Effect of a continuous hot water treatment of potato tubers on seed-borne fungal pathogens

Summary The viability of five pathogens was decreased by treatment with hot water when tested in vitro.Polyscytalum pustulans was most sensitive andRhizoctonia solani least sensitive. Potato tubers were exposed to 55°C for 5 min in a commercial continuous hot water treatment plant using naturally contaminated seed tubers and tubers which had been inoculated by dipping in comminuted cultures. The frequency of eyes colonised byP. pustulans, Helminthosporium solani, andR. solani was reduced to virtually zero and the effect persisted on tubers subsequently stored at 4°C and at 15°C for up to 16 wk. Results withColletotrichum coccodes were inconclusive. Treatment suppressedPenicillium spp. which, however, rapidly recolonised the eyes during storage, leading to higher contamination levels in the treated than in the untreated tubers. With tubers inoculated withPhoma foveata, good control was achieved when the incubation period before treatment was 10 d but not when the period was 42 d.     

Effects of soil-borneRhizoctonia solani Kühn on yield and quality of ten potato cultivars

Summary The effects of soil-borneRhizoctonia solani on yield and quality of potato were studied by an extensive individual plant sampling procedure. From 1983 to 1986, stem canker and stolon pruning were examined in 10 768 plants growing on a sandy soil. Tuber yield and quality and haulm yield per plant were also recorded. The degree of attack byR. solani depended on the cropping frequency of potato. Severe and very severe stem and stolon attacks decreased fresh yield, dry matter yield and dry matter content of tubers and increased the number of deformed and small tubers, whereas the effect on haulm yield and stem number was small. Potato cultivars differed in susceptibility toR. solani, but yield response did not differ significantly between cultivars at the same levels of stem and stolon infection.     

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.     

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.     

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.     

Effects of soil disinfection and potato harvesting methods on stem infection byRhizoctonia solani Kühn in the following year

Summary A two year field experiment was conducted twice to assess effects of chemical soil disinfection at planting and methods of harvesting potatoes on stem infection withRhizoctonia solani in the subsequent year. In the first year of the experiments seven methods. including one with soil disinfection at planting, were applied in August. In the following year,R. solani stem and stolon infection (disease severity) on potato plants were assessed in June. Soil treatment at planting with pencycuron resulted in lowest disease severity in the following year. Compared with chemical haulm killing and haulm pulling. immature-crop-harvesting also resulted in a lower disease severity, but only when black scurf was scarce on tubers at harvest in the preceding year.     

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.     

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.     

Storage of seed potatoes in pallet boxes. 2. Causes of tuber surface wetting

Summary In a paper to establish how different store ventilation systems affect the population of tuber borne Erwinia, periodic reductions in tuber skin resistance occurred (Pringle & Robinson. 1996). To help understand these changes, tuber skin resistance data was collected simultaneously from two stores nearby. All reductions were due to condensation when tubers were colder than the dew-point temperature of the surrounding air. Warm humid air from newly harvested or sprouting potatoes below condensed on the cooler surface tubers in naturally, but not periodically forced ventilated, boxes. Condensation occurred in both naturally ventilated and forced ventilated boxes when ambient air, under both fan pressure or wind induced leakage, and with a dew-point temperature above that of the stored tubers, entered the store. Individual tubers in trays, simulating the surface layer of boxes, received condensation when ventilation stopped. Warmer humid air from the rest of the crop condensed on these preferentially cooled tubers.     

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 Fungicide and Rate of Application on the Development of Isolates of <Emphasis Type="Italic">Polyscytalum pustulans</Emphasis> Resistant to Thiabendazole and on the Control of Skin Spot

Resistance to thiabendazole, sometimes of varying degrees, has occurred in a number of fungal species causing diseases of potato tubers, including skin spot caused by Polyscytalum pustulans. The effect of applying a range of concentrations of thiabendazole, imazalil or a formulated mixture of thiabendazole and imazalil on the development of isolates of P. pustulans partially or fully resistant to thiabendazole and on the control of skin spot over a number of successive generations of seed potato multiplication was examined in a series of experiments over 3 years. Seed tubers were inoculated with a sensitive, partially resistant or fully resistant isolate in the first year of an experiment and combinations of fungicide and a range of concentrations were applied to the seed tubers annually before planting over two or three successive generations. Immersing seed tubers inoculated with the partially resistant isolate in a relatively high concentration of thiabendazole, 0.7 g a.i. l^-1, did not result in the development of fully resistant isolates. Moreover, the effect of all fungicides in controlling eye contamination and skin spot did not differ between partially and fully resistant isolates. For seed tubers inoculated with the sensitive isolate, applying thiabendazole resulted in the occurrence of resistant isolates on first-generation daughter tubers, but the effect of fungicide concentration on the frequency of resistant isolates, partially or fully resistant, was inconsistent. In one experiment, the highest frequency of resistance was found at the greatest concentration, 0.4 g a.i. l^-1, but, in the second experiment, the frequency was unaffected by the concentration of thiabendazole. On second-generation daughter tubers derived from thiabendazole-treated seed tubers, virtually all isolates were resistant and the frequency of partially and fully resistant isolates was similar to that in the first generation. Applying a mixture of thiabendazole and imazalil to seed tubers inoculated with a sensitive isolate resulted in the development of some resistant isolates in the first year but the frequency did not generally increase with successive applications of the mixture. A mixture of thiabendazole and imazalil, and imazalil alone gave similar control of eye contamination when applied at equivalent concentrations. However, increasing the concentration of imazalil from 0.1 to 0.7 g imazalil l^-1 did result in an increasing impairment of plant growth, but the deposits achieved within that range of concentrations were greater than that recommended by the manufacturer. These results demonstrate that control of P. pustulans by thiabendazole, imazalil and a mixture of thiabendazole and imazalil applied at a range of concentrations was similar for isolates partially or fully resistant to thiabendazole.     

The role of stem inoculum in the transmission ofFusarium sulphureum to potato tubers

Summary Over 2 years, seed tubers or stems were inoculated withFusarium sulphureum Schlecht. (F. sambucinum Fuckel f. 6 Wollenw.) and the incidence of dry rot on daughter tubers assessed by standardised wound tests. The amount of stem and soil inoculum was also measured in one year. Inoculating stem bases resulted in greater concentrations of spores on the stem bases, greater soil populations at harvest and more dry rot on tubers than inoculating the seed tuber. Overall, the incidence of dry rot and the amount of stem and soil inoculum was similar for the three methods of haulm destruction: cutting and removing the stems, applying sulphuric acid or applying diquat dibromide to the growing plants.     

Assessing progenies of potato for resistance to early blight

Summary Progenies from crosses between cultivars varying widely in resistance to early blight (Alternaria solani Sor.), were assessed for resistance as true seedlings in a glasshouse in Scotland. The resistance of a representative sample of surviving genotypes from each progeny was compared with samples of the same progenies not previously exposed to the fungus, both in the glasshouse in Scotland and in the field in Israel. The exposed population was more resistant. Resistance was identified more effectively in adult plants from tubers in the glasshouse than in true seedlings and agreement between glasshouse and field assessment was better when progenies were compared rather than individual genotypes. The mid parent and progeny mean scores of the unexposed population were correlated at both sites, thus confirming that the resistance is heritable. Selecting resistant individuals at the seedling stage is suggested as a useful tool for resistance breeding, having first chosen the best parents for crossing.     

Some effects ofRhizoctonia solani on growth and yield of potatoes

Summary Below ground infection byRhizoctonia solani had little effect on the development of potato haulm. Severe infection caused an increase in the number of tubers initiated, but a reduction in the total weight of tubers. Tubers often formed in leaf axils of severely infected plants. It is suggested that these effects occur because assimilate production is not markedly affected by infection, but the compounds need to be redistributed when lesion development on the stolons blocks transport to the usual sinks.     

Light induced bio-control of potato storage pathogens in vitro

Summary The influence of freeze dried potato powder derived from tubers (cvs Désirée and Epicure) exposed to light as a culture medium on the growth, number and spore size of nine fungal pathogens was determined in vitro with particular emphasis on the role of glycoalkaloids. Greater reductions in pathogen growth reflected increased exposure to light of tubers prior to freeze drying. High correlations (% R² adj) were obtained between glycoalkaloid concentration and growth rate of the majority of pathogens tested. Where spores were produced, numbers were in general significantly less when cultured on freeze dried powder derived from tubers exposed to light. No effects on spore size ofFusarium species were recorded, but spore length of remaining pathogens was reduced and spore width increased, with the exception ofC. coccodes where spore length and width was increased following culture onto freeze dried powder derived from tubers exposed to light.     

Influence of stem canker (Rhizoctonia solani Kühn) on tuber yield,tuber size,reducing sugars and crisp colour in cv. Record

Summary In eight experiments in 4 years, inoculating seed tubers withR. solani caused stem canker and in seven experiments decreased the total yield and the yield of tubers 40–70 mm. Yields of larger tubers were usually increased. In most experiments inoculating increased the reducing sugar content of the tubers and darkened the colour of crisps, but did not consistently affect the amounts of sucrose. Treating soil with aldicarb or oxamyl before planting also slightly increased the total reducing sugars.     

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.     

Screening for resistance to early blight (Alternaria solani) in potato (Solanum tuberosum L.) using toxic metabolites produced by the fungus

Summary The phytotoxic components of the culture filtrate of the fungusAlternaria solani Sor, which causes early blight in potato, were used in this study to differentiate between resistant and susceptible genotypes selected to represent a range of reactions when screened in the field. Detached leaflet assays, using spores and toxic metabolites from the culture filtrate, were compared with field ratings and a whole-plant glasshouse test. Rank correlations between several detached leaflet tests were highly significant (P<0.01) but the correlations between these tests and the glasshouse and field tests were poor. The disease ratings obtained in the various tests were clustered into four groups and assigned to resistance classes. For many of the genotypes there was a good correspondence between resistance classes for all test methods. Divergent results between tests were associated with foliage maturity characteristics of the genotypes. Contribution no. 3879050 of the Lethbridge Research Station.     

The effect of crop rotation and granular nematicides on the incidence ofRhizoctonia solani in potato

Summary The incidence ofRhizoctonia solani in potato was studied in two crop rotation experiments from 1981 to 1985 inclusive. The greater the frequency of potato cropping, the more severe the attack on stems and stolons byR. solani. Severity of black scurf on progeny tubers and cropping frequency were also correlated, but less significantly. In fields with antagonists toR. solani, continuous potato cropping caused less black scurf on tubers than did 1∶2 rotations. Application of the nematicides oxamyl, ethoprofos and aldicarb resulted in a marked increase inR. solani infection of stems and stolons.

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Zusammenfassung In zwei Fruchtfolgeexperimenten wurde von 1979 bis 1985 das Vorkommen vonRhizoctonia solani an Kartoffeln untersucht. In Experiment 1 wurde auf sandigem Boden ein kontinuierlicher Kartoffelanbau (P) mit folgenden Fruchtfolgen verglichen: Mais-Kartoffeln (MP), Zuckerrübe-Kartoffeln (SP) und Mais-Zuckerrübe-Gerste-Gerste-Kartoffeln. (MSBBP). In Experiment 2 wurde auf Lehmboden ein kontinuierlicher Kartoffelanbau (P) mit den Fruchtfolgen Weizen-Kartoffeln (WP), Zuckerrübe-Kartoffeln (SP) und Weizen-Zuckerrübe-Hafer-Kartoffeln (WSOP) verglichen. In beiden Versuchen steigerte sich der Stengel- und Stolonenbefall mit zunehmender H?ufigkeit des Kartoffelanbaus (Tabellen 1 und 2). Es ergaben sich jedoch keine deutlichen Unterschiede imRhizoctonia-Befall der Tochterknollen bei einem Vergleich von kontinuierlichem Anbau und dem 1∶2 Fruchtfolgen mit Kartoffeln, w?hrendRhizoctonia weniger h?ufig in den Fruchtfolgen MSBBP und WSOP vorkam (Tabellen 3 und 4). Der HyperparasitVerticillium biguttatum kam im Experiment 1 (Abb. 1) besonders in den kurzen Fruchtfolgen P, MP und SP (Tabelle 5) vor, er konnte jedoch nicht den Stengel- und Stolonenbefall mitR. solani verhindern. Bodendesinfektion mit granulierten Nematiziden (Oxamyl, Ethoprofos und Aldicarb) erh?hte den Stengel- und Stolonenbefall mitR. solani betr?chtlich (Tabellen 1 und 2), diese Chemikalien hatten aber eine nur begrenzte Wirkung aufRhizoctonia an Tochterknollen (Tabellen 3 und 4). Der Mechanismus, der die positive Wirkung der Nematizide aufR. solani verursacht, ist nicht bekannt und erfordert eine detailiertere Untersuchung.

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Résumé La fréquence deRhizoctonia solani sur pomme de terre a été étudiée dans deux essais de rotations culturales de 1979 à 1985. Dans le premier essai, conduit en sol sableux, la culture continue de pomme de terre (P) a été comparée aux rotations suivantes: ma?s — pomme de terre (MP); betterave sucrière — pomme de terre (SP); et ma?s — betterave sucrière — orge — orge — pomme de terre (MSBBP). Dans le second essai implanté en sol argileux, la culture continue de pomme de terre (P) a été comparée aux rotations: blé — pomme de terre (WP); betterave sucrière — pomme de terre (SP); et blé — betterave sucrière — avoine — pomme de terre (WSOP). Dans les deux essais, le niveau des attaques sur tiges et sur stolons croit avec la fréquence de la culture de pomme de terre (tableaux 1 et 2). Toutefois, il n'y a pas de différence évidente sur l'état sanitaire de la descendance (sclérotes sur tubercules fils) entre la culture continue et la rotation alternée; cependant, il se forme moins de sclérotes sur les tubercules dans les rotations MSBBP et WSOP (tableaux 3 et 4). L'hyperparasite,Verticillium biguttatum, s'est manifesté dans l'essai 1 (fig. 1), principalement dans les rotations courtes P, MP et SP (tableau 5) mais pas de fa?on suffisante pour empêcher les attaques deR. solani sur tiges et stolons. Le traitement du sol à l'aide de nématicides granulés (oxamyl, ethoprofos et aldicarb), augmente fortement les attaques deR. solani sur tiges et stolons (tableaux 1 et 2), mais ces traitements chimiques n'ont qu'une influence minime sur l'état sanitaire de la descendance (tableaux 3 et 4). Le mécanisme qui conduit à la stimulation deR. solani par ces nématicides n'est pas clairement expliqué et nécessite des recherches détaillées supplémentaires.

     

Studies on natural repellents against potato tuber moth (Phthorimaea operculella Zeller) in country stores

Summary The effects of the leaves of five plant species, one from each of the generaAmbrosia, Anemone, Eupatorium, Eucalyptus andLantana, on potato tuber moth were investigated under indigenous storage conditions at the Central Potato Research Station, Shillong (1800 m above sea level). Their action was compared with that of a biological insecticide (spores ofBacillus thuringiensis), a chemical insecticide (carbaryl), and an untreated control. The data collected after six months storage on tuber damage, sprout damage and the rotting indicated that the leaves ofLantana aculeata provided most protection to the tubers, reducing damage from over 70% in the check to below 5%, and sprout damage from over 45% to below 3%. Next best wasEucalyptus globulus followed byB. thuringiensis. They may be used on tubers stored for table use or for seed as they had no adverse effect on germination or on the yield of a subsequent crop.