Evaluation of Rice Cultivars under Greenhouse Conditions for Adult-Plant Resistance to Pyricularia oryzae

Six rice cultivars showing various types of resistance or susceptibility to Pyricularia oryzae in the field were compared under controlled environmental conditions. The resistance of the cultivars with adult-plant resistance was race-specificat early growth stage. On all cultivars tested, blast infection became increasingly reduced on either leaves of adult plants or older leaves, as observed in rice plants of different leaf stages infected with different individual races. Their increase inhost resistance was marked by an apparent transition in infection type and reduced blast severity depending on leaf age and developmental stage of plants. The ranking of disease severity of the adult-plantresistant cultivars to different races was constant during plant development, whereas that of the susceptible cultivars was differential. It is suggested that different degrees of resistance in rice cultivars may exist in seedlings and be consistently maintained during plant development, probably becoming more distinct as rice plants mature.     

Genetic characterization of powdery mildew resistance in U.S. hard winter wheat

Powdery mildew significantly affects grain yield and end-use quality of winter wheat in the southern Great Plains. Employing resistance resources in locally adapted cultivars is the most effective means to control powdery mildew. Two types of powdery mildew resistance exist in wheat cultivars, i.e., qualitative and quantitative. Qualitative resistance is controlled by major genes, is race-specific, is not durable, and is effective in seedlings and in adult plants. Quantitative resistance is controlled by minor genes, is non-race-specific, is durable, and is predominantly effective in adult plants. In this study, we found that the segregation of powdery mildew resistance in a population of recombinant inbred lines developed from a cross between the susceptible cultivar Jagger and the resistant cultivar 2174 was controlled by a major QTL on the short arm of chromosome 1A and modified by four minor QTLs on chromosomes 1B, 3B, 4A, and 6D. The major QTL was mapped to the genomic region where the Pm3 gene resides. Using specific PCR markers for seven Pm3 alleles, 2174 was found to carry the Pm3a allele. Pm3a explained 61% of the total phenotypic variation in disease reaction observed among seedlings inoculated in the greenhouse and adult plants grown in the field and subjected to natural disease pressure. The resistant Pm3a allele was present among 4 of 31 cultivars currently being produced in the southern Great Plains. The genetic effects of several minor loci varied with different developmental stages and environments. Molecular markers associated with these genetic loci would facilitate incorporating genetic resistance to powdery mildew into improved winter wheat cultivars.     

Hydroxamic acids affecting barley yellow dwarf virus transmission by the aphid Rhopalosiphum padi

2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), a hydroxamic acid (Hx) occurring in wheat, was shown to deter feeding by the aphid Rhopalosiphum padi (L.), and to reduce BYDV transmission to the plant. Dual choice tests with wheat leaves showed the preferential settlement of aphids on leaves with lower levels of DIMBOA. Electric monitoring of aphid feeding behaviour showed that in seedlings with higher DIMBOA levels fewer aphids reached the phloem and they needed longer times to contact a phloem vessel than in those with lower levels. When aphids carrying BYDV were allowed to feed on wheat cultivars with different DIMBOA levels, fewer plants were infected with BYDV in the higher DIMBOA cultivars than in the lower ones. Preliminary field experiments showed a tendency for wheat cultivars with higher Hx levels to be more tolerant to infection by BYDV than lower Hx level ones.     

Subterranean clover mottle sobemovirus: its host range, resistance in subterranean clover and transmission through seed and by grazing animals

Subterranean clover mottle sobemovirus (SCMV) was transmitted by manual inoculation of sap to 27 cultivars belonging to three sub-species of subterranean clover. The virus readily infected systemically all inoculated plants of five susceptible cultivars of ssp. subterraneum. Ten others showed partial resistance as not all infected plants developed systemic infection; cold winter conditions further delayed or prevented systemic movement in four of them. Two cultivars of spp. brachycalycinum and four of spp. yanninicum failed to develop systemic infection following inoculation and were considered highly resistant. Resistance to SCMV in three of the spp. yanninicum was further confirmed by the failure to establish detectable primary infections in most of the inoculated leaves. Moreover, when the four ssp. yanninicum cultivars were graft-inoculated with SCMV, systemic infection eventually developed in them but the virus concentration was low. SCMV was also transmitted by manual inoculation of sap to a further 23 species of Trifolium, Medicago or Pisum. Three species were non-hosts, five were infected only in inoculated leaves and 18 others developed systemic infection in some or all plants. SCMV reached very high concentrations and was stable in subterranean clover sap. It was transmitted experimentally between subterranean clover plants by brushing infected leaves against healthy ones and in swards was readily transmitted by the trampling and grazing of sheep, but only poorly by mowing. Seed transmission of SCMV to seedlings of five cultivars of subterranean clover was low (0–0.12%). SCMV was not transmitted by Myzus persicae.     

Different Assessment Techniques of Leaf Blotch (Rhynchosporium secalis (Oud.) J. Davis) in Winter Barley after Artificial Inoculation

Seven winter barley cultivars expressing different field resistance against leaf blotch (Rhynchosporium secalis) and 60 doubled haploid (DH) lines descending from and F₁ hybrid resistant x susceptible were used to develop a test for the quantitative determination of disease severity. Differences in the level of resistance could be measured very precisely after artificial infection of seedlings at the 4-leaf-stage and subseqent incubation at 7 C. The protein of six isolates was usedf, or antiserum production which was subsequently supplied for serological testing (Enzyme-Linked-Immunosorbent-Assay) of infected leaves. The results of visible scoring of the seedlings was significantly correlated with measuring with the ELISA reader. The, results of the seedling test also corresponded to field scores after natural infection.     

The influence of temperature on the development and yield of maize in Britain

Sugar beet seedlings were evaluated for resistance to downy mildew by exposing them to natural infection in a glasshouse maintained at a high relative humidity (c. 95–98%). Exposure to natural infection for at least 3 days was required for adequate infection levels which occurred in the majority of tests; failure in others was related to decreases in relative humidity during clear weather. Seedling disease symptoms were different from those of mature plants. In most instances there was adequate differentiation among seedlings in populations with low, but worthwhile, levels of variation for resistance. The application of the method in selection programmes for resistance is discussed.     

The role of cuticle and epidermal cell wall in resistance of rapeseed and mustard to Rhizoctonia solani

A study was conducted to determine whether the cuticles in two genera of the family Cruciferae are effective barriers to infection by Rhizoctonia solani, and whether differences in cuticle and epidermal cell wall thickness and morphology of epicuticular wax exist between resistant and susceptible cultivars. As Canola/rapeseed (Brassica napus) and mustard (Sinapis alba) plants develop from 1 to 3 weeks of age, they become increasingly resistant to R. solani AG2-1 seedling root rot. Seven-day-old seedlings of S. alba cultivars are invariably more resistant than B. napus cultivars. Brassica napus cultivars do not show an obvious cuticle layer at 1 week but at 3 weeks the presence of a cuticle is seen through autofluorescence with a concomitant increase in resistance to R. solani. Removal of the cuticle from 3-week-old hypocotyls by chloroform treatment results in a decrease in cuticular autofluorescence and a significant increase in disease severity in both resistant and susceptible cultivars. Three-week-old plants of S. alba have a much lower percent disease rating and a significantly (p=0.05) thicker cuticle layer than similar-age plants of B. napus. The results suggest that the cuticle plays an important role in the resistance of S. alba and older plants of B. napus to infection by R. solani.     

Biological control of seedling blight of wheat caused by Fusarium graminearum with beneficial rhizosphere microorganisms

Fusarium graminearum is associated with the cereal damping-off complex which reduces germination, seedling stand and yield. Fifty-two bacterial strains and six Trichoderma spp. isolated from the wheat rhizosphere were evaluated for biocontrol of seedling blight of wheat caused by F. graminearum. Their potential as biocontrol agents was tested in vitro and in the greenhouse. Isolates varied in their ability to inhibit the mycelial growth of F. graminearum in agar plate bioassays by 0–79%. This parameter was not related with biocontrol efficacy of in vivo assays. In greenhouse trials, all isolates were initially evaluated for reducing disease on wheat cultivars Klein Centauro (moderately resistant to F. graminearum) and Pro INTA Oasis (susceptible) planted in sterilized soil artificially infested with the pathogen. Among the 25 bacteria and six fungal isolates that exhibited a pronounced suppressive effect, the most efficient 10 for both cultivars were further assayed on eight cultivars (Buck Candil, Buck Catriel, Buck Chambergo, Buck Poncho, Buck Topacio, Klein Cacique, Klein Centauro and Pro INTA Oasis) potted in cultivated–inoculated soil. Three weeks after sowing, plant stand, percentage of diseased emerging seedlings, plant height and dry weight were evaluated. Among the antagonists only Stenotrophomonas maltophilia was significantly better than the control for the average of the eight cultivars for plant stand, height and dry weight. Stenotrophomonas maltophilia also caused a non-significant decrease in the percentage of diseased plants. Three strains of Bacillus cereus and one isolate of Trichoderma harzianum gave also a good control in some cultivars. The ability of these isolates to affect the infection of wheat seedlings by F. graminearum may be of potential value in field trials.     

Rooted leaf cuttings as an aid in screening for resistance to Plasmodiophora brassicae

Rooted leaf cuttings were used to screen a range of genotypes and cultivars from Brassica spp. and Raphanus sativus for resistance to Plasmodiophora brassicae. The optimum conditions for obtaining equivalent disease incidence and disease severity on leaf cuttings and on seedlings were investigated. In the method used leaves were cut near the base of the petiole before the lamina was fully expanded. They were kept in a mist propagator for 6 to 18 days and then transferred to a clubroot test bench and individually inoculated with a suspension of resting spores. There was an optimum period for each cultivar between cutting and inoculation which gave the maximum incidence of infection: c. 15 days for Doon Major (swede); 10–15 days for Civasto (stubble turnip) and Giant Rape (forage rape); c. 22 days for Maris Kestrel (marrow stem kale) and c. 10 days for Slobolt (fodder radish). Gall size on rooted leaf cuttings gave a quantitative measure of disease severity equivalent to that on seedlings in tests using the European Clubroot Differential host set, the five above-mentioned cultivars and four swede cultivars. There was little callus production except on some B. campestris hosts and on cuttings with shoot tissue; microscopic examination was used to confirm the presence or absence of infection in galls. Hormone rooting powder did not increase the rate of root production in two cultivars of forage rape. Root development was less extensive in B. campestris than in B. oleracea or B. napus. Apparently normal shoots developed on 96% of leaf cuttings (inoculated and uninoculated) in which a fragment of axillary bud tissue was included. The results are discussed in relation to screening for resistance in whole plants and in tissue culture.     

Stem canker (Rhizoctonia solani) on five early and seven maincrop potato cultivars I. Infection of shoots, stolons and tubers

In 1983 and 1984, potato seed tubers of five early and seven maincrop cultivars were inoculated with cultures of Rhizoctonia soluni during planting to simulate severe seed infection. Shoot and stolon infection was assessed in June-August and black scurf on tubers recorded after harvest in October. Almost all shoots of all cultivars had stem canker in both years and disease on shoots, stolons and tubers was more severe in 1984 than in 1983. In 1983 similar amounts of disease developed on all early cultivars and between 11% (Ulster Sceptre) and 32% (Maris Peer) shoots were pruned off. Maris Peer had a stem canker score lower than other cultivars in 1984 but more than half the shoots were pruned off. Shoot pruning on Estima, Ulster Prince and Ulster Sceptre was more common on plants from sprouted than non-sprouted seed. Between 30 and 50% of stolons were pruned off. After harvest in 1985, black scurf was least prevalent on Arran Comet and Maris Peer tubers and in 1984 on Arran Comet and Estima tubers from non-sprouted seed. Of the maincrop cultivars, King Edward plants from sprouted seed had many shoots pruned off in both years. Shoot pruning was also prevalent on Maris Piper and Pentland Squire plants from non-sprouted seed. Record had fewest pruned shoots and stolons and the lowest stem canker score. The disease was more severe on Pentland Crown and Maris Piper plants from non-sprouted than sprouted seed. Black scurf was most common on Cara and King Edward tubers in 1983 and on King Edward and Record tubers in 1984. In both years few shoots but many tubers were infected on plants from non-inoculated seed and the significance of this is discussed.     

Colonization of Fusarium Wilt‐Resistant and Susceptible Watermelon Roots by a Green‐Fluorescent‐Protein‐tagged Isolate of Fusarium oxysporum f.sp. niveum

Interactions between watermelon and a green fluorescent protein (GFP)‐tagged isolate of Fusarium oxysporum f.sp. niveum race 1 (Fon‐1) were studied to determine the differences in infection and colonization of watermelon roots in cultivars resistant to and susceptible to Fusarium wilt. The roots of watermelon seedlings were inoculated with a conidial suspension of the GFP‐tagged isolate, and confocal laser scanning microscopy was used to visualize colonization, infection and disease development. The initial infection stages were similar in both the resistant and susceptible cultivars, but the resistant cultivar responded differentially after the pathogen had penetrated the root. The pathogen penetrated and colonized resistant watermelon roots, but further fungal advance appeared to be halted, and the fungus did not enter the taproot, suggesting that resistance is initiated postpenetration. However, the tertiary and secondary lateral roots of resistant watermelon also were colonized, although not as extensively as susceptible roots, and the hyphae had penetrated into the central cylinder of lateral roots forming a dense hyphal mat, which was followed by a subsequent collapse of the lateral roots. The initial infection zone for both the wilt‐susceptible and wilt‐resistant watermelon roots appeared to be the epidermal cells within the root hair zone, which the fungus penetrated directly after forming appressoria. Areas where secondary roots emerged and wounded root tissue also were penetrated preferentially.     

Comparative studies on the activities of chitinase, β-1,3-glucanase, peroxidase and phenylalanine ammonia lyase in the leaves of triticale and wheat infected with Stagonospora nodorum

The activities of chitinase, β-1,3-glucanase, peroxidase and phenylalanine ammonia lyase, constitutive and induced by Stagonospora nodorum were examined in the 10 – 14 day old seedlings of three triticale and two wheat cultivars under controlled environmental conditions and in flag leaves of two triticale cultivars in the field. Two S. nodorum isolates of different virulence were used. Both the constitutive and induced activities in triticale and wheat depended on genotype and in triticale the effect of growth conditions was also evidenced. The constitutive activities of chitinase, β-1,3-glucanase and peroxidase were several fold lower in flag triticale leaves in plants from the field than in the seedlings, growing under controlled conditions, but induction in the infected flag leaves was significantly more pronounced. In triticale genotypic differences in the response to infection were revealed only upon inoculation by S. nodorum isolate of higher virulence. The enzymatic activities increased several fold during successive days after the infection except for phenylalanine ammonia lyase. Induction of this enzyme was only transient and the activity decreased 48 or 96 h after infection when the activities of other enzymes were rising. In flag leaves in the field this activity was differentiated only after infection with more a virulent strain. A tendency appeared in triticale seedlings for association of the resistance to the pathogen with lower enzymatic constitutive activities. This relationship became more evident in triticale infected by S. nodorum and may imply that although the investigated enzymes are certainly involved in general, non-specific defense mechanism, they do not decide on the resistance to pathogen at least in the early stages of infection and cooperate with other factors in the complex pathogen-plant interaction. One can also assume that the enzymatic activities are associated with severity of infection rather than resistance to pathogen.     

Differential interactions among cotton genotypes and isolates of Fusarium oxysporum f. sp. vasinfectum

Abstract

The pathogenicity of nine isolates of Fusarium oxysporum f. sp. vasinfectum (Fov) was evaluated on seedlings of 30 cotton (Gossypium barbadense L.) genotypes in 2005 and 2006. Isolate×genotype interaction was a highly significant (P < 0.01) source of variation in wilt incidence, suggesting that physiologic specialization exists within Fov isolates. Cluster analysis of aggressiveness of isolates and susceptibility of genotypes by the unweighted pair-group method based on arithmetic means (UPGMA) placed the isolates and the genotypes in several groups. Isolates were separated into two distinct groups. One group was closely related to race 5 while the other group was closely related to race 1. Cluster analysis also demonstrated that the Egyptian commercial cultivars had unique susceptibility patterns to Fov isolates remotely related to those of the other genotypes. The interaction between experiments of 2005 and 2006 was mainly due to a differential effect of years on the disease incidence for cotton cultivars.     

Analysis of potential sources of variation in tissue culture derived celery plants

Somaclonal variation derived from tissue culture is a potential source of variation that can be used in crop improvement programmes. The characteristics of this variation are first shown in the regenerant generation and their heritability is then confirmed by examination of the progeny. There would be savings of time, space and labour if this variation could be detected in vitro using easily assessed visual cues. The aim of this study was to relate variation in the source of explant and the morphology of the newly initiated callus to the characteristics of the regenerant plant, of which the most important was resistance to leaf spot disease caused by Septoria apiicola. Associations were investigated by isolating four stem explants from each of 564 surface sterile seedlings, var. Celebrity, on a callus initiation medium (MS medium, 30 g litre‘¹sucrose, 0.5 mg litre’¹2,4-D, 0.6 mg litre‘¹kinetin) and assessing the morphology and colour of the callus. After this initial culture (8 wk), each callus was transferred to a regeneration medium (MS medium, 30 g litre“¹sucrose). Plantlets were regenerated from many of the callus cultures and these were transferred to the glasshouse. When all of the surviving regenerant plants (276) were mature, leaf shape, amount and composition of the essential oils and resistance to late blight were assessed. Statistical analysis revealed that the character of the newly initiated callus (width, height, colour, organogenesis) showed poor correlation with all aspects of the regenerated plant measured. However, it was shown that increased variation resulted from different seedlings more than from plants derived from within seedlings or within callus.     

Changes in the Hill Reaction and the Chlorophyll Fluorescence of Chloroplasts Isolated from Wheat Plants Infected with Stem Rust (Puccinia graminis f. sp. tritici)

Wheat plants with different resistance to stem rust were inoculated with urediospores of Puccinia graminis f. sp. tritici race 32. By means of isolated chloroplasts Hill activity and chlorophyll fluorescence were measured during the first 8–10 days of the infection. Highly resistant cultivars (infection type 0) showed no significant differences in Hill activity whereas resistant (infection type 1) and susceptible plants (infection type 4) exhibited a significant decrease in electron transport. This was associated with an apparent reduction of chlorophyll content. In both the incompatible as well as in the compatible interaction an additional factor had an effect on the inhibition of the Hill activity. Chlorophyll fluorescence was also affected in compatible and in incompatible interactions. However, the two interactions showed different patterns. Whereas the decline in variable fluorescence dominated in resistant varieties, susceptible cultivars showed an additional significant increase in basic fluorescence. The determination of Hill activity and chlorophyll fluorescence demonstrated that photosynthesis was inactivated in different ways in resistant and in susceptible cultivars.     

Germination of Erysiphe gratninis f.sp. hordei conidia on barley leaves

Germination of Erysiphe graminis f.sp. hordei conidia on leaves of several barley cultivars was studied in the laboratory. On both detached leaves and intact plants, within 48 h of inoculation a higher proportion of conidia had germinated on the basal and middle portions of the adaxial leaf surface than on the corresponding portions of the abaxial surface. Such differences between surfaces were not observed near the leaf tip. Similar results were obtained with all the cultivars and growth stages tested, and with five isolates of E. graminis, and are consistent with the observation that there is usually less powdery mildew on the abaxial than the adaxial surface of barley leaves. With most of the barley genotype/mildew isolate combinations tested, within 48 h of inoculation higher proportions of conidia germinated on seedlings and juvenile plants than on older plants. Inherited characteristics which affect spore germination on the leaf surface may be important factors in the development of adult-plant resistance of barley to powdery mildew, particularly in certain genotypes.     

Development of pea bacterial blight caused by Pseudomonas syringae pv. pisi in winter and spring cultivars of combining peas (Pisum sativum) with different sowing dates

Pea bacterial blight occurred by natural infection in a field trial on peas in 1995. Disease development in the winter cultivars Rafale, Frilene and Froidure was compared with that in the spring cultivars Baccara, Conquest and Bohatyr, each sown on six dates in October, November, December, mid-March, late March and April. Disease incidence had reached 100% plants affected in all treatments by mid-July. Disease severity was greater in winter-sown (October, November or December) than in spring-sown peas of each cultivar at each assessment. Significant (P < 0.05) differences in disease severity occurred between cultivars in the winter-sown plots in May and June and the spring cultivars were affected more severely than the winter cultivars. Comparison of areas under the disease progress curves for both disease incidence and severity also showed that the winter-sown peas were more affected by disease than spring-sown peas and that spring cultivars were more severely affected than winter cultivars. Yield was strongly correlated with disease severity. A linear regression model suggested that, for peas sown in October, November or December, a yield loss of 0.5 tha^-1 occurred for each 10% increase in canopy area affected by pea bacterial blight.     

Effects of Inoculation with a Mild Strain of Tomato Aspermy Virus on the Growth and Yield of Tomatoes and the Potential for Cross Protection

Mechanical inoculation of seedlings of 26 tomato cultivars with an attenuated strain of tomato aspermy virus (TAV-M, ATCC # PV361) resulted in development of only mild symptoms, which had largely disappeared by the time the infected plants had begun to set fruit. Yield trials revealed that plants inoculated with TAV-M when transplanted at the 6–8 leaf stage displayed no delay in fruit set and had yields only slightly less than uninoculated plants. Cross protection studies carried out in the greenhouse revealed that TAV-M has the potential to interfere with infection of tomato seedlings by a more severe TAV strain (ATCC # PV127). ‘Pik-Red’ seedlings inoculated with TAV-M showed a significant reduction in symptom expression when challenged with the Blencowe strain of TAV 21 days after the initial inoculation with TAV-M; interference was incomplete at 7 and 14 days.     

Stem canker (Rhizoctonia solani) of maincrop potatoes.

In two years, potato plants were sampled at 1- or 2- weekly intervals from plots planted with seed tubers bearing sclerotia of Rhizoctonia solani (black scurf) and with seed without sclerotia either infested or not with cultures of R. solani at planting. Sprouted King Edward seed was used in 1981 and sprouted and non-sprouted King Edward and Pentland Crown seed in 1982. In both years 60–80% of shoots from seed with sclerotia and 90% of shoots from seed inoculated at planting were affected with stem canker. Most disease developed before shoots emerged although it gradually increased later when new shoots arising both from seed tubers or as branches on shoots with damaged apices (pruned shoots) became infected before they emerged. Sprouting seed tubers bearing sclerotia decreased the disease on both cultivars but with soil-applied inoculum the disease was more severe on plants from sprouted than non-sprouted seed. Some stolons were infected by R. solani soon after they developed and incidence of infection later increased. Thirty to 50% of stolons were infected on plants from infected seed tubers and 60% on plants with soil-applied inoculum. With both cultivars and sources of inoculum about 70% of the infected stolons had their apices killed (pruned).     

Role of mycorrhizal infection in the growth and reproduction of wild vs. cultivated plants

Summary An experiment was conducted to determine whether wild accessions and cultivars ofLycopersicon esculentum Mill. differed in inherent morphological, physiological or phenological traits and whether such differences would result in variation in response to vesicular-arbuscular mycorrhizal infection. While distinctions between wild accessions and cultivars were apparent (the cultivars generally had higher phosphorus use efficiencies and shorter lifespans than the wild accessions) and the cultivars were, as a group, more responsive to mycorrhizal infection than the wild accessions, there was significant variation among wild accessions and among cultivars in response to infection. Regardless of cultivation status, non-mycorrhizal plant root density was significantly negatively correlated with response to infection. Phosphorus use efficiency was generally not significantly correlated with response to infection. Mycorrhizal infection decreased phosphorus use efficiency in all accessions, but had variable effects on root density, depending upon accession and time. Finally, the vegetative response was not necessarily of the same magnitude as the reproductive response.