Apical leaf necrosis as a defence mechanism against pathogen attack: effects of high nutrient availability on onset and rate of necrosis

An outdoor experiment was conducted to increase understanding of apical leaf necrosis in the presence of pathogen infection. Holcus lanatus seeds and Puccinia coronata spores were collected from two adjacent and otherwise similar habitats with differing long‐term N fertilization levels. After inoculation, disease and necrosis dynamics were observed during the plant growing seasons of 2003 and 2006. In both years high nutrient availability resulted in earlier disease onset, a higher pathogen population growth rate, earlier physiological apical leaf necrosis onset and a reduced time between disease onset and apical leaf necrosis onset. Necrosis rate was shown to be independent of nutrient availability. The results showed that in these nutrient‐rich habitats H. lanatus plants adopted necrosis mechanisms which wasted more nutrients. There was some indication that these necrosis mechanisms were subject to local selection pressures, but these results were not conclusive. The findings of this study are consistent with apical leaf necrosis being an evolved defence mechanism.     

Sporulation of Pyrenopeziza brassicae (light leaf spot) on oilseed rape (Brassica napus) leaves inoculated with ascospores or conidia at different temperatures and wetness durations

In controlled environment experiments, sporulation of Pyrenopeziza brassicae was observed on leaves of oilseed rape inoculated with ascospores or conidia at temperatures from 8 to 20°C at all leaf wetness durations from 6 to 72 h, except after 6 h leaf wetness duration at 8°C. The shortest times from inoculation to first observed sporulation ( l ₀), for both ascospore and conidial inoculum, were 11–12 days at 16°C after 48 h wetness duration. For both ascospore and conidial inoculum (48 h wetness duration), the number of conidia produced per cm² leaf area with sporulation was seven to eight times less at 20°C than at 8, 12 or 16°C. Values of Gompertz parameters c (maximum percentage leaf area with sporulation), r (maximum rate of increase in percentage leaf area with sporulation) and l ₃₇ (days from inoculation to 37% of maximum sporulation), estimated by fitting the equation to the observed data, were linearly related to values predicted by inserting temperature and wetness duration treatment values into existing equations. The observed data were fitted better by logistic equations than by Gompertz equations (which overestimated at low temperatures). For both ascospore and conidial inoculum, the latent period derived from the logistic equation (days from inoculation to 50% of maximum sporulation, l ₅₀) of P. brassicae was generally shortest at 16°C, and increased as temperature increased to 20°C or decreased to 8°C. Minimum numbers of spores needed to produce sporulation on leaves were ≈25 ascospores per leaf and ≈700 conidia per leaf, at 16°C after 48 h leaf wetness duration.     

Genetics of resistance to cotton leaf curl disease in Gossypium hirsutum

Twenty-two cotton varieties were screened for resistance to cotton leaf curl disease (CLCuD), a disease of viral origin, using three procedures: field evaluation, whitefly transmission assay and graft inoculation. Viral infection of cotton varieties was determined by visual symptom assessment as well as dot-blot and multiplex PCR diagnostic techniques. Crosses were made between the most susceptible variety (S-12) and highly resistant varieties (CP-15/2, LRA-5166 and CIM-443). All F₁ plants of these crosses were resistant, showing dominant expression of the resistance as well as the absence of extrachromosomal inheritance. The F₂ plants of the crosses CP-15/2 × S12, LRA-5166 × S-12 and CIM-443 × S12 exhibited a ratio of 13 resistant (symptomless) to three susceptible (with symptoms). Screening of the F₂ generation for virus infection by multiplex PCR further subdivided the resistant class into those exhibiting a high level of resistance (HR; PCR-negative) and those exhibiting resistance (R; symptomless, yet showing virus replication by PCR analysis). Hence, the final ratio was 3:10:3 (HR:resistant:susceptible). The F₃ progeny of susceptible F₂ plants segregated for resistance, indicating the probable presence of a suppressor gene ( S ). These findings are consistent with three genes being involved in G. hirsutum resistance to CLCuD, two for resistance ( R _1CLCuDhir and R _2CLCuDhir ) and a suppressor of resistance ( S _CLCuDhir ).     

Estimation of amounts of Phytophthora infestans mycelium in leaf tissue by enzyme-linked immunosorbent assay

A polyclonal antiserum, prepared in a rabbit immunized with a mycelium extract of Phytophthora infestans , reacted in an enzyme-linked immunosorbent assay (ELISA) with mycelial extracts of two Phytophthora species but not with those of 10 other micro-organisms found on potato. P. infestans mycelium in leaf tissue was readily detected by ELISA using either the plate-trapped antigen or F(ab')₂ antibody-fragment techniques. The amount of mycelium in leaf extracts was estimated by comparing the values obtained in ELISA with those for known concentrations of P. infestans mycelium.     

Evaluation of carrot resistance to alternaria leaf blight in controlled environments

The objective of this study was to find a technique for plant resistance screening to alternaria leaf blight (ALB), caused by the fungus Alternaria dauci , in controlled environments. Glasshouse and laboratory screening methods were compared using three cultivars and F₂ genotypes segregating for ALB resistance evaluated against self-pollinated F₃ field-grown plants. Plant disease was assessed through a disease index obtained from the size and number of symptoms on carrot leaves. The results indicated the value of glasshouse evaluation and the inadequacy of detached leaf and hypocotyl assays for carrot screening for ALB resistance. Spearman's rank correlation, applied to results obtained with both F₂ plants and their progeny, indicated that the optimal evaluation stage for ALB resistance in carrot is 20 days after inoculation. This test was powerful enough to be used as a prescreening test in breeding programmes.     

Weed shoot morphology effects on competitiveness for light in direct-seeded rice

The effects of weed shoot morphology on competitiveness for light in rice ( Oryza sativa L.) have not been well described quantitatively and are difficult to study empirically. A rice:weed model was used to analyse the effects of weed leaf area densities (LAD; m² m^–3), leaf angles (as leaf light extinction coefficients, k _leaf) and maximum heights ( H _m, m) on growth and competition with rice. Weed morphologies were hypothetical but empirically based, as follows: LADs were skewed to the bottom or conical, k _leaf values varied from 0.2 (erectophile) to 0.8 (planophile), and H _m values were 0.5 H _R, 1 H _R and 1.25 H _R, where H _R was rice maximum height. Other parameters were equal to those of rice. Growth and competitiveness were evaluated using mature seed dry weights (g m^–2). Short weeds and weeds with conical LADs were weakly competitive, regardless of other traits. For other weed types, interference with rice was positively related to H _m, LAD skewness and more planophile leaves. All three traits were critical determinants of weed interference but no single morphological trait guaranteed competitiveness. All else being equal, weeds with highly skewed LADs produced the most seed dry weight. Planophile leaves were particularly beneficial for short weeds, giving over five times more seed dry weight than erectophile leaves.     

Relationships between temperature and latent periods of rust and leaf-spot diseases of groundnut

The effect of temperature on the latent periods of rust, late leaf spot and early leaf spot diseases of groundnut caused by Puccinia arachidis, Phaeoisariopsis personata and Cercospora arachidicola , respectively, was studied. The latent periods (LP) of rust, late leaf spot and early leaf spot ranged from 12–49 days, 13–38 days and 13–39 days, respectively, between 12°C and 33°C An equation relating the rate of pathogen development (1/LP) to temperature was fitted using daily mean temperatures to provide three cardinal temperatures: the minimum (T_mln), optimum (T_opl), and maximum (T_max), T_mln was about 12°C for rust and about 10°C for the two leaf-spot diseases. T_opt, for all three diseases was close to 25°C. T_max was 31°C for early leaf spot, and extrapolated values for late leaf spot and rust were about 35 and 40°C, respectively.
For P. personata , a temperature response curve was fitted using data only from controlled environment experiments. This curve was used to simulate latent periods from both mean daily and mean hourly temperatures in the field. There was substantially better agreement between observed and simulated latent period with hourly temperatures, provided the developmental rate of the pathogen was determined at a constant temperature.     

Competition between maize and Datura stramonium in an irrigated field under semi-arid conditions

Crop growth of maize ( Zea mays L.) and Datura stramonium L. in monoculture and competition was studied over 4 years in a flood irrigated field in Zaragoza (Spain). Plant density was 8.33 m^–2 for maize and 16.66 m^–2 (1994 and 1995) and 8.33 m^–2 (1996 and 1997) for D. stramonium . Maize yield was decreased by 14–63% when competing with the weed. Yield reduction increased as the time between crop and weed emergence decreased. The development of leaf area per plant during the exponential growth phase was faster in maize primarily because the leaf area of maize seedlings at emergence time was greater than that of the weed. The faster growth of maize in leaf area and height reduced the photosynthetically active radiation received by the weed. Datura stramonium had a lower radiation use efficiency (RUE) than maize. Competition from the weed slightly decreased the maximum leaf area index (LAI) of the crop, and leaf senescence of maize was accelerated. The weed competed with the crop late in the season reducing crop growth rate, grain number per ear and grain weight. Competitive ability of D. stramonium for light was mainly due to its growth habit, with the leaves concentrated in the upper part of the canopy (more than 75% of LAI in the upper 25% of its height), its higher light extinction coefficient (0.89) and its indeterminate growth habit. The N plant content of maize was not influenced by the presence of the weed. The weed had a higher N plant content than the crop throughout the season and took up more N in monoculture.     

Nitrogen per unit leaf area affects the upper asymptote of Puccinia striiformis f.sp. tritici epidemics in winter wheat

Field trials tested which components of epidemic development of Puccinia striiformis , the cause of yellow rust, were affected by nitrogen (N) fertilizer applied to winter wheat. Both timing and amount of N were varied to affect canopy size and leaf N content, and to provide a supply of mobile N to the pathogen, by causing fresh N uptake after leaf expansion was complete. No N was applied to control plots. A logistic disease-progress function was fitted to disease-severity data, which were assessed in absolute units. Leaf area and specific leaf N (g N per m² leaf tissue) were quantified. Large and highly significant effects of N on the upper asymptotes, or 'carrying capacities' ( c ) were found. Effects on rates and points of inflection of the epidemics were not significant. Early N resulted in larger shoot numbers and leaf area, but disease was also more severe, so that by grain filling, the remaining green leaf areas were larger without N than with N. Later N treatments did not increase canopy size, but did increase symptom area compared with the control. These effects differ from the concept that N affects disease as a result of its effect on canopy growth, and therefore canopy microclimate, and suggest instead a substrate effect. Linear regression revealed that 51% of the observed differences in c were explained by variation in specific leaf N, suggesting that growth of the rust fungus may depend directly on particular components of total leaf N.     

THE GENETIC RESPONSE OF BARLEY TO DDT AND BARBAN AND ITS SIGNIFICANCE IN CROP PROTECTION

Summary. The response of eleven barley varieties to DDT and barban was assessed in field and glasshouse trials. Chlorosis following the application of DDT and minimal doses of barban is an all-or-nothing effect, whereas in apical inhibition there are several degrees of resistance. The criteria used to assess the eflect of barban on apical inhibition were fresh weight of seedlings, fertile tiller production at maturity and visual estimates of damage at the seedling stage and at maturity. Inheritance studies showed that resistance to the chlorosis reaction of both DDT and barban is controlled by single recessive genes independently inherited.
Resistance to apical inhibition, on the other hand, appears to be quantitatively inherited. When F₂ and F₃ segregating populations in the field were sprayed with barban, the proportion of resistant plants in successive generations was markedly increased.
The authors conclude that collaboration between plant breeder and pest control specialists could lead to a far more efficient control of weed, pest and pathogen by the development of varieties of crop plants with a high degree of tolerance to the chemical employed, especially under conditions where selectivity is narrow and delicate, such as in the control of other monocotyledonous plants in cereals.     

Comparative ecophysiology of grain sorghum and Abutilon theophrasti in monoculture and in mixture

Selection of crop genotypes that are more competitive with weeds for light interception may improve crop yield stability in the presence of weeds. The effects of interference on ecophysiological characteristics of Abutilon theophrasti Medic. and three morphologically diverse grain sorghum hybrids was evaluated to determine the relative tolerance and suppressive ability of the three hybrids and specific traits that may contribute to those differences. A tall hybrid was more tolerant to A. theophrasti interference than two medium stature hybrids. Early leaf area growth of two medium-stature sorghum hybrids was reduced by A. theophrasti interference, whereas early growth of a tall hybrid was unaffected. The height of A. theophrasti was greater than two moderate-stature hybrids but lower than the tall hybrid. Greatest leaf area density (L_D) of the tall sorghum hybrid was above that of A. theophrasti , whereas greatest L_D of medium-stature hybrids was below that of the weed. In monoculture, the partitioning of new biomass to various plant organs was similar among sorghum hybrids, whereas the tall sorghum hybrid partitioned less biomass to leaves and more to stems than medium hybrids in mixture. The results indicate that the three hybrids differ in their susceptibility to A. theophrasti competition. Crop traits that may contribute to greater crop competitiveness include greater maximum height and its growth rate and greater height of maximum leaf area distribution.     

A host-pathogen simulation model: powdery mildew of grapevine

An epidemiological model simulating the growth of a single grapevine stock coupled to the dispersal and disease dynamics of the airborne conidia of the powdery mildew pathogen Erysiphe necator was developed. The model input variables were either climatic (temperature, wind speed and direction) or related to the pathogen (location and onset of primary infection). The environmental input variables dictated plant growth and pathogen spread (latent period, infection, lesion growth, conidial spore production and release). Input parameters characterized the crop production system, the growth conditions and the epidemiological characteristics of the pathogen. Output described, at each time step, number, age and pattern of healthy and infected organs, infected and infectious leaf area and aerial density of spores released. Validation of the model was achieved by comparing model output with experimental data for epidemics initiated at different times of host growth. Epidemic behaviour for two contrasting years of crop development and 7 phenological stages at the time of primary infection (PI) was examined. For PI occurring at day 115 a vine with late budbreak (1998) showed 58% of primary leaves diseased at flowering compared with only 19% for a vine with early budbreak (2003). Depending on the phenological stage at PI (1–4 leaves), the proportion of diseased primary leaves decreased from 42% to 6% at flowering. Simulations suggested that differences resulted from the interplay between the timing of the first sporulation event, the phenological stage at the time of initial infection, and the age structure and spatial distribution of the leaf population.     

Effects of Ascochyta blight (Mycosphaerella pinodes) on the photosynthesizing leaf area and the photosynthetic efficiency of the green leaf area of dried-pea (Pisum sativum)

The effects of Ascochyta blight (caused by Mycosphaerella pinodes ) on the net photosynthetic rate of glasshouse-grown dried pea were studied on foliar discs and whole plants in pots showing different disease intensities, assessed visually using a disease scale (scores 0 to 5) or by an estimation of the fraction of leaf area with necrosis using an image analyser. The photosynthetic rate of foliar discs was measured using a leaf disc electrode; for plants it was assessed by CO₂ exchange rate measurements in a closed chamber. A reduction in the net photosynthetic rate of diseased plants was correlated with an increase in disease score. A 53–56% reduction was found in foliar discs with a mean disease score of 3. On whole plants, mean scores of 3.5 and 4.0 were associated with reductions of 36% and 98% respectively. The disease also induced a decrease in the photosynthetic efficiency of the non-necrotic leaf area. The photosynthetic rate was zero when 30–40% of foliar area was necrotic on foliar discs and on whole plants. A function was derived from the foliar disc data that described the dependence of photosynthetic rate on the fraction of leaf area without necrosis. The model is discussed with reference to data obtained from entire plants and other pathosystems.     

Purification and Biological Characterization of Host-Specific SV-Toxins from Stemphylium vesicarium Causing Brown Spot of European Pear

ABSTRACT Culture filtrates of a pathogenic isolate (IT37) of Stemphylium vesicarium, causing brown spot of European pear, induced veinal necrosis only on pear leaves susceptible to the pathogen. Two host-specific toxins, SV-toxins I and II, were purified from culture filtrates of IT37 by successively using Amberlite XAD-2 resin adsorption, cellulose thin-layer chromatography, and high-performance liquid chromatography under three different sets of conditions. Susceptible cultivars showed veinal necrosis at a SV-toxin I concentration of 0.01 to 0.1 mug/ml, whereas resistant cultivars were insensitive to the toxin at 1,000 mug/ml. SV-toxins I and II caused a dose-dependent increase in electrolyte loss from susceptible leaf tissues. No increase in electrolyte loss was detected in leaf tissues from resistant cultivars. The results of physiological studies indicated that SV-toxins appear to have an early effect on plasma membranes of susceptible leaves. Spores of a nonpathogenic isolate induced necrotic lesions on susceptible leaves in the presence of a small amount of toxin. SV-toxins were detected in intercellular fluids obtained from diseased leaves after inoculation with the pathogen. The results indicate that SV-toxins I and II produced by S. vesicarium can be characterized as host-specific toxins.     

西北旱区制种玉米灌浆期气孔导度与叶水势对水氮胁迫的响应

为了探究西北旱区制种玉米水氮胁迫条件下的生理节水机理,设置充分灌溉（FI）与亏缺灌溉（DI）处理,以及施氮（N150）与不施氮（N0）处理,分析了不同水氮模式下叶水势与气孔特征的变化规律。结果表明：充分灌溉条件下,施氮主要通过增加气孔开度和气孔数量提高气孔导度,与不施氮处理相比,气孔开度提高19.46%,气孔数量增加8.46%;亏缺灌溉条件下则通过增加单位面积开放气孔数量和降低气孔开度来应对水分消耗,提高抗旱能力,与充分灌溉相比,亏缺灌溉气孔开度平均降低21.38%,气孔数量平均增加14.08%;增加灌水量能极显著增大气孔开度（P＜0.05）和提高玉米叶片黎明和正午叶水势值,促进作物蒸腾作用;FIN150处理较DIN150处理黎明和正午叶水势均值分别上升了3.30%和2.00%,FIN0处理较DIN0处理黎明和正午叶水势均值分别上升了13.41%和14.29%;亏缺灌溉条件下,施氮处理黎明和正午叶水势均值分别上升了2.87%和0.17%。干旱导致气孔导度及正午叶水势的降低可以通过增施氮肥得到部分补偿。     

Effects of ozone exposure and leaf age of wheat on infection processes of Septoria nodorum Berk.

The infection process of Septoria nodorum was studied on the flag and second leaves of wheat plants at the flag-leaf emergence growth stage (GS 39), after pre-inoculative fumigation with different doses of ozone (0, 80, 180 or 240 μg/m³) for 7 h daily over 7 days. On ozone-treated plants, formation of appressoria occurred earlier, leading to a greater number and density of appressoria and faster growth of superficial infection hyphae than on plants grown without ozone amendment. Additionally, on leaves treated with 180 and 240 μg/m³ ozone, the percentage of papillae per appressorium was only half that on control plants. The number of germ tubes per conidium was not significantly affected by any treatment. Effects of increasing leaf age on the infection process were similar to those obtained with ozone. These results provide further evidence for similarities in the physiological effects of ozone and leaf ageing, which support the hypothesis that disease alterations due to ozone are strongly determined by the senescing effects of the pollutant.     

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.     

外源赤霉素对厚皮甜瓜坐果节位叶片早衰与内源激素含量的影响

采用全株叶片喷施外源赤霉素 (GA₃) 的方法,以黄瓜品种‘迎春’为试材,通过对黄瓜坐果节位叶片中的叶绿素、可溶性蛋白质、抗氧化酶活性、活性氧及内源激素含量等各项生理指标及果实品质指标进行测定,探究GA₃对厚皮甜瓜坐果节位叶片早衰及内源激素含量变化的调控作用,为延缓厚皮甜瓜坐果节位叶片早衰并提高其果实产量和品质提供理论依据。结果显示:与对照相比,叶面喷施50、100和150 mg/L的外源GA₃水溶液可不同程度地增加坐果节位叶片中的叶绿素和可溶性蛋白含量,提高其抗氧化酶活性,降低超氧阴离子产生速率,减缓丙二醛含量的增长,同时抑制脱落酸的增长,提升生长素、赤霉素和玉米素核苷水平,最终显著改善果实产量和品质。通过叶面喷施适宜浓度外源GA₃,对厚皮甜瓜坐果节位叶片抗氧化酶系统和内源激素含量的调控产生积极影响,从而延缓叶片早衰并提高果实产量和品质。在本试验条件下,喷施100 mg/L的外源GA₃水溶液延缓早衰效果最佳,且在各个时期能够使坐果叶片内脱落酸含量分别平均降低19.22%,玉米素核苷、生长素和内源赤霉素含量分别平均升高14.60%、23.50%和56.13%。     

番茄灰叶斑病原菌的鉴定及抗性种质资源的筛选

<正>番茄灰叶斑病是一种世界性病害,近年来,该病在北京、台湾、海南、山东和浙江等地均有发生,极大地限制了我国番茄生产的竞争力~([1])。该病害主要由Stemphylium spp.不同的4个种引起~([2])。本研究采用形态观察法、ITS和gpd序列分析法对从番茄病株分离得到的病原菌进行鉴定,并对番茄灰叶斑病抗性鉴定方法进行研究,以期为番茄灰叶斑病的抗病育种研究及其利用提供理论依据。     

Effect of water on germination of Plasmopara viticola oospores

The effect of moisture in grape leaf litter holding overwintering Plasmopara viticola oospores was investigated. Oospores were incubated under different regimes of water activity ( a_W 0·991 to 0·123) for 2 to 15 days and their ability to germinate and cause infection was determined using a sensitive leaf disk assay. Reduction of a_W caused a significant shift in the infection dynamics, with maximum effect when a_W  ≤ 0·56. Dynamics of a_W in the leaf litter under natural conditions were estimated from moisture data using a Chen-Clayton equation. Daily patterns of leaf litter moisture (M in % weight) were determined in non rainy periods between mid February and mid June, while the Chen-Clayton equation was calculated using data of a_W and M measured in both sorption and desorption conditions, at different temperatures. Water activity was highest at 08·00 hours, decreased progressively until 14·00 hours, and then increased. Water activity was favourable for oospores to develop in about 25% of the measurements, all made between 18·00 hours and 08·00 hours. A close relationship was found between vapour pressure deficit (VPD in hPa) and a_W of the leaf litter, so that when VPD is lower than 2·13 hPa there is sufficient water for oospores to develop. Results showed that leaf litter moisture due to water from the atmosphere makes oospore development possible during non rainy periods.