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.     

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.     

Testing the resistance to silver scurf in commercial potato cultivars under controlled environmental conditions

Summary Optimum storage conditions to identify resistance to silver scurf among commercially grown potato cultivars were determined in a series of experiments. Inoculation of field-grown tubers with a conidial suspension ofH. solani (concentration 10⁴ conidia per ml of water) and incubation at 15°C with 95% RH for 1 month followed by 2 months at 85% RH produced the greatest differences in disease severity between potato cultivars. Results with glasshouse-grown tubers were comparable with those from field-grown tubers, indicating that the test could be used to identify resistance in wild species of potato and to understand the inheritance of this resistance.     

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.     

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.     

The effect of condensation on sporulation ofHelminthosporium solani on potato tubers infected with silver scurf and held in simulated store conditions

Summary Chambers were designed to simulate environmental conditions present in commercial potato stores and were used to introduce condensation to tubers infected with silver scurf (Helminthosporium solani). The electrical resistance measured across the skin of tubers from the top of the chambers was consistently lower than that from the bottom of the chamber, and this was reflected in an increased number ofH. solani spores produced by the top tubers. An increase in the length of the condensation period resulted in the skin resistance remaining lower for a longer period and also resulted in higher number of spores. Tubers held at relatively high ambient temperatures (15 °C) required a shorter period of condensation to cause an increase in spore count. This investigation highlighted the need for close monitoring of store environments so that condensation events can be quickly identified and the tubers dried.     

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.     

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.     

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.     

Drycore Appears to Result from an Interaction between Rhizoctonia solani and Wireworm (Agriotes ssp.)—Evidence from a 3-Year Field Survey

Drycore is an important quality deficiency in Europe especially in organic potato production and after grass clover leys. The drycore symptom is attributed to Rhizoctonia solani Kühn (teleomorph: Thanatephorus cucumeris (Frank) Donk). In the framework of a 3-year survey (2001–2003), data concerning potato quality, crop rotation, management, and site parameters were analysed from 185 potato fields. The hypothesis was tested that injuries on potatoes caused by wireworms facilitate the penetration of R. solani and favour the formation of drycore. Analysis of variance showed a significant influence of wireworm damage, seed quality, and grass clover leys in the crop on the level of drycore damage. On fields which had both a low occurrence of black scurf on the seed tubers and a low occurrence of wireworm damage at harvest, significant drycore damage was never observed. The relative risk for drycore damage on tubers was significantly higher if black scurf or wireworm damage was on the same tuber. In contrast, no higher risk for drycore was observed on tubers with slug damage. Abiotic factors like farm manure application, organic matter content, texture, and pH of the soil also had no significant influence on the level of drycore. Thus, the wounding of potatoes by wireworm could be confirmed as the major variable for drycore. The mode of action has to be clarified under controlled conditions.     

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.     

Control ofRhizoctonia solani in potato by biological,chemical and integrated measures

Summary The effects of biological, chemical and integrated control on the formation of selerotia ofRhizoctonia solani on new potato tubers were studied in experimental fields. Sprouts of seed tubers, sprouted in daylight, were inoculated withVerticillium biguttatum, an ecologically obligate mycoparasite ofR. solani. Fungicides were mixed with the top soil. Biological control byV. biguttatum increased the percentage of harvests with less than 5% loss due to grading from 24% (non-inoculated) to 56%. Fungicides at the recommended rates usually gave good results but lower doses were less effective in sand than in loam soils. When fungicides were combined with biological control, the results were often, better: harvests with less than 5% sorting loss increased from 56 to 81%. Integrated control with pencycuron at 25% of the recommended rate was about equal to chemical control with pencycuron at full rate. The advantages of integrated control are discussed. Subsequently to the paper being provisionally accepted by the processing Editor, a shorter version of this paper was published in the Proceedings of the First Conference of the European Foundation for Plant Pathology, ‘Biotic Interactions and Soil-borne diseases’ held at Wageningen, 1990.     

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.     

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.     

Detection ofClavibacter michiganensis subsp.sepedonicus in potato stems and tubers grown from seed pieces with various levels of inoculum

Summary An indirect sandwich enzyme-linked immunosorbent assay (ELISA) was used to monitor potato stems of three cultivars for the presence of the ring rot pathogen,Clavibacter michiganensis subsp.sepedonicus, during the growing season and in progeny tubers after harvest. The highest ELISA values were obtained with the highest concentration of bacteria used to inoculate seed pieces in all cultivars tested. Low ELISA values were obtained for stems and progeny tubers selected from plants grown from seed inoculated at lower bacterial concentrations. Estimates of bacterial densities in stems and progeny tubers by immunofluorescence indicated that low ELISA values were most probably caused by low bacterial numbers. It is suggested that the sensitivity of ELISA for detecting the ring rot pathogen in potato stems and progeny tubers is a function of the concentration of bacteria in individual seed pieces.     

Effects of wounding fungicide-treated potato seed tubers on silver scurf disease on daughter tubers at harvest

Summary Silver scurf on daughter tubers at harvest was decreased by treating seed tubers with imazalil or thiabendazole in March but significantly increased when the treated tubers were wounded the day before planting in May. The disease was also increased by wounding seed tubers not treated with fungicide. Black dot was decreased by seed treatment with imazalil but was not affected by wounding.     

Relationships between disease levels on seed tubers,on crops during growth and in stored potatoes. 3. Silver scurf

Summary Incidence of silver scurf (Helminthosporium solani) was assessed macroscopically (silver scurf lesions) or microscopically (H. solani infection of tuber eyes) on seed tubers, on progeny tubers during growth or at harvest, and on stored tubers, from up to 26 commercial crops·cv. King Edward in each of 5 years, and up to 13 crops grown from ‘healthier’ seed in 4 years. Infection of eyes was frequent on seed in all years, scace on progeny tubers during growth, but had increased by harvest. Silver scurf was prevalent in stored tubers, except in 1975, and more developed at 10°C than at 3 °C. Infection of tubers during growth, at harvest, or after storage, was not related to incidence of eye infection on seed. In 3 years, infection at harvest was significantly related to infection during growth, but only in 1975 was infection at harvest related to silver scurf in store. In 2 years amounts of silver scurf on crops grown from seed derived from stem cuttings (‘healthier’ seed) were positively correlated with disease incidence in commercial crops grown on the same fields.

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Zusammenfassung Zwischen 1971 und 1975 wurde in bis zu 26 Proben von Marktware der Sorte King Edward das Auftreten von Silberschorf an gelagerten Kartoffelknollen untersucht sowie die Infektion mitHelminthosporium solani von Knollenaugen des Pflanzgutes, von Tochterknollen w?hrend des Wachstums und zur Ernte (Adams et al., 1980). Zwischen 1972 und 1975 wurden diese Erhebungen auch an Erntegut aus ‘gesünderem’ Pflanzgut (das von Stecklingen abstammte) das benachbart zur Marktware aufwuchs, gemacht. Das durchschnittliche Auftreten von Infektion oder Krankheitsbesatz auf Marktware (Tab. 1) zeigte, das in allen Jahren die Infektion der Knollenaugen des Pflanzgutes h?ufig war, seltener an den Tochterknollen w?hrend des Wachstums, aber vorallem 1971 zur Ernte wieder verst?rkt war. Ausser 1975 trat Silberschorf h?ufig auf gelagerten Knollen auf, und entwickelte sich bei 10°C mehr als bei 3°C. Eine Wundheilung von 2 Wochen bei 15°C hatte keinen Einfluss auf diese Krankheit. Bei ‘gesünderem’ Pflanzgut waren weniger Augen infiziert als im Durchschnitt der Marktware, aber w?hrend des Wachstums und zur Ernte war der Befall der Augen genau so gross oder gr?sser als von Marktware (Tab. 2). Die Signifikanz der Regressionen zwischen den Feststellungen des Krankheitsbesatzes oder der Infektion, die zu verschiedenen Zeiten gemacht wurden (Tab. 3) zeigte, dass die Bonitur der Infektion des Pflanzgutes im allgemeinen mit den folgenden Bonituren nicht in Beziehung stand. In drei Jahren war das Auftreten von Augeninfektionen zur Ernte mit dem w?hrend des Wachstums verbunden (Abb. 1). 1975 wurden Pflanzkartoffeln und geerntete Knollen unter feuchten Bedingungen inkubiert und die Sporulation vonH. solani nach dem Waschen und dem Zentrifugieren der Flüssigkeit untersucht. Die Sporulation war abh?ngig vom Auftreten von Augeninfektionen des Pflanzgutes (Abb. 2c). Untersuchungen des Inokulums vonH. solani bei denen Schalenstückchen, ausgenommen Augen, verwendet wurden, ergaben Beziehungen zum Ergebnis von Augenstückchen des Pflanzgutes (Abb. 2b) und zur Sporulation auf geerntetem Nachbau (Abb. 2d). Das Auftreten von Silberschorf nach der Lagerung war mit allen verschiedenen Messungen des Inokulums verbunden und am besten mit den Ergebnissen der Sporulation (Abb. 2e). Ein Vergleich der Ergebnisse ‘gesünderer’ Ernte mit den Ergebnissen benachbarter Marktware ergaben nur wenig signifikante Beziehungen. 1975 waren jedoch das Ausmass der Augeninfektion zur Ernte (Abb. 3a) und des Krankheitsbesatzes nach der Lagerung (Abb. 3b) verbunden, was auf eine allgemeine Quelle des Inokulums, eine Verbreitung zwischen benachbarten Ernten oder die Bedeutung der allgemeinen Bodenbedingungen hinweist

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Résumé L'incidence de la gale argentée sur les tubercules en conservation et l'infection parHelminthosporium solani des yeux des tubercules de semence et des tubercules-fils durant la croissance et à la récolte, ont été notées, de 1971 à 1975, dans 26 cultures issues de lots de semence du commerce de la variété King Edward (Adams et al., 1980a). Entre 1972 et 1975, des évaluations ont également été faites sur des lots issus de semences ‘plus saines’ (dérivées de boutures) cultivées près de certains lots commerciaux. L'incidence moyenne de la contamination ou de la maladie sur les lots commerciaux (tableau 1) montre que la contamination des yeux a été fréquente sur les semences tous les ans, rare sur les tubercules-fils en végétation mais plus fréquente à la récolte, surtout en 1971. La gale argentée est apparue fréquemment en conservation, sauf en 1975, et s'est plus développée à 10 °C qu'à 3°C. La cicatrisation des blessures pendant deux semaines à 15°C n'a pas eu d'effet marqué sur l'incidence de la maladie. Les semences ‘plus saines’ avaient moins d'yeux atteints que la moyenne des semences des autres lots, mais la contamination des yeux en végétation et à la récolte atteignait ou dépassait celle des tubercules des autres lots (tableau 2). La signification des régressions entre les notations de maladie ou d'infection réalisées à différentes époques (tableau 3) a montré que les notations de contamination des tubercules de semence n'étaient pas généralement en relation avec les suivantes. Pour 3 années, l'incidence de l'infection des yeux à la récolte était en relation avec celle notée durant la croissance (Fig. 1). En 1975, on a aussi placé les tubercules de semence et la récolte en conditions humides et on a contr?lé la sporulation deH. solani après lavage et centrifugation du liquide. La sporulation était liée à l'incidence de l'infection des yeux des tubercules de semence (Fig. 2a) et celle des tubercules-fils récoltés. Des analyses d'inoculum deH. solani sur des fragments de peau sans yeux se sont montrés corrélées aux résultats obtenus sur les yeux des tubercules de semence (Fig. 2b) et à l'importance de la sporulation sur la récolte (Fig. 2d). L'incidence de la gale argentée en conservation s'est montrée liée à toutes les différentes mesures d'inoculum effectuées sur la récolte et en particulier aux mesures de sporulation (Fig. 2e). La comparaison entre les notations sur les cultures ‘plus saines’ et leurs homologues de la série commerciale n'a révélé que peu de relations significatives. Cependant, en 1975, il y avait relation entre la contamination des yeux à la récolte (Fig. 3a) et la maladie après conservation (Fig. 3b) sont liées, laissant supposer soit une source commune d'inoculum, soit une propagation d'une culture à l'autre, soit l'importance des conditions habituelles de sol.

     

Detecting resistance to the root-knot nematodesMeloidogyne hapla andM. chitwoodi in potato and wildSolanum spp.

Summary Various methods of screening for resistance to root-knot nematodes were compared and evaluated. Seedling populations ofSolanum spp., grown in clay pots and plastic tubes with silver sand and inoculated with juveniles ofMeloidogyne chitwoodi andM. hapla, showed large differences in the number of egg masses on roots 7 weeks after inoculation. The differences were reproducible when re-testing was done with cuttings and plants from tubers. No resistance toMeloidogyne spp. was observed with ten potato cultivars when grown in clay pots, plastic tobes or closed containers. Plants from tubers in growth pouches developed a large two-dimensional root system, and after inoculation with juveniles the infection process could be observed over 8 weeks. A method of infecting potato tuber tissue withMeloidogyne is described, using tuber slices in Petri dishes as a potential screening test for tuber resistance.     

Optimisation of sample size for the detection of latent infection byRalstonia solanacearum in potato seed tubers in the highlands of Peru

Summary A sampling strategy was evaluated in the Andean highlands of Peru to optimise the detection ofRalstonia solanacearum in seed tubers harvested from symptomless crops. A sensitive and specific serological method developed at CIP was used to detect the pathogen in latently infected tubers. Optimum sample size was evaluated for symptomless crops after analysing various numbers of composite samples and using a binomial distribution model to calculate the detection probabilities.R. solanacearum was detected in all lots from fields with visible symptoms, so validating the detection technique. About half of the seed lots from apparently healthy fields at altitudes of up to 3,100 m were found positive for the pathogen.R. solanacearum was detected with 99% probability in samples of 350 tubers from seed lots from symptomless crops. This number of seed tubers could feasibly be processed in a seed-health test without incurring too high a cost for labour and materials.     

Incidence, appearance and development of potato mop-top furovirus-induced spraing in potato cultivars and the influence on yield, distribution in Denmark and detection of the virus in tubers by ELISA

Summary 23 potato cultivars were grown in fields infested with potato mop-top virus and spraing was assessed on tubers at harvest and after storage. Large differences in spraing susceptibility were found between cultivars. Compared with other countries, spraing in Denmark is characterised by a high incidence at harvest, a low increase during storage and a very low proportion of superficial spraing. Decrease in spraing during storage was recorded in two cultivars. DASELISA tests for PMTV in tubers revealted a high reliability. Mapping of PMTV in important Danish potato growing areas showed that the virus is widespread. Occurrence of spraing did not influence total yield or dry matter content. Soil acidity did not influence incidence of spraing but it was more common on coarse-grained soil than on finer sandy soils. PMTV in viruliferous resting spores ofSpongospora subterranea was inactivated by heating to 90°C for 15 minutes.