Epidemiology of eyespot (Pseudocercosporella herpotrichoides) on winter wheat, with particular reference to the period of infection

The period of infection by Pseudocercosporella herpotrichoides was investigated by transplanting winter wheat from uninfested to infested sites at Cambridge. Maximum infection was found on plants moved between November and March; most conidia were trapped during winter but infection was not closely related to the numbers of conidia. Plants exposed at intervals on infested land and incubated in a standard environment became infected throughout the period from October to July; infection was not closely related to the numbers of conidia trapped or plant age, but was positively correlated with the number of wet days/wk. Thus, while enough P. herpotrichoides conidia were dispersed throughout the life of winter wheat crops to cause substantial infection, environmental factors limited the development of new infections from April onwards. Practical implications of these results for the choice of sowing date, the development of resistant varieties, and fungicide use, are discussed.     

MBC-resistente Freilandpopulationen von Pseudocercosporella herpotrichoides in langjährigem Überwachungsversuch

MBC resistant field populations of Pseudocercosporella herpotrichoides in a long-term monitoring experiment In, long-term field experiment, since 1975/1976 winter whaet was grown continuously for eight years. One half of the 1-ha field was sprayed once a year with carhendazim against eyespot (Pseudocercosporella, herpotrichoides); another application of MBC/TBZ was, after heading. From 1978/1979 seed was treated with MBC also. The other half of the field remained untreated all the time. During the first five years, in the sprayed plot the relative proportion of MBC-resistant conidia increased steadily, but on a very low level only. During the next three years, however, there was a sharp increase in the level of MBC-resistance in the sprayed plot. — In the unsprayed plot, at the end of the eight year period the level of MBC-resistance in the field population of the fungus still was low.     

The roles of ascospores and conidia of Pyrenopeziza brassicae in light leaf spot epidemics on winter oilseed rape (Brassica napus) in the UK

Ascospores of Pyrenopeziza brassicae were produced in apothecia (cup‐shaped ascomata) on oilseed rape debris. The conidia, which were morphologically identical to the ascospores, were produced in acervular conidiomata was greater than for lesions caused by ascospores. In June 2000, on the ground under a crop with light on the surface of living oilseed rape tissues. Ascospores were more infective than conidia on oilseed rape leaves. The proportion of lesions caused by conidia located on leaf veins leaf spot, numbers of petioles with apothecia decreased with increasing distance into the crop from the edge of pathways. Air‐borne ascospores of P. brassicae were first collected above debris of oilseed rape affected with light leaf spot on 5 October 1998 and 18 September 1999,12 or 23 days, respectively, after the debris had been exposed outdoors. P. brassicae conidia were first observed on leaves of winter oilseed rape on 6 January 1999 and 15 February 2000, respectively, after plots had been inoculated with debris in November 1998 and October 1999. In 1991/92, numbers of ascospores above a naturally infected crop were small from January to April and increased in June and July. P. brassicae conidia were first observed in February and the percentage plants with leaves, stems or pods with light leaf spot increased greatly in May and June. In 1992/93, in a crop inoculated with debris, numbers of airborne ascospores were small from October to January and increased from April to June. P. brassicae conidia were first observed on leaves in late November and light leaf spot was seen on stems and pods in March and June 1993, respectively.     

Control of Rhizoctonia solani and Cotton Seedling Disease by Laetisaria arvalis and a Mycophagous Insect Proisotoma minuta (Collembola)

Population changes in Pseudocercosporella herpotrichoides with and without fungicide selection A conidium mixture of W (wheat-type) and R (rye-type) strains of Pseudocercosporella herpotrichoides (W:R = 1:1) was incubated on PD-agar without fungicide or amended with the imidazole prochloraz or the triazole cyproconazole. The W:R ratio was assessed over four generations using the different benomyl sensitivity of the strains as marker. In untreated mixtures, the R strain completely dominated the population already after two generations. Similarly, in cyproconazole-treated mixtures, the R strain increased to 100% after the second generation, whereas in prochloraz-treated mixtures the R strain either followed the same pattern as in untreated mixtures or fluctuated around the initial proportion of 50% over four generations. The W and R strains displayed different fitness properties. The average hyphal growth rate was 8.5 mm/10 d and 4.5 mm/10 d for the W and the R strains, respectively; the sporulation capacity was 80 and 400 conidia/colony, respectively, for the two strains. The different fitness resulted in a strong dominance of the R strains both in vitro and in field populations towards the end of the vegetation period. This shift towards R-dominance occurred about equally fast with or without fungicide selection.     

Effects of water potential and temperature on the development of eyespot lesions in wheat

Growth of Pseudocercosporella herpotrichoides on potato dextrose agar at water potentials from -0.5 to -6.9 MPa was optimal at 20°C. At 12 and 20°C, six isolates of P. herpotrichoides grew more rapidly at -0.5 to -2 MPa than at -6.9 MPa. Wheat plants inoculated with P. herpotrichoides and grown in columns of soil at either 15 or 20°C developed more severe eyespot lesions under a heavy watering regime than under medium or light watering regimes. P. herpotrichoides penetrated leaf sheaths of inoculated plants grown in compost more rapidly at 10°C night/15°C day temperatures than at 5/10°C; death of leaf sheaths was also more rapid and consequently there were fewer living infected leaf sheaths at 10/15°C than at 5/10°C. Irrigating for 5 wk before harvest increased the severity of eyespot lesions in a 1983 wheat crop.     

Pathogenicity to Wheat of Isolates of Pseudocercosporella herpotrichoides (Fron) Deighton from Five Graminaceous Hosts

Eleven isolates of Pseudocercosporella herpotrichoides were obtained from field material of the Gramineae Agropyron repens, Alopecurus myosuroides, Apera spica-venti, Hordeum vulgare and Triticum aestivum. Four of these isolates belonged to the variety acuformis and 7 isolates to the variety herpotrichoides of the fungus. In a greenhouse experiment, winter wheat was inoculated with these isolates. All isolates were pathogenic on wheat. They differed in virulence, but these differences could not be related to the nature of their original hosts nor to their classification as variety acuformis or herpotrichoides.     

Untersuchungen zur Bildung extrazellulärer Hemicellulasen durch Pseudocercosporella herpotrichoides in vitro

Investigations on the production of extracellular hemicellulases by Pseudocercosporella herpotrichoides in vitro For all 15 isolates of Pseudocercosporella herpotrichoides investigated, xylanase as well as arabanase activity could be demonstrated. After cultivation of 3 weeks, the activity of the enzymes reached a peak. The activity of xylanase was considerably increased by addition of xylan in comparison to Maltzin as the sole source of carbohydrate. Also the arabanase activity could be increased significantly by addition of araban or xylan as compared to the Maltzin variant. The optimum temperature with regard to activity and stability of xylanase ranged at 50°C. The pH-optimum for xylanase activity was found to be at pH 5.0, and the enzyme was stable in ° range between pH4.0 and 8.0 (9.0). In case of arabanase, the temperature optimum varied between 40 and 50°C; up to this temperature, the enzyme was also stable. At pH 5.0, the arabanase activity reached its optimum; stability was observed in - pH range between 4.0 and 9.0. In extracts prepared from autoclaved wheat coleoptiles which were inoculated with Pseudocercosporella herpotrichoides, the presence of the enzymes xylanase, arabanase, cellulase and polymethylgalacturonase could be demonstrated. The enzyme activities of the inoculated samples were considerably higher than those of non-inoculated controls. The differences, in most cases, were statistically significant. Der Deutschen Forchungsgemeinschaft danken wir für finanzielle Unterstützung.     

Negative cross-resistance between benomyl and MDPC in British isolates of Botrytis cinerea, Pseudocercosporella herpotrichoides and Verticillium fungicola var. fungicola

Fifteen isolates of Pseudocercosporella herpotrichoides, four isolates of Botrytis cinerea and four isolates of Verticillium fungicola var. fungicola were examined on potato dextrose agar amended with benomyl or methyl N-(3, 5-dichlorophenyl)-carbamate (MDPC). Negatively correlated cross-resistance was clearly demonstrated with the isolates of P. herpotrichoides and B. cinerea. There were indications that the same phenomenon might also operate with the isolates of V. fungicola var. fungicola.     

Development of Pseudocercosporella herpotrichoides (Fron) Deighton var. herpotrichoides and var. acuformis on Wheat Plants Measured by ELISA

Using the ELISA method, the development of Pseudocercosporella herpotrtchoides var. herpotrichoides and var. acuformis in a susceptible cultivar of winter wheat was compared under controlled and held conditions. In the greenhouse, var. acufornis grew less vigorously, was slower in penetrating the coleoptile and the successive leaf sheaths and in colonizing the stem tissue than var. herpotrichoides. In the field, these differences were confirmed on the last leaf sheaths and the stem. At ripening stage, however, identical ELISA values were measured for both varieties. Moreover, a significant variation was observed between the individual isolates of each variety. Comparison of the effect of both varieties of P. herpotrichoides on 20 wheat cultivars characterized by different resistance levels showed significant interactions. The cultivars carrying the Pch-1 gene always remained the less diseased genotypes. In general, var. acuformis developed less antigen in the cultivars than var. herpotrichoides. It is concluded, that in tests for resistance to P. herpotrichoides mixtures of many highly pathogenic isolates of both fungus varieties should be used. Less complex mixtures or single isolates may result in wrong estimates of resistance.     

Sensibilité des plantules de Triticinées àľagent Pathogène du Piétin-Verse [Pseudocercosporella herpotrichoïdes (Fron) Deighton]

Sensitiveness of seedlings of Triticinae against the pathogenic agens of Pseudocercosporella herpotrichoides (Fron) Deighton The study of the sensitiveness of 16 varieties or lines of Triticinae against two races of Pseudocercosporella herpotrichoides shows that, whatever the strain used, the importance of the attacks is much less serious on seedlings of Ae. ventricosa no. 11, Ae. squarrosa no. 15, Ae. squarrosa no. 33 and Roazon, Roazon was the least sensitive among the Triticum. It is obvious that Ae. ventricosa is the most resistant one in all varieties or lines used.     

Effects of shading and powdery mildew infection on senescence of the root cortex and coleoptile of wheat and barley seedlings,and implications for root- and foot-rot fungi

Summary Nuclear and cytoplasmic staining methods were used to study natural senescence of the root cortex and coleoptile of wheat and barley seedlings grown in glasshouse conditions. Coleoptiles of barley senesced more slowly than those of wheat, paralleling the known difference in rates of root cortex senescence in these cereals. The coleoptiles and root cortices of both cereals senesced more slowly in shaded than in unshaded conditions, but infection of the shoots of barley byErysiphe graminis had little effect on root cortex senescence. The results are discussed in relation to infection by root- and foot-rot fungi. Previous reports on the effects of illumination on take-all infection (Gaeumannomyces graminis) are explained. It is suggested that natural senescence of the coleoptile might affect establishment of infection by the eyespot fungus,Pseudocercosporella herpotrichoides, either directly or through the activities of competing microorganisms.     

Attempts to define and mimic the effects of pollen on the development of lesions caused by Phoma betae inoculated onto sugarbeet leaves

Expanding lesions resulted when conidia of Phoma betae Frank, mixed with rye pollen, were inoculated on to sugarbeet leaves by a standard technique. Conidia without pollen generally caused non-expanding necrotic spots; these could be made to spread later by covering them with pollen or orange juice, but not with water. Germ-tube growth was quicker on water agar than on sugarbeet leaves. Pollen extract stimulated germ-tube growth on leaves 10 h after inoculation and resulted in the production of knots of hyphae overlying areas where intercellular hyphae could be discerned and where expanding lesions developed. Some inorganic salts mimicked the stimulatory effect of pollen on germtube growth on agar slides, but only a mixture of hexose sugars with boric acid reproduced the effect of pollen on both numbers and size of expanding lesions caused by P. betae on sugarbeet leaves, and numbers of expanding lesions caused by Botrytis cinerea Pers. ex Fr. on bean leaves.     

Development of Eyespot and Brown Foot Rot in Wheat Plants Inoculated with Mixtures of Pseudocercosporella herpotrichoides Types and Fusarium Species

The effects of Fusarium avenaceum, F. culmorum and Microdochium nivale on eyespot development and of types of the eyespot fungus Pseudocercosporella herpotrichoides on brown foot rot caused by the Fusarium or Microdochium spp. were investigated by sequential inoculation of wheat plants grown in pots in a controlled environment. The W-type of the eyespot fungus induced more severe disease than the R-type, but its symptons were suppressed to a greater extent by Fusarium spp., especially F. avenaceum. Brown foot rot symptoms were sometimes suppressed by P. herpotrichoides but were occasionally more severe when the Fusarium inoculum was applied after the P. herpotrichoides. There, sults are discussed in relation to observations of natural infections.     

Effect of Microsphaeropsis ochracea on Production of Sclerotia-borne and Airborne Conidia of Botrytis squamosa

Onion leaf blight, caused by Botrytis squamosa (Walker), is a destructive disease of onion. Conidia produced on overwintered sclerotia are the main source of initial inoculum, and those produced on lesions are responsible for secondary inoculum build-up. The biological control agent Microsphaeropsis ochracea (Carisse &; Bernier) was evaluated for its ability to control sclerotia-borne inoculum, to colonize onion leaves and reduce the production of conidia under field conditions. Colonisation by M. ochracea of onion leaves at different growth stages was monitored and its effect on B. squamosa sporulation on necrotic leaves was evaluated. Onion plots were treated with either Dithane® or with M. ochracea at 7–10-day intervals and according to inoculum production index (IPI). The concentration of airborne conidia and the number of lesions per leaf, on 20 plants per plot, were evaluated throughout the cropping season. The number of conidia produced per sclerotium treated with M. ochracea, was reduced by 75.5%. In the field, M. ochracea colonised only senescent or necrotic leaves and reduced the production of conidia on these leaves by an average of 82% as compared with untreated leaves. Best disease control was obtained by Dithane®, followed by M. ochracea applied at 7–10-day intervals. For the three years of the study, there were no significant differences in airborne concentrations of conidia in plots treated at 7–10-day intervals with Dithane® or M. ochracea. Fall application of M. ochracea could be used as a sanitation practice to reduce initial inoculum or as a part of an IPM program during the season.     

Effect of Light, Temperature and Substrate during Spore Formation on the Germinability of Conidia of Colletotrichum falcatum

This paper presents results on the effect of light, temperature and substrate during spore formation on the germinability of conidia in Colletotrichum falcatum. Light seems to have no effect on the germination of conidia unless the cultures were exposed to a high intensity of light during sporulation, in which case the spores showed a reduced germination and an increased appressoria formation. Conidia produced at temperatures higher than the optimum showed better germination and less appressoria formation than the spores produced at the temperature optimum for the growth and sporulation of the fungus. A similar increase in germination was also observed in conidia obtained from inoculated sugarcane leaves as compared to those produced on culture media. The light type virulent isolates of C. falcatum showed greater sensitivity to all these treatments than the dark type weakly pathogenic isolates.     

Dispersal of Pseudocercosporella herpotrichoides Spores from Infected Wheat Straw

Evidence from spore samples collected amongst infected straw spread on fallow ground supported the conclusion that spores of Pseudocercosporella herpotrichoides are dispersed mostly by rainsplash. Most spores travelled a short distance in the larger ballistic splash droplets, although some may have travelled further in smaller airborne droplets. Weekly spore counts from microscope slides under rainshields, a funnel and an impinger, evaluated as samplers for spores of P. herpotrichoides, showed a similar seasonal pattern. The funnel, as the largest sampler, generally collected most spores, but the impinger collected more spores per unit area of sampling surface. Slides sometimes collected spores when none was recovered from other samplers. Young wheat plants, exposed with the samplers and changed weekly, subsequently developed eyespot symptoms for most of the season.     

Production and Pathogenicity to Wheat of Pseudocercosporella herpotrichoides Conidia

Weekly estimates of numbers of Pseudocercosporella herpotrichoides conidia on naturally infected wheat straw, made from February to July 1982, showed there were most conidia (8.1 × 10⁶ per straw) in February and least (1.9 × 10⁴ per straw) at the end of June. The viability of these spores remained high throughout this period, with an average of 85 % germination after 24 h.
After removal of spores produced in the field, straws were incubated at 5, 10, 15, 20 or 25°C and subsequent sporulation assessed after 3 or 5 weeks. The optimum temperature for spore production was 5°C and very few spores were produced at 25°C. There was no difference in viability between spores produced at different temperatures.
Wheat seedlings placed amongst infected straw collected and retained spores on the upper and lower surfaces of all leaf blades and on outer leaf sheaths. Both naturally dispersed spores and spores sprayed on to plants were not removed by subsequent rainfall.
When wheat seedlings were inoculated between the coleoptile and outer leaf sheath with different numbers of P. herpotrichoides spores, lesion development was most rapid in seedlings inoculated with the greatest numbers of spores. However, after incubation for 12 weeks visible lesions were present on all plants inoculated with > c. 10 spores.     

The resistance to Oidium lycopersici conferred by ol-2 gene in tomato

The powdery mildew caused by Oidium lycopersici is one of the most destructive diseases in glass-house-grown tomato and is widespreading all over the world. A high level of resistance to O. lycopersici was found in an accession of Lycopersicon esculentum var. cerasiforme at the Department of Biology and Plant Pathology, University of Bari. The genetic analysis of F1, F2 and BC plants indicated that the resistance is conferred by a single recessive gene, designed as ol-2. Studies on the infection process of O. lycopersici on susceptible and ol-2 gene resistant tomatoes were carried out at 24 °C and 90 % relative humidity. Light microscope observations on conidia germination, formation of primary appressoria, elongation of hyphae and sporulation were made on artificially inoculated basal, intermediate and apical leaves. Inoculation was made by shaking mildewed tomato leaves over each test plant. Disease development were assessed by removing the fungal structure from the leaf surface with the ceroidin film technique and by direct observations of stained inoculated leaves. The rate of conidial germination and the appressoria formation was not affected by host genotype. Mycelia growth and sporulation on leaf surface of resistant plant was strongly restricted and influenced by the leaf age. The results indicated that the resistance in ol-2 tomato is postinfectional and is not associated with a hypersensitive response. This work was supported by the MURST and CNR (Paper no. 331)     

Surveys of stem base diseases and fusarium ear diseases in winter wheat in England, Wales and Scotland, 1989–1990

An investigation into the incidence of stem base diseases and the pathogens associated with them was undertaken on winter wheat samples collected at growth stages 31 and 73–75 in 1989 and 1990. Symptoms at growth stage 31 were classified into 10 different types based on visual characteristics. Although Pseudocercosporella herpotrichoides was associated mainly with eyespot lesions and Rhizoctonia spp. with sharp eyespot lesions, a significant number of isolates of each pathogen were taken from lesions classified as fusarium. Different types of fusarium lesion on stem bases at both growth stages were not associated with any one particular Fusarium or Microdochium species. 98% of P. herpotrichoides isolates obtained from eyespot lesions at growth stage 31 in 1989, and 87% of those obtained in 1990 were identified as the sub-species P. herpotrichoides var. acuformis, Microdochium nivale was more common than any of the Fusarium species at both growth stages, and was particularly prevalent in samples from Scotland at the earlier growth stage in 1989. Isolations from the top internodes at growth stage 73–75 indicated that systemic infection by Fusarium species, although present in some stems, was of little importance. Fusarium ear blight affected 0.4% of ears in 1989 and 0.5% in 1990. Glume spot lesions on the ears in 1990 were predominantly associated with Fusarium poae.