The effect of environmental factors on infection of blueberry fruit by Colletotrichum acutatum

Anthracnose fruit rot of blueberries caused by Colletotrichum acutatum is a serious problem in humid blueberry‐growing regions of North America. In order to develop a disease prediction model, environmental factors that affect mycelial growth, conidial germination, appressorium formation and fruit infection by C. acutatum were investigated. Variables included temperature, wetness duration, wetness interruption and relative humidity. The optimal temperature for mycelial growth was 26°C, and little or no growth was observed at 5 and 35°C. The development of melanized appressoria was studied on Parafilm‐covered glass slides and infection was evaluated in immature and mature blueberry fruits. In all three assays, the optimal temperature for infection was identified as 25°C, and infections increased up to a wetness duration of 48 h. Three‐dimensional Gaussian equations were used to assess the effect of temperature and wetness duration on the development of melanized appressoria (R² = 0·89) on Parafilm‐covered glass slides and on infection incidence in immature (R² = 0·86) and mature (R² = 0·90) blueberry fruits. Interrupted wetness periods of different durations were investigated and models were fitted to the response of melanized appressoria (R² = 0·95) and infection incidence in immature (R² = 0·90) and mature (R² = 0·78) blueberry fruits. Additionally, the development of melanized appressoria and fruit infection incidence were modelled in relation to relative humidity (R² = 0·99 and 0·97, respectively). Three comprehensive equations were then developed that incorporate the aforementioned variables. The results lay the groundwork for a disease prediction model for anthracnose fruit rot in blueberries.     

Effects of temperature,inoculum concentration,leaf age,and continuous and interrupted wetness on infection of olive plants by Spilocaea oleagina

Experiments were conducted on olive plants in controlled environments to determine the effect of conidial concentration, leaf age, temperature, continuous and interrupted leaf wetness periods, and relative humidity (RH) during the drier periods that interrupted wet periods, on olive leaf spot (OLS) severity. As inoculum concentration increased from 1·0 × 10² to 2·5 × 10⁵ conidia mL^?1, the severity of OLS increased at all five temperatures (5, 10, 15, 20 and 25°C). A simple polynomial model satisfactorily described the relationship between the inoculum concentration at the upper asymptote (maximum number of lesions) and temperature. The results showed that for the three leaf age groups tested (2–4, 6–8 and 10–12 weeks old) OLS severity decreased significantly (P < 0·001) with increasing leaf age at the time of inoculation. Overall, temperature also affected (P < 0·001) OLS severity, with the lesion numbers increasing gradually from 5°C to a maximum at 15°C, and then declining to a minimum at 25°C. When nine leaf wetness periods (0, 6, 12, 18, 24, 36, 48, 72 and 96 h) were tested at the same temperatures, the numbers of lesions increased with increasing leaf wetness period at all temperatures tested. The minimum leaf wetness periods for infection at 5, 10, 15, 20 and 25°C were 18, 12, 12, 12 and 24 h, respectively. The wet periods during early infection processes were interrupted with drying periods (0, 3, 6, 12, 18 and 24 h) at two levels of RH (70 and 100%). The length of drying period had a significant (P < 0·001) effect on disease severity, the effect depending on the RH during the interruption. High RH (100%) resulted in greater disease severity than low RH (70%). A polynomial equation with linear and quadratic terms of temperature, wetness and leaf age was developed to describe the effects of temperature, wetness and leaf age on OLS infection, which could be incorporated as a forecasting component of an integrated system for the control of OLS.     

PUCREC/PUCTRI – a decision support system for the control of leaf rust of winter wheat and winter rye*

During a three‐year project from 2003 to 2006, two models have been developed to predict leaf rust (Puccinia recondita and P. triticina) occurrence and to simulate disease incidence progress curves on the upper leaf layers of winter rye (PUCREC) and winter wheat (PUCTRI). As input parameters the models use air temperature, relative humidity and precipitation. PUCREC and PUCTRI firstly calculate daily infection favourability and a cumulative infection pressure index and, in a second step, disease incidence is estimated. An ontogenetic model (SIMONTO) is used to link disease predictions to crop development. PUCREC and PUCTRI have been validated with data from 2001 to 2005. Both models give satisfactory results in simulating leaf rust epidemics and forecasting dates when action thresholds for leaf rust control are exceeded.     

Environmental Factors Affecting Production, Release, and Field Populations of Conidia of Alternaria alternata, the Cause of Brown Spot of Citrus

ABSTRACT Alternaria brown spot, caused by Alternaria alternata pv. citri, affects many tangerines and their hybrids, causing loss of immature leaves and fruit and reducing the marketability of the remaining fruit. Conidial production of A. alternata was greatest on mature leaves moistened and maintained at near 100% relative humidity (RH) for 24 h, whereas leaves that had been soaked or maintained at moderate RH produced few conidia. Conidial release from filter paper cultures and infected leaves was studied in a computer-controlled environmental chamber. Release of large numbers of conidia was triggered from both substrates by sudden drops in RH or by simulated rainfall events. Vibration induced release of low numbers of conidia, but red/infrared irradiation had no effect. In field studies from 1994 to 1996, air sampling with a 7-day recording volumetric spore trap indicated that conidia were present throughout the year with periodic large peaks. The number of conidia captured was not closely related to rainfall amounts or average wind speed, but was weakly related to the duration of leaf wetness. Likewise, disease severity on trap plants placed in the field weekly during 1995 to 1996 was not closely related to conidial numbers or rainfall amounts, but was weakly related to leaf wetness duration. Sufficient inoculum appears to be available to allow infection to occur throughout the year whenever susceptible host tissue and moisture are available.     

Rain-driven epidemics ofPhytophthora porri on leek

White tip disease of leek (Allium porrum), caused byPhytophthora porri, was studied in field experiments. On fields infested by soil-borne inoculum (oospores), relatively short periods of explosive disease increase alternated with periods in which apparently no new infections occurred. The analysis of rain data and disease data, using a degree-day model for incubation periods at constant temperatures, confirmed the hypothesis that disease increase ofP. porri is significantly correlated with rain; R _adj ² was 0.91, 0.41 and 0.51 in 1992, 1993 and 1994, respectively. Correlations were highest early in the season. Lack of correlation later in the season may be ascribed to the effect of lesion death, which may be caused by total or partial leaf death, by desiccation or by other fungi overgrowingP. porri, and to the effect of secondary infection by zoosporangia, which appears to be not so strongly rain-driven as primary infection. Zoosporangia were observed in fields on water-logged light-green lesions. High lesion densities of leaf tips and leaf units at 10–20 cm above the leaf axils indicated that most infections depend on free water, either in puddles or in a water basin near the leaf axils. Although disease correlates well with rain data, disease forecasts will be unreliable as long as rain forecasts are unreliable.     

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.     

Environmental Factors Affecting the Release and Dispersal of Ascospores of Mycosphaerella citri

ABSTRACT Greasy spot, caused by Mycosphaerella citri, produces a leaf spot disease affecting all citrus species in Florida and the Caribbean Basin. M. citri produces pseudothecia and ascospores, which are considered the principal source of inoculum, in decomposing leaves on the grove floor. In studies using a computer-controlled environmental chamber, a single rain event triggered release of most mature ascospores beginning 30 to 60 min after the rain event. Additional rain events did not bring about further release. High relative humidity without rain triggered release of low numbers of ascospores, but vibration and red/infrared irradiation had little or no effect on ascospore release. After three to four cycles of wetting and drying of leaves, all pseudothecia had matured and released their ascospores. In the field, ascospores were detectable starting about 2 h after the beginning of a rain or irrigation and most ascospores were released within 16 h. Ascospore release was greatest following rain events and somewhat less following irrigations, and low numbers of ascospores were detectable on days without precipitation. Ascospore numbers declined linearly with horizontal distance from the source and as a function of the logarithm of ascospore numbers with vertical distance. Low numbers of ascospores were detected 7.5 m above the ground and 90 m downwind from the grove. Ascospore release can be advanced by irrigating frequently during dry, nonconducive conditions to stimulate ascospore release when environmental conditions are unfavorable for infection, but the eventual effects on disease severity are uncertain.     

气象因子对黄土高原间套作豌豆和春小麦叶片水势的影响

在甘肃省农业科学院会宁农业试验站,对春小麦和豌豆间套作模式下,不同气象因子对春小麦和豌豆叶水势的影响进行了田间对比观测。结果表明:在半干旱地区,春小麦和豌豆的叶水势与气象因子的关系在不同生育期的表达式不同,与大气温度呈线性关系,与太阳辐射、大气相对湿度、大气水势均呈二次方程模型,与综合气象因子也呈良好的线性关系。不同气象因子对作物叶水势影响的通径分析表明,影响春小麦叶水势日变化作用最强的气象因子是大气水势,其次是大气相对湿度、大气温度和太阳辐射。大气相对湿度、大气温度和太阳辐射对春小麦叶水势日变化的直接影响小于它们通过大气水势的间接影响。对豌豆叶水势日变化直接影响最大的气象因子是大气温度,其次是大气水势、太阳辐射和大气相对湿度。大气水势、太阳辐射和大气相对湿度的直接通径系数均小于各自通过大气温度的间接通径系数,这三者对豌豆叶水势日变化的直接影响小于它们通过大气温度的间接影响。     

葡萄白腐病(Coniothyrium diplodiella(Speg.)Sacc.)的研究——Ⅰ:田间白腐病的最初发生与葡萄园微气候结构的关系

篱架葡萄穗白腐病最初多出现于6月中、下旬,病穗首先开始于架下,呈多点散发,7月份雨季以后,隨着雨水渐勤,病害乃逐蔓延而散见于架面。观察指出;造成白腐病发生的以上诸特点,除了最初侵染源来自表层土壤这一重要因素外,也与果园微气侯结构的关系至为密切。在枝叶茂密的葡萄园里,架下往往相当郁蔽而潮湿,白天地面温度较架上高,但夜间却常常产生显著的逆温。由于夜间地面湿度高,温度低,尤其在6月份葡萄座果后,正值华北干旱的季节,此时倘有微雨或灌溉,每引起架下偶然的水份凝聚,而架上却远较干燥。水份凝聚是有利于病害的侵染和发生的。因此6月份中、下旬架下先出现病害,及至7—8月份雨季里,架上下的湿度都高,故差异不显著,此时病害也就散见于架面,而且雨量愈多,流行愈炽。     

Environmental and irrigation conditions can mask the effect of Magnaporthiopsis maydis on growth and productivity of maize

Maize production in temperate countries is threatened by late wilt, caused by Magnaporthiopsis maydis. Plant infection occurs early after sowing, but symptoms appear from flowering onwards. The disease is mainly controlled by genetic resistance, which is often partially expressed in the field. Development of disease symptoms is also highly dependent on environmental conditions. This study looked at whether production and growth of susceptible maize are affected by M. maydis under environmental conditions that are suboptimal for disease development. In addition, the effect of water availability on disease development under optimal conditions was determined. Pot experiments were conducted in an open-air enclosure in 2013, 2015 and 2016. Under unfavourable conditions for disease (low air temperature and relatively high air humidity), aboveground symptoms did not appear in the plants despite growth and production variables being clearly altered by the fungus. When air temperatures and humidity were optimal for disease development (air temperatures relatively high and humidity rather low), leaf symptoms on inoculated plants became apparent but with secondary importance compared to decreases in growth and production. The pathogen also affected the root:aboveground biomass ratio to a greater extent when the plants were under good water conditions than under deficit irrigation. Under optimal conditions and with good soil water content, the infected crop may end its cycle without symptoms, with the disease undetected, although reductions in yield and aboveground biomass can occur.     

环境因子对草茎点霉SYAU-06菌株侵染鸭跖草的影响

以病叶率、病情指数和鲜重抑制率为评价指标,研究了环境条件包括温度、相对湿度、露水期和光照对草茎点霉SYAU-06菌株侵染鸭跖草的影响。结果表明,SYAU-06菌株侵染鸭跖草达到理想的除草效果,要求最适的温度范围是28～32℃,环境相对湿度保持在80%以上,接种后露水持续时间至少需要48h。保湿期的光照时间与病害发生程度成反比,光照时间越短,病害发生越严重。     

A dynamic model forecasting infection of pear leaves by conidia of <Emphasis Type="Italic">Venturia nashicola</Emphasis> and its evaluation in unsprayed orchards

A dynamic model, called VenInf, was developed to forecast infection of pear leaves by conidia of Venturia nashicola. By simulating conidial infection processes following a rain event, the model estimates % conidia that successfully infected leaves at the end of an infection period. The model is mainly derived from logistic models developed from recent laboratory and glasshouse experimental results on infection of pear seedlings to estimate the rates of infection and mortality. It simulates the conidial infection process at 5 min intervals using temperature, relative humidity (RH), surface wetness and rainfall as input. The model was evaluated against pear scab in four unsprayed orchards in China over a 4-year period. In all orchards, all significant disease increases were associated with infection periods predicted by the model. In one orchard, in 2004 the incidence of leaf infection remained very low (<3%) during the entire season despite the model forecasting several severe infection periods. Results of orchard evaluation suggest that the model is able to identify all important potential infection periods. Thus, further field studies should be carried out to determine whether and how the model can be used in practice to assist farmers in making decisions on fungicide applications.     

Influence of Light, Relative Humidity, and Maturity of Populations on Discharge of Ascospores of Venturia inaequalis

ABSTRACT Ascospore release in 20 populations of Venturia inaequalis was generally suppressed in wind tunnel tests during darkness and simulated rain, but the following relieved this suppression: (i) exposure to low relative humidity during simulated rain and (ii) protracted incubation of leaf samples and the consequent senescence of the pathogen population. No counterpart to (i) was observed under orchard conditions. Although V. inaequalis also released a high percentage of ascospores during darkness in field studies under simulated rain late in the season of ascospore release, this phenomenon has not been reported for natural rain events. A threshold value of 0.5 muW/cm(2) at 725 nm was identified as the minimum stimulatory light intensity. Ascospore release increased with increasing light intensity from 0.5 to 5.2 muW/cm(2) at 725 nm. There was also an intrinsic increase in ascospore release as duration of rain increased. In orchards, the combined impact of both processes is probably responsible for a delay in reaching peak ascospore release at several hours after sunrise. Ascospore release during darkness will generally constitute a small proportion of the total available supply of primary inoculum. Significant ascospore release, and therefore infection periods, can be assumed to begin shortly after sunrise, when rain begins at night in orchards with low potential ascospore dose (PAD). A PAD level of 1,000 ascospores per m(2) of orchard floor per season is suggested as a threshold, above which the night-released ascospores should not be ignored.     

Temperature and relative humidity shape white leaf spot (Neopseudocercosporella capsellae) epidemic development in rapeseed (Brassica napus)

Studies were undertaken to determine the combined environmental effects of two temperature regimes (14/11 ℃, 17/14 ℃ day/night) and duration of postinoculation high humidity on progression of white leaf spot, caused by Neopseudocercosporella capsellae, in rapeseed (Brassica napus) at each of two different plant growth stages. Overall, percentage disease indices were significantly affected by temperature, high humidity duration, plant growth stage, and host cultivar (all p < 0.001). There were significant two-way interactions of temperature regime with plant age (p < 0.001) and with host cultivar (p < 0.01), and significant three-way interactions of high humidity duration with growth stage and host cultivar (p = 0.01) and with temperature and host cultivar (p = 0.03). At cotyledon stage, mean percentage cotyledon disease index was 38.2 at 17/14 ℃ day/night, and 33.1 at 14/11 ℃. At fourth leaf stage, mean percentage leaf disease index was 32.0 at 14/11 ℃ but 17.5 at 17/14 ℃. Disease severity increased with increasing duration of high humidity. Results explain why this disease is more prevalent and severe in Australia following longer durations of high humidity; why disease at cotyledon stage is largely independent of temperature if seasonal temperature ranges between 11 and 17 ℃; and why more severe leaf infection (e.g., at fourth leaf stage) can be attributed to cooling winter conditions between autumn and winter. Studies suggest current/predicted climate changes across southern Australia of increasingly warmer autumn-winter temperatures and decreasing growing-season (May–August) precipitation will lessen severity of future epidemics.     

Leaf blotch severity on spring barley infected by isolates of Rhynchosporium secalis under different temperature and humidity regimes

The infection efficiency and severity of leaf blotch on spring barley inoculated with three pathotypes of Rhynchosporium secalis from central Norway were studied under different temperature and humidity regimes. Seedlings of the cultivar Arve were subjected to two constant temperatures, 13° or 18°C. Dry periods of 8 h or longer before or after a wet period of 4 h, carried out in the first 48 h postinoculation, reduced disease severity assessed 16 days after inoculation. The effect of dry periods of up to 24 h was nullified when plants were subjected to high humidity for 48 h after the dry treatment. The disease developed most rapidly when the wet period was 48 h and the temperature 18°C. At or near the optimum temperature for R. secalis (18°C), leaf wetness duration as short as 2 h resulted in considerable disease. Isolates reacted differently to temperature. The most aggressive isolate caused severe disease irrespective of temperature (56–70% of the leaf area infected); however, disease severity caused by the least aggressive isolate was significantly higher at the optimum temperature compared with a lower temperature (13°C). This information can facilitate evaluation of weather data in relation to predicting leaf blotch for advisory purposes.     

Pathogenicity of <Emphasis Type="Italic">Mycochaetophora gentianae</Emphasis>, causal fungus of gentian brown leaf spot,as affected by host species,inoculum density,temperature, leaf wetness duration,and leaf position

When the influence of host species, inoculum density, temperature, leaf wetness duration, and leaf position on the incidence of gentian brown leaf spot caused by Mycochaetophora gentianae, was examined, the fungus severely infected all seven Gentiana triflora cultivars, but failed to infect two cultivars of G. scabra and an interspecific hybrid cultivar. Inoculum density correlated closely with disease incidence, and a minimum of 10² conidia/mL was enough to cause infection. In an analysis of variance, temperature and leaf wetness duration had a significant effect upon disease incidence, which increased with higher temperature (15–25°C) and longer duration of leaf wetness (36–72 h). No disease developed at temperatures lower than 10°C or when leaf wetness lasted <24 h. At 48-h leaf wetness, disease incidence was 0, 28, 77, and 85% at 10, 15, 20, and 25°C, respectively. Middle and lower leaves on the plant were more susceptible than upper leaves. In microscopic observations of inoculated leaves, >50% of conidia germinated at temperatures >15°C after 24-h leaf wetness. More appressoria formed at higher temperatures (15–25°C) with extended duration of leaf wetness (24–72 h). At 48-h leaf wetness, appressorium formation was 0, 8, 26, and 73% at 10, 15, 20, and 25°C, respectively. These results suggest that temperature and leaf wetness duration were important factors for infection of gentian leaves.     

空气湿度对灰霉菌侵染番茄叶片组织结构分析及抗氧化系统的影响

空气湿度对番茄灰霉病的发生有显著性影响。为了探明空气湿度对灰霉菌侵染番茄叶片的过程与机理,本研究以‘金棚14-6’番茄为材料,观察分析了高空气相对湿度(80%～95%)和低空气相对湿度(65%～80%)对灰霉菌侵染番茄叶片表型变化、细胞学差异、形态结构变化、活性氧含量和抗氧化酶的影响。结果表明：高湿接种60 h大量芽管伸长出现在叶片下表皮细胞并分化菌丝,叶肉细胞间隙分布了大量的菌丝并伴随病斑出现,灰霉菌在60 h完成侵染;低湿接种108 h芽管伸长出现在叶片下表皮并分化菌丝,叶肉细胞间隙有少量菌丝分布,没有明显病斑出现。随着灰霉菌的侵染,低湿与高湿相比栅栏组织和海绵组织结构从整齐紧密变为排列疏松的时间滞后;高湿和低湿处理的叶片厚度、栅栏组织和海绵组织厚度均呈先上升后下降的变化趋势,侵染后期低湿处理的叶片组织结构厚度显著高于高湿。随着接菌时间延长,高湿和低湿的活性氧含量和抗氧化酶活性处于相对活跃的调整、适应的变化过程,大致呈先上升后下降趋势,而对照和接菌相比无显著差异,变化趋势维持在基本的振幅上。研究显示灰霉菌发生的环境条件：湿度80%～95%和侵染完成时间60 h,即控制高空气湿度的持...     

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.     

Coincidence of maximum severity of powdery mildew on grape leaves and the carbohydrate sink‐to‐source transition

Grapevine leaves infected with powdery mildew are a source of inoculum for fruit infection. Leaves emerging on a single primary shoot of Vitis vinifera cv. Cabernet Sauvignon were exposed to average glasshouse temperatures of 18°C (0·23 leaves emerging/day) or 25°C (0·54 leaves emerging/day). All leaves on 8–10 shoots with approximately 20 leaves each were inoculated with Erysiphe necator conidia to assess disease severity after 14 days in the 25°C glasshouse. Two photosynthetic ‘source’ leaves per shoot on the remaining 8–10 shoots were treated with ¹⁴CO₂ to identify, by autoradiography, the leaf position completing the carbohydrate sink‐to‐source transition. There was a clear association between the mean modal leaf position for maximum severity of powdery mildew (position 3·7 for 18°C; position 4·4 for 25°C) and the mean position of the leaf completing the sink‐to‐source transition (position 3·8 for 18°C; position 4·7 for 25°C). The mean modal leaf position for the maximum percentage of conidia germinating to form secondary hyphae was 4·2 for additional plants grown in the 25°C glasshouse. A higher rate of leaf emergence resulted in a greater proportion of diseased leaves per shoot. A Bayesian model, consisting of component models for disease severity and leaf ontogenic resistance, had parameters representing the rate and magnitude of pathogen colonization that differed for shoots developing in different preinoculation environments. The results support the hypothesis that the population of leaves in a vineyard capable of supporting substantial pathogen colonization will vary according to conditions for shoot development.