Wind-mediated spread of Rice yellow mottle virus (RYMV) in irrigated rice crops

A study was carried out to demonstrate that Rice yellow mottle virus (RYMV), a virus known to be transmitted by beetles, can spread between rice plants by direct leaf contact caused by wind. Almost all healthy plants surrounding an infected plant became infected when exposed to a fan blowing for 15 min at a distance of 50 cm. Spread of RYMV by plant contact, mediated by wind, was also demonstrated in field experiments, the extent of spread depending on plant density. Infection was almost 10 times higher in plots with a density of 33 plants m⁻² than in plots with 16 plants m⁻². Less spread was observed in plots protected by 1·5 m high windscreens. It is suggested that wind-mediated spread of RYMV may result from abrasive contact between leaves of plants.     

Spatio-temporal development of pycnidia and perithecia and dissemination of spores of Mycosphaerella pinodes on pea (Pisum sativum)

The pattern of development of pycnidia and perithecia of Mycosphaerella pinodes was studied in the glasshouse on pea plants (cv. Solara) sprayed with a pycnospore suspension and in field plots inoculated with barley grains colonized by the fungus. The numbers of pycnidia and perithecia were estimated on each stipule and internode of infected plants, and were related to ratings of disease severity (0–5 scale). Pycnidia were produced on both green and senescent organs, whereas perithecia only appeared on senescent organs. The development and quantity of pycnidia were related to initial inoculum concentration and the physiological stage of the plants. The formation of fruiting bodies progressed from the bottoms to the tops of plants during crop development. Spore trapping showed that both pycnospore dispersal and ascospore discharge were initiated by rainfall or dew. Pycnospores were principally trapped in the first 20 cm above the soil surface while ascospores were also trapped above the crop canopy. Pycnospores and ascospores were dispersed throughout the growing season, suggesting that ascospores also play an important role in secondary infections.     

关于北也门的一种高梁、玉米的病害——高梁霜霉病

 高梁霜霉病(Sclerospora sorghi)是一个世界性的重要病害。目前国内尚无确切的报道。作者在北也门重新描述了当地的病害症状与病原,研究了该病的流行与防治。明确病害的初侵染主要靠土壤中的卵孢子。高梁幼苗出土时即可被害,田间出现少量系统病株。在适合的条件下,病株产生孢子囊进行局部侵染,并使病害逐渐扩展至全田。早期的局部侵染可转为系统性侵染,大大加重了高梁的受害程度。生长后期局部侵染减少,系统病株叶片破碎,向土中散出大量卵孢子,为次年流行提供了初侵染源。当地海拔800~1800米的山区,孢子囊和卵孢子均可对高梁造成严重的损失。在当地高梁霜霉病流行的气象要素是:日平均气温25℃至29℃,日平均相对温度56%以上,月结露日期多于18天,累积月降雨在30毫米以上。全年仅12月至次年4月间的气候条件不利于病害的流行。在当地玉米主要被土壤中的卵孢子侵染为害。病害的流行往往与降水量有关,一般苗期积水的田块,病害较重。高梁品种间抗病性差异明显。虽中国的品种材料一般较感病,但也有一些抗病的材料。     

Spread of Phoma exigua var. foveata to healthy potato plants

Contamination of stem-cutting bait plants exposed at various locations in and around a potato crop was greatest within the crop, less outside it and least at a site c. 20 km from commercial crops. Within the crop about half the stem infection and tuber contamination detected had occurred before haulm destruction. The incidence of contamination on tubers of bait plants placed within the crop was similar to that on tubers from the surrounding plants. Phoma exigua var. foveata was recovered from green leaves sampled at intervals between June and early September. In one year contamination by P. exigua var. foveata of tubers sampled from healthy plants surrounding a diseased plot was greatest in samples adjacent to the plot and decreased progressively away from it. This pattern was less apparent in two other years. These results are discussed in relation to the spread of airborne inoculum.     

Predicting light leaf spot (Pyrenopeziza brassicae) risk on winter oilseed rape (Brassica napus) in England and Wales, using survey, weather and crop information

Data from surveys of winter oilseed rape crops in England and Wales in growing seasons with harvests in 1987–99 were used to construct statistical models to predict, in autumn (October), the incidence of light leaf spot caused by Pyrenopeziza brassicae on winter oilseed rape crops the following spring (March/April), at both regional and individual crop scales. Regions (groups of counties) with similar seasonal patterns of incidence (percentage of plants affected) of light leaf spot were defined by using principal coordinates analysis on the survey data. At the regional scale, explanatory variables for the statistical models were regional weather (mean summer temperature and mean monthly winter rainfall) and survey data for regional light leaf spot incidence (percentage of plants with affected pods) in July of the previous season. At the crop scale, further explanatory variables were crop cultivar (light leaf spot resistance rating), sowing date (number of weeks before/after 1 September), autumn fungicide use and light leaf spot incidence in autumn. Risk of severe light leaf spot (> 25% plants affected) in a crop in spring was also predicted, and uncertainty in predictions was assessed. The models were validated using data from spring surveys of winter oilseed rape crops in England and Wales from 2000 to 2003, and reasons for uncertainty in predictions for individual crops are discussed.     

Temporal Dynamics of Phytophthora Blight on Bell Pepper in Relation to the Mechanisms of Dispersal of Primary Inoculum of Phytophthora capsici in Soil

ABSTRACT The effect of components of primary inoculum dispersal in soil on the temporal dynamics of Phytophthora blight epidemics in bell pepper was evaluated in field and growth-chamber experiments. Phytophthora capsici may potentially be dispersed by one of several mechanisms in the soil, including inoculum movement to roots, root growth to inoculum, and root-to-root spread. Individual components of primary inoculum dispersal were manipulated in field plots by introducing (i) sporangia and mycelia directly in soil so that all three mechanisms of dispersal were possible, (ii) a plant with sporulating lesions on the soil surface in a plastic polyvinyl chloride (PVC) tube so inoculum movement to roots was possible, (iii) a wax-encased peat pot containing sporangia and mycelia in soil so root growth to inoculum was possible, (iv) a wax-encased peat pot containing infected roots in soil so root-to-root spread was possible, (v) noninfested V8 vermiculite media into soil directly as a control, or (vi) wax-encased noninfested soil as a control. In 1995 and 1996, final incidence of disease was highest in plots where sporangia and mycelia were buried directly in soil and all mechanisms of dispersal were operative (60 and 32%) and where infected plants were placed in PVC tubes on the soil surface and inoculum movement to roots occurred with rainfall (89 and 23%). Disease onset was delayed in 1995 and 1996, and final incidence was lower in plants in plots where wax-encased sporangia (6 and 22%) or wax-encased infected roots (22%) were buried in soil and root growth to inoculum or root-to-root spread occurred. Incidence of root infections was higher over time in plots where inoculum moved to roots or all mechanisms of dispersal were possible. In growth-chamber studies, ultimately all plants became diseased regardless of the dispersal mechanism of primary inoculum, but disease onset was delayed when plant roots had to grow through a wax layer to inoculum or infected roots in tension funnels that contained small volumes of soil. Our data from both field and growth-chamber studies demonstrate that the mechanism of dispersal of the primary inoculum in soil can have large effects on the temporal dynamics of disease.     

Effect of wind on the dispersal of oospores of Peronosclerospora sorghi from sorghum

The effect of wind on the dispersal of oospores of Peronosclerospora sorghi , cause of sorghum downy mildew (SDM) is described. The oospores are produced within the leaves of aging, systemically infected sorghum plants. These leaves typically undergo shredding, releasing oospores into the air. Oospores are produced in large numbers (6.1 × 10³ cm⁻² of systemically infected leaf) and an estimate of the settling velocity of single oospores (0.0437 m s⁻¹) of P. sorghi indicated their suitability for wind dispersal. In wind tunnel studies wind speeds as low as 2 m s⁻¹ dispersed up to 665 oospores per m³ air from a group of leaves previously exposed to wind and displaying symptoms of leaf shredding. The number of oospores dispersed increased exponentially with increasing wind speed. At 6 m s⁻¹, up to 12 890 oospores per m³ air were dispersed. Gusts increased oospore dispersal. A constant wind speed of 3 m s⁻¹ dispersed a mean of 416 oospores per m³. When gusts were applied the mean was 15 592 oospores per m³. In field experiments in Zimbabwe, oospores were sampled downwind from infected plants in the field and at a height of 3.8 m above ground level immediately downwind of an infected crop. These data indicate that wind could play a major role in the dispersal of oospores from infected plants in areas where SDM infects sorghum, perhaps dispersing oospores over long distances.     

Development of Equipment to observe the Weather and calculate Crop Disease Risks1

Equipment has been developed to make automatic measurements of temperature, wet-bulb depression, rainfall, duration of surface wetness and run of wind and to give warning of the probable development of a number of crop diseases. Field trials of the battery-powered microprocessor-controlled instrument have shown it to be reliable and simple to use.     

The Distribution and Spread of Sorghum Downy Mildew in Sorghum and Maize Fields in Nigeria and Zimbabwe

Sorghum downy mildew (Peronosclerospora sorghi, SDM) is a damaging disease of sorghum and maize crops in Africa. Runs analysis was used to study the distribution of systemically infected sorghum and maize plants in Nigeria and Zimbabwe. The temporal and spatial development of local lesions of SDM on sorghum in Zimbabwe was investigated by assessing the local lesion symptoms caused by conidia in plots with a single point source of inoculum. With ordinary runs analysis, there was evidence of clustering of disease in some fields in the humid areas of Nigeria and the semi-arid areas of Nigeria and Zimbabwe. Clustering was found in two of the eight runs analyses performed on maize in the humid south of Nigeria, and in only one of the eight runs in Zimbabwe, which was interpreted as a predominance of random infection at the time of assessment and at the spatial scales assessed. Symptoms of local lesions of SDM developed rapidly across plots from an introduced point source of infection. After 9 days-exposure to the source of inoculum, the incidence of diseased leaves was 1.2%, and after 50 days it was 74.5%. A disease gradient which initially developed flattened as the plot became uniformly diseased. The predominant wind direction was NNE, and most rapid spread of disease was towards the SSW and WSW. In conclusion, local lesions can spread rapidly in sorghum crops, suggesting that they may be an important source of conidial inoculum for further local and systemic infections during the growing season.     

Effect of the duration of inoculum exposure on development of citrus canker symptoms on seedlings of Swingle citrumelo

Citrus canker (Xanthomonas citri subsp. citri, Xcc) is one of the most serious diseases citrus in Florida, and elsewhere in the world. The disease causes yield loss and some fresh fruit trade restrictions may apply. Cultural management techniques such as windbreaks may work by not only reducing wind speed, but also reducing the period of exposure of susceptible foliage or fruit to those wind speeds that support infection from incoming inoculum. To investigate the effect of exposure period to inoculum of Xcc, seedlings of canker-susceptible Swingle citrumelo were exposed to sprayed inoculum for increasing periods at different wind speeds. The incidence and severity of citrus canker was assessed. In three experiments the incidence and severity of citrus canker most often increased with longer periods of exposure to inoculum, especially so at wind speeds of ≥16 m/s compared to wind speeds of ≤5 m/s (wind speed also increased disease incidence and severity). Regression analysis demonstrated relationships between period of exposure to inoculum and the percent infected leaves per plant, the number of lesions per plant, the number of lesions per infected leaf, and for the percent of infected leaves with lesions on the petioles at wind speeds of ≥16 m/s (R²?=?0.16–0.72). Due to the effect of inoculum exposure period and wind speed, attempts should be made to minimize exposure of canker-susceptible citrus when wind speed is highest and inoculum is available. Windbreaks should help minimize periods of exposure to splashed inoculum in high winds.     

Sorghum ergot can develop without local Claviceps africana inoculum from nearby infected plants

Batches of glasshouse-grown flowering sorghum plants were placed in circular plots for 24 h at two field sites in southeast Queensland, Australia on 38 occasions in 2003 and 2004, to trap aerial inoculum of Claviceps africana. Plants were located 20–200 m from the centre of the plots. Batches of sorghum plants with secondary conidia of C. africana on inoculated spikelets were placed at the centre of each plot on some dates as a local point source of inoculum. Plants exposed to field inoculum were returned to a glasshouse, incubated at near-100% relative humidity for 48 h and then at ambient relative humidity for another week before counting infected spikelets to estimate pathogen dispersal. Three times as many spikelets became infected when inoculum was present within 200 m of trap plants, but infected spikelets did not decline with increasing distance from local source within the 200 m. Spikelets also became infected on all 10 dates when plants were exposed without a local source of infected plants, indicating that infection can occur from conidia surviving in the atmosphere. In 2005, when trap plants were placed at 14 locations along a 280 km route, infected spikelets diminished with increasing distance from sorghum paddocks and infection was sporadic for distances over 1 km. Multiple regression analysis showed significant influence of moisture related weather variables on inoculum dispersal. Results suggest that sanitation measures can help reduce ergot severity at the local level, but sustainable management will require better understanding of long-distance dispersal of C. africana inoculum.     

Glasshouse trials of Coniothyrium minitans and Trichoderma species for the biological control of Sclerotinia sclerotiorum in celery and lettuce

Coniothyrium minitans, Trichoderma harzianum (HH3) and Trichoderma sp. (B1) were tested for ability to control disease caused by Sclerotinia sclerotiorum in a sequence of a celery crop and two lettuce crops in the glasshouse. In control plots, over 80% of celery and 90 and 60% of lettuce in first and second crops, respectively, were infected at harvest. Only the C. minitaris treatment in the first lettuce crop decreased disease and increased marketable yield. Nevertheless, C. minitans reduced the number of sclerotia recovered at harvest in the celery and first lettuce crops and decreased sclerotial survival over the autumn fallow periods following the celery and second lettuce crop. C. minitans survived in soil for over 1 year and spread to infect sclerotia in virtually all other plots. C. minitans infected sclerotia at all times of the year but sclerotia still failed to degrade during the summer months when the soil was dry. The Trichoderma species tested had no effect on disease and almost no effect on the survival of the sclerotia. even though they could be recovered from soil for the duration of the experiments.     

Effect of Irrigation Regimes on Disease Expression in Melon Plants Infected with Monosporascus cannonballus

The effect of irrigation regimes on disease expression in melon plants infected with Monosporascus cannonballuswas studied during two summer growing seasons (1998 and 1999) in the Arava region of southern Israel. Less frequent and reduced irrigation postponed the onset of plant collapse and lowered disease incidence. Delaying disease development in infested fields by reducing irrigation frequency allowed crop harvest before plant collapse. However, reduced irrigation regimes reduced yields, as shown in methyl bromide fumigated plots. Fruits from melon plants grown under reduced irrigation in the infested plots were also of lower quality due to water shortage. The delay in plant collapse under the reduced irrigation treatments was attributed to a combined effect of reduced fruit load and the development of a deeper root system, which could support the increased water demand of the mature, fruiting plant.     

Relationships between Weather Variables, Airborne Spore Concentrations and Severity of Leaf Blight of Garlic Caused by Stemphylium vesicarium in Spain

Ascospores and conidia released into the air were recorded around plots on which garlic debris infected by Stemphylium vesicarium were fixed onto the soil surface. Symptoms in garlic trap plots located in the vicinity of infected debris, started in March and developed during April–May to reach disease incidence close to 100%, final disease severity values being lower in 1993 and 1995 than in 1994 and 1996. Whereas daily concentrations of ascospores were rather erratic, with 30% of captures between 0 and 6 h, conidia showed a daily periodicity with highest concentrations between 12 and 18 h, with a pronounced peak between 14 and 16 h, and lowest values at night. Ascospore release occurred mainly in February and March. It coincided with rainfall periods, 14 h with vapour pressure deficit 5 mb and solar radiation <145 W m^–2 on the current day of the capture. In contrast, greatest captures of conidia started in late April and were prevalent in May, and were associated with rainfall in days previous to the capture in which rather high temperature occurred and solar radiation was 109–345 W m^–2. Among the weather variables considered, rainfall appeared directly related to the aerial concentration of ascospores and conidia. The role of relative humidity seemed essential when rainfall did not occur. There was a relationship between conidia concentration in the air and number of hours with temperature in the range 12–21 °C. Ascospore production was not essential for infections to take place, since primary infection from conidia may occur and disease can develop from them readily.     

Incidence of Verticillium wilt of artichoke in eastern Spain and role of inoculum sources on crop infection

Surveys of 94 artichoke fields throughout the artichoke production areas of Comunidad Valenciana (eastern Spain) were conducted from 1999 to 2002 to determine the incidence and distribution of Verticillium wilt.Verticillium dahliae was isolated from 80.9% of the sampled fields, and detected in all artichoke-growing areas, with a mean disease incidence of 53.8% infected plants. The disease was found to cause severe damage to cv. ‘Blanca de Tudela’, which is the most important artichoke cultivar grown in Spain, and was also observed on the seed-propagated cv. ‘Imperial Star’. In field trials to study the role of infected planting material and soil inoculum on infection of artichoke plants during the cropping season,V. dahliae was transmitted from infected stumps to the plants, confirming that the use of infected stumps could have greatly contributed to the dissemination of the pathogen. Inoculum density ofV. dahliae in soil had an effect on crop infection, in that a higher number of microsclerotia per gram of soil resulted in a higher percentage of infected plants. In addition, yield of cv. Blanca de Tudela was significantly affected byV. dahliae infection, showing that a higher percentage of infection corresponded with lower yield. http://www.phytoparasitica.org posting July 21, 2005.     

Inoculum potential of Pythium arrhenomanes in soils from three crop rotation experiments

Soil from crop rotation experiments at three different sites was tested by using bait plants and a most probable number (MPN) method to estimate the inoculum potential of Pythium arrhenomanes in terms of the density of infective units. Recorded values varied between 0 and 85 infective units/kg soil. In soil from one of the sites the inoculum densities following barley monoculture were significantly higher than those from crop rotations, whether sampled in the spring or autumn. At the other two sites, which had consistently lower inoculum densities, small effects of rotation treatment were also recorded when they were sampled in the autumn. However, the results did not support the tentative conclusions of earlier studies, that inoculum of P. arrhenomanes could be a major cause of yield reduction in barley monoculture. The MPN method cannot be recommended for predicting expected disease levels of P. arrhenomanes , unless the infective unit can be carefully defined and its relation to yield loss reliably established.     

Yellow mottle of tomatillo (Physalis ixocarpa) caused by physalis mottle virus

Tomatillo ( Physalis ixocarpa ) is being evaluated as a food crop in Louisiana. Preliminary studies indicate that virus diseases may be the major factor limiting production. A foliar mosaic and yellow mottle disease was found commonly affecting plants in experimental plots. The cause of the disease was identified as physalis mottle virus (PhyMV). The virus was identified by host reaction, serology and dsRNA analysis. Other viruses found less frequently included cucumber mosaic virus and tomato spotted wilt virus. The flea beetle, Epitrix cucumeris , transmitted PhyMV experimentally. The annual weed Physalis pubescens was found naturally infected with PhyMV near infected tomatillo.     

Effect of alfalfa mosaic virus isolates on forage yield of lucerne (Medicago sativa) in Britain

The effect of alfalfa mosaic virus (A1MV) on lucerne forage yield was investigated in experimental plots in which all plants were indexed for freedom from seed-borne A1MV. Plants which were artificially inoculated with A1MV were tested for successful infection prior to transfer to the plots. Inoculated plots yielded 14-8-22-8% (fresh weight) and 15.0 18.1% (dry weight) less than uninoculated plots. Isolate A1MV-I reduced the yield of five lucerne cultivars and yield was decreased by several A1MV isolates in cv. Vertus. Yield decreases tended to be greatest at the first harvest and progressively smaller at the second and third. Despite aphicide treatments and a cereal barrier crop, ELISA indexing showed natural infection developed in control plots. Most known infected plants failed to produce symptoms and yield decreases were associated with predominantly symptomless infection.     

薄土坡耕地石坎反坡水平阶水沙调控效果

为研究石坎反坡水平阶对于薄土坡耕地的水沙调控效果,设置了两个标准天然降雨径流小区（措施小区和对照小区）分别对次降雨径流和泥沙进行了观测,结果表明：石坎反坡水平阶措施能够有效地调控坡耕地地表径流,年均径流削减率为22.5%,且在短历时、大雨强降雨条件下的调控效果好于长历时、小雨强降雨;该措施对于坡耕地泥沙调控效果显著且稳定,年均泥沙削减率95.5%,措施小区的坡面产沙量随降雨特征值变化的波动幅度远远小于对照小区;石坎反坡水平阶措施通过减少坡面侵蚀,间接提高了坡耕地作物产量,春玉米年均增产6.2%。     

Cassava Bacterial Blight in Uganda

Abstract

Cassava bacterial blight (CBB) is a major problem of cassava in Uganda. It was first recorded in Terego County, Nile Province in 1976, and a disease survey indicated that CBB is now present in over half of the country. Losses of 90–100% have been observed on susceptible varieties in parts of the Nile, North and Eastern provinces. Symptoms include angular leaf spots, blight, wilt dieback and gum exudations. Planting of infected materials, rainfall, hailstorms, wind and insects are major factors in the spread of the disease. Restriction of movement of infected cassava cuttings to non-infected areas, planting of resistant varieties and healthy cuttings, encouragement of mixed cropping and crop rotation are recommended for control.