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

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

Glycoalkaloids and resistance to the Colorado potato beetle inSolanum chacoense Bitter

Resistance of 20 F₂ Solarium chacoense Bitter clones to the Colorado potato beetle,Leptinotarsa decemlineata (Say), was measured in a field test. Levels of total glycoalkaloids (TGA) and composición of the glycoalkaloid mixtures in foliage of the clones were also determined. Clones with either commersonine or dehydrocommersonine as the major foliar glycoalkaloid were significantly more resistant (lower damage ratings, fewer larvae and adult insects) than clones with solanine and chaconine. Damage ratings were negatively correlated (r = -0.67, p = 0.01) with foliar TGA levels. The results indicate that the types of glycoalkaloids present in the foliage ofS. chacoense may be as important as the level of TGA in limiting damage and numbers of insects.     

Evaluation ofLaetisaria arvalis as a biological control agent ofRhizoctonia solani on white potato

Laetisaria arvalis, a soil-inhabiting basidiomycete, reducedRhizoctonia solani infection of white potato. A greenhouse study that monitored population levels of both fungi showed that soil infested withL. arvalis suppressed the growth ofR. solani after eight wks. Populations ofL. arvalis andR. solani increased after 11 wks. when both fungi were infested simultaneously and whenL. arvalis was introduced to soil previously infested withR. solani. Five years of field studies showed that variousL. arvalis seed treatments and soil amendments reducedR. solani infection of stems and stolons and sclerotial formation on tubers equal to the standard chemical treatment of pentachloronitrobenzene (PCNB) + thiabendazole. No relationship was shown between populations ofL. arvalis andR. solani in field plot soils, but native or wild strains ofL. arvalis were shown to be present at detectable levels in field soils.     

Soil survival and thiabendazole sensitivity ofHelminthosporium solani isolates from Alberta,Canada

Summary Survival of the pathogen causing silver scurf of potato (Helminthosporium solani) in Alberta soils was studied by evaluatingH. solani infection in the progeny ofH. solani-free nuclear seed potato tubers planted in fields where potatoes had never been grown or were grown 1, 2, 3, or 4 years previously. Daughter tubers from all the fields developed silver scurf lesions, andH. solani was isolated from infected tubers. This is the first report of survival ofH. solani in Alberta soils. Soil-borne inoculum appears to have a role in the epidemiology of the disease and in the introduction of the pathogen into silver scurf-free potato seed stock. Of 31 plant species tested, only potato was found to be a host ofH. solani. Most of theH. solani isolates from north central Alberta were more sensitive to thiabendazole than those from southern Alberta, where thiabendazole is much more commonly used.     

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.     

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.     

Contamination of soil and transmission of seedborne potato dry rot fungi (Fusarium spp.) to progeny tubers

A survey of selected potato soils in Maine was made to determine the species and populations ofFusarium spp. present.Fusarium solani ‘Coeruleum’ was most often isolated butFusarium roseum ‘Sambucinum’ was not recovered from any soil. Crops or cropping sequences in general showed no direct relationship to observed Fusaria populations or tuber dry rot. Field studies were conducted to determine the effect ofFusarium contaminated seed on soil and daughter tuber contamination. Whole and cut seed were inoculated withFusarium roseum ‘Sambucinum’ and half of each lot treated with thiabendazole (1500 ppm) to controlFusarium seed decay. Soil populations ofF. roseum ‘Sambucinum’ were higher near plants produced from non-treated, cut seed tubers. No increase inFusarium populations was found between plants or near plants produced from whole or thiabendazole treated seed. Daughter tuber contamination was greatest from plants produced from non-treated contaminated cut seed, and lowest from whole or treated seed. Whole non-treated seed produced daughter tubers with contamination equal to treated whole seed. This indicates that the use of whole seed could be a method to reduce daughter tuber contamination and reduce reliance on chemical treatments. Cut seed contaminated withFusarium spp. is an important source of daughter tuber contamination which can result in a high amount of tuber dry rot in storage.     

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.     

The effect of tuber-borne sclerotia ofRhizoctonia solani Kühn on the potato crop

Black scurf on seed tubers did not result in seed decay or stem cankers and did not affect the tuber yield in two field experiments at Ottawa and Fredericton. Seed pieces were classified into three disease categories: 0 to 1%, 1.1 to 5%, 5.1 to 15%, according to the percentage surface area covered by sclerotia ofRhizoctonia solani. The difference between disease categories, in the amount of black scurf on the seed, was perpetuated on the progeny tubers although the actual amount decreased.     

Glandular pubescence,glycoalkaloid composition,and resistance to the green peach aphid,potato leafhopper,and potato fleabeetle inSolanum berthaultii

Fifteen accessions ofSolarium berthaultii Hawkes andS. berthaultii xS. tarijense Hawkes were assessed for resistance to field infestations of the green peach aphid,Myzus persicae (Sulzer), potato leafhopper,Empoasca fabae (Harris), and potato fleabeetle,Epitrix cucumeris (Harris). Accessions bearing both Type A and Type B glandular trichomes were much more resistant to the green peach aphid and potato leafhopper than accessions bearing Type A hairs alone. All accessions had significantly smaller populations of these 3 pests than S.tuberosum cultivars. Total glycoalkaloid (TGA) content of foliage and tubers was not correlated with insect populations. Foliar TGA levels of field-grown plants varied among accessions, ranging from < 2–240 mg/100 g fresh wt. Solasonine and solamargine were the major foliar glycoalkaloids while solamarines predominated in tubers.     

Growth and yield of potatoes as affected by severity of stem canker (Rhizoctonia solani)

Summary Different amounts ofRhizoctonia solani inoculum were applied to sprouted and non-sprouted seed tubers cultivar Désirée during planting in field experiments at Rothamsted in 1984 and 1985. Severity of stem canker and stolon infection increased with increasing amounts of inoculum; with small or medium amounts these infections were most severe on plants from sprouted seed. In 1985 plants grown from non-sprouted, seed had most stem canker when large amounts of inoculum were applied. Severe infection following high inoculum levels delayed shoot emergence, increased the variability in stem height and decreased the mean height of stems and weight of foliage but did not affect the number of stems per plant or final plant populations. Tuber initiation was delayed and tuber numbers slightly decreased by severe infection and at harvest the size distribution of tubers was altered. Tuber yields from severely infected plants were decreased by 15% (sprouted seed) and 19% (non-sprouted seed) in 1984 and by 17 and 28% respectively in 1985.     

Seed Potato Performance after Storage in Light at Elevated Temperatures

In regions with short growth seasons, it is of great importance to use potato (Solanum tuberosum L.) seed tubers with a high growth vigour and a short growth cycle. Such qualities may be obtained by treatments advancing the physiological age of the seed tubers. In this study, we have exposed tubers from four cultivars to various combinations of temperature and light conditions (green-sprouting) for 3–7 months in controlled climate. Subsequent sprout quality, seed tuber health and performance were studied in laboratory, greenhouse and field trials. Satisfactory short, sturdy and leafy sprouts were produced even after 7 months storage at 15 °C under light exposure. An assay of black scurf (Rhizoctonia solani) on the tuber skin showed that light exposure significantly reduced the occurrence compared with dark-stored tubers, while the average effect of storage temperatures was insignificant. In general, green-sprouting advanced emergence and plant growth by 1–2 weeks, and showed early tuber initiation and growth, compared to untreated material. Yields, 107 days after planting in the field trial, did not deviate significantly from untreated tubers. However, plant development at harvest was in accordance with general responses to physiological ageing of potato seed tubers, i.e. still tall and immature plants from untreated tubers, and short and mature plants from aged tubers. Results demonstrated the possibility of successful long-term storage of potato seed tubers in light at elevated temperatures and a potential for earlier harvests and higher early yields from such treatments.     

Steroid glycoalkaloids and related compounds as potato quality factors

The steroid glycoalkaloids are triterpenoid derivatives which are found in all tissues of the potato plant including the tubers. The compounds are largely localized in the peel of tubers, but tissue beneath the peel rapidly accumulates the steroid glycoalkaloids to levels equal to or greater than those in the peel as a result of injury or environmental stress. The accumulation is restricted to the outer 1–2 mm of injured or stressed tuber. Potatoes containing over 0.02% steroid glycoalkaloids are considered toxic to man, and at this concentration they would impart a distinctly bitter flavor. The accumulation of steroid glycoalkaloids is suppressed and the accumulation of sesquiterpenoids is elicited in tubers infected by various pathogens and nonpathogens including the late blight pathogen,Phytophthora infestans. Arachidonic acid and eicosapentaenoic acids, two polyunsaturated fatty acids isolated fromP. infestans, are potent inhibitors of steroid glycoalkaloid accumulation. Both acids elicit the localized accumulation of sesquiterpenoids including rishitin, lubimin, phytuberin, phytuberol and solavetivone. Rishitin and lubimin generally comprise 85–90% of the total sesquiterpenoids which accumulate. The steroid glycoalkaloids and sesquiterpenoids appear to have a role in disease resistance to some fungal pathogens. Both groups of compounds are synthesized via the acetate-mevalonate pathway. Arachidonic and eicosapentaenoic acids appear to inhibit steroid glycoalkaloid accumulation at the level of the conversion of farnesyl pyrophosphate to squalene and they activate the biosynthesis of sesquiterpenoids. The reduction of steroid glycoalkaloids in potato foliage and tubers for health and flavor considerations should be considered relative to the ability of tubers and foliage to accumulate sesquiterpenoids in response to infection and its influence on disease and insect resistance.     

Evaluation of potato cultivars,clones and a true seed population for resistance toRhizoctonia solani

Field and laboratory studies were conducted in 1985–1986 to evaluate the relative severity ofRhizoctonia solani damage among 26 clones and five potato (Solanum tuberosum) cultivars. A test with true potato seeds showed that resistance toR. solani could be identified in two years. One highly resistant line was identified by this method. Moderate degrees of resistance were found in lines from the USDA Potato Breeding Program (BARC) with russet types producing a higher percentage of highly resistant lines than round whites. A laboratory test revealed that apical sprout damage (nipping) was variable within and between lines and that it was an important phase of disease development to measure when determining resistance because of its effects on the resulting plant growth and production of marketable tubers. It was also determined that to accurately define resistance toR. solani in potato, all phases of the disease must be evaluated to insure against resistance “breakdown” at some point in plant development.     

Production of phytoalexins, glycoalkaloids and phenolics in leaves and tubers of potato cultivars with different degrees of field resistance after infection withPhytophthora infestans

Summary The kinetics of accumulation of phytoalexins, glycoalkaloids and phenolics was studied in two potato cultivars differing in their degrees of field resistance when infected withPhytophthora infestans. Tuber slices and leaves of cvs Pampeana INTA (high degree of field resistance, free of R genes) and Bintje (susceptible) were infected with race C (complex race 1, 3, 5, 7, 11) ofPhytophthora infestans. Phytoalexins and phenolics accumulated in tuber and leaf tissues which had been inoculated. The levels of these compounds in the susceptible cv. Bintje were relatively low and similar to those found before inoculation. Leaves of cv. Pampeana INTA had a very high glycoalkaloid content, suggesting that glycoalkaloids may play a role in protection of leaves against the fungus. However, we could find no correlation between resistance and glycoalkaloid content of tubers. Our results suggest a major role of phytoalexins, phenolics and glycoalkaloids in the complex mechanisms of field resistance.     

Isolation of solanidine from haulm and tuber sprouts of potato (Solanum tuberosum L.)

Summary The kinetics of enzymatic transformation of total glycoalkaloids (TGA) in haulm and tuber sprouts of potato to solanidine and the kinetics of solanidine isolation from plant material were studied. The optimum reaction conditions for the enzymatic TGA transformation were found for haulm to be: plant material to liquid phase 1:4 w/v, temperature 35°C, pH 5.4; and for tuber sprouts: plant material to liquid phase 1:3 w/v, temperature 37°C, pH 5.4. The. maximum degree of enzymatic transformation of TGA in haulm and tuber sprouts of 65 and 75%, respectively, was achieved after 50h. Carbon tetrachloride was the best solvent for solanidine extraction from the plant materials. The optimum ratio of plant material to carbon tetrachloride for the isolation of solanidine was 1:8 w/v for both extraction cycles, ensuring a yield of solanidine of approximately 55 and 65% from haulm and tuber sprouts, respectively, after 30 min.     

Screening tuber-bearingSolanum species for resistance toHelminthosporium solani

Silver scurf, caused byHelminthosporium solani, is an important disease of table stock and processing potatoes, for which few management strategies exist. Two hundred-twelve accessions of tuber-bearingSolarium species were screened for their response toH. solani. Tubers were inoculated in the laboratory with a spore suspension, incubated for 1 mo, and evaluated for infection using the number of conidiophore groups per square millimeter of sporulating tuber surface. Tubers with relatively low sporulation in the laboratory were retested using natural inoculum present in potato storages and assessed for infection. There were significant differences (P < 0.05) between accessions with both methods of inoculation. Some accessions ofSolanum demissum, S. chacoense, S. acaule, S. stoloniferum, S. oxycarpum, andS. hondelmannii consistently demonstrated low sporulation in laboratory and storage, suggesting partial resistance. Both methods of inoculation, laboratory and storage, were found to be correlated (ranging from r = 0.582,P= 0.006; to r = 0.925,P < 0.003).     

Change in Some Glycoalkaloids of Potato under Different Storage Regimes

The potato (Solanum tuberosum L.) is an important food crop worldwide. Potato tubers can be stored to provide continuity of supply between production seasons, but it is important that they be stored under appropriate conditions as incorrect storage will result in deterioration in end user quality and may increase glycoalkaloid levels. We have investigated the effects of types of household storage on potato glycoalkaloid content (total glycoalkaloids [TGA]; α-solanine; α-chaconine) in Turkey. Tubers of potato cultivars (cvs.) Agria and Bettina were stored under four types of storage conditions (indirect sunlight for 10 h per day and dark storage for the remaining 14 h per day; storage under continuous fluorescent light; storage in constant darkness; storage in the dark in a refrigerator) for 56 days. Samples of tubers were taken at the beginning of the storage period and after 14, 28 and 56 days of storage for tuber glycoalkaloid measurement. Tubers stored in the three light environments showed an increase in glycoalkaloid levels; however, none of the cv. Agria tubers reached the critical level of 200 mg/kg tuber. On day 56 the TGA level of cv. Bettina tubers stored under fluorescent light reached 234.31 mg/kg.     

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.     

Occurrence,development, and losses associated with silver scurf and black dot on Colorado potatoes

Silver scurf, caused byHelminthosporium solani, and black dot, caused byColletotrichum atramentarium, are pathogens of tuber periderm whose presence in Colorado was only recently reported. A field survey conducted in September 1977 revealed thatC. atramentarium was more prevalent (21.8% tuber infection) and had a wider distribution thanH. solani (5.4% tuber infection). A greater incidence of both pathogens was observed on thin skinned tubers of chipping cultivars (49.0% infection) than on thicker skinned tubers of table stock cultivars (9.1% infection). Conidial development ofH. solani is tretic, pleurogenous, and requires 17–21 hours per conidium. Conidial septations appear while conidial elongate, and require 3–5 hours per septum. Light microscopy revealed that at least 11 conidia per conidiophore are produced in culture in 54 hours at 20–25 C (68–77 F) and humidity >90%. Scanning electron microscopy showed that fructifications ofH. solani (conidiophores-conidia) arise from beneath infected tuber periderm. Histological studies indicate some peridermal loosening and sloughing. Heavy deposition of unidentified compounds was observed in infected periderm, and hyphae were restricted to periderm cells. Fresh weight loss of tubers naturally infected withC. atramentarium was significantly greater than fresh weight loss of nearly noninfected (< 1% surface area infected) control tubers. Periderm infected with eitherH. solani orC. atramentarium appeared similar, i.e. shriveled, suggesting infections from either pathogen may result in increased fresh weight loss through alteration of the periderm.