Showing papers in "Plant Pathology in 2001"
TL;DR: Breeding for resistance (genetic, disease escape or tolerance), stubble management, crop rotation and fungicide seed treatments are important strategies for control of phoma stem canker in all areas.
Abstract: Phoma stem canker (blackleg), caused by Leptosphaeria maculans, is an important disease on oilseed rape (canola, rapeseed, Brassica napus, Brassica juncea, Brassica rapa) causing seedling death, lodging or early senescence in Australia, Canada and Europe, but not in China. The two forms of L. maculans (A group and B group) that occur on oilseed rape are now considered to be separate species. The epidemiology and severity of phoma stem canker differs between continents due to differences in the pathogen population structure, oilseed rape species and cultivars grown, climate and agricultural practices. Epidemics are most severe in Australia, where only the A group occurs, and can be damaging in Canada and western Europe, where both A and B groups occur, although their proportions vary within regions and throughout the year. Epidemics are slight in China, where the A group has not been found. Dry climates (Australia, western Canada) lengthen the persistence of infected debris and may synchronize the release of airborne ascospores (after rain) with seedling emergence. L. maculans spreads from cotyledon and leaf infections down petioles to reach the stem, with infections on cotyledons and leaves early in the season producing the most damaging stem cankers at the stem base (crown). Development of both crown cankers and phoma stem lesions higher up stems is most rapid in regions with high temperatures from flowering to harvest, such as Australia and Canada. Breeding for resistance (genetic, disease escape or tolerance), stubble management, crop rotation and fungicide seed treatments are important strategies for control of phoma stem canker in all areas. Fungicide spray treatments are justified only in regions such as western Europe where high yields are obtained, and accurate forecasts of epidemic severity are needed to optimize their use.
433 citations
TL;DR: It is proposed that the greater inhibition of infection attempts from Mycosphaerella spp.
Abstract: The effects of the fungicides azoxystrobin (a strobilurin) and epoxiconazole (a sterol biosynthesis inhibitor) on phyllosphere fungi, senescence and yield were studied in winter wheat in field trials free of visible disease and under controlled environmental conditions. In two field trials, treatments with each of the two fungicides prolonged green leaf area retention and increased yield compared with untreated control plots. Azoxystrobin maintained green leaf area for longer than epoxiconazole and, in one trial, treatments with azoxystrobin gave a greater yield response than epoxiconazole. Mycelial growth on leaf surfaces, mainly originating from saprophytic fungi, was reduced by each of the fungicides. Papilla formation and hypersensitive reactions, almost exclusively against infection attempts by Mycosphaerella spp. (most probably M. graminicola), occurred with high frequency in the leaves. These defence reactions presumably incurred a significant energy cost, accelerating plant senescence. Fewer defence reactions were recorded in azoxystrobin-treated leaves than in epoxiconazole-treated and untreated leaves. Inoculation in a glasshouse experiment with the saprophytic fungi Alternaria alternata and Cladosporium macrocarpum accelerated wheat senescence. Control of the saprophytes by azoxystrobin or epoxiconazole treatments caused a delay in the accelerated senescence, but without significant increase in above-ground biomass and yield. Neither fungicide influenced senescence, above-ground biomass or yield in noninoculated wheat plants. In growth chamber experiments azoxystrobin inhibited spore germination and mycelial growth of A. alternata and C. macrocarpum. Epoxiconazole had little inhibitory effect on spore germination, but strongly inhibited mycelial growth of both saprophytes. Both fungicides reduced A. alternata-induced papilla formation in wheat leaves, with epoxiconazole being more effective. Inoculation with either of the two saprophytes did not significantly increase wheat leaf respiration, in contrast to inoculation with the nonhost pathogen Erysiphe graminis f.sp. hordei. Treatment with azoxystrobin did not affect this latter increase in respiration whereas it was reduced by epoxiconazole treatment. It is proposed that the greater inhibition of infection attempts from Mycosphaerella spp. by azoxystrobin, compared with epoxiconazole, may account for the greater yield given by azoxystrobin in field plots.
176 citations
TL;DR: Samples from 360 to 450 randomly selected winter wheat crops in England and Wales were collected annually during the milky ripe development stages from 1989 to 1998, with significant regional differences in levels of septoria leaf blotch, brown rust, eyespot, sharp eyespot (Rhizoctonia cerealis) and BYDV.
Abstract: The above article was published without a number of the author's corrections included. The publisher offers their unreserved apology for this and the sections affected are reprinted below in their entirety.Samples from 360 to 450 randomly selected winter wheat crops in England and Wales were collected annually during the milky ripe development stages (GS 73–75) from 1989 to 1998. The number of samples from each region was proportional to the area of winter wheat grown. The percentage area affected by disease was assessed on the top two leaves and the ear, and the incidence and severity of stem base diseases were also recorded. An estimate of the percentage area of the crop affected by Barley yellow dwarf virus (BYDV) and take-all (Gaeumannomyces graminis) was made in the field. Septoria leaf blotch (Septoria tritici, teleomorph Mycosphaerella graminicola) was the major foliar disease recorded, with an average maximum severity of 7·8% of the area of leaf 2 affected in 1998. Eyespot (Tapesia spp.) was the major stem base disease, with the highest incidence of stems falling into the damaging moderate plus severe categories (18·9%) in 1998. Levels of powdery mildew (Blumeria graminis) showed a decline from 0·4% of the area of leaf 2 in 1989 to 0·1% in 1998. This fall was associated with a reduction in the proportion of disease-susceptible cultivars grown. There were significant regional differences in levels of septoria leaf blotch, brown rust (Puccinia recondita), eyespot, sharp eyespot (Rhizoctonia cerealis) and BYDV. The percentage of crops treated with a fungicide rose from 96% in 1989 to 98% in 1998 and the mean number of spray applications per crop rose during this period from 2·1 to 2·5. A higher proportion of crops was treated with fungicides between the end of tillering and fifth node detectable (GS 29–35) than around flag leaf emergence (GS 36–48) or ear emergence (GS 49–71). Prior to 1994, the majority of late fungicide sprays was applied at, or after, ear emergence, but from 1994, the majority was applied around flag leaf emergence. The value and socioeconomic implications of the results are discussed.
132 citations
TL;DR: The standard hybrid was nonpathogenic to the bark of four other hardwood and two conifer species, indicating that it is relatively host specific, and P. cambivora was an aggressive pathogen on live bark of Quercus and Castanea.
Abstract: Pathogenicity tests were carried out on the bark of Alnus glutinosa with 19 isolates of the standard (near-tetraploid) hybrid alder phytophthora, nine isolates representing its known heteroploid variants and 11 isolates of P. cambivora, a probable parent species of the hybrid. Over a 4-year period, 12 experiments were conducted on living alder logs incubated at 20°C. Most isolates of the standard hybrid and those of the ‘Dutch variant’ were highly aggressive to alder bark. Isolates of the ‘Swedish’, ‘UK’ and ‘German variants’, and of P. cambivora, were only weakly pathogenic. Also, isolates of P. fragariae, P. cinnamomi, P. sp. ‘O-group’, P. cryptogea, P. megasperma, P. gonapodyides and P. citricola were either weakly or nonpathogenic. Rates of lesion development were greatest on logs cut during July–October, slower on logs cut between November and March and zero on logs cut during April, indicating a strong seasonal effect. Other evidence indicated that lesion development was subject to critical thresholds of host resistance. The standard hybrid was nonpathogenic to the bark of four other hardwood and two conifer species, indicating that it is relatively host specific. In contrast, P. cambivora was an aggressive pathogen on live bark of Quercus and Castanea. The significance of these results is discussed.
125 citations
TL;DR: Evidence presented suggests that CBSD is caused by Cassava brown streak virus, a tentative member of the genus Ipomovirus, as this virus is consistently found associated with CBSD.
Abstract: A partial sequence of 1114 nucleotides of a virus from cassava brown streak diseased (CBSD) material was obtained. Alignment of the predicted amino acid sequence with those of other members of the Potyviridae showed closest identity with the coat protein of Sweet potato mild mottle virus (genus Ipomovirus). The predicted amino acid sequence has one open reading frame with a 3′ untranslated region of 144 nucleotides and a poly(A) tail. The expressed protein was shown to cross-react with an antiserum raised previously to a virus isolated from CBSD material. Evidence presented suggests that CBSD is caused by Cassava brown streak virus, a tentative member of the genus Ipomovirus, as this virus is consistently found associated with CBSD.
121 citations
TL;DR: The lack of molecular variation in the Australian and New Zealand collection is consistent with the stepwise mutation theory of pathotype evolution from a single introduction.
Abstract: Since its initial detection in Australia in 1979, wheat yellow (stripe) rust (Puccinia striiformis f.sp. tritici) has evolved in Australia and New Zealand into more than 20 pathotypes with assorted virulence characteristics. This evolution is believed to have occurred in a stepwise fashion from an original single pathotype, with no subsequent new introductions. A combination of random amplified polymorphic DNA (RAPDs) and amplified fragment length polymorphisms (AFLPs) was used to examine the level of molecular variation in Australian and New Zealand isolates, and to compare this with variation amongst other isolates of P. striiformis. Using 60 RAPD primers on seven Australian isolates representing seven different pathotypes collected between 1979 and 1991, more than 300 potentially polymorphic loci were analysed and no polymorphisms were detected. Using the same primers on two UK isolates, 3% of loci showed a polymorphism. A similar level of polymorphism was found between UK isolates using AFLP primers, and between 5 and 15% of fragments were polymorphic between an isolate from the UK, an isolate from Denmark, and one from Colombia. However, no AFLP polymorphisms were found amongst 14 Australian and New Zealand isolates tested, at over 100 potentially polymorphic loci. The lack of molecular variation in the Australian and New Zealand collection is consistent with the stepwise mutation theory of pathotype evolution from a single introduction.
112 citations
TL;DR: Bean yield was more closely related to the new variables ‘photosynthesizing leaf area index duration’ (PAD) and ‘ photosynthesized leaf areaIndex absorption” (PAA) than to those variables previously published, but only for diseases with a large β-value and at high levels of disease severity.
Abstract: The relationships between the net photosynthetic rate of bean leaves and severity of rust, angular leaf spot and anthracnose were quantified at different temperatures of plant incubation, stages of disease development, and phenological stages of the crop, in two bean cultivars. Several experiments were performed in controlled environment chambers and in the field. The virtual lesion concept was used to quantify the reduction in photosynthetic efficiency of the green leaf tissue surrounding the lesions. The β parameter that expressed the ratio between areas of the virtual and visual lesions was estimated according to the model Px /P0 = (1 − x)β, where Px was the net photosynthetic rate of a leaf with severity x and P0 was the average net photosynthetic rate of healthy leaves. The β-value was 2·17 ± 0·02 (R2 = 0·88) for rust, 3·81 ± 0·04 (R2 = 0·91) for angular leaf spot, and 7·97 ± 0·13 (R2 = 0·94) for anthracnose. For each disease, the parameter β was consistent regardless of incubation temperature, stage of disease development, bean phenological stage and bean cultivar. In addition, the relationships between bean yield and the variables of area under the disease progress curve (AUDPC), healthy leaf area index duration (HAD) and healthy leaf area index absorption (HAA), published in the literature for rust, angular leaf spot and anthracnose, were recalculated with the virtual disease severity. Bean yield was more closely related to the new variables ‘photosynthesizing leaf area index duration’ (PAD) and ‘photosynthesizing leaf area index absorption’ (PAA) than to those variables previously published, but only for diseases with a large β-value and at high levels of disease severity.
102 citations
TL;DR: Three species of Phytophthora,P.
Abstract: Three species of Phytophthora, P. cambivora, P. citricola and P. cactorum, were found to be associated with a recent outbreak of ink disease causing high mortality of chestnut trees in central Italy. Phytophthora cambivora was isolated from 11·6% of the soil samples taken around symptomatic trees, and was mainly associated with heavily diseased trees. It was the most aggressive species to Castanea sativa, but survived poorly in the soil. Phytophthora citricola and P. cactorum showed a limited ability to induce disease on chestnut, but could be recovered from soil during most of the year. A fourth species, P. gonapodyides, was recovered only from mud of stream beds within the chestnut stands. Involvement of these species in the development of disease is discussed.
101 citations
TL;DR: The detached leaf technique could be a useful complement to field trials and an alternative to whole seedling assays in assessing cultivar resistance and investigating the genetics of the host–pathogen interaction.
Abstract: A detached seedling leaf technique was developed to screen for resistance to septoria tritici blotch of wheat and to detect specific interactions between cultivars and isolates. Wheat seedlings were inoculated with spore suspensions of Mycosphaerella graminicola. Detached primary leaves were then placed in a clear plastic box such that their cut ends were sandwiched between layers of agar containing benzimidazole, with a gap below the middle of the leaves. Mean levels of disease were affected by light and temperature, and also by the concentration of benzimidazole, such that higher concentrations resulted in less disease. Second leaves were more susceptible than seedling primary leaves. However, none of these factors affected ranking of disease among cultivars or cultivar-by-isolate interactions. Kavkaz–K4500 1.6.a.4, Synthetic 6x and Triticum macha showed specific susceptibility and resistance to different isolates. The detached leaf technique could be a useful complement to field trials and an alternative to whole seedling assays in assessing cultivar resistance and investigating the genetics of the host–pathogen interaction.
97 citations
TL;DR: The effects of the highly aggressive isolate KB-2 of the Karnal bunt pathogen (Neovossia indica) on phenol metabolism, peroxidase (POX) and its isoenzymes were studied in wheat and the potential use of the additional band at a relative mobility of 0·42 in seeds and 0·28 in seedlings as markers for Karnalbunt resistance is discussed.
Abstract: The effects of the highly aggressive isolate KB-2 of the Karnal bunt pathogen (Neovossia indica) on phenol metabolism, peroxidase (POX) and its isoenzymes were studied in wheat. Two resistant genotypes, HD 29 of bread wheat (Triticum aestivum) and DWL 5023 of durum wheat (T. durum), and one susceptible bread wheat, WL 711, were used. In the bread wheats, total phenols reached a maximum 2 days after inoculation (d.a.i.). In the resistant durum line, total phenols did not change significantly for 6 d.a.i., but declined significantly at 10 d.a.i. Three phenolic compounds, caffeic acid, l-tyrosine and hydroquinone, were detected using thin-layer chromatography. The first two were detected at all times at and after inoculation, but hydroquinone was detected only in the resistant wheats at 6 d.a.i. The activity of POX was highest at 2 d.a.i. in the two resistant wheats, but increased more slowly to a peak at 6 d.a.i. in the susceptible wheat. The number of isoenzymes of POX detected by polyacrylamide gel electrophoresis (PAGE) changed after inoculation with KB-2. The maximum number of isoenzymes occurred at 2 d.a.i. in the two resistant wheats and at 6 d.a.i. in the susceptible wheat. Although the isoenzymes detected in seedlings were not identical to those detected in seeds, the PAGE banding patterns of seeds and seedlings were the same for the two resistant wheats. The potential use of the additional band at a relative mobility of 0·42 in seeds and 0·28 in seedlings as markers for Karnal bunt resistance is discussed.
97 citations
TL;DR: An RT-PCR based detection method for Cassava brown streak virus (CBSV)-infected cassava has been developed and can detect the virus in the new growth of cassava sticks before any disease symptoms are visible.
Abstract: An RT-PCR based detection method for Cassava brown streak virus (CBSV)-infected cassava has been developed. The RT-PCR detection method described includes RNA extraction methods for cassava leaves, a distinct primer set for the virus and RT-PCR conditions. The primers were designed to the virus coat protein gene and generate a virus-specific product of 231 bp from infected cassava. The test can detect the virus in the new growth of cassava sticks before any disease symptoms are visible. This test was used successfully with infected cassava from both Tanzania and Mozambique. Three isolates from Tanzania were found to exhibit different symptoms on the secondary host plants Nicotiana benthamiana and N. tabacum SR1. They have nucleotide sequence variation within the coat protein region of up to 8% and amino acid differences of up to 6%.
TL;DR: Analysis of VOCs collected from potato tubers inoculated with Phytophthora infestans, Fusarium coeruleum or sterilized distilled water provides key information for developing a sensor-based early warning system for the detection of postharvest diseases in stored potato Tubers.
Abstract: Volatile organic compounds (VOCs) collected from potato tubers inoculated with Phytophthora infestans (late blight), Fusarium coeruleum (dry rot) or sterilized distilled water (as a control) were analysed using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-flame ionization detection (GC-FID). A total of 52 volatiles were identified by GC-MS in the headspaces above P. infestans- and F. coeruleum-inoculated tubers after incubation for 42 days in the dark at 10°C. Of these VOCs, the six most abundant were common to both pathogens. These were benzothiazole (highest abundance), 2-ethyl-1-hexanol (second highest abundance), and at approximately equal third abundance, hexanal, 2-methylpropanoic acid-2,2-dimethyl-1-(2-hydroxy-1-methylethyl)-propyl ester, 2-methylpropanoic acid-3-hydroxy-2,4,4-trimethyl-pentyl ester and phenol. In addition, styrene also occurred at approximately equal third abundance in the headspace of F. coeruleum-inoculated tubers, but at lower abundance in the headspace of P. infestans-inoculated tubers. Some VOCs were specific to each pathogen. Butanal, 3-methylbutanal, undecane and verbenone were found at low levels only in the headspace of tubers inoculated with P. infestans, while 2-pentylfuran and copaene were found only in the headspace of tubers inoculated with F. coeruleum. Additionally GC-FID analysis identified ethanol and 2-propanol in the liquid exudate from both P. infestans- and F. coeruleum-inoculated tubers after incubation for 35 days, and in the headspace after incubation for 42 days. These data provide key information for developing a sensor-based early warning system for the detection of postharvest diseases in stored potato tubers.
TL;DR: Improved diagnosis and production of virus-free plants of SCYLV-susceptible cultivars will facilitate research to quantify the effect of the virus on yield and to analyse the processes involved in disease development.
Abstract: Sugarcane yellow leaf virus (SCYLV), a member of the Luteoviridae, is implicated in the sugarcane disease known as yellow leaf syndrome (YLS), which is characterized by yellowing of the leaf midrib followed by leaf necrosis and possible growth suppression. YLS is distributed worldwide and susceptible cultivars are commonly infected with SCYLV. However, not all cultivars infected with SCYLV show symptoms of YLS and some cultivars that show symptoms do so sporadically. Since it is difficult to obtain virus-free plants of susceptible cultivars, it has not been possible to study the factors involved in SCYLV infection nor the effects of infection on plant growth and yield. A tissue blot immunoassay was used to visualize in vivo presence of the virus so that virus-infected and virus-free plants could be distinguished. Meristem tip cultures were used to produce virus-free plantings of six SCYLV-susceptible sugarcane cultivars. Nearly all of the regenerated sugarcane lines remained virus-free over a period of up to 4 years, whether grown in isolated fields or in the glasshouse. Experimental re-infection of the virus-free plants by viruliferous aphids demonstrated that meristem tip culture did not affect susceptibility of sugarcane to SCYLV. Improved diagnosis and production of virus-free plants of SCYLV-susceptible cultivars will facilitate research to quantify the effect of the virus on yield and to analyse the processes involved in disease development.
TL;DR: In this article, the authors reported that Australian isolates formed a continuum in their response to phosphite, but could be divided into sensitive (9% of isolates), intermediate (82%) and tolerant (9%) groups.
Abstract: Seventy-one Australian isolates of Phytophthora cinnamomi (68 from Western Australia) were tested for sensitivity to phosphite on Ribeiro’s modified medium. Isolates formed a continuum in their response to phosphite, but could be divided into sensitive (9% of isolates), intermediate (82%) and tolerant (9%) groups. Sensitivity varied between isolates, with EC50 values ranging from 4 to 148 mg phosphite mL 21 . Phytophthora cinnamomi A1 mating-type isolates were at the upper end of the range of tolerance shown by the A2 mating-type isolates.
TL;DR: A molecular diagnostic method for the early in planta detection of D V. dahliae pathotype D DNA in infected root and stem tissues of young olive plants was developed, aimed especially at nursery-produced olive plants.
Abstract: Spread of Verticillium wilt into newly established olive orchards in Andalucia, southern Spain, has caused concern in the olive industry in the region. This spread may result from use of Verticillium dahliae-infected planting material, which can extend distribution of the highly virulent, defoliating (D) pathotype of V. dahliae to new areas. In this study, a molecular diagnostic method for the early in planta detection of D V. dahliae was developed, aimed especially at nursery-produced olive plants. For this purpose, new primers for nested PCR were designed by sequencing a 992-bp RAPD marker of the D pathotype. The use of the specific primers and different nested-PCR protocols allowed the detection of V. dahliae pathotype D DNA in infected root and stem tissues of young olive plants. Detection of the pathogen was effective from the very earliest moments following inoculation of olive plants with a V. dahliae pathotype D conidia suspension as well as in inoculated, though symptomless, plants.
TL;DR: The results suggest that BHA, which is a food-grade chemical, might have the potential to enhance the activity of fungicides currently used to control C. musae on bananas, allowing lower concentrations of fungicide to be used.
Abstract: Two isolates of Colletotrichum musae (CM100 and CM103) which differed significantly in their sensitivity to the fungicides thiabendazole (TBZ) and imazalil were grown on malt extract agar amended with the following antioxidants: ascorbic acid, benzoic acid, butylated hydroxyanisole (BHA), butylated hydroxytoluene, dimethyl sulphoxide, propionic acid, propyl gallate (PG), propyl paraben (PP) and thiourea. The compounds showing the greatest antifungal activity were BHA, benzoic acid, PG and PP. At concentrations of 15 mm or less, these chemicals inhibited conidial germination (at 72 h) and mycelial growth (by day 14) of isolate CM100 and gave high levels of inhibition of isolate CM103. Combinations of low concentrations of BHA (0·5 mm) and imazalil (1·8 µm) completely prevented mycelial growth and germination of the two isolates tested in vitro and showed some synergism when combined at even lower concentrations of BHA. BHA, PP and benzoic acid, in combination with each other and with imazalil and TBZ, applied to banana stalk tissues and crowns previously inoculated with C. musae, were less effective at controlling the fungus in vivo compared with inhibiting mycelial growth and conidial germination in vitro. BHA at 5 mm inhibited fungal growth at 25°C by 35–41%. When BHA (5 mm) was combined with imazalil (0·45 mm), better control (63–80% inhibition) of simulated crown rot was achieved than when imazalil (1·78 mm) or TBZ (2·46 mm) were applied on their own (53–58% and 43–54% inhibition, respectively). The results suggest that BHA, which is a food-grade chemical, might have the potential to enhance the activity of fungicides currently used to control C. musae on bananas, allowing lower concentrations of fungicide to be used.
TL;DR: Results showed that the relationship between PLWP and gs was modified by infection with P. cinnamomi, which acted synergistically on the water relations of Q. ilex plants, and in some Q. suber plants exhibiting a severe root loss.
Abstract: The combination of soil infestation with Phytophthora cinnamomi and repetitive flooding was studied on 1-year-old plants of Quercus ilex (holm oak) and Q. suber (cork oak). In a second experiment, using 2-year-old plants of the same species and of red oak (Q. rubra), the soil infestation was followed by two drought-rewatering cycles. Oak predawn leaf water potential (PLWP) and stomatal conductance (gs) were monitored during both experiments. Root infection, root loss, wilting and mortality were assessed at the end of the experiments. Q. ilex exhibited the highest susceptibility to P. cinnamomi, and Q. rubra the lowest. Root infections caused by P. cinnamomi were more severe in the flooding than in the drought experiment. The most noticeable effect of the infection on plant water relations was a decrease in stomatal conductance. This occurred at different times after inoculation, varying with species susceptibility and experiment. Inoculation with P. cinnamomi induced a decrease of PLWP in Q. ilex plants, and in some Q. suber plants exhibiting a severe root loss. The results further showed that the relationship between PLWP and gs was modified by infection with P. cinnamomi. The combination of flooding and infection with P. cinnamomi acted synergistically on the water relations of Q. ilex. By contrast, there was no significant increase in disease severity due to the postinoculation water stress imposed on the oaks.
TL;DR: The results confirm that line-by-isolate interactions in septoria tritici blotch of wheat are effective in adult plants in field conditions, and are not simply confined to seedlings.
Abstract: Isolate-specific resistance of 71 cultivars and breeding lines of wheat (Triticum aestivum) to septoria tritici blotch was evaluated in six field trials in the Netherlands, Switzerland and the UK between 1995 and 1997. Each plot was inoculated with one of six single-pycnidium isolates of the pathogen Mycosphaerella graminicola. There were strong interactions between wheat lines and M. graminicola and the line-by-isolate interactions were stable over the six trials. Lines with specific resistance or specific susceptibility to each of the isolates were identified. Specific resistance to isolate IPO323 was especially common, being carried by 22 lines from 10 countries. The results confirm that line-by-isolate interactions in septoria tritici blotch of wheat are effective in adult plants in field conditions, and are not simply confined to seedlings. Wheat lines with good, quantitative resistance to all or most isolates were identified, including lines from Brazil, the USA and seven European countries. These may be useful as sources of resistance in wheat breeding.
TL;DR: The finding that genetic differences can occur in P. brassicae field isolates will be an important consideration in resistance genetic studies and in choosing breeding strategies to develop durable clubroot resistance.
Abstract: Plasmodiophora brassicae is an obligate biotroph that causes clubroot, one of the most damaging diseases of crucifers. Differential cultivars and random amplified polymorphic DNA markers were used to assess the extent of genetic diversity among nine single-gall populations of P. brassicae and 37 single-spore isolates (SSI) derived from four of those field samples. Isolates were classified into eight pathotypes, and each isolate was associated with a unique molecular genotype. Virulence and DNA polymorphisms were detected within and between field isolates, and among SSIs from different pathotypes, hosts and geographical origins. The relatively high level of genetic diversity among field isolates was similar to that among SSIs derived from a single-club field isolate. Molecular and pathogenicity-based classifications were not clearly correlated, but isolates belonging to pathotype P1 were clustered. Two RAPD markers were specific to pathotype P1. The finding that genetic differences can occur in P. brassicae field isolates will be an important consideration in resistance genetic studies and in choosing breeding strategies to develop durable clubroot resistance.
TL;DR: Low temperatures in January and February reduced yellow rust on the susceptible cv.
Abstract: Disease severity of wheat yellow rust, Puccinia striiformis f.sp. tritici, was analysed in Denmark from 1985 to 1999 in relation to the effects of weather on winter survival, distribution of host cultivars and pathotype dynamics. Below-average temperatures in January and February (midwinter) reduced yellow rust on the susceptible cv. Anja, and in three of four growth seasons following cold winters no yellow rust was observed on any cultivar under natural conditions. The agronomic consequences of dispersal of yellow rust urediniospores from external sources to Denmark, in a period during which large areas were planted with relatively few wheat cultivars, were demonstrated in several cases, most evidently when the Yr9 and Yr17 resistance genes became ineffective. Yr9 was overcome by the pathogen in a period with severe yellow rust epidemics on commercial cultivars, while virulence for Yr17 was first observed in a year with almost no yellow rust. In contrast, the resistance in cv. Kraka (Yr1, CV) was increasingly effective in controlling yellow rust, because pathotypes with the matching combination of virulence declined in the pathogen population. Pathotype frequency dynamics were thus influenced by selection forces within the country, and by selection forces in areas where spores were spread to Denmark from outside. The importance of a sufficient level of partial resistance in the wheat germplasm to prevent too much damage by yellow rust epidemics, in the event that the resistance genes are overcome by the pathogen population, is emphasized.
TL;DR: Sexuality in the oomycete Plasmopara viticola, the causal agent of grapevine downy mildew, was studied using isolates from five populations from North America and Europe, indicating that the pathogen was heterothallic with two mating types.
Abstract: Sexuality in the oomycete Plasmopara viticola, the causal agent of grapevine downy mildew, was studied using isolates from five populations from North America and Europe. Leaf discs of Vitis vinifera cv. ‘Chardonnay’ were inoculated with either individual single-sporangiophore isolates, or in all possible pairwise combinations of 25 isolates from New York State, USA. The occurrence of oospores in leaf discs indicated that the pathogen was heterothallic with two mating types, P1 and P2 in a ratio of 11 : 14 for this population. Heterothallism was confirmed when three representative isolates of each mating type from New York were coinoculated with each of 40 isolates from populations of P. viticola from Michigan, Missouri (USA), Germany and Italy. For each isolate tested, oospores formed with either test isolates of P1 or test isolates of P2 mating types, indicating that the isolates were exclusively P1 or P2 only. For these same isolates, no oospores formed as a result of self-crosses. The ratio of P1 : P2 mating types for all isolates in the study was 27 : 38, statistically equivalent to a 1 : 1 ratio according to χ2 analysis (P = 0·68).
TL;DR: This is the first in vivo study that has used Streptomyces phage to significantly disinfest seed potatoes of StrePTomyces scabies and thereby reduce contamination of soil from seed-tuber-borne inoculum and reduce infection of daughter tubers.
Abstract: A highly virulent and polyvalent Streptomyces phage was isolated from a potato field near Albany, Western Australia. The efficacy of the isolated phage to disinfest seed potato tubers artificially inoculated with a common scab-causing streptomycete was evaluated. The phage suspension was prepared in a mini-bioreactor. Diseased potatoes were bathed in a phage suspension (1 × 109 plaque-forming units per mL) for 24 h. The suspension was constantly circulated within a novel 25 L phage bath by means of an air-sparging pipe driven from an air compressor. Phage-treated scab-affected seed potatoes planted into free-draining polystyrene boxes containing steam-pasteurized field soil produced tuber progeny with significantly (P < 0·05) reduced levels of surface lesions of scab (1·2%) compared with tubers harvested from nonphage-treated tubers (23%). The number of scab lesions was also significantly reduced (P < 0·05) by phage treatment of mother tubers. No significant differences were recorded in weight, size or number of harvested tubers from phage-treated or nontreated mother tubers. This is the first in vivo study that has used Streptomyces phage to significantly disinfest seed potatoes of Streptomyces scabies and thereby reduce contamination of soil from seed-tuber-borne inoculum and reduce infection of daughter tubers.
TL;DR: The different virulence of the blue-stain fungi associated with the two pine shoot beetles may explain the lower level of aggressiveness in T. minor.
Abstract: The pine shoot beetles Tomicus minor and T. piniperda are common in the Nordic countries. Of these, T. piniperda may attack and kill living but severely stressed trees, whereas T. minor has never been reported to be individually responsible for killing live trees. Both species are associated with blue-stain fungi: T. minor with Ophiostoma canum and T. piniperda with Leptographium wingfieldii and Ophiostoma minus. The growth of these fungi was studied in phloem and sapwood of live Scots pine trees, on malt agar, and on malt agar under oxygen-deficient conditions. Leptographium wingfieldii was more virulent (i.e. caused more extensive host symptoms) grew more quickly on malt agar, and was less affected by oxygen-deficient growth conditions than either O. minus or O. canum. Ophiostoma canum was least virulent. In low-density inoculations it induced lesions similar to those induced by sterile control inoculations; it grew very slowly on malt agar and stopped growing after <30 mm under oxygen-deficient conditions. Ophiostoma minus was intermediate in all respects. The different virulence of the blue-stain fungi associated with the two pine shoot beetles may explain the lower level of aggressiveness in T. minor.
TL;DR: Using the polymerase chain reaction, reverse-transcriptase–PCR, and double-antibody-sandwich enzyme-linked immunosorbent assay, a phytoplasma and a virus were detected in sugarcane with yellow leaf syndrome (YLS) in Mauritius and results indicate that the presence of both organisms enhanced the syndrome.
Abstract: Using the polymerase chain reaction (PCR), reverse-transcriptase–PCR (RT–PCR) and double-antibody-sandwich enzyme-linked immunosorbent assay (DAS–ELISA), a phytoplasma (sugarcane yellows phytoplasma, ScYP) and a virus (Sugarcane yellow leaf virus, ScYLV) were detected in sugarcane with yellow leaf syndrome (YLS) in Mauritius. Samples were collected from clones undergoing quarantine, in a variety-collection plot and in commercial fields. A 1·25 kb DNA fragment encoding the phytoplasma 16S rRNA was consistently amplified by nested PCR. Of 134 samples with and without symptoms derived from 113 varieties, 111 were infected by either ScYLV or ScYP. The phytoplasma was detected in 63 samples by PCR. Restriction fragment-length polymorphism (RFLP) analysis of the phytoplasma 16S rDNA amplified product indicated that sugarcane yellows phytoplasma group III, which is related to Western X phytoplasma, is present in Mauritius. ScYLV was detected by RT–PCR and ELISA. The virus was more widely distributed than the phytoplasma, and was found in 70 and 100 samples by ELISA and RT–PCR, respectively. There was a significant correlation between the presence of the phytoplasma and YLS symptoms, while such correlation was not significant for ScYLV detected by RT–PCR. ELISA was less sensitive than RT–PCR for detection of ScYLV. Forty-one samples were coinfected with both microorganisms. Eighty-five per cent of the samples displayed symptoms when ScYLV and SCYP coexisted, while 55 and 38% were observed when ScYP or ScYLV, respectively, was present alone. The results indicate that the presence of both organisms enhanced the syndrome.
TL;DR: The biology of the pathogen, epidemiology of the disease, detection of the Pathogen and integrated control measures for the management of silver scurf in both field and potato tuber stores are discussed.
Abstract: During the 1990s, silver scurf (causal agent Helminthosporium solani) emerged as an economically important disease of table stock and processing potatoes (Solanum tuberosum). The pathogen attacks the periderm of the potato tuber causing blemishes. The disease cycle of silver scurf has both field and storage phases. Primary infection occurs in the field and high relative humidity favours the spread and increase of silver scurf in potato stores. Control of the disease by chemical and cultural practices remains difficult. Increase in disease has been attributed to H. solani isolates resistant to the postharvest fungicide thiabendazole (TBZ). Polymerase chain reaction (PCR)-based detection methods for H. solani and TBZ-resistant isolates are rapid and more specific than traditional identification. This review discusses the biology of the pathogen, epidemiology of the disease, detection of the pathogen and integrated control measures for the management of silver scurf in both field and potato tuber stores.
TL;DR: Results suggest that phosphite reduces, but does not prevent, the production of viable zoospores on infected trees, and phosphite application may not remove the risk of P. cinnamomi spreading from infested, sprayed areas.
Abstract: The efficacy of phosphite to control the production of zoospores of Phytophthora cinnamomi on infected trees grown in a glasshouse and in a revegetated mined area was examined. Banksia grandis and Eucalyptus marginata seedlings in the glasshouse and E. marginata seedlings in the minepit were sprayed with 0, 5 and 10 g phosphite L−1. In both trials, zoospores were produced from infected tissue of plants treated with all concentrations of phosphite. In the glasshouse, spray application of 5 and 10 g phosphite L−1 significantly reduced the production of zoospores from both B. grandis and E. marginata seedlings. In the mined area there was a similar, though nonsignificant, reduction in the number of zoospores produced from phosphite-treated and nontreated E. marginata seedlings. However, the average number of zoospores produced was greater in plants not treated with phosphite (1·75 zoospores mL−1) than from plants treated with 5 or 10 g phosphite L−1 (0·04 and 0·09 zoospores mL−1, respectively). Pimelea ferruginea leaves were used to bait the water surrounding the plants in the mined area to determine if zoospores produced from phosphite-treated plants were able to infect plant material. Significantly more baits were infected by zoospores from plants not treated with phosphite compared with plants treated with 5 or 10 g phosphite L−1. These results suggest that phosphite reduces, but does not prevent, the production of viable zoospores on infected trees. Thus phosphite application may not remove the risk of P. cinnamomi spreading from infested, sprayed areas.
TL;DR: Trichoderma koningii (strain Tr5) grew in the epidermal mucilage of onion roots without entering healthy epider mal tissue, then branched and spread throughout the root cortical tissues damaged by enzymes and toxins which diffused ahead of S. cepivorum hyphae, and impeded the path of the infection.
Abstract: Trichoderma koningii (strain Tr5) grew in the epidermal mucilage of onion roots without entering healthy epidermal tissue. When placed on the epidermis of Sclerotium cepivorum-infected roots, T. koningii colonized epidermal passage cells, with little colonization of other epidermal tissues, then branched and spread throughout the root cortical tissues damaged by enzymes and toxins which diffused ahead of S. cepivorum hyphae, and impeded the path of the infection. When T. koningii colonized infected tissue, many S. cepivorum hyphae became detached at septa, cell walls dissolved and many hyphal apices burst. Contact between hyphae was not necessary for lysis to occur. T. koningii produced two endochitinases (Rf 0·15 and 0·24) and two exo-acting chitinolytic enzymes (Rf 0·46 and 0·62) during degradation of crabshell chitin and S. cepivorum cell walls. The Rf 0·24 and 0·46 proteins were detected when T. koningii colonized S. cepivorum-infected roots and are likely to be a component of the antagonism process.
TL;DR: In controlled-environment experiments, ascospores of both A-group and B-group Leptosphaeria maculans were able to infect leaves of oilseed rape and produce phoma leaf spot lesions at temperatures from 5 to 20°C and wetness durations from 8 to 72 h after inoculation.
Abstract: In controlled-environment experiments, ascospores of both A-group and B-group Leptosphaeria maculans were able to infect leaves of oilseed rape and produce phoma leaf spot lesions at temperatures from 5 to 20°C and wetness durations from 8 to 72 h after inoculation. Lesions formed on leaves inoculated with B-group ascospores had few pycnidia and were darker, smaller and less noticable than the larger, pale grey lesions with many pycnidia produced by A-group ascospores. Lesions formed by A-group or B-group L. maculans on naturally infected winter oilseed rape experimental crops were similar to lesions produced by the two groups on inoculated plants. The greatest numbers of lesions were produced with a leaf wetness duration of 48 h and at temperatures of 15–20°C for both A-group and B-group ascospores. As leaf wetness duration and temperature decreased below the optimal values, the number of lesions decreased. The incubation period, estimated as the time from inoculation to the appearance of the first lesions (t1), or the time to the appearance of 50% of the lesions (t50), of B-group was often shorter than that of A-group L. maculans. As temperature decreased below 20°C, the length of the incubation period of both A-group and B-group L. maculans increased.
TL;DR: An epidemiological model was developed in which transmission and loss rates were attributed to the different virus infection possibilities and it was shown that, in order to ensure virus survival in a mixed infection, the basic reproductive number should exceed a critical value which is larger than unity.
Abstract: Many virus diseases of plants are caused by a synergistic interaction between viruses within the host plant Such synergism can induce symptoms more severe than would be caused by additive effects In a synergistic interaction, the virus titre of both, one, or neither virus may be enhanced and, as a consequence, the rate of disease spread may be affected An epidemiological model was developed in which transmission and loss rates were attributed to the different virus infection possibilities Sharing the same host population implies competition, and this imposes an increased constraint on the survival of both viruses It was shown that, in order to ensure virus survival in a mixed infection, the basic reproductive number should exceed a critical value which is larger than unity (R0 > Rc > 1) Here R0 is used in the same sense as in the absence of superinfection Increased virulence (equivalent to disease severity) in dually infected plants decreases the opportunities for both viruses to coexist, while increased virus transmission from dually infected plants increases such opportunities The net effect of increased virulence and increased virus transmission on virus persistence was neutral if synergism caused the same proportional effect on both Total host abundance was, however, reduced The opportunity for virus persistence was increased if the enhancement of transmission exceeded that of virulence Indeed, by this mechanism a virus which was nonviable alone could invade and persist in a chronic epidemic of another virus Where the effect on virulence is greater than that on transmission, the viruses are likely to exclude each other, especially when the transmission rates of both viruses have intermediate values In such cases, the final outcome is determined by both the parameter values and the initial state