Foliar and tuber assessment of late blight (Phytophthora infestans (Mont.) de Bary) reaction in cultivated potato (Solanum tuberosum L.)

Summary Host plant resistance is an important component to the management of potato late blight,Phytophthora infestans (Mont.) de Bary. Assessment of potato lines (Solanum tuberosum L.) with various levels of resistance toP. infestans (US8, A2 genotype) were evaluated in field trials, greenhouse controlled environment chambers and inoculated tuber reactions. Five lines (AWN86514-2, B0692-4, B0718-3, Jacqueline Lee, and B0288-17) with strong foliar resistance to late blight were identified in these inoculated field trials. Greenhouse controlled environment chamber studies allowed resistant and susceptible lines to be distinguished, but the 1998 greenhouse results did not correlate well with field data. Four lines (A084275-3, Bzura, MSG007-1, and MSG297-4RD) evaluated by a digital image analysis technique demonstrated tuber resistance based upon average reflective index values in the inoculated tuber studies. Tuber resistance did not correlate with field foliar resistance. Based upon these results, field assessment of foliar reaction toP. infestans provides the best measure for assessing late blight resistance in potato. Tuber resistance to late blight can be identified among lines with varying levels of foliar resistance.     

A seedling progeny test for resistance to potato foliage blight (Phytophthora infestans (Mont.) de Bary)

Summary Batches of 25 seedlings from progenies representing a range of resistance to foliage blight were exposed toP. infestans in a glasshouse seedling test, the surviving seedlings grown on and the tubers harvested. Two tubers of 15 clones of 51 such progenies chosen to represent a range of resistance from high to low were grown in a field trial. An equal number of clones from unexposed populations of the same progenies were included. The mean level of resistance of each exposed progeny was higher than its unexposed counterpart, and there was a high correlation between the seedling score and the field score of unexposed progenies. The mean field scores were higher and variances lower in exposed populations, indicating that truly susceptible seedlings had been eliminated by the seedling test and confirming the validity of this test for assessing horizontal resistance to late blight.

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Zusammenfassung Anhand eines umfangreichen Kreuzungsplanes wurden Samen aus 220 Nachkommenschaften, die sowohl gegen Krautf?ule resistente Klone als auch Sorten mit unterschiedlichen Resistenzgraden enthielten, erzeugt. Zwei T?pfe, von denen jeder 25 S?mlinge derselben Nachkommenschaft enthielt, wurden besprüht mit der Rasse 1, 2, 3, 4, 5, 7, 8, 9, 10, 11 vonPhytophthora infestans, wie bei Stewart et al. (1983) beschrieben. Nach 7 Tagen wurde der Gesamt-Infektionsgrad bewertet mittels einer 1–4 Skala mit sinkender Anf?lligkeit, und wurden die stark infizierten und abgestorbenen S?mlinge entfernt. Die überlebenden Pflanzen wurden mit Fungizid besprüht und danach in Einzelt?pfe verpflanzt, in denen sie bis zur Abreife blieben. Die Knollen wurden geerntet. Zwei T?pfe mit S?mlingen wurden ohne Behandlung mitP. infestans in gleicher Weise verpflanzt. Mit Hilfe der mittleren Werte der beiden behandelten Wiederholungen wurde jede Nachkommenschaft den Kategorien 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 oder 4.0 zugeordnet. Acht Nachkommen in jeder Kategorie (ausser der ersten beiden) wurden in einem Feldversuch ausgepflanzt. Jede Nachkommenschaft war durch zwei Knollen von 30 Klonen vertreten: 15 überlebende aus dem Krautf?uletest und 15 nicht-behandelte Klone. Die Knollen wurden in zwei Bl?cken randomisiert, jeder enthielt 1340 Pflanzen in 17 Reihen. Jede dritte Reihe wurde mit der Sorte King Edward bepflanzt, die Anfang Juli mitP. infestans inokuliert worden war. Der Befallsgrad an Krautf?ule wurde im August in Intervallen bewertet nach der 1–9 Skala von Malcolmson (1976) mit ansteigender Resistenz. Die am 30. August ermittelten Daten von 30 Knollen der acht über den Versuch verteilten Kontrollsorten sind in Tabelle 1 dargestellt. Im Vergleich zu ver?ffentlichten Daten zeigen sie eine zufriedenstellende Rangordnung. Tabelle 2 zeigt die Varianzanalyse der Krautf?ule-Werte der nicht-behandelten Proben von Nachkommen. Sie zeigt signifikante Variationen zwischen Nachkommenschaften innerhalb der S?mlingstest-Kategorien, zwischen Geschwistern innerhalb der Nachkommenschaften und zwischen Kategorien. In Tabelle 3 ist der durchschnittliche Krautf?ule-Index behandelter und nicht-behandelter Populationen jeder Nachkommenschaft aufgelistet, entsprechend ihrer Klassifizierung im S?mlingstest. Feld- und S?mlingstest korrelieren im hohen Masse, und der durchschnittliche Grad an Resistenz jeder behandelten Probe ist h?her als der von entsprechenden, nicht-behandelten Proben, zumal ihre Variation geringer ist. Auf diese Weise ergab ein S?mlingstest zuverl?ssige Bewertungen der Resistenz reifer Pflanzen und kann zur Eliminierung anf?lliger S?mlinge in frühen Stadien von Züchtungsprogrammen herangezogen werden. Er kann zudem als Methode zur Prüfung von Nachkommenschaften mit umfangreichem Elternanteil dienen, um die besten Kombinationen für weitere Züchtung zu identifizieren, wobei die Selektion eher auf dem Genotyp als dem Ph?notyp beruht.

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Résumé Des semences de 220 lignées sont obtenues par un programme de croisements de fa?on à obtenir des cl?nes résistants au mildiou du feuillage et des variétés présentant différents degrés de résistance. Deux pots, contenant chacun 25 plantules de la même lignée, sont pulvérisés avec les races 1, 2, 3, 4, 5, 7, 8, 9, 10, 11, dePhytophthora infestans suivant la méthode décrite par Stewart et al. (1983). Après 7 jours, les différentes contaminations sont notées pour chaque pot, selon une échelle l à 4 de sensibilité décroissante; les plantules détruites ou sévèrement atteintes sont retirées. Les plantes restantes sont traitées avec un fongicide et transplantées dans des pots individuels puis laissés jusqu'à maturité. Les tubercules sont ensuite récoltés. Deux pots de plantules ont été transplantés en l'absence de contamination parP. infestans. A partir de la note moyenne des deux répétitions, on attribue à chaque lignée une catégorie 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 ou 4.0. Huit lignées dans chaque catégorie (sauf pour les deux premières où il y en a moins) sont ensuite plantées dans un essai de plein champ. Chaque lignée est représentée par 30 clones de 2 tubercules: 15 cl?nes ayant survécu au test mildiou et 15 cl?nes non contaminés. Les tubercules sont plantés de fa?on aléatoire dans 2 blocs comprenant chacun 1340 plantes sur 17 rangs. La variété King Edward plantée sur chaque troisième rang, a été inoculée avecP. infestans en début juillet. La gravité du mildiou sur feuille est mesurée au cours du mois d'ao?t, suivant l'échelle 1 à 9 de résistance croissante de Malcolmson (1976). Les résultats obtenus le 30 ao?t à partir de tubercules provenant de 8 variétés prises comme témoins et réparties à travers l'essai, sont regroupés dans le tableau 1 et sont en concordance avec des résultats déjà publiés. Le tableau 2 donne l'analyse de variance résultant des notations de mildiou sur les lots non contaminés. On observe une différence significative entre les lignées au sein d'une même catégorie, entre les individus d'une même lignée et entre les catégories. Le tableau 3 regroupe les notations de mildiou sur les populations contaminées et non contaminées de chaque lignée, selon leur classification par le test de résistance à partir des plantules. Les notes obtenues en plein champ et par le test de résistance à partir des plantules sont correlées de fa?on ha⫲tement significative et le niveau moyen de résistance de chaque lot contaminé est supérieur à celui du lot correspondant non contaminé, bien que la variance soit plus faible. Le test de résistance à partir des plantules donne une bonne estimation de la résistance des plantes à maturité et il peut être utilisé pour supprimer les plantules sensibles à un stade précoce dans un programme de sélection. Il peut être un moyen rapide pour tester un nombre important de lignées en identifiant les combinaisons les meilleures dans la poursuite de la sélection, cette dernière étant fondée sur le génotype plut?t que le phénotype.

     

Resistance to late blight (Phytophthora infestans (Mont.) de Bary) of potato plants regenerated fromin vitro selected calli

Summary Cells of seven potato cultivars were selectedin vitro with culture filtrate (CF) ofPhytophthora infestans (Mont.) de Bary. Regenerated plants were tested for late blight resistance. The aim of the study was to check the efficacy of CF in the selection of potatoes for resistance to late blight and to evaluate the effects of additive factors on general resistance. One selection cycle, applied to a cell culture system, selected cells resistant to toxic metabolites of CF.In vivo screening of clones regenerated from selected cells was done in two steps: on whole plants, assessing foliage late blight, and on detached leaves, assessing single factors of horizontal resistance. In general, the frequency of resistant variants selected with CF did not differ from that of resistant somaclonal variants. Nevertheless, high concentrations of CF in the growth medium seemed to induce an improvement of some partial resistance factors as compared with the source plants.     

Confirmatory evidence for the efficacy of a seedling progeny test for resistance to potato foliage blight (Phytophthora infestans (Mont.) de Bary)

Summary Examination of the field resistance to foliage late blight of a population of selected potato clones has clearly demonstrated the efficiency of a seedling blight screen in eliminating susceptible material from that population. The subsequent selection from amongst survivors of the screen of agronomically superior material compares favourably with an unexposed sample of the same population, confirming the applicability of a seedling screen in a breeding programme.     

Mutation breeding for yield andPhytophthora infestans (Mont.) de Bary foliar resistance in potato (Solanum tuberosum L. cv. Golden Wonder) using computerized image analysis in selection

Summary Using visual selection for off-types and image analysis to select against maturation mutants, a line combining improved late blight resistance and yield was isolated in an in vitro mutagenesis programme from a population of 2101 putative mutants. The adventitious regenerants from mutagenesis treatment, including spontaneous mutants (somaclonal variants), were subjected to serial subculture of the apical bud in vitro and two cycles of apical cuttings in vivo to breakdown chimeras. The selected line was stable in trials over five years; others were unstable or failed to combine improvement in late blight resistance with high yield. Leaf and tuber image analysis was used to characterize selected lines. Lines that differed most from cv. Golden Wonder in yield and late blight resistance were more easily distinguished from the control by image analysis. The role of image analysis in negative selection and in determination of the relatedness of mutants to the control is discussed.     

Sites of entry ofphytophthora infestans (Mont.) de Bary into potato tubers in assessment of tuber blight resistance

Summary During assessment of tuber blight resistance using three test methods, the sites of entry ofPhytophthora infestans into the tuber was estimated for 41 cultivars. The trials were conducted using a mix of three highly virulent isolates at Gro? Lüsewitz 1991–1993. The influence of date of harvest and intermediate storage before inoculation was examined with two cultivars in 1993. Two levels of soil moisture and five densities of inoculum were tested in 1994. Methods of assessment of tuber blight resistance, cultivars, years and all their interactions showed a significant effect on sites of entry. Eye infections identified the highest the level of resistance and correlated best with tuber blight resistance in net bag test, r=-0.58. Soil moisture and temperature before harvest varied the ontogenic development of resistance and the relation of sites of entry of the pathogen from July to September. Intermediate storage changed the resistance and the preferred site of cultivar-specific infection. Interactions cultivar x soil moisture and cultivar x density of inoculum were significant and should be considered in the resistance assessment of cultivars.     

Susceptibility of potato varieties and advanced breeding lines (Solanum tuberosum L.) toPhytophthora infestans (Mont.) de Bary in greenhouse screenings

Late blight (Phytophthora infestons (Mont.) de Bary) has re-emerged as an important pathogen of the cultivated potato (Solanum tuberosum subsp.tubero-sum L.) in North America. The purpose of this study was to evaluate the relative susceptibility of potato germplasm in the greenhouse in order to initiate a breeding program for resistance to the US-8/A2 mating type which is the more aggressive and prevalent strain of late blight. Whole plants of 147 cultivars and breeding lines were evaluated. Percent plant area infection was visually assessed. Seven days after inoculation, infection ranged from 0 to 100% and the overall mean was greater than 50%. Two-thirds of the cultivars and breeding lines tested were very susceptible to the US-8 genotype. The highest resistance was identified in the somatic hybrids betweenS. tuberosum andS. bulbocastanum and their backcross derivatives. Pike and Snowden were less susceptible than the other North American cultivars. Zarevo was most resistant among the European cultivars. Seven of the advanced breeding lines were equivalent to Zarevo in infection levels. The host plant resistance identified among the material tested in this study can be used by breeding programs to develop improved cultivars with resistance to US-8 genotypes of late blight.     

Assessment of the field resistance of potato genotypes with major gene resistance to late blight (Phytophthora infestans (Mont.) de Bary) using inoculum comprised of two complementary races of the fungus

Summary Two races ofPhytophthora infestans, differing in virulence against four major (R) genes for resistance, were used to assess the field resistance of differential (R) genotypes, both singly and mixed together in equal proportions. The mixed inoculum led to an over-estimation of the foliage resistance of genotypes with which only one of the races was compatible, by up to two scoring categories of a 1–9 scale in field trials and, to a lesser extent, in glasshouse tests depending on the differential and the test. Assessment of tuber resistance was not significantly affected. The R₁₀ and R₂ differentials showed a higher level of field resistance in the foliage and tubers, respectively, than the other genotypes. It is concluded that assessments of genotypes which may possess R genes, using mixed inoculum, should be interpreted with caution, and apparent high levels of field resistance confirmed through genetic studies of the inheritance of the resistance.     

Control of late blight (Phytophthora infestans (Mont.) de Bary) with fungicide applied by hydraulic and electrostatic sprayers

Summary Similar amounts of metalaxyl were deposited onto potato leaves on the middle and lower parts of the stem by different tractor-mounted sprayers, but much more was deposited onto apical leaves by an electrostatic sprayer. Late blight spread into plots from inoculated guard plants but was controlled by Fubol (metalaxyl+mancozeb) at least as well when applied electrostatically (4.4 or 91/ha) as hydraulically (200l/ha).     

The Pathosystem Solanum tuberosum L.-Phytophthora infestans (Mont.) de Bary in Chapingo,Mexico. Expected,Observed, and Simulated

The 1:1 ratio of the Phytophthora infestans A1 and A2 mating types favors the sexually derived genetic diversity of the late blight pathogen of potatoes, which is widespread in the central highlands of México. This ratio guarantees the successful infection of this pathogen, even in resistant potato hosts. However, wild Solanum species present in the region serve as alternative hosts to the pathogen. Knowledge about the external factors that influence the dynamics of this disease facilitates the assessment and selection for genetically resistant potato cultivars to late blight, in addition to providing the capacity to predict epidemics. This work aimed to assess the expected, observed, and simulated progress of natural P. infestans infection of potatoes during two epiphytotic field seasons (2009 and 2010) at Chapingo, México. Using 8-years of weather datasets, six ideal situations were predicted with four to eight infection cycles of 6 to 12 h each. In comparison to the predictions, the observed effect of the area under the disease progress curve, and its components (AUDPC, RAUDPC, RaRAUDPC), was highly significant, with a low coefficient of variation among the potato cultivars used in the study. In conclusion, we confirm that the LATEBLIGHT-LB2004 model is useful for simulating and predicting late blight epidemics based on the weather conditions of Chapingo, except for the magnitude of the relative humidity threshold variable (RH_threshold), which requires calibration for each cultivar.     

Foliage resistance toPhytophthora infestans (Mont.) de Bary in the Ecuadorian natinal collection ofSolanum phureja ssp.phureja Juz. & Buk

Summary The Ecuadorian national collection ofS. phureja ssp.phureja Juz. & Buk. was evaluated in two parts in consecutive years in the field for resistance toPhytophthora infestans (Mont.) de Bary. Levels of resistance higher than that of the moderately-resistant tetraploid control were found. Preliminary studies indicate that this resistance is a result of minor genes. Three components of resistance: lesion diameter, lesion number and latent period were all correlated with field resistance as measured by the area under the disease progress curve.     

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 3. Physiological races ofPhytophthora infestans

Summary A survey of races ofPhytophthora infestans was made in 1985 and 1986 on the western slopes of the volcano Nevado de Toluca, Mexico. Isolates were collected from infected plants of wildSolanum species found in five long (7–10 km) and some short (300–500 m) transects. The study are of 15 000 ha was between 2900 and 3900 m altitude. Isolates were collected fromSolanum demissum (35 isolates),verrucosum (11),iopetalum (2),brachycarpum (7),stoloniferum (2), r-genotype (9) and from differential hosts (11). No isolate was identified as race 0. Ten virulence genes were identified with a set of differential hosts and the most common were numbers 2 and 4. Isolates with eight virulence genes were the most common and those with ten were found on eight occasions in the natural habitat and on three in the experimental fields.     

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 2. Distribution ofPhytophthora infestans

Summary An area of 15 000 ha on the Western slopes of the volcano Nevado de Toluca was surveyed during 1982–1986. The occurrence of late blight (Phytophthora infestans) was studied in 1986 along five long (7–10 km) transects from an altitude of 2900 m to 3900 m, the upper limit ofSolanum demissum (dms) and in all years in some other sites. Late blight was confined to scattered pockets, 100–500 m in diameter, separated by several kilometers. In 789 colonies of wildSolanum spp., each consisting of 4 to >100 plants per colony ofdms, verrucosum (ver), iopetalum, brachycarpum, × edinense, stoloniferum or one unidentified species, symptoms were found in 10% of the colonies or less than 1% of the plants in each colony. Plants had few (1–2) lesions which did not affect vigour. No infected tubers were found. Most (64%) of the infected colonies were ofdms, the predominant species (69%). Colonies ofver were less frequent (11%) and 17% were infected. No infected plants were found in the unidentified species, thought to bedms×ver.     

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 5. Type of resistance toP. infestans

Summary Plants of wildSolanum species (S. demissum (dms), S. verrucosum (ver), S. iopetalum (iop), S. brachycarpum (bra), and an unidentified species (unsp), from the natural habitat were inoculated in the laboratory withPhytophthora infestans race 0, to see if major genes for resistance were present, and with the most complex and aggressive race available in an attempt to reveal the level of general resistance. No plant with a susceptible reaction to race 0 was found in 120 plants ofdms, and 34 plants ofunsp. Plants with a hypersensitive reaction to race 0 and susceptible plants were found inver. When infected with a complex aggressive isolate of the fungus 73.5% of the WSS plants were susceptible. It was concluded that WSS in the area possess both complex race specific resistance and a high level of general resistance, whose effects cannot yet be separated.     

The degree of starch phosphorylation as influenced by phosphate deprivation of potato (Solanum tuberosum L.) plants

Summary The levels of starch phosphorylation in potato tubers were investigated in relation to fertilization with phosphorus (P) of field-grown and greenhouse-grown plants. The field-grown plants received 0, 15 or 30 kg P ha⁻¹. Starch from plants grown without P-fertilizer contained 15.6 nmol P (mg starch)⁻¹ whereas starch from plants grown with 30 kg P ha⁻¹ contained 20.6 nmol P (mg starch)⁻¹. The greenhouse-grown plants were cultured in inert media and received nutrient solutions containing 0, 3, 12 or 24 mg P l⁻¹, respectively. Plants grown with no P-fertilizer produced tuber starch with phosphorylation levels reduced to approximately 30% of the level found in plants grown with an ample supply of P. Thus, the level of starch phosphorylation can be modified by limiting the P-supply through the root system. Application of foliar P-fertilizer to the greenhouse-grown plants had no significant effect on the phosphorylation of the tuber starch.     

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the western slopes of the volcano Nevado de Toluca. 1.Solanum species

Summary During the period 1982–87, wildSolanum species were surveyed in an area of 15000 ha by observations and collections along five long (10 km) transects covering altitudes from 2900 to 3900 m, and twelve short (300 m) transects. The sevenSolanum species,demissum (dms), verrucosum (ver), iopetalum, brachycarpum, x edinense, stoloniferum and an unidentified species, possibly a sterile hybrid betweendms andver, grew patchily and their many small colonies formed part of a complex plant community that changed in composition with altitude and topography. The most frequent species wasdms (80%), while other species often grew in mixed colonies with it. As the altitude increased, so did the distances between colonies, whereas species diversity decreased and above 3500 m onlydms was found. Most species propagated both clonally, through tubers on long stolons, and sexually through botanical seed.     

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 4. Plant development in relation to late blight infection

Summary Development of plants ofSolanum demissum (dms) andSolanum verrucosum was followed in three sites at altitudes of 2950, 3250 and 3500 m, and the incidence of late blight and physiological races determined. No plants were found that had grown from true seed although berry formation was abundant indms. Tubers sprouted during the dry period and most shoots emerged at the beginning of the rainy season in late April. Emergence continued for about two months indms, and new plants were later formed from stolons, so that plants of different ages were present at the same time. Late blight was first found in cultivated fields adjacent to the site at 2950 m, and three weeks later lesions were found in wildSolanum species (WSS) simultaneously at all sites. Few plants per colony of WSS were infected and none was seriously damaged. Complex races (6–10 virulence genes) were found early in the season at one locality.     

Effects of shoot,root and stolon temperature on the development of the potato (Solanum tuberosum L.) plant. I. Development of the haulm

Summary Cultivars Bintje and Désirée were grown in a set-up that allowed temperatures of the shoot, root, and stolon environments to be separately varied. Shoot and root temperatures were either low, 18°C, or high, 28°C. Stolon temperatures were either ‘normal’, i.e. allowed to equilibrate at an ambient that depended on the temperatures of the shoot and root compartments, or ‘in-creased’ by fixed supplementary heating units in the stolon chambers. Haulm longevity was increased by the combination high shoot temperature and low root temperature, but greatly decreased by high shoot temperature combined with high root temperature. The increased stolon temperature also advanced senescence. The number of branches per stem (and thereby the number of leaves) was increased by an increase in stolon temperature, but the interaction with air temperature was significant. A high root temperature reduced the number of leaves significantly. The effects on stem yield were complex; several interactions proved significant. The total drymatter yield was affected by all single factors except shoot temperature.