Leaf position prevails over plant age and leaf age in reflecting resistance to late blight in potato

ABSTRACT The effects of plant age, leaf age, and leaf position on race-nonspecific resistance against Phytophthora infestans were investigated in a series of field and controlled environment experiments with five different potato (Solanum tuberosum) cultivars. Leaf position proved to be the most significant factor; apical leaves were far more resistant to late blight than basal leaves. Plant age and leaf age had only minor effects; therefore, the resistance of a specific leaf remained about the same during its entire lifetime. The gradual increase in late blight resistance from basal leaves to apical leaves appeared to be a general effect, irrespective of cultivar, growing conditions, or resistance test. Therefore, it is important to consider leaf position in tests for late blight resistance, because contrasts in resistance may be ascribed erroneously to differences between genotypes or treatments, whereas they are actually caused by differences in leaf position.     

Comparative analysis of powdery mildew development on leaves, seedlings and flower panicles of different genotypes of cashew

Resistance of cashew ( Anacardium occidentale ) genotypes to powdery mildew ( Oidium anacardii ) was assessed on detached leaves, seedlings and flower panicles. Leaves were more susceptible before the development of a waxy cuticle, which confers a dull appearance. Differences in susceptibility/resistance were observed on detached leaves inoculated in the laboratory. Some genotypes supported dense growth of mildew, whereas on others only slight infection was observed under comparable conditions. Mature leaves were immune to colonization. In the field the rate of mildew infection, as estimated from disease progress curves on both seedlings and flower panicles, varied significantly between genotypes allowing their categorization into susceptible and partially resistant clones. A similar rank order for degree of susceptibility/resistance was determined from studies on detached leaves, seedlings and flower panicles. Brazilian introductions of A. occidentale and the AZA2 and AC6 genotypes proved to be partially resistant, while AM6, ATA19 and AIN62 were highly susceptible. The relationship between leaf and flower mildew infection showed positive and highly significant correlations. It is concluded that each of the three approaches to the assessment of disease susceptibility can be used to screen genotypes against mildew attack. Screening in the laboratory provides a rapid indication of potential resistance providing that leaves are compared at the same stages of maturity.     

Variation in adult plant resistance to powdery mildew in spring oats under field and laboratory conditions

Ten oat genotypes were assessed for adult plant resistance (APR) to powdery mildew ( Erysiphe graminis f.sp. avenae ) in small field plots and on detached leaf segments in the laboratory using glasshouse-grown plants. Significant genotype differences were observed in the field and on leaf segments for latent period, infection frequency and spore production, components used to assess APR. The genotype OM1711, which was selected because it expressed a higher level of mildew resistance than its most resistant parent, cv. Maldwyn, showed this transgressive resistance in both field and laboratory tests. By contrast, the genotype OM1387, also selected for high mildew resistance, only showed this enhancement in the field. It is concluded that quantitative assessment of mildew in detached leaves is not sufficiently precise to be used alone in selecting segregants for high levels of APR, and that field data are also needed.     

The role of aggressiveness and competition in the selection of Phytophthora infestans populations

Selection within populations of Phytophthora infestans was investigated by comparing the aggressiveness of single‐lesion isolates on detached leaflets of four potato cultivars with differing levels of race‐nonspecific resistance to P. infestans. The isolates included 23 representative of Northern Ireland genotypes from the early 2000s, used to inoculate previously reported field trials on competitive selection (2003–2005), plus 12 isolates recovered from the 2003 trial. The cultivars were those planted in the previous trials: Atlantic (blight‐susceptible) and Santé, Milagro and Stirling (partially resistant). Very highly significant variation for latent period, infection frequency and lesion area was found between genotypes and cultivars; differences between genotypes were more marked on the more resistant cultivars, but no one genotype was the most aggressive across all. Detached leaflets were also inoculated with mixtures of isolates from each genotype group at three sporangial concentrations: differences in aggressiveness between genotypes were more apparent at lower concentrations and on the more resistant cultivars. Genotype groups that were the most aggressive on the more resistant cultivars tended to be those selected by the same cultivars in the field. A mixture of all isolates of all genotypes was used to inoculate detached leaflets of the same cultivars. With one exception, single spore isolates recovered from any one leaflet belonged to a single genotype, but different genotypes were recovered from different cultivars. Phytophthora infestans isolates from Northern Ireland showed significant variation for foliar aggressiveness, and pathogen genotypes exhibited differential aggressiveness to partially resistant cultivars and interacted competitively in genotype selection.     

Effects of temperature and light on non-race-specific resistance of potato leaflets to late blight

Effects of temperature and illumination on colonization by Phytophthora infestans of detached leaflets of five potato cultivars differing in field resistance to blight were investigated using an ELISA system to quantify the pathogen. Leaflets of cvs Teena and Shelagh, and in one experiment cv. Brodick, were more resistant to colonization when infected leaflets were incubated at 10°C than at 20°C, but temperature conditions before inoculation had little effect. Both photoperiod and light intensity during illumination of intact plants before inoculation interacted with genotype to determine subsequent colonization of infected leaflets incubated in darkness. Leaflets from plants of cv. Teena were more resistant to colonization after exposure to low, rather than to high, light intensities but photoperiod had no apparent effect. Leaflets of cv. Shelagh grown in a 20 h day were more resistant than those grown in a 10 h day but light intensity had no effect. Leaflets of cv. Brodick were more resistant after a 20 h day than a 10 h day and after exposure to low, rather than to high, light intensities. Leaflets of cv. Bintje were extensively colonized and those of cv. Torridon remained relatively resistant to colonization, irrespective of temperature and lighting conditions. There was a high level of unexplained variation in all the experiments.     

Foliar resistance of cacao (Theobroma cacao) to Phytophthora palmivora as an indicator of pod resistance in the field: the effect of light intensity and time of day of leaf collection

Resistance of cacao leaves to Phytophthora palmivora was studied with regard to the time of leaf collection (morning, afternoon) and the degree of exposure of the leaves to light in the field (low, medium and high). The efficiency of leaf disc inoculations in predicting field resistance of nine clones was compared with that of detached and attached pod inoculations. Significant effects were observed, with leaves exposed to high light intensity and collected early in the afternoon showing highest susceptibility. The effect of time of leaf collection was reduced when leaves were stored overnight and leaf discs prepared and inoculated the following day, as compared to inoculations on the day of collection. Interactions between the main factors were significant, though less substantial than the clone effects. The most significant correlations with pod resistance ( r  = 0·70 to 0·97) were obtained for leaves collected early in the morning and exposed to intermediate shade conditions in the canopy. For other treatments, the correlations with pod resistance were still positive ( r  = 0·23 to 0·83) but often not significant. Pod inoculations in the laboratory were better correlated with field resistance ( r  = 0·92) than pod inoculations in the field ( r  = 0·72). Detached pod inoculations were also better correlated with leaf disc inoculations than those of attached pods. The results confirm the validity of laboratory inoculations of leaves and pods to assess field resistance to Phytophthora . Standardization of the leaf disc test is essential to obtain reliable results.     

Differential expression of resistance to powdery mildew incited by race 1 or 2 ofsphaerotheca fuliginea inCucumis melo genotypes at various stages of plant development

Thirty-nine genotypes ofCucumis melo (plant introduction entries, open-pollinated cultivars and F₁ hybrids) were evaluated for resistance to powdery mildew under either natural field conditions or artificial inoculation in growth chambers at the cotyledonary stage and the 2-true-leaf stage. Results confirmed that susceptibility in cotyledons was not necessarily associated with susceptibility in either true leaves in growth chambers or adult plants in the field. However, resistance at the 2-true-leaf stage in growth chambers was highly correlated with resistance of field-grown plants. Results also showed that 20 muskmelon genotypes resistant to race 1 at the cotyledonary stage were also resistant at the 2-leaf-stage and as adult plants in the field. The same was true for ten genotypes with race 2 inoculations. Because muskmelon genotypes expressing resistance in cotyledons were also resistant in true leaves in growth chambers or the field, the use of plants at the cotyledonary stage is recommended for screening for powdery mildew resistance caused by race 1 or race 2 ofS. fuliginea. When cotyledons are susceptible, screening should be done at the 2-true-leaf stage.     

The response of different celery genotypes to infection by septoria apiicola

A rapid and useful method of disease assessment, based on a visual key, was developed for leaf spot of celery, caused by Septoria apiicola . Details were provided by an assessment of percentage infection, number and type of lesions per plant and number of pycnidia per lesion. The lesion response was graded from 0 (resistant with no lesions) to 3 (susceptible with grey lesions, leaf chlorosis and numerous pycnidia). Assessments were performed on leaves attached to the plants as well as on detached leaves incubated in sealed Petri dishes, after inoculation with a standard spore suspension. Tests were carried out on currently popular celery varieties, lines of wild celery, hybrids of celery × wild celery and celery × parsley. Lesion response 0 was found on some lines of wild celery, lovage and parsley; response 1 on lines of wild celery; response 2 on hybrid celery × wild celery and on the commercial celery Giant Red; and response 3 on all of the remaining celery varieties tested (Monarch, Fenlander, Tall Utah and Galaxy). Detached leaves from the individual genotypes showed responses in an order similar to that of the attached leaves but the onset of the response in detached leaves was always more rapid, with fewer lesions and pycnidia. Analysis of leaf chlorophyll content from the different sources was made to establish any relationship between chlorophyll and resistance but none was found.     

Influence of competition and host plant resistance on selection in Phytophthora infestans populations in Michigan, USA and in Northern Ireland

Competition between genotypes of Phytophthora infestans was studied by inoculating potato cultivars with differing susceptibility to late blight in field experiments over three years in Northern Ireland, UK, and Michigan, USA. Multiple isolates of six genotype groups of P. infestans were chosen from the local populations in both N. Ireland and Michigan for inoculation of separate field trials planted in 2003, 2004 and 2005. Four cultivars were used in each trial; two (susceptible cv. Atlantic and the partially resistant cv. Stirling) were common to both locations, whereas the two additional cultivars (with partial resistance to late blight) were cvs Santé and Milagro in N. Ireland and cvs Pike and Jacqueline Lee in Michigan. Single-lesion isolates of P. infestans were obtained from leaves at 1% level of infection, characterized using pre-assigned markers and re-assigned to their respective genotype groups. Extreme selection occurred within the population of genotypes of P. infestans in N. Ireland in each year, with different genotype groups dominating the infection of different cultivars. Selection was observed on all cultivars tested, but was greatest on the more resistant cultivars. Over the 3 years, all of the 114 isolates obtained from cv. Milagro belonged to a single group, whereas among the 118 isolates from cv. Atlantic all six groups were represented. By contrast, in Michigan, the US-8 genotype dominated infection in all cultivars in each year; only 12 of 374 isolates characterized belonged to other genotypes (11 US-14 and a single US-10 isolate).     

Intensity of infection of tomato by Phytophthora infestans in central Ukraine in relation to varietal resistance

Phytophthora infestans is widespread in the central part of Ukraine, infecting tomatoes everywhere but not to the same extent in different years. The disease can completely destroy the crop in epiphytotic years. We have studied commercial cultivars and hybrids of tomato with different degrees of resistance to P. infestans. The degree of disease development and the date of its appearance were found to depend on the resistance group of the genotype investigated. In weakly resistant tomato cultivars and hybrids, first disease symptoms were noticed 3–4 days earlier than in more resistant ones. The degree of disease development reached 40–80% on weakly resistant genotypes at the end of the growing season, but only 5–40% on resistant ones (depending on the suitability of conditions for development of P. infestans ). The development of P. infestans on fruits also depended on the degree of resistance. Differences between moderately and weakly resistant tomatoes in the degree of disease development and the date of its appearance were not significant. The data was corroborated in the laboratory conditions in the way of artificial inoculation.     

Oospore Production of Phytophthora infestans in Potato and Tomato Leaves

ABSTRACT Fungal, host, and environmental factors affecting sexual reproduction of Phytophthora infestans in planta were studied. Intact and detached leaves were coinoculated with sporangia of various combinations of A(1) and A(2) mating-type isolates; leaves were incubated under various conditions, and oospore production was estimated microscopically within whole, clarified leaflets. Some A(1) + A(2) isolate combinations were more reproductive than others, whereas some potato genotypes better supported oospore formation than others. Tomato usually supported more oospore formation than potato. To induce oospore formation, A(1) and A(2) sporangia were usually mixed at a 1:1 ratio. Ratios of 1:19 to 19:1, however, also allowed abundant production of oospores. Optimal temperatures for sexual sporulation ranged from 8 to 15 degrees C, but oospores also were produced at 23 degrees C. Oogonia developed 5 to 6 days after sporangial coinoculation, and oospores developed after 8 to 10 days. Light had little effect on oospore formation in both tomato and potato leaves provided that initial lesions were established under photoperiodic conditions. Although A1 and A(2) sporangia usually were mixed before inoculation on leaves to obtain oospores, we found that discrete A(1) and A(2) lesions produced on opposite sides of the midvein of tomato leaves also induced oospore formation in the midvein and adjacent tissues. Oospores also formed when the two halves of the leaves were cut and separated at 3 days after sporangial coinoculation, which corresponded with the appearance of late blight lesions. The continuous supply of moisture to infected leaves was essential to oospore production. No oospores or oogonia formed in severely diseased plants kept at 50 to 80% relative humidity. Such plants did allow some oospore formation when kept continuously wet for 2 weeks in plastic boxes or tents. Detached leaves floated on water supported the highest sexual sporulation. Under optimal conditions of wetness and temperature, as many as 100 oospores per mm(2) of tissue were observed.     

Rapid Screening of Cacao Genotypes for Field Resistance to Phytophthora palmivora Using Leaves, Twigs and Roots

Black pod, caused by Phytophthora spp. is one of the most important diseases of cacao occurring worldwide. Losses due to black pod caused by P. palmivora are still moderate in Côte d'Ivoire but P. megakarya causes high losses in Ghana and other Central African countries. Variation in field attack has been observed between cacao genotypes, but evaluation of pod losses is unsuitable for obtaining rapid progress in breeding. Results of inoculation tests using young detached leaves, twigs and roots, obtained from field and nursery plants, are presented here and compared to field resistance of similar genotypes observed over a 10-year period. Nine different Upper Amazon Forastero genotypes were tested together with progenies obtained by crossing these with the susceptible check IFC5 (Amelonado genotype). Rank correlations between the early screening tests and the level of field attack were positive and mostly significant (r=0.58–0.95). The coefficient of correlation was slightly higher for leaves (r=0.88) and roots (r=0.89) than for twigs (r=0.76). Also, resistance of the different plant organs was correlated (r=0.6–0.9). Resistance of the Upper Amazon parents was well correlated with the resistance of their cross progenies (r=0.7–0.9), suggesting that resistance is highly heritable. Resistance of leaves and twigs from the nursery was better correlated with field resistance than resistance of leaves and twigs from the field, which might result from more uniform growing conditions in the nursery. Inoculation of leaves appears the most suitable early screening method for black pod resistance. Application of this test in breeding more resistant cacao cultivars is discussed.     

Development of an effective screening method for partial resistance to Alternaria brassicicola (dark leaf spot) in Brassica rapa

In order to develop a method to measure resistance to Alternaria brassicicola (cause of dark leaf spot disease) in Brassica rapa, the effects of inoculum concentration, leaf stage, leaf age and incubation temperature of inoculation on infection were studied under controlled conditions using several B. rapa genotypes. Three inoculation methods (cotyledon, detached leaf and seedling inoculation) were evaluated for this purpose. The detached leaf inoculation test was the most suitable for screening B. rapa genotypes because clear symptoms were observed on the leaves in less than 24 h, and there was a significant positive correlation between the results from the detached leaf inoculation test and the seedling inoculation test, an established method considered to yield reliable results. In addition, it was very easy to screen plants for resistance on a large scale and to maintain standard physical conditions using detached leaves. For successful infection, inoculum concentration should be adjusted to 5 × 10⁴ conidia ml⁻¹, and incubation temperature should be between 20 °C and 25 °C. The 3rd/4th true leaves from 30 day-old plants were optimal for inoculation. In a screening test using 52 cultivars of B. rapa, the detached leaf test effectively discriminated between various levels of partial resistance among cultivars. As a result, we identified two cultivars, viz Saori and Edononatsu, as highly resistant and five cultivars, viz Tokinashi Taisai, Yajima Kabu, Purara, Norin-F₁-Bekana and Tateiwa Kabu, as having borderline resistance.     

Screening Lactuca for resistance to Myzus persicae

A macrotest was developed to screen rapidly many genotypes ofLactuca in the greenhouse for resistance toMyzus persicae. In this test the estimated number of aphids on certain leaves of plants artificially exposed to aphid infestation was representative for the total number of aphids per plant. Significant differences in resistance between genotypes were observed and several partially resistant PIVT numbers were found among 645 accessions tested in 1973 and 1974. Also a microtest with leaf cages was developed in which biomass after seven to eight days of larval development and net larvae production during four to seven days functioned as criteria of resistance. Besides differences between genotypes, also within plants and between plants of the same genotype significant differences were found to occur. After testing nineteen genotypes with different levels of resistance it appeared that there was a clear relationship between results from both types of tests.     

Evaluation and analysis of reduction of late blight disease in a diploid potato progeny

The rate of late blight disease was analysed for individuals of a diploid Solanum phureja – Solanum tuberosum dihaploid hybrid population (PD), using three different assessment techniques, in the laboratory, screenhouse, and field. These hybrids expressed low disease rates in the field, comparable to resistance based on intact R genes. However, none of the parents of PD expressed any R genes and the pattern of segregation within the PD population was not indicative of R-gene inheritance. The foliage (or leaflet) area diseased had the largest broad-sense heritability of all criteria analysed, in all tests performed. In the field evaluations, the PD population showed intermediate levels of broad-sense heritability for foliage area diseased, relative to the much larger heritability detected for the group of controls possessing R genes. Resistance in the field of the PD hybrids had very little genotype-environment (G × E) interaction, indicating stability of its expression. All genotypes without R genes exhibited heritable, reduced rates of late blight disease in the field, but they were susceptible with low heritability in screenhouse and laboratory tests. This differential expression of disease indicates that the plants' indirect response to unknown environmental stress in the field may have been measured. The value of the PD hybrids for breeding of late blight resistant potato and the use of the disease rate data for detection of the underlying quantitative trait loci are discussed.     

致病疫霉对缬菌胺抗性突变体生物学特性及抗性风险量化评估

 通过研究致病疫霉对缬菌胺抗性突变体的生物学特性,评估致病疫霉对缬菌胺产生抗性的风险。采用菌丝生长速率法检测田间菌株对缬菌胺的敏感性;通过比较抗性突变体在无药条件下继代培养前后对缬菌胺的敏感性来确定其抗性稳定性;采用离体叶片法测定抗性突变体及其亲本敏感菌株的适合度和竞争力;通过量化7个基本抗性因素的风险值评估致病疫霉对缬菌胺的基本抗性风险。结果表明:在2018年从内蒙、河北、贵州、四川、黑龙江等五省(自治区)采集分离的153个菌株和2019年从河北、内蒙采集分离的40个菌株中低抗菌株分别占31.4%和35.0%,敏感菌株分别占68.6%和65.0%,抗性指数均为0.34,抗性倍数分别为1.5和2.1,未发现中抗和高抗菌株;8个抗性突变体在无药条件下继代培养10代后,少数抗性突变体的抗药性不能稳定遗传;6个抗性突变体的适合度均显著低于其亲本敏感菌株;4个抗性突变体与其亲本菌株的孢子囊不同比率混合物分别在离体叶片上继代培养1、3、7代的抗性频率显著低于初始抗性频率或与初始抗性频率无显著差异;致病疫霉对缬菌胺7个基本抗性因素的风险值之和为9,推测致病疫霉对缬菌胺的基本抗性风险为低至中等。建议加强致病疫霉菌群体对缬菌胺的抗性监测,将缬菌胺与不同作用机理的杀菌剂交替或混合使用,以延缓抗药性产生和发展。     

Towards the Development of a Novel In Vitro Strategy for Early Screening of Fusarium Ear Blight Resistance in Adult Winter Wheat Plants

A novel in vitro bioassay is described for screening Fusarium ear blight (FEB) resistance in adult winter wheat plants. Seven winter wheat cultivars were assessed for components of partial disease resistance as 28 day-old detached leaf segments in the laboratory using isolates of Microdochium nivale var. nivale and M. nivale var. majus. Results were compared with disease data obtained at anthesis using the same cultivars as whole plants and the same isolates under glasshouse conditions. Significant cultivar differences were observed using detached leaves, with cv. Avalon (a Fusarium culmorum ear susceptible cultivar) having the shortest leaf incubation period, greatest leaf lesion development and shortest leaf latent period compared to cv. Spark (a Fusarium culmorum ear resistant cultivar), which had the longest leaf incubation period, least leaf lesion development and longest leaf latent period. Using whole plants, cv. Avalon had the shortest ear incubation period and greatest ear disease severity, whilst cv. Spark had the longest incubation period and least ear disease severity. Overall, cultivars of intermediate F. culmorum ear resistance expressed intermediate responses to M. nivale isolates, using both detached leaves and whole plants. Significant correlations were found with ear disease severity and ear incubation period in whole plants and components of partial disease resistance in detached leaves, with significant correlations obtained between leaf incubation period and ear disease parameters using the M. nivale var. nivale isolate. In addition, leaf latent period and leaf lesion size showed significant correlations with whole plant reactions using M. nivale var. nivale and var. majus isolates. The in vitro screening of cultivars as detached leaves using M. nivale isolates may offer a real possibility of a rapid bioassay for the early screening of FEB resistance in wheat and other cereals.     

Host–pathogen interaction between Phytophthora infestans and Solanum nigrum, S. villosum, and S. scabrum

Potato and tomato are the two major hosts for Phytophthora infestans causing late blight. The susceptibility of leaves and whole plants of Solanum nigrum, S. villosum, and S. scabrum to infection by P. infestans was tested under laboratory conditions. Out of 39 plants representing 38 different S. nigrum accessions, 16 were highly resistant (seven accessions did not show any symptoms of infection, nine were highly resistant showing necrotic lesions in the place of infection), and 23 plants of S. nigrum were colonized by, at least, 1 of the 2 isolates of P. infestans (17 accessions were infected with two P. infestans isolates, and 6 accessions showed different reactions depending on the isolate used for inoculation). Three accessions of S. villosum, and one accession of S. scabrum were tested and did not show any symptoms of infection. The majority of S. nigrum accessions infected by P. infestans in a detached leaf assay were also infected in the whole plant assay. The reaction of field- and greenhouse-grown plants to inoculation with P. infestans in detached leaf assays was similar, but in some cases leaves from field-grown plants reacted as resistant in comparison with the leaves from greenhouse-grown plants, which were susceptible.     

Stability of partial resistance in potato cultivars exposed to aggressive strains of Phytophthora infestans

Potato cultivars were evaluated for their resistance responses to aggressive strains of Phytophthora infestans in field and laboratory experiments. Analysis of variance revealed differential cultivar-by-isolate interactions for both foliar and tuber blight resistance. Differential responses occur as revealed by specific susceptibilities of cultivars to certain pathogen genotypes and changing rank order. In general, severity of late blight epidemics as observed in the haulms did not correlate well with foliar blight resistance ratings as presented in the National List of Recommended Potato Varieties. No significant correlation was found between tuber blight incidence under field conditions and the tuber blight rating in the National List. Also, there was no relation between the field and laboratory tuber blight resistance assessments. A significant association was demonstrated between late blight infection in the foliage and tuber blight incidence under field conditions. The presence of differential interaction, independent of R-gene-based resistance, indicates some adaptation of P. infestans to partial resistance and consequently adverse effects on the stability and durability of partial resistance to potato late blight.