five successive crops in the greenhouse to induce suppressiveness to Verticillium wilt. Potatoes were harvested 9–12 wk after planting and disease incidence was determined by recovery ofV. dahliae from stem segments on selective medium. Six of the nine soils had high disease incidence during the first cropping and remained conducive to Verticillium wilt throughout the experiment. Disease incidence increased in soils 3 and 9 over time, while disease incidence decreased nonlinearly in soil 1. Viability of microsclerotia buried in soil 9 for 4 wks was reduced compared to the viability of microsclerotia buried in soils 1, 3, and 6. Addition of 50 or 100 g hydrated lime/10 kg soil to soil 9 eliminated its suppressiveness to Verticillium wilt. Repeated cropping of the same soil can induce suppressiveness to Verticillium wilt. Soils exhibiting induced suppressiveness may be a source of new antagonists againstV. dahliae.     

The effect of storage environment on the infection of potato tubers byAlternaria solani

Manipulation of storage environment to provide favorable conditions for wound healing immediately after harvest reducedAlternaria solani tuber infection. Prestorage of tubers at 15.6°C for three weeks before final storage at 10°C normally maintained for commercial storage of potato chip stocks resulted in fewer and smaller lesions than constant storage at 10°C or 4.4°C or prestorage for varying periods at these temperatures. Prestorage of potatoes at a low temperature (4.4°C) for 1–3 months resulted in moreA. solani infection suggesting that low temperatures may inhibit the wound healing process more than they reduce rates ofA. solani infection or disease development. Variations in relative humidities above 75% did not have an appreciable effect on the wound healing process nor onA. solani infection.     

Effect of potato seed tuber glycoalkaloid content on subsequent infection byRhizoctonia solani

Since glycoalkaloids have been shown to have fungitoxic properties, it is important to know what impact this might have on potato plant breeding programs which select for low levels of total glycoalkaloids (TGA) in the tuber. Potato clones with TGA levels ranging between 1.6 and 32.8 mg/100 g fresh tissue were planted in two trials to evaluate the relationship between seed tuber TGA and incidence ofRhizoctonia solani infection in the developing plant. An inoculation procedure was followed in the greenhouse and field to establish uniform infection of potato seedlings. Plants were examined for lesion development on stems and stolons and assigned disease ratings. In the field, yield of malformed tubers was recorded and included in the overall disease ratings. The results reported herein indicate that TGA levels in the seed tuber are unrelated to the severity ofR. solani infection in the seedling.     

Pre-harvest burning for control of tuber infection byAlternaria solani

Pre-harvest propane burning of vines and soil surface significantly reduced populations ofAlternaria solani spores on the soil surface in field studies. Infection of potato tubers harvested from burned fields was reduced in most cases. Laboratory studies indicated a minimum temperature of 200°C for 1–2 seconds was required to significantly (P = 0.05) reduce germination ofA. solani spores. This temperature was achieved at the soil surface in the field when plots were burned at tractor speeds of 2, 3, 4 and 5 mph. In practice burning at only 2, 3 and 4 mph significantly reduced spore populations and tuber infection in the field. Recontamination of plots after burning occurred relatively rapidly. The data show that propane burning at tractor speeds of 2–4 mph can effectively reduceA. solani infection of potato tubers by lowering fungus populations on the soil surface.     

Root deterioration in the potato early dying syndrome: Causes and effects of root biomass reductions associated with colonization byVerticillium dahliae

Russet Burbank potatoes were grown in the greenhouse in a split-root culture system to assess the impact of pathogens associated with the Potato Early Dying Syndrome (PED) on root deterioration. Plants were grown with all, half, or none of the root system in steamed soil infested withVerticillium dahliae, or in field soil infested withV. dahliae and other PED pathogens. The direct impact of pathogens on root deterioration versus indirect effects resulting from premature senescence of the whole plant was evaluated. Root biomass did not differ significantly between root halves from the same plant in infested and uninfested soil. Root biomass from plants with both halves of the root system in uninfested soil was usually greater than from plants with one or both halves growing in infested soil. Root biomass decreased rapidly after plant stems were inoculated with conidial suspensions ofV. dahliae. Root deterioration in PED was associated with premature senescence of foliar tissue and occurred independently of root colonization by root-colonizing pathogens.     

Control of Rhizoctonia solani fruit rot of tomatoes by Trichoderma harzianum Rifai

Application of Trichoderma harzianum, to soil or by coating tomato fruits, reduced Rhizoctonia solani fruit rot by up to 43% and 85%, respectively, under laboratory conditions. When mixed with naturally infested soil, Trichoderma reduced R. solani inoculum potential by 86% in field trials. It also significantly reduced fruit rot by 27–51%.     

Influence of seed tuber infestation,chemical seed treatment,and pre-harvest climate on incidence of gangrene and dry rot of potato (Solanum tuberosum L.)

Summary In each of the five years 1978 to 1982, seed lots of cv. Bintje, variously infested byPhoma foveata Foister andFusarium solani var.coeruleum (Sacc.) Booth, as found by wound and cut tests, respectively, were planted in a series of field experiments at six sites in Sweden. A statistically significant relationship between seed and progeny infestation was found forP. foveata each year, and in three years forF. solani. Benomyl and thiabendazole mist treatments of seed tubers at grading in spring controlled the development of gangrene and dry rot in treated seed tubers, but did not consistently reduce the levels ofP. foveata andF. solani infestation in progeny from treated seed tubers. The site of cultivation markedly influenced mean levels ofP. foveata infestation, which were found to be related to temperature and rainfall during a 40 day pre-harvest period at the trail sites; however, mean levels ofF. solani infestation were not so related.     

Resistance toSpongospora subterranea in tuber-bearing and non tuber-bearingSolanum spp.

Summary The incidence of infection withSpongospora subterranea was studied in the non tuber-bearingSolanum brevidens andS. etuberosum and the tuber-bearingS. acaule, S. sucrense andS. tuberosum cvs Olympia and Pito. Shoot cuttings were grown in soil naturally infested withS. subterranea, or the roots were inoculated with a zoospore suspension. Logit models were used to analyse the data. The incidence ofS. subterranea was higher in plants inoculated with zoospore suspension than in those grown in infested soil (odds ratio (OR) 10.65). Ageing the inoculum reduced the incidence of infection in the plants (OR 0.30) without altering the interspecific differences. The ORs of infection (compared to cv. Olympia) were 0.07, 0.29, 0.60 and 2.88 forS. acaule, S. sucrense, S. brevidens andS. etuberosum. OnlyS. acaule was significantly more resistant to infection than cv. Olympia. No infection was detected in cv. Pito.     

The role ofErwinia carotovora in the epidemiology of potato blackleg. II. The effect of soil temperature on disease severity

Relative disease severity (seedpiece decay or post emergence blackleg) incited byErwinia carotovora var.carotovora (Ecc) andErwinia carotovora var.atroseptica (Eca) was studied in the field and the greenhouse. When inoculated tubers were planted in cool soils in the field or greenhouse (7.0–18.5°C average minimum and 16–26°C average maximum temperature during the first 30 days after planting)Eca caused significantly more disease thanEcc. Under these experimental conditionsEcc did not cause significantly more disease than that found in non-inoculated controls. When soil temperatures were high at planting time (21.4–24.0°C average minimum and 29.6–35.0°C average maximum temperature for the first 30 days after planting), inoculation withEcc resulted in severe disease whileEca produced no more infection than that found in uninoculated controls. At intermediate soil temperatures both were infective. Seedpiece decay by both organisms was correlated with soil temperatures during the first 30 days after planting. As the soil temperature increased, inoculation withEcc resulted in more disease than inoculation withEca. At lower soil temperatures the opposite result was obtained. Field and greenhouse studies showed that bothEca andEcc were able to cause typical post emergence blackleg infection, the former at low soil temperature and the latter at high temperature conditions. The data suggest that bothEcc andEca are components of the potato blackleg and soft rot disease complex. Environmental conditions, especially soil temperature, determine which organism is causative under a particular set of conditions.     

Effect of infection byRhizoctonia solani on the quality of tubers for processing

The effect ofRhizoctonia solani infection on the yield and quality of tubers destined for processing was evaluated in a 10 × 2 factorial experiment. Inoculation of 10 cultivars withR. solani significantly decreased total and marketable yields and significantly increased the number of malformed and fissured tubers as well as the number of tubers with black scurf. Specific gravity was significantly decreased in 3 of the 4 years. Chip color at harvest was adversely affected in 2 of the 4 years and over the 4 years combined; whereas chip color three months after harvest was adversely affected in all 4 years.     

Prevention,with Trichoderma harzianum Rifai aggr., of reinfestation by Sclerotium rolfsii Sacc. and Rhizoctonia solani Kühn of soil fumigated with methyl bromide,and improvement of disease control in tomatoes and peanuts

Application of Trichoderma harzianum Rifai aggr. after soil fumigation with methyl bromide improved the control of Sclerotium rolfsii Sacc. and Rhizoctonia solani Kühn in a peanut field. Although soil fumigation controlled the diseases caused by these pathogens, it was followed by rapid reinfestation by S. rolfsii and R. solani. The biological control agent T. harzianum prevented reinfestation of the fumigated soil by the pathogens (88% reduction) both in a controlled environment and in field conditions. In soil treated with T. harzianum, survival of sclerotia was considerably less than in the untreated control. The combined treatment, of fumigation and T. harzianum applications, caused almost total mortality of sclerotia in soil in the laboratory and in the field. Application of T. harzianum to the root zone of tomatoes effectively controlled S. rolfsii in a field naturally infested with S. rolfsii and R. solani. Transplanting plants treated with T. harzianum into soil fumigated with methyl bromide reduced disease incidence by 93% and increased yield by 160%.     

Effect of Brassicaceae seed meals with different glucosinolate profiles on Rhizoctonia root rot in wheat

The soil-borne pathogen Rhizoctonia solani AG 8 causes major yield losses in wheat (Triticum aestivum. L) production worldwide. Plant tissues of Brassicaceae species contain glucosinolates that are hydrolyzed in the presence of the enzyme myrosinase into products with pesticidal properties. Growth chamber studies were conducted to determine the effect of the Brassicaceae seed meals (SMs) from Brassica juncea, Brassica napus and Sinapis alba on the suppression of the R. solani AG 8 infection of winter wheat. Pasteurized sandy soils were amended with intact and denatured SMs of rape seed and mustard at a rate of 0.5% by soil weight. Regardless of the glucosinolate type and content, all intact and denatured Brassicaceae significantly reduced the infection of winter wheat seedlings by R. solani AG 8 compared to the un-amended control. However, soils amended with S. alba SMs had the lowest severity of Rhizoctonia root rot relative to other amended soils. Phytotoxicity arising from the use of Brassicaceae SMs was observed particularly in soils amended with high glucosinolate-containing SMs. These studies demonstrate that Brassicaceae SMs can be used to manage disease caused by R. solani AG-8. However, future studies will need to focus on strategies for diminishing the crop growth-reducing effects associated with Brassicaceae SM amendment to fully maximize these fungicidal benefits.     

Mycoflora and nutritional value of shelled melon seeds (Citrulus vulgaris Schrad.) in Nigeria

Thirteen fungi were isolated from mouldy shelled melon seeds. The fungi more frequently isolated included species ofMucor (11.25%),Rhizopus (13.75%),Aspergillus (36.25%),Macrophoma (2.50%),Penicillium (8.75%),Alternaria (5.00%),Fusarium (8.75%),Botrytis (6.25%),Torula (3.75%) andGeotrichum (3.75%). Healthy, shelled melon seeds inoculated withFusarium solani for 7 or 14 days caused increases in the free fatty acid (f.f.a.) content of the healthy seeds from an initial value of 1.06% to 2.19% after 7 days and 4.23% after 14 days. Similar results were obtained when other fungal isolates were used as inocula butFusarium solani effected the greatest increase in the f.f.a. content followed byAspergillus niger andPenicillium notatum. The fungi also caused decreases in the crude protein from 35.51% to 25.16%, crude fibre from 4.30% to 1.35% and total carbohydrate content from 4.28% to 3.01% of the seeds after 14 days infection. Aflatoxin was detected (0.20µg/g) in the infected seeds being sold in the markets and also from seeds inoculated with spores (0.40µg/g) ofA. flavus and (0.50µg/g) from a mixture of spores of variousAspergillus strains after incubation for 14 days.     

Lesion nematode involvement in potato early dying disease

Ohio field studies in microplots in the past 6 years have demonstrated a synergistic interaction of the lesion nematodePratylenchus penetrans withVerticillium dahliae in potato early dying. Tests with various populations ofP. penetrans andV. dahliae, alone and in combinations, have shown that disease occurs when both pathogens are present at populations which individually have little or no effect. Yield loss is not necessarily associated with foliar symptom development, but occurs with the addition of high temperature stress during tuberization. Comparative studies with two other species ofPratylenchus commonly found in Ohio potato soils have shown thatP. crenatus does not interact withV. dahliae and thatP. scribneri interacts only slightly. Since all three species feed and reproduce well on potato,Verticillium interactions probably are not due to wounding. The involvement ofPratylenchus withV. dahliae in potato early dying is an important factor that must be considered in the development of future crop-loss predictive systems.