The selective control of Poa trivialis by dalapon in perennial ryegrass crops grown for seed

The effects of dalapon applied in March and April at doses between 1·7 and 2·8 kg/ha a.e. on Poa trivialis present in crops of perennial ryegrass grown for seed, were investigated. Good control of P. trivialis was achieved at all doses sprayed at both times. Reductions in numbers of inflorescences and yield of seed of perennial ryegrass were however recorded, particularly after applications in March. Differences in the response of perennial ryegrass cultivars were also observed; cultivar S.23 was more susceptible to the herbicide than cultivar S.24. The results also suggest that the variation in response of cultivar S.24 in different experiments within the study may be due to the influence of climatic factors. Lutte sélective contre Poa trivialis avec le dalapon dans des cultures porte-graines de ray-grass vivace. Les effets du dalapon, appliqué en mars et en avril à des doses comprises entre 1,7 et 2,8 kg/ha (é.a.) sur Poa trivialis présent dans des cultures porte-graines de ray-grass vivace, ont étéétudiés. Une bonne efficacité sur Poa trivialis a été obtenue à toutes les doses appliquées aux deux époques. Toute fois, des réductions du nombre des inflorescences et des rendements en graines du ray-grass vivace, ont été constatés, particuliérement aprés les applications faites en mars. Des différences entre les réactions des cultivars ont été aussi observées: le cultivar S.23 s'est révélé plus sensible que le cultivar S.24. Les résultats suggèrent que les réactions différentes du cultivar S.24 dans diverses expériences comprises dans ce travail, peuvent également êre dues à l'influence de facteurs climatiques. Selektive Bekämpfung durch Dalapon von Poa trivialis in Deutschem Weidelgras für Samenproduktion Die Wirkung von Dalapon gegen Poa trivialis in Kulturen zur Samengewinnung von Deutschem Weidelgras wurde untersucht. Im März und April wurden Dosierungen zwischen 1,7 und 2,8 kg/ha appliziert. Poa trivialis konnte in alien Dosierungen und an beiden Behandlungszeitpunkten bekämpft werden. Minderungen in der Zahl der Blutenstände und Grassamenerträge von Deutschem Weidelgras wurden jedoeh festgestellt, insbesondere nach Applikation im März. Ebenfalls reagierten die Weidelgrassorten untersehiedlich. Sorte S.23 war gegen das Herbizid empfindlicher als Sorte S.24. Die Resultate deuten auch an, dass die unterschiedliche Reaktion von Sorte S.24 in den Experimenten dieser Untersuchung auf den Einfluss von Klimafaktoren zurückgeführt werden könnte.     

Requirements of leaf wetness and temperature for infection of groundnut by rust

Experiments are described to quantify the effects of temperature and duration of leaf wetness on infection of groundnut by Puccinia arachidis. After inoculation, a minimum period of leaf wetness, m. was necessary for infection. When leaf wetness duration was greater than m, lesion density increased with increasing wetness duration to an asymptote, D_max. The principal effects of temperature were on m and D_max- The value of m decreased linearly from 6 h, as temperature increased from 15 to 25 C and increased slightly at temperatures greater than 25 C D_max increased with temperature from zero at 8 C to a maximum at 22 C. and decreased to zero again at about 30 C. The experimental results were used to produce a set of curves relating an infection index to leaf wetness duration at different temperatures. The implications for infection of groundnut crops are discussed in relation to the climate at Patancheru in southern India.     

Effects of three herbicides used to control volunteer ryegrass on tillering and seed yield of two second year perennial ryegrass crops

Atrazine, chlorpropham and diuron gave excellent control of volunteer perennial ryegrass seedlings and reduced vegetative tillers of perennial ryegrass grown in the glasshouse and for seed in the field. However, there was no compensation in terms of the fertile tillers or seed weight during the second year. The results suggest that the lack of positive effect on seed yield may be due to direct effects on the crop from the residual herbicides.     

Role of phenology in host susceptibility and within-plant spread of stem rust during reproductive development of perennial ryegrass

ABSTRACT Perennial ryegrass (Lolium perenne) was inoculated with urediniospores of Puccinia graminis subsp. graminicola at four stages of reproductive tiller development. All developmental stages, from expansion of the penultimate leaf through anthesis, were equally susceptible to infection measured as number of pustules per total inoculated plant area. However, within each stage from boot through anthesis, there was a negative correlation of susceptibility with age of host tissue, the highest disease severities occurring on expanding or newly expanded leaves or inflorescences. Within-plant spread of the disease from primary lesions on the flag leaf sheath to the elongating stem of the inflorescence was observed and verified. A single primary infection on the leaf sheath can produce an elongated strip of contiguous secondary infections on the inflorescence head and stem, which is a characteristic sign of stem rust. Position and timing of eruption of the secondary pustules on the stem can be explained by assuming that infections occur on the stem slightly distal to the overlying sheath lesion and erupt one latent period later, by which time tiller elongation has moved the stem infection site upward. Repeated infections occur as the stem elongates, producing contiguous lesions. Expansion rate and final lesion area on the stem were correlated with elongation rate of the inflorescence. Microscopic observation of plant tissue sections revealed that the fungus sporulates on the inner surface of the sheath, and germinating urediniospores are attached to the stem surface under the covering sheath starting 1 to 2 cm distal to the location of the sheath infection site.     

A temperature and leaf wetness duration-based model for prediction of gray leaf spot of perennial ryegrass turf

ABSTRACT Gray leaf spot is a serious disease of perennial ryegrass (Lolium perenne), causing severe epidemics in golf course fairways. The effects of temperature and leaf wetness duration on the development of gray leaf spot of perennial ryegrass turf were evaluated in controlled environment chambers. Six-week-old Legacy II ryegrass plants were inoculated with an aqueous conidial suspension of Pyricularia grisea (approximately 8 x 10(4) conidia per ml of water) and subjected to four different temperatures (20, 24, 28, and 32 degrees C) and 12 leaf wetness durations (3 to 36 h at 3-h intervals). Three days after inoculation, gray leaf spot developed on plants at all temperatures and leaf wetness durations. Disease incidence (percent leaf blades symptomatic) and severity (index 0 to 10; 0 = leaf blades asymptomatic, 10 = >90% leaf area necrotic) were assessed 7 days after inoculation. There were significant effects ( alpha = 0.0001) of temperature and leaf wetness duration on disease incidence and severity, and there were significant interactions ( alpha = 0.0001) between them. Among the four temperatures tested, 28 degrees C was most favorable to gray leaf spot development. Disease incidence and severity increased with increased leaf wetness duration at all temperatures. A shorter leaf wetness duration was required for disease development under warmer temperatures. Analysis of variance with orthogonal polynomial contrasts and regression analyses were used to determine the functional relationships among temperature and leaf wetness duration and gray leaf spot incidence and severity. Significant effects were included in a regression model that described the relationship. The polynomial model included linear, quadratic, and cubic terms for temperature and leaf wetness duration effects. The adjusted coefficients of determination for the fitted model for disease incidence and severity were 0.84 and 0.87, respectively. The predictive model may be used as part of an integrated gray leaf spot forecasting system for perennial ryegrass turf.     

Fusarium-induced diseases of tropical, perennial crops

ABSTRACT The world's oldest ecosystems are found in the tropics. They are diverse, highly evolved, but barely understood. This and subsequent papers describe diseases of tropical, perennial plants that are caused by Fusarium spp. Many of these are economically significant, difficult to manage, and of scientific interest. Some represent coevolved patho-systems (e.g., Panama disease, tracheomycosis of coffee, fusariosis of pineapple, and Fusarium wilt of oil palm), whereas others may be new-encounter diseases or are caused by generalist pathogens (cushion gall of cacao). New vector relationships are evident in other pathosystems (e.g., mango malformation), and two or more pathogens have been shown to cause some of the diseases (Panama disease and tracheomycosis of coffee). More work on these pathosystems is warranted as they could reveal much about the evolution of plant pathogens and the important diseases they cause.     

Effect of temperature and leaf wetness duration on infection of sweet oranges by Asiatic citrus canker

Asiatic citrus canker, caused by Xanthomonas smithii ssp. citri , formerly X. axonopodis pv. citri , is one of the most serious phytosanitary problems in Brazilian citrus crops. Experiments were conducted under controlled conditions to assess the influence of temperature and leaf wetness duration on infection and subsequent symptom development of citrus canker in sweet orange cvs Hamlin, Natal, Pera and Valencia. The quantified variables were incubation period, disease incidence, disease severity, mean lesion density and mean lesion size at temperatures of 12, 15, 20, 25, 30, 35, 40 and 42°C, and leaf wetness durations of 0, 4, 8, 12, 16, 20 and 24 h. Symptoms did not develop at 42°C. A generalized beta function showed a good fit to the temperature data, severity being highest in the range 30–35°C. The relationship between citrus canker severity and leaf wetness duration was explained by a monomolecular model, with the greatest severity occurring at 24 h of leaf wetness, with 4 h of wetness being the minimum duration sufficient to cause 100% incidence at optimal temperatures of 25–35°C. Mean lesion density behaved similarly to disease severity in relation to temperature variation and leaf wetness duration. A combined monomolecular-beta generalized model fitted disease severity, mean lesion density or lesion size as a function of both temperature and duration of leaf wetness. The estimated minimum and maximum temperatures for the occurrence of disease were 12°C and 40°C, respectively.     

Effects of relative humidity on infection,colonization and conidiation of Magnaporthe orzyae on perennial ryegrass

Grey leaf spot, caused by Magnaporthe oryzae, causes severe damage on perennial ryegrass (Lolium perenne) turf. In this study, the effects of relative humidity (RH, 88 to 100% at 28°C) on infection, colonization and conidiation of M. oryzae on perennial ryegrass were investigated in controlled humidity chambers. Results showed that the RH threshold for successful M. oryzae infection was ≥92% at 28°C. The advancement of infection on the leaf tissue was further examined with a green fluorescent protein (GFP)‐tagged M. oryzae strain. No appressorium formation was found when the inoculum was incubated at RH ≤ 88%. Additionally, the GFP‐tagged staining provided a rapid method to quantitatively compare the fungal colonization from leaf tissue at different levels of RH. The fluorescence intensity data indicated that the fungal biomass was highest at 100% RH and there was no fluorescence intensity observed at 88% RH or below. Conidiation was only observed when RH was ≥96%, with the most abundant conidiation occurring 8 days after inoculation. Reduced conidiation was associated with decreasing RH, and no conidiation occurred at RH ≤ 92%. This study indicates that infection and conidiation of M. oryzae on perennial ryegrass required different thresholds: 92% and 96% RH for infection and conidiation, respectively. The quantitative data from this research will assist in prediction of grey leaf spot disease outbreaks and of secondary infection of perennial ryegrass.     

Effects of leaf wetness duration, relative humidity, light and dark on infection and sporulation by Didymella rabiei on chickpea

No infection occurred at less than 95% relative humidity (r.h.) when chickpea plants were dried after inoculation with conidia of Didymella rabiei. Infection was significant when the dry leaves were exposed to 98% r.h. for 48 h. When inoculated plants were subjected to different leaf wetness periods, some infection occurred with 4 h wetness, and disease severity increased with wetness duration according to an exponential asymptote, with a maximum value after about 18 h. Germination of conidia and germ tube penetration increased linearly with increasing wetness periods when recorded 42 h after inoculation. With a 24-h wetness period, germination of conidia was first observed 12 h after inoculation and increased linearly with time up to 52 h (end of the experiment). Dry periods immediately after inoculation, followed by 24-h leaf wetness, reduced disease severity; as the dry period increased the severity decreased. Disease severity increased with increasing periods of darkness after inoculation. The number of pycnidia and the production of conidia on infected leaves increased only slightly with high r.h. (either in the light or in the dark), but large increases occurred over an 8-day period when the leaves were kept wet.     

The incidence of strains of barley yellow dwarf virus in perennial ryegrass crops in south-west and central Scotland

The incidence of barley yellow dwarf virus (BYDV) in perennial ryegrass ( Lolium perenne ) crops in four areas of south-west and central Scotland was investigated between March 1988 and February 1989. BYDV was detected in 93·8% of the grass swards using an indirect enzyme-linked immunosorbent assay (ELISA). This enabled the seasonal periodicity of the BYDV strains to be monitored over 12 months for the first time in Scotland. The incidence of the RPV, PAV and MAV strains of BYDV declined between March and July 1988, before gradually rising in August. Incidence increased markedly in September, especially of the RPV and MAV strains, and then gradually decreased over the winter months, before stabilizing in February 1989. The incidence of the different strains in perennial ryegrass leys varied between geographical areas and between fields within areas. Most ryegrass samples contained a mixture of the three strains of BYDV. RPV was the most common strain in Ayrshire, while the incidence of PAV was highest in Wigtownshire and that of MAV was highest in Dumfriesshire and Stirlingshire. The incidence of BYDV increased with the age of the sward. The role of perennial ryegrass as a source of virus for the infection of cereals is discussed.     

Effect of temperature, wetness duration, and planting density on olive anthracnose caused by Colletotrichum spp

The influence of temperature, wetness duration, and planting density on infection of olive fruit by Colletotrichum acutatum and C. simmondsii was examined in laboratory and field experiments. Detached olive fruit of 'Arbequina', 'Hojiblanca', and 'Picual' were inoculated with conidia of several isolates of the pathogen and kept at constant temperatures of 5 to 35°C in humid chambers. Similarly, potted plants and stem cuttings with fruit were inoculated and subjected to wetness periods of 0 to 48 h. Infection occurred at 10 to 25°C, and disease severity was greater and the mean latent period was shorter at 17 to 20°C. Overall, C. acutatum was more virulent than C. simmondsii at temperatures <25°C. When temperature was not a limiting factor, disease severity increased with the wetness period from 0 to 48 h. Disease severity was modeled as a function of temperature and wetness duration; two critical fruit incidence thresholds were defined as 5 and 20%, with wetness durations of 1.0 and 12.2 h at the optimum temperature. In the field, anthracnose epidemics progressed faster in a super-high-density planting (1,904 olive trees/ha) than in the high-density plantings (204 to 816 olive trees/ha) and caused severe epidemics in the super-high-density planting even with the moderately resistant Arbequina. Data in this study will be useful for the development of a forecasting system for olive anthracnose epidemics.     

The control of Holcus lanatus in perennial ryegrass swards by asulam.

Asulam was evaluated in three fields experiments for the control of Yorkshire fog (Holcus lanatus L.) in perennial rygrass(Lolium perenneL). It was used in 1976 on a 3 year-old ryegrass/timothy (Phleum pratense L.) sward in comparison with propyzamidc, in 1977 on a 9 year-old ryegrass pasture grazed at two stocking rates and in 1978 on an intensively managed ryegrass dominant sward. Asulam at 1.75 kg ha^?1 gave good selective control in all the field experiments and was more effective under higher than lower slocking rates. In laxly grazed swards it was most effective when sprayed in October. Propyzamide also controlled H. lanatus and increased yields of while clover (Trifolium repens L.) even at 0.25 kg ha^?1 but it damaged the sown grasses and increased the growth of broad-leaved weeds.     

The influence of herbicides, nitrogen fertilizer, seed rate and method of sowing, on the establishment and long-term composition of a perennial ryegrass ley.

Changes in ihe species composition of a ryegrass pasture were examined over a 2 year period from a spring establishment. Seed of S23 perennial ryegrass was sown in main plots, eilher broadcast or drilled, at rates of 15, 25 or 35 kg/ha in a field experiment at Oxford. The whole area was oversown withfour weed grasses (Agrostis siolonifera.Holcus lanatus. Poa annua and P trivialis). Five cultural and three herbicide treatments were applied across the plots. These consisted of three levels of nitrogen (0. 50 and 100 kg N per ha), two companion species (Italian ryegrass and white clover) and 2.4-D, benazolin and methabenzihiazuron applications. Ten weeks after sowing, broad-leaved weeds accounted for nearly half the lotal dry-matter yield, except where controlled by the three herbicides. Grass yields were nol increased by herbicide treatment. The only cultural treatment checking broad-leaved weeds was ihe 35 kg/ha seeding rale. Seven months after sowing, the only treatment which had decreased the abundance of weed grasses was melhabenzthiazuron especially againsi A. stolonifera and Poa spp. Two years afler sowing, perennial ryegrass had only 34% ground cover and had been replaced largely by H. lanatus (38%), Poa spp, (15%) and A. stolonifera (7%), Where methabenzthiazuron had been used the ryegrass ground cover was 48%. with less P. trivialis and A. stolonifera. but H. lanatus was nol altered. White clover decreased H. lanatus cover. In two supplementary pot experiments, methabenzthiazuron sprayed pre-emergence damaged perennial ryegrass unless the seed was covered with at least 4 mm of soil. Only H. lanatmand F. rubra out of sixteen grasses were relatively resistant to methabenzihiazuron when applied at ihe tillering stage, confirming that this herbicide has potential for preventing the ingress of many unsown grasses in new leys.     

Effects of temperature and wetness duration on conidial infection, latent period and asexual sporulation of Pyrenopeziza brassicae on leaves of oilseed rape

Experiments in controlled environments were carried out to determine the effects of temperature and leaf wetness duration on infection of oilseed rape leaves by conidia of the light leaf spot pathogen, Pyrenopeziza brassicae . Visible spore pustules developed on leaves of cv. Bristol inoculated with P. brassicae conidia at temperatures from 4 to 20°C, but not at 24°C; spore pustules developed when the leaf wetness duration after inoculation was longer than or equal to approximately 6 h at 12–20°C, 10 h at 8°C, 16 h at 6°C or 24 h at 4°C. On leaves of cvs. Capricorn or Cobra, light leaf spot symptoms developed at 8 and 16°C when the leaf wetness duration after inoculation was greater than 3 or 24 h, respectively. The latent period (the time period from inoculation to first spore pustules) of P. brassicae on cv. Bristol was, on average, approximately 10 days at 16°C when leaf wetness duration was 24 h, and increased to approximately 12 days as temperature increased to 20°C and to 26 days as temperature decreased to 4°C. At 8°C, an increase in leaf wetness duration from 10 to 72 h decreased the latent period from approximately 25 to 16 days; at 6°C, an increase in leaf wetness duration from 16 to 72 h decreased the latent period from approximately 23 to 17 days. The numbers of conidia produced were greatest at 12–16°C, and decreased as temperature decreased to 8°C or increased to 20°C. At temperatures from 8 to 20°C, an increase in leaf wetness duration from 6 to 24 h increased the production of conidia. There were linear relationships between the number of conidia produced on a leaf and the proportion of the leaf area covered by 'lesions' (both log₁₀-transformed) at different temperatures.     

Genetics of resistance to wheat leaf rust, stem rust, and powdery mildew in Aegilops sharonensis

Aegilops sharonensis (Sharon goatgrass) is a wild relative of wheat and a rich source of genetic diversity for disease resistance. The objectives of this study were to determine the genetic basis of leaf rust, stem rust, and powdery mildew resistance in A. sharonensis and also the allelic relationships between genes controlling resistance to each disease. Progeny from crosses between resistant and susceptible accessions were evaluated for their disease reaction at the seedling and/or adult plant stage to determine the number and action of genes conferring resistance. Two different genes conferring resistance to leaf rust races THBJ and BBBB were identified in accessions 1644 and 603. For stem rust, the same single gene was found to confer resistance to race TTTT in accessions 1644 and 2229. Resistance to stem rust race TPMK was conferred by two genes in accessions 1644 and 603. A contingency test revealed no association between genes conferring resistance to leaf rust race THBJ and stem rust race TTTT or between genes conferring resistance to stem rust race TTTT and powdery mildew isolate UM06-01, indicating that the respective resistance genes are not linked. Three accessions (1644, 2229, and 1193) were found to carry a single gene for resistance to powdery mildew. Allelism tests revealed that the resistance gene in accession 1644 is different from the respective single genes present in either 2229 or 1193. The simple inheritance of leaf rust, stem rust, and powdery mildew resistance in A. sharonensis should simplify the transfer of resistance to wheat in wide crosses.     

Effects of inoculum density, leaf age, moisture, temperature, and wetness duration on black streak of edible burdock

Inoculum density, temperature, leaf age, and wetness duration were evaluated for their effects on the development of black streak (Itersonilia perplexans) on edible burdock (Arctium lappa L.) in a controlled environment. The effect of relative humidity (RH) on ballistospores production by I. perplexans was also evaluated. Symptoms of black streak on leaves increased in a linear fashion as the inoculum density of I. perplexans increased from 10² to 10⁶ ballistospores/ml. Rugose symptoms on young leaves were observed at densities of ≥10⁴ ballistospores/ml. Disease severity of I. perplexans in relation to leaf age followed a degradation curve when the leaves were inoculated with ballistospores. Disease severity was high in newly emerged leaves up to 5 days old, declined as leaf age increased to 29 days, and was zero when leaf age increased from 30 to 33 days. Disease development of edible burdock plants exposed to ballistospores of I. perplexans was evaluated at various combinations of temperature (10°, 15°, 20°, 25°C) and duration of leaf wetness (12, 24, 36, 48, and 72 h). Disease was most severe when plants were in contact with the ballistospore sources at 15° or 20°C. The least amount of disease occurred at 25°C regardless of wetness duration. Ballistospores required 24–36 h of continuous leaf wetness to cause visible symptoms by infection on edible burdock. Ballistospores production in infected lesions required at least 95.5% RH.