A glasshouse test for assessing resistance to early blight (Alternaria solani)

Summary Fourteen genotypes of potato were assessed for resistance toAlternaria solani Sor., both in the glasshouse at the Scottish Crop Research Institute in Scotland and in the field in the Negev, Israel. Glasshouse assessments agreed well with field results, identifying the most resistant and susceptible genotypes. The effects on the glasshouse test of plant age, inoculum concentration, and length of time plants were kept damp after inoculation, were examined. The best agreement with field results was shown by plants inoculated at flowering, and the best discrimination between genotypes was achieved by keeping the foliage damp for 24 h after inoculating with a spore suspension containing 2.5×10³ spores/ml.     

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.     

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.     

Assessing the tolerance toGlobodera pallida of resistant potato genotypes by means of field and pot tests

Summary The tolerance toGlobodera pallida of 15 potato genotypes with a high level of resistance toG. pallida was assessed by measuring their tuber yield in heavily infested fields and in nematicide treated strips in the same fields. Tolerance was also assessed in pots in the glasshouse by measuring the total biomass after 35 and 70 days growth in heavilyG. pallida infested soil and in uninfested soil. Large differences in tolerance between the genotypes were observed. High correlations were observed between tolerance assessments made in the field experiments and between field and pot experiments. No relationship was found between tolerance and maturity. Shoot and root dry weight were about equally affected by the nematode.     

Comparison of resistance toFusarium spp. of glasshouse-and field-grown tuber progenies of potato

Summary Field-grown tubers of 22 progenies ofSolanum tuberosum L. generated in a crossing programme involving seven parents differing in resistance toFusarium coeruleum Lib. ex Sacc. andF. sulphureum Schlect. (=F. sambucinum Fuckel, teleomorphGibberella pulicaris (Fr.) Sacc.) were wound-inoculated with a cornmeal + sand culture of each pathogen. Parental genotypes were also included. The mean lesion size of each progeny was compared in 2 years of tests, as well as with published data on glasshouse-grown tubers. ForF. coeruleum there was a high correlation between years as well as with the glasshouse data, but no such correlations were apparent withG. pulicaris. Furthermore, parental and GCA values, as well as progeny means and mid-parent scores, also correlated highly forF. coeruleum but not forG. pulicaris. Glasshouse-grown tubers of 11 wildSolanum spp. were also inoculated with both pathogens. Some resistance to one or other, or both, was apparent, particularly inS. chacoense.     

Inheritance of resistance to late blight in foliage and tubers of progenies from parents differing in resistance

Summary TenSolanum tuberosum genotypes differing in resistance to late blight (Phytophthora infestans (Mont.) de Bary) in foliage and tubers were intercrossed to determine the inheritance of disease resistance in their progenies. Plots of 10–15 clones per progeny were established in each of 2 years and resistance assessed by field or laboratory tests. The parental genotypes were similarly tested each year. The parents differed in general combining ability (GCA) for both foliage blight (FB) and tuber blight (TB). The parental and GCA scores were significantly correlated for both aspects of the disease, but the correlations between foliage and tuber scores for parents and for GCAs were not significant. Three parental genotypes were highly resistant in both foliage and tubers, and the genotype with the highest GCA for resistance to both FB and TB (cv. stirling) is recommended as the best parent. There was no evidence of strong genetic correlation between both aspects of resistance, and it is suggested that both be selected for in a breeding programme.     

Susceptibility toPhytophthora infestans of field-and glass-house-grown potato tubers

Summary Tests for resistance toPhytophthora infestans were done twice in each of 2 years on field- and glasshouse-grown tubers of ten cultivars. The cultivar rankings, were almost identical for the means of the four glasshouse tests and the means of the four field tests. However, for the field tests but not for the glasshouse tests there were cultivar x harvest date and cultivar x year interactions. It is concluded that resistance can be assessed on glasshouse-grown tubers and that such assessments should prove more consistent over years than those done on field-grown tubers. R.L. Wastie died 16.1.96     

Inheritance of resistance toFusarium sulphureum in crosses betweenS. tuberosum potato cultivars measured on field and glasshouse grown tubers

Summary Solanum tuberosum potato cultivars with differing levels of resistance to dry rot caused byFusarium sulphureum were hybridised in a half diallel crossing programme. Glasshouse grown tubers from true seedlings and field-grown tubers of the resulting progenies were assessed for their resistance toF. sulphureum. Statistical analysis revealed differences between the progenies forF. sulphureum resistance on both field and glasshouse grown tubers. On the field grown tubers, differences between progenies were due to differences in the general combining ability (GCA) of the parents. Parental phenotypes and GCA values were highly correlated. Overall. GCA was consistent for resistance or susceptibility, and glasshouse grown progenies showed a good correlation for GCA between years (r=0.85^***), and with the results of the field grown progenies (r=0.61^* in 1998 and r=0.69^** in 1999). It appears that resistance toF. sulphureum is heritable, and that this heritability is better assessed on field grown rather than glasshouse grown tubers, although the two are largely in agreement.     

Inheritance of tolerance ofGlobodera pallida and the relationship between tolerance and resistance toG. pallida in potatoes

Summary The tolerance of and resistance toGlobodera pallida of two potato progenies which segregated for these characteristics were assessed in pots in the glasshouse. The level of tolerance varied widely between genotypes within the progenies, with several genotypes being significantly more tolerant than the intolerant parent. The levels of resistance and tolerance in the progenies were not significantly correlated. No indications were obtained that the ranking of genotypes for tolerance is influenced by the density ofG. pallida in the soil. The results confirm that selection for tolerance in aG. pallida resistance breeding programme is useful and feasible.     

Acquisition of potato leafroll virus byMyzus persicae from secondarily-infected potato plants of different genotypes

Summary The acquisition of potato leafroll virus (PLRV) byMyzus persicae nymphs from the top leaves of potato plants was studied throughout a growing season in relation to the antigen titre in those leaves and the feeding behaviour of the aphid. Secondarily-infected plants of eight potato genotypes with different levels of field resistance served as virus sources. Early in the growing season, plants were efficient sources for virus acquisition. The amount of viral antigen detected inM. persicae nymphs fed on the top leaves was strongly correlated with the titres of viral antigen in these leaves. Virus acquisition from the top leaves of older potato plants was markedly impaired and could not be correlated with their virus titre. With increasing age of the potato plants and the development of virus symptoms, the virus titre in the leaves declined and the initial weak correlation between the virus titre and field resistance ratings disappeared. Thus, screening secondarily-infected potato plants for field resistance to PLRV based on the concentration of viral antigen in leaves or in aphids fed on them should be avoided later in the growing season. The feeding rate ofM. persicae, measured by the number of honeydew droplets excreted, did not account for the reduced uptake of virus from older plants since it was not influenced by the age of the plant. Throughout the growing season, the feeding rate ofM. persicae nymphs on PLRV-infected plants was higher on genotypes with low levels of field resistance to PLRV than on genotypes with high ones.     

Assessing progenies of potato for resistance to skin spot (Polyscytalum pustulans) in the glasshouse

Summary Crosses were made between six parents differing in susceptibility to skin spot and glasshouse-grown tubers from true seedlings of the nine resulting progenies assessed for resistance to the disease. The assessment was evaluated by a two-year comparison with the behaviour of field-grown tubers of the progenies and their parents. The correlations between progenies in the glasshouse and field were r=0.911 and 0.753 and those between the mid-parental values and the progeny means were high (r>0.864), indicating that parental differences were heritable. It is concluded that correlations were sufficient for glasshouse-grown tubers to be used for assessing large numbers of progenies in genetical studies and for eliminating highly susceptible progenies at an early stage in a breeding programme. R.L. Wastie died 16.01.96.     

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.     

The susceptibility of potato cultivars to gangrene in laboratory tests in relation to origin of tubers,damage, method of inoculation and test centre

Summary Undamaged and damaged tubers of 10 cultivars were inoculated withPhoma foveata by two methods at five test centres over three years. Damaging tubers before inoculation increased the discrimination between cultivars. While this increase was small with tubers rolled in cornmealsand cultures, it was often substantial with those dipped in a suspension of comminuted agar cultures. Correlations between centres for the four tests were significant (P<0.05) and were highest for tubers inoculated with cornmeal-sand with or without damage. The gangrene scores of the cultivars in the four tests were correlated (P<0.05) with their field susceptibility ratings but there was a strong interaction between cultivar reaction and year of test. When tubers grown in Cumbria and Midlothian were tested at one centre over two years, the gangrene scores were also correlated (P<0.05).     

Selection for resistance to Phytophthora pod rot of cocoa (Theobroma cacao L.) in Cameroon: Repeatability and reliability of screening tests and field observations

Phytophthora pod rot (Ppr) disease caused by Phytophthora megakarya is the major constraint to cocoa production in Cameroon. The development of resistant varieties requires the use of effective resistance testing methods. The repeatability and reliability of the leaf disc and detached pod tests, as applied in selection activities in Cameroon, were studied. Repeatability of the tests was estimated by calculating the correlation (r) between resistance scores of clones, progenies and individuals within progenies in different inoculations rounds of the leaf disc and detached pod tests. Such correlations were generally significant for both tests. For the leaf disc test, as expected, correlations were lower for individuals within seedling progenies than for the average of seedling progenies or of clones. This suggests that a higher number of replicate observations are required for correct evaluation of individual seedlings than for evaluation of the average level of resistance of progenies. Observations carried out 5 or 7 days after inoculation was highly correlated, suggesting that scoring in the leaf disc test may be done only once at 5, 6 or 7 days after inoculation. In one experiment the ranking of leaf disc and detached pod inoculation test results could be compared statistically, with data being significantly correlated (r 0.78). The reliability of the tests was evaluated by the correlations between results of the tests and the level of field infection. These were generally positive and significant, for the leaf disc and detached pod test. A variation between mean scores of 5 and 8 in the detached pod test appeared to be related to a 40% difference in field infection with P. megakarya in years with medium disease pressure. However, correlations with field resistance were not always significant, suggesting the influence of uncontrolled environmental factors affecting field observations or the results in the screening tests. It is concluded that the leaf disc and detached pod tests, if applied under standardized conditions, can be of great value to speed up selection for Ppr resistance.     

Leaf effects of wild species ofSolanum and interspecific hybrids on growth and behaviour of the potato tuber moth,Phthorimaea operculella Zeller

Summary Four introductions ofSolanum berthaultii wild species and sevenS. tuberosumxS. berthaultii hybrids were evaluated by antibiosis and antixenosis laboratory tests with potato tuber moth. Some morphological and chemical traits related to the trichomes were analysed. Two clones of the wild speciesS. sparsipilum and five ofS. pinnatisectum, characterised by high tuber resistance, were tested for leaf resistance, both by antibiosis and antixenosis assays. The genotypes with high density of trichomes A and with high exudate PPO activity, showed a moderate negative effect on pupal weight and fecundity in the antibiosis tests. Trichomes B exudate showed a strong repulsive effect on oviposition of adult in the antixenosis tests. TheS. pinnatisectum clones revealed a good leaf resistance level, whereas theS. sparsipilum clones showed lower levels of antibiosis and antixenosis.     

Monoxenischer Resistenztest für Kartoffelgenotypen gegenüber dem ZystennematodenGlobodera pallida. 3. Anwendung und Prüfung des Tests

Zusammenfassung Zur Resistenzselektion von Kartoffelgenotypen gegenGlobodera pallida (Stone) Beharens (Pa3) wurde ein monoxenischerin vitro-Test entwickelt, der die Auswertung geringer quantitativer Unterschiede in der vermutlich polygen-vererbten Resistenzreaktion erlaubt. Hierzu wurde das aussagekr?ftigste Resistenzkriterium an 30 unterschiedlich resistenten Genotypen bestimmt. An den anf?lligen undG. rostochiensis-resistenten Genotypen entwickelte sich immer ein h?herer Anteil Weibchen als M?nnchen, im Gegensatz zu denG. pallida-resistenten Genotypen, bei denen die M?nnchen überwogen. Eine weitere Unterteilung derG. pallida-resistenten Genotypen war durch das erh?hte Auftreten nekrotisierter Wurzelbereiche und durch Larven mit stagnierender Entwicklung m?glich. Zur Bewertung des Einflusses der Genotypen auf die charakteristische Populationsentwicklung des Nematoden wurde ein Schema mit vier Gruppen erstellt. Die überprüfung der untersuchten Genotypen in einem Topfkulturetest zeigte eine übereinstimmung mit den Ergebnissen aus dem monoxenischen Resistenztests.

---

Summary A monoxenicin vitro test, which was developed for selecting resistance in potato genotypes toGlobodera pallida (Stone) Behrens, can distinguish small differences in the resistance reaction, presumed to be polygenically controlled. To establish meaningful criteria for resistance, a number of characters which could be assessed rapidly were determined (number of necrotic root areas, number of invading juveniles but not yet sexually differentiated, number of juveniles developing as males and females, number of juveniles with arrested development). Assessments were made 21, 35 and 49 days after the introduction of 1000 freshly hatched juveniles ofG. pallida Pa3- population ‘Frenswegen’ per test Petri dish. Because of differences in levels of juvenile invasion per genotype (number of necrotic root areas) and differentiated individuals (Table 1), to make genotypic comparisons, it was necessary to consider other resistance criteria along with percentage juvenilie invasion. The ratio of males to females was shown to be the most reliable criterion of resistance. There was a higher proportion of females than males on susceptible, andG. rostochiensis resistant genotypes than onG. pallida resistant genotypes on which males predominated (Table 1). A further subdivision of theG. pallida resistant genotypes was possible using the increased occurrence of necrotic root areas, and juveniles with arrested development. A scheme was constructed for rating the effect of genotypes on the characteristic development of the nematode populations. It contained four groups: susceptible reaction (Figure 1), weakly expressed resistance (Figure 2), expressed resistance (Figure 3) and strongly expressed resistance (Figure 4). Female counts from pot tests with particular genotypes were in general agreement with the results from the monoxenic resistance test (Table 1).

     

Monoxenischer Resistenztest für Kartoffelgenotypen gegenüber dem ZystennematodenGlobodera pallida. 1. Kultivierung der Genotypen

Zusammenfassung Zur routinem?ssigen Resistenzbestimmung von Kartoffelgenotypen gegenüberGlobodera pallida (Stone) Behrens (Pa3) wurde ein monoxenischesin vitro-Verfahren entwickelt, dessen Grundlage die keimfreie Kultivierung der Testgenotypen in einem N?hragarmedium unter definierten Bedingungen war. Dadurch konnte die vollst?ndige Entwicklung der Nematoden sowie die Dimension ihres N?hrzellensystems in den transparenten Kartoffelwurzeln kontinuierlich beobachtet werden. Der Test erm?glichte die Selektion resistenter Genotypen anhand geringer quantitativer Unterschiede in der vermutlich polygen-vererbten Resistenzreaktion. Für die Anzucht keimfreier Pflanzen aus Kartoffelsamen und- knollen wurden spezielle Verfahren entwickelt und an mehreren Genotypen geprüft. Die dabei festgestellte Variabilit?t, insbesondere bei der Wurzelbildung, erforderte für den geeigneten Zeipunkt der Nematodenzugabe die Ausarbeitung eines Wurzelboniturschemas. Die Genotypen konnten nach dem Test in T?pfen weiterkultiviert werden.

---

Summary A monoxenic resistance test for routine use was developed to determine the resistance of potato genotypes to the cyst nematodeGlobodera pallida (Stone) Behrens. The basis of the procedure is the aseptic cultivation of the genotypes under test in a nutrient medium in defined conditions. It was possible with it to observe invasion, the development of the nematodes, as well as the dimensions of their feeding cells in the transparent potato roots. Small, quantitative differences, due presumably to the polygenically determined resistance reactions of the genotypes, could be distinguished, and selection for resistance was feasible. For routine trials special procedures were developed using surface sterilisation of potato seed and etiolated sprouts which gave high levels of sterility. The genotypes from seeds could be tested directly, but those from etiolated sprouts had to be prepared by stem cutting. A root system was obtained which correlated with known ratings. Each genotype established from seed and etiolated shoots, developed at different rates, as did their root systems, and no definite timespan could be set before nematode introduction. Growing the cultivars under light before the test resulted in faster root development (Table 2), and a higher nematode infestation than in darkness (Figure 4). The time for nematode inoculation was determined by a rating system based on the location of the perimeter of the root system (Figure 5). In this way physiological factors and the individual growth rate of each genotype were allowed for, with the aim of assessing possibly high levels of nematode infection. At the end of the test the seedling genotypes can be removed from sterile culture and grown on in pots. Where genotypes form minitubers (Table 1) it is possible to proceed early to the next stage in cultivation or to store them trouble-free.

     

Evidence of genetic transmission of antibiosis and antixenosis resistance of sorghum to the spotted stemborer, Chilo partellus (Lepidoptera: Pyralidae)

Spotted stemborer, Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae), is the most important pest of sorghum in Asia and south and eastern Africa. Host plant resistance is an important control tactic for controlling this pest. Two breeding lines 27B × PB 15881-3 and 463B × PB 15881-3 with their parents, resistant and susceptible genotypes were evaluated in the field, glasshouse and laboratory for different resistance parameters. Breeding lines and genotypes varied significantly in foliar damage ratings, percentage of stem length tunneled, percentage of plants with deadhearts, larval survival, larval and pupal weights, larval and pupal duration, and percentage pupation and adult emergence in diets amended with leaf powder of different sorghum genotypes. The breeding lines 27B × PB 15881-3 and 463B × PB 15881-3 showed antixenosis and antibiosis to C. partellus in terms of reduced eggs per plant, larval survival and development. The levels of antixenosis and antibiosis of both breeding lines were similar to their resistant parents. Results indicate that transmission of characteristics responsible for resistance to the progeny from the resistant parent occurred.     

Die Effizienz der Nassfäuleabwehr bei der Wundheilung von Kartoffelknollen. 2. Der Einfluss auf die Resistenzbewertung von Genotypen

Zusammenfassung Die wundheilbedingte Infektionsabwehr beeinflusst das Resistenzverhalten der Knollen. Sie ist bei f?uleanf?lligen Genotypen effizienter als bei widerstandsf?higen. Die Resistenzbewertung von Genotypen nach der H?he der befallsausl?senden Erregerdichte wird von den Vegetationsbedingungen beeinflusst. Sie korreliert mit einer Bewertung nach dem Ausmass der F?ulesymptome, nicht aber mit dem Ausmass einer enzymatischen Mazeration.

---

Summary A cultivar collection was assessed for tuber resistance to the soft rot organism,Erwinia carotovora subsp.atroseptica (=Eca), and for the strength of the defence reaction associated with wound healing, by measuring the inoculum density needed to initiate infection (effective dose). Wound healing was more efficient in susceptible genotypes than in resistant ones (Fig. 1b), and resulted in changes of the ranking of cultivars, and a narrowing of the variation for soft rot resistance (Fig. 1a). No relationship was found between enzymic maceration of tissue and effective dose (Fig. 2a). The resistance ratings of cultivars were in general agreement with the effective dose and with symptom expression (Fig. 2b). Resistance behaviour was modified by growing conditions (harvest year) (Table 1), and cultivars need therefore to be assessed over several years.

     

The resistance of potato plants toErwinia carotovora subsp.atroseptica in relation to their calcium and magnesium content

Summary The effect of adding gypsum (CaSO₄) to growing media on the resistance toErwinia carotovora subsp.atroseptica (van Hall) Dye of potato (Solanum tuberosum L.) stems and mother and daughter tubers was examined in field and glasshouse experiments. The decayin situ of inoculated mother tubers from field plots treated before planting with gypsum was significantly delayed compared with tubers sampled from non-amended plots. Blackleg development was also significantly delayed in the gypsum-amended plots; however, for both mother tubers and stems the increased resistance associated with the application of gypsum later disappeared. Stem resistance tended to be related to tissue calcium and magnesium concentrations which were higher in plots treated with gypsum than in untreated plots. Daughter tubers produced in plots amended with gypsum generally contained a higher concentration of calcium in their cortex and were more resistant to decay byE. carotovora subsp.atroseptica but the effect was not consistent.