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.     

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

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

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 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.     

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.     

Occurrence of Skin Blemish Diseases (Scab and Scurf) in Norwegian Potato Production

A survey of the prevalence of skin blemish diseases in potatoes after the growing seasons of 2008 and 2009 was carried out on 247 potato lots representing different cultivars and production regions in Norway. The results showed the presence of silver scurf (Helminthosporium solani) in all lots. Skin spot (Polyscytalum pustulans) and black scurf (Rhizoctonia solani) were found in 80% of the lots, and black dot (Colletotrichum coccodes) and common scab caused by Streptomyces spp. were present in 50–70%. Also, powdery scab (Spongospora subterranea) occurred in 65–80% of the lots, and root-lesion nematodes (Pratylenchus spp.) were detected in 60% of the sub-samples that exhibited symptoms of common scab.     

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

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

Metabolite profiling of sheath blight disease resistance in rice: in the case of positive ion mode analysis by CE/TOF-MS

Rice sheath blight is an important disease caused by Rhizoctonia solani. The resistant and susceptible rice lines (32R and 29S, respectively) showed different responses to R. solani infection in metabolite levels. The aim of this study was to characterize the metabolite levels in rice lines during R. solani infection using capillary electrophoresis equipped with time of flight mass spectrophotometry (CE/TOF-MS) in positive ion mode. Hundred metabolites were identified and classified into six clusters by hierarchical cluster using Mass Profiler Professional software. Changes in metabolite level at inoculated 32R and 29S were mapped on branches of tricarboxylic acid and glycolysis pathway. Volcano plot successfully filtered the metabolites based on fold change and p-value. The volcano plot result showed that 10 metabolites were up and down regulated in inoculated 32R relative to 29S. One metabolite, chlorogenic acid, showed a positive response in 32R. Meanwhile, pipecolic acid showed as the highest magnitude of fold change and p-value significance level in 29S. In addition, eight amino acids; glutamate, γ-aminobutyric acid, glycine, histidine, phenylalanine, serine, tryptophan, and tyrosine showed increase in 29S after R. solani inoculation.     

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.     

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.     

Naturally occurring viral infections inSolanum brevidens andS. fernandezianum

Summary Two species of wild potato,S. brevidens andS. fernandezianum, were surveyed for six potato-infecting viruses in their natural habitats in the Puerto Montt Region and Robinson Crusoe Island of Chile, respectively. Potato viruses S (PVS), M (PVM) and X (PVX) and potato yellowing virus (PYV) were found in some clones of the Chilean Potato Germplasm Collection, whereas only one population ofS. brevidens out of six was virus-infected, namely by PVS. The cultivated potatoes on Robinson Crusoe Island were infected by PVS, PVM and PVX and infested byAulacorthum solani andMyzus persicae, whereasS. fernandezianum was infected by PYV and infested byA. circumflexum, A. solani andAphis spp.     

Susceptibility assessment of potato cultivars toFusarium dry rot species

Summary The susceptibility of 43 commercially available potato cultivars to dry rot caused byF. sulphureum, F. solani andF. oxysporum under similar conditions was examined over two years. The cultivars reacted with differing levels of resistance to the dry rot species.F. sulphureum was more aggressive than the other two species and the resistance to each species was independent. The overall cluster analysis classified the cultivars into six groups. Saturna, in an individual group, is the most resistant variety to all tested species, and also toF. surphureum. Panda, Fregate, Folva, Arian and a few others in a group are resistant toF. solani. AlthoughF. oxysporum was the least pathogenetic, almost half of the cultivars are included in a resistant group to this species. There was little correlation between the rank orders of susceptibility of the cultivars to the two main pathogens.     

Comparative susceptibility of some potato cultivars to dry rot caused byFusarium sulphureum andF. solani var.coeruleum

Summary The susceptibility of 15 potato cultivars to dry rot caused byFusarium sulphureum andf. solani var.coeruleum was examined over 8 years. Tubers were wounded, inoculated, incubated at 10°C for 7–8 weeks and the size of the rot assessed.F. sulphureum was the more aggressive species. There was little correlation between the rank order of susceptibility of the cultivars to the two pathogens, but a higly significant correlation between years. Two years' tests are deemed sufficient to assess susceptibility to both species.     

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

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

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.

     

Levels of antinutritional factors in pearl millet as affected by processing treatments and various types of fermentation

Pearl millet (Pennisetum typhoideum) was fermented with Lactobacilli or yeasts alone and in combination, and with natural microflora after various processing treatments, as grinding, soaking, debranning, dry heat treatment, autoclaving and germination. Fermentation was carried out at 30°C for 48 hours withLactobacillus plantarum (LP) andRhodotorula (R) isolated from naturally fermented pearl millet andLactobacillus acidophilus (LA),Candida utilis (CU) and natural microflora (NF). Germination and autoclaving, and debranning and autoclaving were the most effective processing treatments to reduce the phytic acid, amylase inhibitors and polyphenols. There was a further reduction in these antinutrients due to fermentation. Phytic acid and amylase inhibitors were completely eliminated after fermentation in some of the samples especially in soaked, debranned and germinated ones. Polyphenols were altered non-significantly in general but fermentation with Lp+R and NF caused a significant increased in polyphenols.     

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.     

Evaluation of thiabendazole and pentachloronitrobenzene for control of the Rhizoctonia disease complex on white potato (Solanum tuberosum L.)

Thiabendazole (TBZ) and pentachloronitrobenzene (PCNB) alone and in combination were laboratory, greenhouse, and field tested during a five-year period (1977-1982) for their effectiveness in reducing incidence and severity of damage caused byRhizoctonia solani to white potato (Solarium tuberosum). Growth ofR. solani on amended potato dextrose agar was prevented at 7 ppm TBZ and 80,000 ppm PCNB. Tuber-borne sclerotial germination and hyphal growth were prevented following seed treatment with 6,000 ppm TBZ, and severely inhibited by combination treatments of 12,500 or 50,000 ppm PCNB and 1,500 ppm TBZ. Seed treatments which provided the most effective field control ofR. solani were combinations of PCNB (50,000, 25,000 ppm) and TBZ (1,500 ppm), followed by TBZ alone (6,000 ppm) and PCNB alone at rates of 50,000 ppm or above.     

Contamination byPolyscytalum pustulans andPhoma exigua var.foveata of seed stocks derived from stem cuttings in Scotland

Summary The incidence ofPolyscytalum pustulans andPhoma exigua var.foveata on virus-tested stocks of seed potatoes derived from stem cuttings (VTSC) was assessed on samples from more than 15 commercial farms in Scotland from 1974 to 1978. Lower class seed stocks (FS3 and AA1) were also examined in 1978 in relation to the region of Scotland in which they were grown. P. pustulans was detected on tubers grown at the DAFS nuclear stock farm only in 1977 butP. exigua var.foveata was present at very low levels in most years. Contamination by both pathogens was much greater on clones grown on commercial VTSC farms than on the DAFS nuclear stock farm. Both fungi were more prevalent on tubers after the third year of multiplication than on those of second year clones. While there was variation between farms in the amount of tuber contamination of VTSC stocks, contamination was reduced considerably by an annual post harvest fungicide treatment. The incidence ofP. pustulans was largely unaffected by the geographical location of the farms.P. exigua var.foveata was more prevalent in the northern counties of Scotland than in Angus, Perth, Fife and Kinross. This difference was not related to either date of haulm destruction or interval between haulm destruction and lifting.

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Zusammenfassung Das Vorkommen vonPolyscytalum pustulans undPhoma exigua var.foveata in virusgetesteten Pflanzkartoffelbest?nden, die von Stengelschnitten (VTSC) stammen, wurde in den Jahren 1974 bis 1978 auf mehr als 15 kommerziellen Landwirtschaftsbetrieben in Schottland festgestellt. Im Jahre 1978 wurden auch Pflanzgutbest?nde niedrigerer Anbaustufen (FS3 und AA1) in bezug auf ihre Anbauregion in Schottland geprüft. 1974 war der Nachweis vonP. pustulans an Knollen ungef?hr dreimal gr?sser bei Anwendung des Selektivmediums FHC von Bannon (1975) als bei Anwendung der Augenstecklingsmethode nach Hide et al. (1968). Es werden nur Ergebnisse bei Anwendung der ersten Methode aufgeführt. P. pustulans wurde nur 1977 an Knollen, die auf dem DAFS-Saatgutbetrieb erzeugt wurden, festgestellt,P. exigua var.foveata war dagegen in den meisten Jahren (Tabelle 1) in sehr schwachem Umfang vorhanden. Die Verseuchung durch beide Erreger war an Klonen aus den anerkannten VTSC-Betrieben viel st?rker als auf den DAFS-Pflanzgutbetrieben (Tabellen 1 und 2). Beide Pilze waren an Knollen nach dem dritten Jahr der Vermehrung weiter verbreitet als an jenen von Zweijahresklonen. 1974 und 1975 warP. pustulans auf Knollen der Farm E viel weniger verbreitet als auf jenen der vier andern Farmen. Die Knollenverseuchung durchP. exigua var.foveata war dagegen auf Farm D (Tabelle 3) am geringsten. Nachdem die Farmen C, D und E seit 1976 bei der Ernte j?hrliche Fungizidbehandlungen anwandten, um diese Erreger zu bek?mpfen, war die Verseuchung der unbehandelten Knollen 1977 und 1978 viel geringer als auf den Farmen A und B, aber keiner der Erreger war eliminiert. Der Knollenbefall durchP. pustulans undP. exigua var.foveata war bei in FS3 und AA1 klassifizierten Best?nden ?hnlich und war nur schwach geringer an VTSC-Best?nden des vierten und fünften Jahres (Tabelle 4). Das Auftreten vonP. pustulans war durch die geographische Lage der Betriebe weitgehend unbeeinflusst.P. exigua var.foveata war in den n?rdlichen Regionen Schottlands weiter verbreitet als in den Grafschaften von Angus, Perth, Fife und Kinross (Tabelle 4). Die Krautvernichtung fand in Angus, Perth, Kinross und Fife eine Woche früher statt als in den n?rdlicheren Grafschaften (Tabelle 5). Dies erkl?rt den regionalen Unterschied im Auftreten vonP. exigua var.foveata nicht. Der Prozentsatz an durchP. exigua var.foveata befallenen Knollen betrug in Best?nden, die in der 6, und 7. Woche abgebrannt wurden, in der n?rdlichen Region 51,1 und 52,8, in Angus, Perth, Fife und Kinross 17,5 und 12,4.

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Résumé L'influence dePolyscytalum pustulans et dePhoma exigua var.foveata sur des lignées de semences de pommes de terre provenant de boutures et testées vis-à-vis des virus (VTSC) a été évaluée dans plus de 15 exploitations, en Ecosse, de 1974 à 1978. Des lignées de pommes de terre de classe inférieure (FS3 et AA1) ont aussi été examinées en 1978 en relation avec la région d'Ecosse où elles étaient cultivées. En 1974, la détection deP. pustulans sur tubercules était approximativement trois fois plus grande en utilisant le milieu sélectif FHC de Bannon (1975) qu'en utilisant la méthode de Hide et al. (1968). Seuls les résultats utilisant la première méthode sont présentés.P. pustulans a été détecté sur les tubercules cultivés dans les exploitations dépendant de la DAFS uniquement en 1977 maisP. exigua var.foveata était présent à des niveaux très bas la plupart des années (tableau 1). La contamination par les deux pathogènes a été plus grande pour les clones cultivés dans les exploitations agrées pour le VTSC que dans celles dépendant du Département de l'Agriculture et de la Pêche Ecossais (DAFS) (tableaux 1 et 2). Les deux champignons étaient plus répandus sur tubercules après la 3ème année de multiplication que sur ceux provenant de clones de 2ème année. En 1974 et 1975,P. pustulans était moins important sur les tubercules provenant de l'exploitation E que sur ceux provenant des 4 autres exploitations tandis que la contamination des tubercules parP. exigua var.foveata était moindre dans l'exploitation D (tableau 3). Après que les exploitations C, D et E aient adopté en 1976, les traitements fongicides à la récolte pour contr?ler ces pathogènes, la contamination des tubercules non traités en 1977 et 1978 était beaucoup moins importante que dans les exploitations A et B où aucun pathogène n'était éliminé. La contamination des tubercules parP. pustulans etP. exigua var.foveata était identique, sur les lignées classées FS3 et AA1 et était faiblement moindre sur les lignées de VTSC du 4ème et de 5ème année (tableau 4). L'incidence deP. pustulans n'était pas affectée par la localisation géographique des exploitations. P. exigua var.foveata était plus important au Nord de l'Ecosse que dans les régions d'Angus, Perth, Fife et Kinross (tableau 4). La destruction des fanes a eu lieu une semaine plus t?t à Angus, Perth, Kinross et Fife que dans les régions du Nord (tableau 5). Ceci n'explique pas la différence régionale dans l'incidence deP. exigua var.foveata: le pourcentage de tubercules contaminés parP. exigua var.foveata provenant de cultures défanées à 6 et 7 semaines était de 51,1 et 52,8 respectivement dans la région Nord et 17,5 et 12,4 à Angus, Perth, Fife et Kinross.