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.     

Reduction of silver scurf on potatoes by pre- and post-storage treatment of seed tubers with imazalil

Control of silver scurf on daughter tubers was obtained by imazalil applied to seed tubers as a pre- or post-storage treatment or as a furrow treatment (at time of planting), in three field experiments. Post-storage and pre- and post-storage combination treatments provided consistent results. The most effective treatment in the first and second experiments was the combined pre- and post-storage application of imazalil. However, separate pre- and post-storage treatment also significantly reduced the incidence compared with the non-treated control. No differences were observed between dosages of 15 and 30 g a.i. of imazalil per ton of seed tubers. Although furrow treatment at planting time (1.5 1/ha) significantly reduced silver scurf incidence on daughter tubers, it was less effective than seed treatments. However, the combined treatment of pre-storage application and furrow treatment significantly reduced the disease and was more effective than the single pre-storage seed treatment. In the third experiment, a reduction of disease incidence and severity index was obtained by post-storage treatments or with the combination of pre- and post-storage applications.     

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.     

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.     

The control of silver scurf and development of thiabendazole resistance inHelminthosporium solani as affected by the rate of fungicide applied to potato seed tubers

Summary Seed tubers were immersed in suspensions of thiabendazole (Storite) or thiabendazole plus imazalil (Extratect) at different concentrations and planted in a field experiment. During the following 2 years samples of the produce were planted after treatment with the same fungicide formulation used on the seed. Silver scurf on daughter tubers decreased as concentrations of fungicide increased and Extratect gave better disease control than Storite applied at the same thiabendazole concentration. Isolates ofH. solani resistant to thiabendazole were found on seed tubers 6 weeks after treatment with Storite, and also on daughter tubers after the first application of the higher rates of Storite; their incidence increased with further annual treatments. In 3 years no thiabendazole-resistant isolates were found after treatment with Extratect. Chlamydospores ofH. solani developed when sensitive isolates were subcultured onto agar containing thiabendazole. Isolates sensitive to thiabendazole were slightly more sensitive to imazalil than resistant ones.     

Effects of watering potato plants before harvest and of curing conditions on development of tuber diseases during storage

Summary Plots of potato plants were watered to field capacity one day before harvest on three dates, and tubers from watered and non-watered plots were stored at 7°C and 95% RH immediately or after curing at 15°C either at 80% or 95% RH. Watering increased the severity of silver scurf after storage when tubers were not cured. Curing at 80% RH decreased the disease and amounts were similar from watered and non-watered plants. Black dot was increased by watering whereas black scurf and golding blotch were decreased.     

Effect of a combination of chlorine dioxide and thiophanate-methyl pre-planting seed tuber treatment on the control of black scurf of potatoes

The pre-planting seed tuber treatments of chlorine dioxide (ClO₂), thiophanate-methyl (TPM), and a combination treatment of ClO₂, followed by TPM, were evaluated for control of stem canker and black scurf (Rhizoctonia solani), and storage rots; black scurf, common scab (Streptomyces scabies), dry rot (Fusarium spp.) and silver scurf (Helminthosporium solani) on potato (Solanum tuberosum L.) cv. Kennebec. Field experiments were conducted in two consecutive years, 1999 and 2000, at the Agriculture and Agri-Food Canada Research Farm in Harrington, Prince Edward Island (PEI), Canada. The combination treatment of ClO₂ (200 μg g⁻¹) and TPM (50 g active ingredient100 kg⁻¹ of tubers) significantly (P=0.05) reduced stem canker and black scurf on progeny tubers at harvest and after storage. The low incidence of scab and dry rot in untreated controls hindered the evaluation of the efficacy of the combination treatment and also showed that conditions were not favourable for disease development during this period at Harrington. The combination treatment was not effective on silver scurf in storage. A comparison among control, ClO₂, TPM, and the combination treatment indicates that pre-planting ClO₂ treatment may have killed the majority of the black scurf sclerotia on the tuber surface and that the combination of TPM fungicide treatment following ClO₂ treatment gave protection to progeny tuber by suppressing the growth of the R. solani. A higher marketable yield was observed in the combination treatment as compared with the untreated control. Phytotoxicity was not observed in tubers treated with the combination treatment.     

Relationship between silver scurf levels on seed and progeny tubers from successive generations of potato seed

The level of silver scurf on potato seed tubers on successive generations of potato seed tubers and their progeny tubers was investigated during 3 years in the field. The objective was to determine the importance of seed-borne inoculum on silver scurf development on the subsequent progeny tubers. Silver scurf incidence and severity increased with each generation. Coefficients of determination for disease levels among generations were significant and ranged from 0.89 to 0.97, indicating that seed tuber source accounted for a large proportion of silver scurf on progeny seed tubers. Incidence and severity of silver scurf also increased with decreasing time periods between potato crops in the field. In a field near Paterson, WA, where potatoes had not been previously grown, the severity of silver scurf increased on progeny tubers of cvs Russet Norkotah, Ranger Russet, and Shepody as disease severity increased on seed tubers of successive generations. Disease severity index significantly increased as disease incidence increased. The relationship between the two was best described using a curvilinear regression model.     

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.     

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.     

Survival ofHelminthosporium solani in soil andin vitro colonization of senescent plant tissue

Silver scurf, caused byHelminthosporium solani, is considered to be a tuberborne disease of potato. The ability ofH. solani to overwinter in soil was evaluated in field studies at two locations in New York during 1992. Field plots were established at sites whereH. solani infected potatoes had been produced in 1991. Plants produced from pathogen-free tubers of four cultivars yielded tubers infected with silver scurf at both locations. Up to 61% of tubers were infected in some plots. The ability ofH. solani to colonize leaf tissue was evaluated inin vitro assays. Detached leaves of ten crops were sprayed with a spore suspension ofH. solani and incubated for 20 days.H. solani colonized and sporulated on senescent leaf tissue of alfalfa, sorghum, rye, oats, corn and wheat, and only colonized senescent tissue of rapeseed, red clover and buckwheat. No growth was observed on potato leaf tissue. These results indicate that soil survival and saprophytic ability may be important in the epidemiology of silver scurf of potato.     

Fungicide treatment of seed tubers infected with thiabendazole-resistantHelminthosporium solani andPolyscytalum pustulans for controlling silver scurf and skin spot on stored progeny tubers

Summary Seed tubers, cv. Désirée, derived from stocks treated annually with thiabendazole were infected with thiabendazole-resistant strains ofHelminthosporium solani andPolyscytalum pustulans. Samples of seed tubers were either untreated or immersed for 5 min in fungicide suspensions of thiabendazole, imazalil or thiabendazole plus imazalil and planted on four farms in 1988 and 1989. After harvest, tuber samples from each treatment were treated with thiabendazole and stored for 6 months. Applying imazalil or thiabendazole plus imazalil to seed tubers decreased the severity of silver scurf and skin spot on stored progeny tubers. Thiabendazole applied to seed tubers or to progeny tubers after harvest did not affect the severity of either disease, but post-harvest treatment decreased the incidence of black scurf after storage.     

Characterization of forty-four potato varieties for resistance to common scab,black scurf and silver scurf

Common scab, black scurf and silver scurf belong to serious diseases of cultivated potato that especially impair tuber quality. Tuber infection in 44 potato (Solanum tuberosum ssp. tuberosum L.) varieties was evaluated using two measures – severity and incidence of infected tubers. Both measures were highly correlated. An important result derived from this study supports fungicide application against potato late blight secondarily acts on reducing tuber infection by all evaluated pathogens. Therefore, the conventional potato growing seems to be more effective compared to ecological growing. The pedigree analysis of 44 varieties indicated the presence of Solanum demissum genotypes in the variety genome may negatively affect variety response to Helminthosporium solani presence.     

Efficacy of metam sodium for controllingVerticillium dahliae prior to potato production in sandy soils

The efficacy of metam sodium (MS) for controlling Verticillium wilt of potato was evaluated in two field experiments conducted in a sandy soil, heavily infested withVerticillium dahliae, and previously treated with MS. Compared to control plots where AUDPC value was 3141 (in 1999) and stem infection was 71.2% (in 2000), in plots treated with 600 L MS/ha AUDPC values were reduced by 57% to 80%, and stem infection was reduced by 49–54%. The incidence of infected dry stems, evaluated at the end of each season, was significantly reduced by all MS treatments, especially by 900 L/ha applied to a 60-cm depth (1999). Disease incidence in daughter tubers was also significantly reduced by all MS treatments. Yields obtained with 600 L/ha MS applied to a 30-cm depth were 32% and 21% higher than in the control in 1999 and 2000, respectively. In plots treated with 600 L/ha MS applied to a 60-cm depth yields were 17% and 28% higher than in the control in 1999 and 2000, respectively. With 900 L/ha MS applied to a 60-cm depth, yield was 21% higher than in the control in 1999. The yield of the low MS dose (300 L/ha), applied only in 2000, was not significantly different from the control. The differences in yield were due to the greater percentage of tubers larger than 45 mm. Yield reduction was highly correlated with disease severity expressed as AUDPC (in 1999 experiment). The net income in the 600 L MS/ha treatments was higher than in the control in both experiments. No indication of enhanced biodegradation was observed in the present study.     

Use of Biocontrol Organisms and Compost Amendments for Improved Control of Soilborne Diseases and Increased Potato Production

Soilborne potato diseases are persistent problems in potato production and alternative management practices are needed. In this research, biocontrol agents (Bacillus subtilis GB03 and Rhizoctonia solani hypovirulent isolate Rhs1A1) and compost amendments (from different source material), were evaluated alone and in combination, for their potential to reduce soilborne diseases and increase tuber yield over three field seasons in Maine. Both biocontrol organisms reduced multiple soilborne diseases, stem and stolon canker by 20–38 %, black scurf by 30–58 %, and common scab by 10–34 % relative to the nontreated control treatment, and the combination treatment of both biocontrol organisms together provided nominally better control than individual treatments. However, biocontrol treatments had no direct effect on tuber yield. Compost amendments from different sources all increased total and marketable tuber yield substantially (11–37 % and 17–51 %, respectively) relative to nontreated controls. However, except for some reduction of Rhizoctonia canker, compost amendments did not reduce soilborne diseases (black scurf and commons scab) in any year, and resulted in increased levels of common scab in some years (20–45 % increase). The combination compost-biocontrol treatment, although did not perform significantly better than individual component treatments, still provided indications of combined beneficial effects from both component treatments. This research demonstrated the usefulness of these approaches and combinations as additional options for reduction of soilborne diseases and increased tuber yield and can be implemented for enhanced sustainability and productivity in potato production systems.     

Treatment of seed potatoes with prochloraz for simultaneous control of silver scurf and black dot on progeny tubers

Summary Potato seed tubers infected withHelminthosporium solani andColletotrichum coccodes were treated with prochloraz (as Omega 450 g a.i. l⁻¹ EC) and/or prochloraz managanese chloride (as Octave 25 g/kg DP), and planted in two separate fields not previously cultivated with potatoes. Dipping seed 28 d prior to planting in 2,4 or 6 g a.i. l⁻¹ Omega, and dusting with 500 or 750 g Octave per 100 kg seed significantly reduced the incidence of silver scurf and black dot on progeny tubers. In one field, the higher concentrations of Omega and Octave gave slightly better control than the lower Omega concentrations. At the second site, postharvest application of 4 g a.i. l⁻¹ Omega or 750 g Octave per 100 kg seed limited development of both diseases on progeny tubers, with slightly better control when an additional Octave dusting was applied 14 d before planting. None of the prochloraz treatments adversely affected plant stand or tuber yield.     

云南省马铃薯银腐病(Helminthosporiumsolani)的研究

20 0 1年我们从云南省薯产区市场上和仓库内采集到马铃薯银腐病标本 ,研究表明 ,该病冬季发病率达 33. 3%。受感染部位多分布于薯块的茎基端 ,重病薯块因失水过多 ,块茎皱缩。病菌生长后期变为棕色。分生孢子梗无分枝 ,大小为 15 0～ 5 5 0× 7 0～ 10 0 μm (平均 35 0× 8 5 μm ) ,1～ 6根成簇着生于球形基部。分生孢子棕色 ,单个或成簇地自分生孢子梗基端往上呈轮状排列 ,直或稍弯 ,顶端小 ,略呈锥形 ,有 3～ 8个横隔 ,大小为 2 0～ 80 μm× 7 5～ 10 μm (平均 5 0× 8 5 μm )。根据该病病原真菌的形态学和培养性状 ,鉴定为茄长蠕孢 (Helminthosporiumsolani)。黑暗和光照下培养 ,分生孢的萌发率无明显差异。通过人工接种于薯块表面 ,表现出较强的致病性 ,马铃薯银腐病在中国属首次报道     

几种杀菌剂对马铃薯黑痣病的防效

马铃薯黑痣病（Rhizoctonia solani K＆#252;hn）在内蒙古发生普遍而又严重。2013年通过田间小区方法进行了4种杀菌剂防治试验。结果表明：24%满穗悬浮剂90 mL/667 m2、50%BAS70301F悬浮剂33.3 mL/667 m2、30%BAS7004F悬浮剂40 mL/667 m2和25%阿米西达悬浮剂60 mL/667 m2处理,防效分别为93.0%、90.4%、89.3%和86.6%,增产分别为17.5%、15.3%、13.1%和11.9%,均具有优良的防治效果。     

Reducing the potential for bacterial soft rot in potato tubers by chemical treatments and drying

The potential for bacterial soft rot caused byErwinia carotovora in freshly inoculated potato tubers was reduced up to 99% by immersion for 5 min in solutions of sodium hypochlorite (chlorine bleach) containing up to 10,000 ppm chlorine. Reductions up to 93% were achieved using a combination treatment of immersion in 1% citric acid for 5 min followed by airdrying. Immersion treatment alone in 1% aqueous solutions of citric, acetic, ascorbic, or malonic acid also significantly reduced the soft rot potential. In contrast, no reductions accompanied treatment with solutions of potassium or calcium acetate. All treatments including the combination immersion/airdrying treatment were much less effective if tubers had been infiltrated initially with soft rotErwinia or had numerous mechanical injuries. If tubers had not been infiltrated with the causal organism, immersing them in 1% citric acid reduced the potential nearly as much as a similar treatment with 1000 ppm chlorine. A 30-sec immersion in the latter was less effective than a similar treatment with 500 ppm a.i. CGA 78039, an experimental bactericide. However, air-drying, coupled with provisions for keeping tuber surfaces free from moisture, remains the most effective means of reducing losses to bacterial soft rot.     

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.