Removal of urediniospores of brown (Puccinia recondita f.sp. tritici) and yellow (P. striiformis) rusts of wheat from infected leaves submitted to a mechanical stress

The passive spore removal from colonies due to mechanical stress was compared in the brown (Puccinia recondita f.sp. tritici) and yellow (P. striiformis) rusts of wheat. Mechanical stress was applied using either a miniaturized wind tunnel or a centrifuge. In wind-tunnel experiments, a wind of minimum velocity of 1.3 and 1.8 m s^-1 for P. recondita f.sp. tritici and P. striiformis, respectively, applied for at least 10 seconds, was necessary to remove spores. The interaction between wind velocity and cumulated duration was significant for both rusts. At low wind velocity, a longer duration was required to remove the spores than at high wind velocity, and vice versa. In centrifugation experiments, the maximum spore removal occurred for angular velocities of 10³ and 2 10³ rotations min^-1, for P. recondita f.sp. tritici and P. striiformis, respectively, applied for 5 min. Calculation of the aerodynamic and centrifugal forces showed that the forces necessary to remove spores are greater for P. striiformis than for P. recondita f.sp. tritici. This difference can be related to the size of the dispersal unit, which is larger in P. striiformis than in P. recondita f.sp. tritici due to spore clustering. These observations are consistent with the differences in the mean spore dispersal distance, which is usually smaller in P. striiformis than in P. recondita f.sp. tritici.     

The incorporation of pathogen spores into rain-splash droplets: a modelling approach

Simple, theoretical, physical principles and existing experimental data were used to derive an analytical model to describe the incorporation of plant pathogen spores into splash droplets. Data were obtained from experiments on splash dispersal of spores of Pseudocercosporella herpotrichoides (cereal eyespot), Pyrenopeziza brassicae (oilseed rape light leaf spot) and Septoria nodorum (wheat glume blotch). In these experiments, incident drops of diameter 4–5 mm were allowed to fall onto spore suspensions 0.5 mm deep with 1.2 × 10⁵ to 6.5 × 10⁵ spores/mL. The analytical model was constructed as the product of three functions of droplet diameter which described, respectively, the frequency distribution of droplet diameters, the proportion of droplets carrying spores and the mean number of spores in spore-carrying droplets in each diameter category. The frequency distribution of droplet sizes was described by a log-normal distribution, the proportion of droplets carrying spores was described by an exponential function and the adimensional spore concentration in spore-carrying droplets was described by a power law. The cumulative proportions of spores in droplets in diameter categories of increasing diameter were calculated to compare observed and fitted data.     

Dispersal of Pseudocercosporella herpotrichoides and Pyrenopeziza brassicae spores in splash droplets

Splash dispersal of conidia which differ in shape and size, those of Pseudocercosporella herpo ichoides (needle-shaped, 52×2 μm) and Pyrenopeziza hrassicae (cylindrical, 12×3 μm), was studisd by allowing 5 mm drops to fall 13 m on to suspensions (depth 0.5 mm, concentration 120, 000 spores/ml) of each. The resulting splash droplets were collected on photographic film. The pattern of spore dispersal with distance and the distribution of spore-carrying droplets and spores within droplet-size categories were similar for both fungi. Regressions of square root spore number) on droplet diameter gave lines with slopes of 0.0138 for P. herpotrichoides and 0.0167 for P. brassicae.     

Studies on dispersal of Septoria tritici pycnidiospores in wheat–clover intercrops

Four experiments studied the effects of a clover understorey on pycnidiospore dispersal of Septoria tritici in a wheat–clover intercrop under simulated rain. Clover significantly reduced the dispersal of spores in a horizontal direction by 33% at a distance of 15 cm from a line inoculum source compared with a wheat monocrop. The clover also reduced the vertical movement of spores from infected leaves at the base of wheat plants by an average of 63% compared to the monocrop, and this suggests that the main movement of spores was from the base upwards. Splash experiments using blue colour marker showed the vertical decline of splash and the number of drops per cm² with height caught on paper adjacent to trays of clover was described by the exponential decline model. The effect of clover in reducing vertical splash approached an asymptote as the leaf area index of the understorey increased. Simulated rain-splash increased the level of disease on the flag leaf and, in one experiment, there was a significant interaction between rain-splash and clover in reducing the number of lesions on the flag leaf. The level of disease resulting from one splash event was low, indicating that subsequent pathogen multiplication is probably required to bring about high severities of disease.     

西洋参疫病菌的水滴飞溅传播规律

本项研究通过实验模拟手段揭示了水滴飞溅散布对西洋参疫病菌传播的机制。从定量的角度确立了西洋参疫病菌传播数量与水平距离、水滴碰撞速率的数学模型: Ln(Y)=b_0 b_1D b_2V 在自然雨滴的直径范围内,垂直下落高度在3.5m以内,飞溅传播直径约1m,在三维空间里,以低于碰撞源水平面的传播效果最为明显。     

落叶松褐锈病的研究1.夏孢子的发生及其形态鉴定

 落叶松褐锈病[Triphragmiopsis laricinum(Chou) Tai],是由落叶松拟三孢锈菌引起的一种叶锈病。现已知在黑龙江、吉林、辽宁三省的落叶松人工林内,均有不同程度的发生。发病率一般70~80%,重病区可达90%以上。病原菌能为害各种落叶松,以苗期和落叶松人工幼林较为严重。     

Hyperspectral signal decomposition and symptom detection of wheat rust disease at the leaf scale using pure fungal spore spectra as reference

This study establishes a method to detect and distinguish between brown rust and yellow rust on wheat leaves based on hyperspectral imaging at the leaf scale under controlled laboratory conditions. A major problem at this scale is the generation of representative and correctly labelled training data, as only mixed spectra comprising plant and fungal material are observed. For this purpose, the pure spectra of rust spores of Puccinia triticina and P. striiformis, causal agents of brown and yellow rust, respectively, were used to serve as a spectral fingerprint for the detection of a specific leaf rust disease. A least-squares factorization was used on hyperspectral images to unveil the presence of the spectral signal of rust spores in mixed spectra on wheat leaves. A quantification of yellow and brown rust, chlorosis and healthy tissue was verified in time series experiments on inoculated plants. The detection of fungal crop diseases by hyperspectral imaging was enabled without pixel-wise labelling at the leaf scale by using reference spectra from spore-scale observations. For the first time, this study shows an interpretable decomposition of the spectral reflectance mixture during pathogenesis. This novel approach will support a more sophisticated and precise detection of foliar diseases of wheat by hyperspectral imaging.     

Infection of ryegrass by three rust fungi (Puccinia coronata, P. graminis and P. loliina) and some effects of temperature on the establishment of the disease and sporulation

Experiments were conducted to examine the processes leading up to the infection of Lolium temulentum by crown rust ( Puccinia coronata ), stem rust ( P. graminis ) and brown rust ( P. loliina ), and the effects of temperature on these processes and sporulation. Uredia of all three rusts were produced freely if the adaxial leaf surface was inoculated, but did not form following inoculation of the abaxial surface. Light and scanning electron microscopy revealed abnormal growth of germlings on the abaxial surface which had amorphous sheet-like epicuticular waxes and very few stomata. On the adaxial leaf surface germ tubes of all the rusts orientated at right angles to the long axis of the leaf. However, the directional growth of germ tubes was often disrupted when they contacted the surface of bulliform cells at the base of leaf grooves. For P. loliina the optimum temperatures for urediospore germination and sub-stomatal vesicle formation were 12–16°C, and 8–20°C for appressorium formation. The optimum temperatures, for the same stages of fungal development, for P. coronata and P. graminis were higher. Urediospore production of P. loliina was higher at 10°C than at 25°C, but was similar at both temperatures for P. coronata .     

Identification and transmission of Piper yellow mottle virus and Cucumber mosaic virus infecting black pepper (Piper nigrum) in Sri Lanka

Sri Lankan black pepper with symptoms of yellow mottle disease contained a mixture of viruses: Piper yellow mottle virus (PYMV) particles (30 × 130 nm), Cucumber mosaic virus (CMV, 30 nm diameter isometric particles), and unidentified, isometric virus-like particles (30 nm diameter). An effective purification procedure is described for PYMV. Immunosorbent and conventional electron microscopy successfully detected badnavirus particles only when at least partially purified extracts were used. PYMV was confirmed as the cause of the disease, with the other two viruses apparently playing no part in producing symptoms. PYMV was transmitted by grafting, by the insect vectors citrus mealy bug ( Planococcus citri ) and black pepper lace bug ( Diconocoris distanti ), but not by mechanical inoculation or through seeds. The CMV isolate was transmitted to indicator plants by mechanical inoculation and by the vector Aphis gossypii , but not by Myzus persicae ; but neither mechanical nor insect transmission of CMV to black pepper was successful. A sensitive polymerase chain reaction assay was developed to detect PYMV in black pepper.     

诱杀球对橘大实蝇诱捕效果的研究

为开发一种新型的橘大实蝇诱捕器,研究了不同颜色、直径的诱杀球和不同颜色的粘虫板对橘大实蝇的诱集效果。结果表明,绿色诱杀球的诱杀数量显著大于黄色和黄绿色诱杀球(P0.05);诱杀球上橘大实蝇的诱集数量,随诱杀球直径的增大而显著增加,但在直径7cm以上时,诱杀球的诱集数量增速变缓,其中以直径7cm的绿色诱杀球生产成本和诱集效果最佳;诱杀球能显著提高黄板对橘大实蝇的诱集效果(P0.05)。     

Toxicity of pyrethroids to larvae of the Egyptian cotton leafworm,Spodoptera littoralis (Boisd.). II. Factors determining the effectiveness of permethrin at ultra-low volume

Factors determining theeffectiveness of permethrinat ultra-low volume against larvae of Spodoptera littoralis (Boisd.) were investigated using a modified Potter tower. Higher mass application rates for 95 % control were necessary when permethrin was used as a residual deposit compared with direct contact plus residual deposit action. Mortality was increased by adjusting droplet density, although the optimum number of drops/ unit area depended on the mass application rate. When mass application rate and droplet density were held constant, mortality increased inversely with the mean droplet diameter. The implications of these results for field applications are discussed.     

Interaction of virus-infected celery and Septoria apiicola

Inoculation of celery plants with viruses CV036 (celery mosaic) and CV506 (parsnip yellow fleck) decreased blight on leaves inoculated later with Septoria apiicola by 17–39% and 54–91%, respectively. There was a significant negative correlation between the amount of blight and the furanocoumarin content of the celery petioles. The antifungal activity of these furanocoumarins was demonstrated by their in vitro inhibition of the germination of spores of S. apiicola and Botrytis cinerea . Scanning electron microscopy of virus-infected leaves sprayed with spores of S. apiicola also showed slight but significant reductions in percentage germination and in germ-tube length, and considerable reductions in the proportion of germ-tubes which produced an appressorium, compared with spores on virus-free leaves.     

Effect of density and age of lesions on sporulation capacity and infection efficiency in wheat leaf rust (Puccinia recondita f.sp. tritici )

The effects of lesion ageing and crowding on spore production and infection efficiency of an isolate of wheat leaf rust (caused by Puccinia recondita f.sp. tritici  ) were studied under controlled conditions. Spore production per lesion per day decreased exponentially with increasing lesion density for lesions older than 13 days after inoculation. Spore production of younger lesions was not affected by crowding. A synthetic formula relating the accumulated spore production to lesion age and density was derived, allowing the evaluation of spore production per lesion on a daily basis. Infection efficiency was comparatively much less affected by lesion ageing and crowding than was spore production. The most efficient spores were produced by 'mature' lesions (14 to 21 days old). At high density, there was no compensation for reduced spore production by an increase in infection efficiency, except for 14-day-old lesions. The Daily Multiplication Factor (the product of spore production per lesion and infection efficiency) was higher at low lesion density and may contribute to a rapid increase of populations of sporulating rust lesions in wheat fields. The detrimental effect of lesion crowding, occurring later in the course of the epidemics, may have negligible effects on disease progress.     

Effects of climatic factors on native arbuscular mycorrhizae and Meloidogyne exigua in a Brazilian rubber tree (Hevea brasiliensis) plantation

The root-knot nematode Meloidogyne exigua and arbuscular mycorrhizal (AM) fungi may both occur in the roots of Brazilian rubber trees ( Hevea brasiliensis ). AM fungi may stimulate plant growth whereas nematodes usually reduce it. Variations of native AM fungi and M. exigua populations in soil and roots of rubber trees were studied for one year in a Brazilian plantation. The number of AM spores in the soil was generally greater in the rainy season than in the dry season, although AM colonization of roots was unaffected by season. During the dry season, numbers of juveniles and eggs of M. exigua in roots were lower than in the rainy season. A site without nematodes in the soil or roots showed the greatest numbers of AM spores in soil and highest AM colonization of roots. A negative correlation was observed between the percentage of AM colonization and the number of second-stage juveniles in soil and second-stage juveniles and eggs in roots. Microscope observations revealed (i) tissue specificity for each of the microorganisms in the roots, with a cortical location of mycorrhizae and a mainly vascular cylinder location of nematodes, and (ii) that Gigaspora was the most abundant AM genus in the plantation soil.     

The life cycle of <Emphasis Type="Italic">Stemphylium vesicarium</Emphasis>, the causal agent of Welsh onion leaf blight

Symptoms of Welsh onion leaf blight, caused by Stemphylium vesicarium, are divided into two types, i.e., brown oval lesions and yellow mottle lesions. Yellow mottle lesions exert considerable economic damage on Welsh onion in northern Japan. In this study, we investigated the life cycle of the pathogen in terms of seasonal fluctuation of spore dispersal and its relationship with development of disease, formation period of pseudothecia and overwintering of the pathogen based on field surveys, spore trapping and fungal isolation. Conidia were trapped throughout the cropping season except before mid June, when no ascospores were trapped. Brown oval lesions, which contained a large number of conidia, usually occurred in July followed by yellow mottle lesions with an increasing number of conidia trapped. These observations suggest that conidia released from brown oval lesions play an important role as a secondary inoculum source of the disease, leading to the development of yellow mottle lesions. Pseudothecia on leaves were first observed at the end of the cropping season or immediately after harvest (late October). The pathogen overwintered in the form of pseudothecia produced on leaves with or without symptoms. Ascospores failed to be trap in the field during the interval between before and beginning of the cropping season in April–May. However, pot experiments demonstrated that ascospores were released from leaf debris in November and rapidly increased in number after snow melt. From this circumstantial evidence, we hypothesize that ascospores are the primary inoculum source of Welsh onion leaf blight.     

Splash dispersal of Pseudocercosporella herpotrichoides spores in wheat monocrop and wheat–clover bicrop canopies from simulated rain

Simulated rain was allowed to fall onto spore suspensions of Pseudocercosporella herpotrichoides . The resulting splash droplets were collected on horizontal traps at the bottom of a canopy and at 12 cm above treatments comprising no-crop, wheat monocrop and a wheat–clover bicrop. The number of conidia collected on horizontal traps declined exponentially with distance from the inoculum source. The mean number of spores collected in the absence of any crop was twice that in a monocrop; in the monocrop it was twice that in the bicrop. Both splash droplet and spore deposition gradients were steeper in the monocrop treatment than in no-crop, and shallower in bicrop than in monocrop. Evidence is presented that suggests the clover canopy acts as a secondary source for the redistribution of previously dispersed droplets and spores.     

Miscellaneous Items

Abstract

The volume median diameter (vmd) of a spray deposit is normally found by converting the drop diameters to their cubes, finding the total volume of the drops and thence the cumulative percentage volume of the drops. The results are plotted against drop diameter to determine the drop size corresponding to the 50% volume. Using the most concise mathematical method such an operation takes half an hour. Using a simple graphical method the vmd can be estimated with the same accuracy in approximately five minutes. The method of drawing the necessary cube curves and their subsequent use are described. Ways of avoiding unnecessary measurement of small drops are also discussed.     

Henneguya cynoscioni sp. n. (Myxosporea: Bivalvulida), an agent of severe cardiac lesions in the spotted seatrout, Cynoscion nebulosus (Teleostei: Sciaenidae)

A new myxosporean species, Henneguya cynoscioni sp. n., is described from the spotted seatrout, Cynoscion nebulosus (Cuvier) (Sciaenidae) as a causative agent of cardiac henneguyosis. This new myxosporean species is characterized by the morphology of spores and the sequence of SSU rDNA. Examination of 227 spotted seatrout from four South Carolina estuaries in 2008-2010 revealed a 33.5% total prevalence of H. cynoscioni. Henneguya cynoscioni produces lesions in the bulbus arteriosus, its specific site of infection. The severity of lesions and their impact on the bulbus arteriosus is proportional to the number of plasmodial stages developing in this segment of the heart, being most pronounced in host reaction directed against spores liberated from plasmodia.     

Splash dispersal of Leptosphaeria maculans pycnidiospores and the spread of blackleg on oilseed rape

The fungus Leptosphaeria maculans causes blackleg (phoma stem canker), one of the most serious diseases of oilseed rape. The role of pycnidiospores produced during asexual reproduction is poorly documented and limits the understanding of the pathogen's population dynamics. The objectives of this study were to assess rain-splash dispersal of pycnidiospores of L. maculans from phoma leaf spots, and transmission of the disease from oilseed rape stubble carrying pycnidia. The work was conducted in still air with either a drop generator or a rain simulator. The impact of simulated incident drops on phoma leaf spots resulted in the dispersal of L. maculans pycnidiospores within splash droplets. Ninety per cent of the spores were collected within 14 cm of the source and a few were regularly observed up to 40 cm. Pycnidiospores produced on oilseed rape stubble and dispersed by simulated rain infected oilseed rape trap plants in a spatial pattern that matched the spatial dispersal of the pycnidiospores. In the field, rain-splash dispersal of pycnidiospores could increase the pathogen population and may enhance sexual reproduction by facilitating the mating of initially spatially separated isolates of opposite mating type.     

Microscopical observations of the primary zoospores of Spongospora subterranea f.sp. subterranea

A solution culture test system with Spongospora subterranea f.sp. subterranea spore ball inoculum and tomato bait plants was used to create a pulse of primary zoospore production and subsequent host-root infection. Spore balls and zoospores were examined by light, fluorescence, and transmission and scanning electron microscopy. Most of the resting spores with a developing exit pore did not show any changes in cytoplasmic content typical of zoospore formation. A few empty resting spores and some with developing exit pores were also observed in the absence of host-root exudates. The average diameter of exit pores of empty resting spores was 1.5 μm and they were often encircled by a ring-like fusion of wall layers. Mature zoospores were never found inside resting spores. Primary and secondary zoospores are identical in morphology. The infection process is similar to that of other Plasmodiophoromycetes with internal 'Rohr'-like structures in encysted zoospores which were attached by an adhesorium to tomato root hairs. Post-infection papillae and uninucleate plasmodia were observed.