Viability and release of Neonectria ditissima ascospores on apple fruit in Brazil

During European canker monitoring in an apple experimental orchard, 14 mummified fruit (two and three trees with 10 and four positive records in 2018 and 2019, respectively) showed perithecia. Perithecium production on apple fruit, confirmation of pathogenicity of Neonectria ditissima isolated from mummified fruit, and ascospore release from fruit tissues has rarely been reported, and their role in the epidemiology of European canker has been largely overlooked. Thus, the objectives of our study were to (a) prove the presence of both conidia and ascospores of N. ditissima in mummified fruit in an experimental field, confirming pathogenesis in different apple cultivars, and (b) monitor production of the two types of inoculum in infected apple fruit over time. Canker incidence in this orchard was 47% of trees with symptoms in 2018 and 48% in 2019. Molecular and morphological tests confirmed that the fungus detected in the mummified apple fruit was N. ditissima. Apple fruit with sporodochia and perithecia washed immediately after collection from the orchard showed conidia but no ascospores of N. ditissima. However, after 4 days’ incubation, perithecia on mummified fruit showed many ascospore cirri. Koch's postulates were fulfilled on apple plants and mature fruit. Fruit inoculated with N. ditissima released spores for over a year under Brazilian field conditions. The release of both spore types peaked in May (Brazilian leaf fall) and October (spring); release of conidia also peaked in February (early harvest). These results support our hypothesis that fruit can serve as primary inoculum for European canker in Brazilian apple orchards.     

Comparative pathogenicity of sexual and asexual spores of Zymoseptoria tritici (septoria tritici blotch) on wheat leaves

Zymoseptoria tritici ascospores and pycnidiospores are considered the main forms of primary and secondary inoculum, respectively, in septoria tritici blotch epidemics. The pathogenicity of the two types of spores of the same genotypic origin were compared through a two‐stage inoculation procedure in controlled conditions. Adult wheat leaves were inoculated with ascospores collected from field sources, yielding 119 lesions; pycnidiospores collected from 12 lesions resulting from these ascospore infections were then used for inoculation. Lesion development was assessed for 5 weeks; latent period, lesion size, and pycnidium density were estimated for different isolates. The latent period was calculated as the maximum likely time elapsed between inoculation and either the appearance of the majority of the sporulating lesions (leaf scale) or the appearance of the first pycnidia (lesion scale). The latent period was significantly longer (c. 60 degree‐days, i.e. 3–4 days) after infection with ascospores than with pycnidiospores. No difference was established for lesion size and density of pycnidia. A comparison with other ascomycete fungi suggested that the difference in latent period might be related to the volume of spores and their ability to cause infection. Fungal growth before the appearance of lesions may be slower after inoculation with an ascospore than with a pycnidiospore. The mean latent period during the very beginning of epidemics, when first lesions are mainly caused by ascospores, may be longer than during spring, when secondary infections are caused by pycnidiospores. Disease models would be improved if these differences were considered.     

Role of Cylindrosporium-type conidia of Mycosphaerella pomi (Pass.) Lindau: cause of Brooks fruit spot of apple, as an infection source in apple orchards

Ascospores of Mycosphaerella pomi, the pathogen of Brooks fruit spot of apple, were produced in pseudothecia on previously infected and overwintered apple leaves from late April through early August in Aomori Prefecture, Japan. In June 2003, the ascospores were germinating and producing Cylindrosporium-type conidia on apple fruit and leaf surfaces in an orchard. After ascospores were sprayed on apple leaves, Cylindrosporium-type conidia developed on the leaf surfaces. Such Cylindrosporium-type conidia caused typical symptoms of Brooks fruit spot on apple trees after inoculations. These results suggested that the Cylindrosporium-type conidia also serve as an infection source, in addition to the ascospores, for Brooks fruit spot in apple orchards.     

A dynamic model simulating infection of apple leaves by Venturia inaequalis

A new dynamic model of the infection of apple leaves by Venturia inaequalis is described. The model begins with the release of spores by rain and incorporates the effect of light on the discharge of ascospores from pseudothecia. The model then simulates infection through the sub-processes of germination, appressorium formation and penetration, separately for ascospores and conidia landed concurrently on wet leaves. The rate of the infection process is determined using different equations for ascospores and conidia. Spore mortality when leaves dry is determined by the stage of infection and RH in the dry period. The infection process is driven by surface wetness, temperature and RH. The progress of each infection period is measured as infection efficiency (IE), namely the percentage of landed spores which have penetrated and thereby infected leaves. The final IE quantifies the favourability of weather in each infection period. In orchard tests in each of three years, the new model detected crucial infection periods in spring and early summer which accounted for outbreaks of leaf scab. These periods were not detected by a static model based on Mills'criteria. The models performed similarly in detecting infection periods later in summer.     

The widespread occurrence of overwintered conidial inoculum of <Emphasis Type="Italic">Venturia inaequalis</Emphasis> on shoots and buds in organic and integrated apple orchards across the Netherlands

A 2-year study was conducted to determine the widespread occurrence of overwintered conidial inoculum of Venturia inaequalis and its impact on the apple scab control in 18 apple orchards (organic and integrated) with various levels of scab in the Netherlands. Autumn assessments of scab lesions showed that the integrated orchards had a significantly lower scab incidence (<20%) compared to that of the organic orchards (>60%). At the bud-break phenological stage, the mean numbers of nonviable and viable conidia on 1 cm pieces of shoots ranged from 1 to about 90 and from 6 to more than 1000 in the integrated and the organic orchards, respectively, for both years. However, viable conidia on shoots were found only in 2 integrated and 6 organic orchards out of the 18 and the viability of conidia was below 2%. The mean numbers of viable and nonviable conidia per 100 buds ranged from 24 to more than 1000 and from 230 to almost 5000 in the integrated and the organic orchards, respectively, for both years. In both years, some 60–85% of the conidia was found on the outer bud scales. The percentage viability associated with the outer bud tissues was below 2% for all the orchards. However, the percentage of viable conidia within the inner bud tissues ranged from 0% to 6% in the integrated and from 2% to 11% in the organic orchards for both years. Differences between the organic and the integrated orchards were clearly demonstrated for overwintered conidia associated with both shoot and bud samples. The relationship between autumn scab incidence and numbers of overwintered conidia associated with shoots or buds was exponential. If the autumn scab incidence was above 40%, then the number of overwintered conidia markedly increased. We conclude that specific treatments for overwintering conidia of Venturia inaequalis may not be necessary in integrated orchards with a low scab incidence in the previous autumn. However, the risk of early scab epidemics initiated by overwintered conidia potentially is high in organic orchards. Preventative measures in early spring and also in the previous year must be established in these orchards.     

苹果黑星病在辽宁省的分布及其发生规律

苹果黑星病是辽宁省植物检疫对象。目前主要分布于小苹果栽植区及大、小苹果混栽区,已接近辽宁南部的大苹果主产区。经自然感病和人工接种证明,大、小苹果上的黑星病菌可以交互侵染,目前一些主栽的大苹果品种均能感染此病。辽宁地区苹果黑星病的初次侵染来源是落地越冬病叶上于翌春产生的子囊孢子。分生孢子不能越冬成活。苹果枝条及芽鳞不带菌。黑星病菌的两种孢子随气流传播,使病害逐步扩大蔓延。经过6年的观察,苹果黑星病发生始期早晚及发病轻重,与早春和夏季的降水量多少成正相关。田间药剂防治试验结果,以托布津、乙磷铝、多菌灵、特克多等药剂防治效果良好。     

渭北旱塬苹果黑星病的初侵染来源

 苹果黑星病是世界各苹果产区的重要病害之一,也是我国渭北旱塬苹果产区的新病害。近5年调查研究证明,渭北旱塬苹果落叶上分生孢子数量随时间推移呈急剧下降趋势,在春季苹果芽鳞萌动之前,分生孢子呈无色透明薄片状,均已失去存活力,芽鳞内外检测到分生孢子的数量很少,且均已丧失萌发力。果园中孢子捕捉结果表明,子囊孢子出现在先,分生孢子在后,两者出现时间相差15d以上,且捕捉到分生孢子的日期是在黑星病发病以后。对洛川县、白水县、长武县、永寿县、泾川县和平凉市等地苹果黑星病普查中未发现发病枝条,以菌丝体在枝条上越冬的可能性极小。综合以上试验结果,初步确定子囊孢子是渭北旱塬苹果黑星病的初侵来染,分生孢子不能越冬。     

Differentiation in populations of the apple scab fungus Venturia inaequalis on cultivars in a mixed orchard remain over time

The ascomycete Venturia inaequalis causes annual epidemics of apple scab worldwide. Scab development is reduced in mixed cultivar orchards compared with monocultures. In order to use mixtures in commercial production, how the population of scab changes in a mixed orchard needs to be understood, together with how likely a super race, with virulence factors overcoming multiple resistance factors in the mixed orchard, is to emerge and become dominant. This study used simple sequence repeat (SSR) markers to investigate the temporal change of scab populations in two mixed cultivar orchards in the UK to infer the likelihood of emergence of a scab super race. There were no significant differences between the populations at the two sampling times (6 or 7 years apart) in either of the two mixed orchards. In one of the orchards, apple scab populations on different cultivars were significantly different and the differences did not diminish over time. These results suggest that it is not inevitable that a super race of V. inaequalis will become dominant during the lifetime of a commercial apple orchard.     

A model simulating deposition of Venturia inaequalis ascospores on apple trees*

Based on existing physical theories and models, a dynamic model estimating the concentration of Venturia inaequalis ascospores in the orchard air and their deposition on apple leaves was elaborated. The model produces two main outputs: number of ascospores deposited per leaf and proportion of ascospores discharged from pseudothecia deposited onto the leaves. The model has a relatively simple structure, and computations are based on few algorithms, which are implemented on an electronic data sheet of common use. Nevertheless, it preserves the accuracy of more complex physical models reasonably well. The model includes the effect of meteorological conditions and horticultural characteristics, and thus provides information for each type of orchard. Few input variables are required: wind speed and rainfall rate can be measured in standard meteorological stations; horticultural characteristics of the orchard can be determined for each type of orchard. The model produces conservative estimates of ascospore deposition, because it assumes a complete retention of the spores deposited by rainfall and does not consider either deposition on stems and flowers or the spatial distribution of plant surfaces. After further validation under orchard conditions, the model will be used to obtain better estimates of scab infection risk in current scab control strategies.     

Moisture Sources in Relation to Conidial Dissemination and Infection by Cladosporium carpophilum Within Peach Canopies

ABSTRACT Cladosporium carpophilum, the causal agent of peach scab, overwinters in lesions on 1-year-old twigs, from which conidia infect the developing fruit during spring and early summer. Twig lesions constitute the sole source of initial inoculum; therefore, the mode of dissemination of conidia from such lesions to the fruit is of considerable interest. In a 4-year study, we determined the relative importance of air- versus water-borne conidia and their interaction with different fruit wetness sources (splash, twig runoff, and dew) in a peach orchard with areas that had been treated or not treated with fungicide the previous year. The rareness of scab twig lesions in the previously sprayed trees implied that fruit infection in these trees would occur primarily by airborne conidia from unsprayed trees nearby (located within the same tree row or the adjacent row). In the unsprayed areas, additional infections could occur by short-distance waterborne dissemination of conidia from locally abundant twig lesions via splashing or runoff. Beginning at calyx fall, individual fruit were protected from splash by rain shields, protected from runoff by cotton wicks placed proximal to the peduncle, or left untreated. Rain shields were adjustable, allowing rain or dew to be excluded selectively. Various combinations of the shield and wick treatments were implemented in the previously sprayed and unsprayed areas, and statistical comparison of fruit scab severity between individual treatments by linear contrasts allowed us to untangle the relative contributions of the various sources of inoculum and fruit wetness. Results showed that aerial dissemination of conidia contributed little to fruit scab development, even in the presence of fruit surface wetness caused by splashing, runoff, or dew. In contrast, waterborne conidia contributed considerably and significantly (P < 0.0001) to disease development. This was due primarily to the importance of splash in disseminating conidia from twig lesions to the fruit, given that exclusion of splashing via rain shields decreased disease severity by >90%. Runoff water from the twig to the fruit via the peduncle also contributed to scab development, as evidenced by the fact that exclusion of runoff by cotton wicks reduced disease severity by 31.6 to 44.9%; however, this effect was not always statistically significant. The exclusion of dew did not reduce scab severity (P > 0.4), suggesting that it played a limited role in infection in the presence of other fruit wetness sources.     

Population variation of apple scab (Venturia inaequalis) within mixed orchards in the UK

Apple scab, caused by Venturia inaequalis, is one of the most important apple diseases worldwide. To investigate between- and within-orchard fungal variability, 212 isolates were sampled from two mixed orchards, one of 10?years of age and the other of 45?years of age, in the UK and genotyped with AFLP and SSR markers. Populations of isolates from the two orchards did not differ significantly in terms of allele frequencies at the screened AFLP and SSR loci. However, groups of isolates from individual cultivars differed significantly within each orchard and there were also significant differences between groups of isolates from individual trees of the same cultivar in the same orchard. These differences were less pronounced in the younger mixed orchard than in the older one. The existence of tree-to-tree fungal variability indicates a possible role for conidia as a source of primary inoculum. Non-random mating may be one of the factors causing the significant differences among fungal populations from different cultivars. These results suggest that apparently ??susceptible?? cultivars have different background genetic resistance factors, which can be exploited for disease management in mixtures.     

Sources and seasonal dynamics of inoculum for brown spot disease of pear

The dynamics of the production of Stemphylium vesicarium conidia and Pleospora allii ascospores from different inoculum sources on the ground were compared in a model system of a wildflower meadow mainly composed of yellow foxtail, creeping cinquefoil and white clover. The meadow was either inoculated (each October) or not inoculated with a virulent strain of S. vesicarium, and either covered or not covered with a litter of inoculated pear leaves. Spore traps positioned a few centimetres above the ground were exposed for 170 7-day periods between October 2003 and December 2006. Ascospores and conidia were trapped in 46 and 25% of samples, respectively. Ascospore numbers trapped from the pear leaf litter were about five times higher than those from the meadow, while conidial numbers were similar from the different inoculum sources. The ascosporic season was very long, with two main trapping periods: December–April, and August–October; the former was most important for the leaf litter, the latter for the meadow. The conidial season lasted from April to November, with 92% of conidia caught between July and September. The fungus persistently colonized the meadow: the meadow inoculated in early October 2003 produced spores until autumn 2006. The present work demonstrates that orchard ground is an important source of inoculum for brown spot of pear. Thus, it is important to reduce inoculum by managing the orchard ground all year long.     

Effect of Timing of Application of the Biological Control Agent Microsphaeropsis ochracea on the Production and Ejection Pattern of Ascospores by Venturia inaequalis

ABSTRACT Field and in vitro trials were conducted to establish the influence of the biological control agent Microsphaeropsis ochracea on the ejection pattern of ascospores by Venturia inaequalis and on apple scab development, and to establish the best timing of application. The ejection pattern of ascospores was similar on leaves sprayed with M. ochracea and on untreated leaves. Fall application of M. ochracea combined with a delayed-fungicide program was evaluated in orchards with intermediate and high scab risk. For both orchards, it was possible to delay the first three and two infection periods in 1998 and 1999, respectively, without causing significant increase or unacceptable leaf and fruit scab incidence. To evaluate the best timing of application, sterile leaf disks were inoculated with V. inaequalis and then with M. ochracea 0, 2, 4, 6, 8, 10, 12, 14, and 16 weeks later. After incubation under optimal conditions for pseudothecia development, the number of ascospores was counted. Similarly, M. ochracea was sprayed on scabbed leaves on seven occasions from August to November 1999 and 2000. Leaves were overwintered on the orchard floor and ascospore production was evaluated the following spring. Ascospore production was reduced by 97 to 100% on leaf disks inoculated with M. ochracea less than 6 weeks after inoculation with V. inaequalis, but ascospore production increased with increasing period of time when M. ochracea was applied 8 to 16 weeks after the inoculation with V. inaequalis. In the orchard, the greatest reduction in production of ascospores (94 to 96% in 2000 and 99% in 2001) occurred on leaves sprayed with M. ochracea in August. The production of ascospores was reduced by 61 to 84% in 2000 and 93% in 2001 on leaves sprayed with M. ochracea in September, reduced by 64 to 86% in 2000 and 74 to 89% in 2001 on leaves sprayed in October, and reduced by 54 and 67% in 2000 and 2001, respectively, on leaves sprayed in November. It was concluded that M. ochracea should be applied in August or September and that ascospore maturation models and delayed-fungicide program could be used in orchards treated with this biological control agent.     

Spatial and temporal increase of eastern filbert blight in European hazelnut orchards in the pacific northwest

ABSTRACT Since its first detection in southwest Washington state 30 years ago, eastern filbert blight, caused by Anisogramma anomala, has spread slowly southward ( approximately 2 km/year) into the Willamette Valley of Oregon, an important hazelnut production region. Experiments were conducted to measure gradients of disease spread, rates of disease increase as affected by distance from an inoculum source and variation in host plant resistance, and dispersal of ascospores of A. anomala from diseased orchards. In each of 3 years, 1-year-old hazelnut trees placed from 0 to 150 m north of diseased orchards were infected uniformly and slopes of disease gradients were not significantly different from zero. In 1 year when trees also were placed south of an orchard, the disease gradient was significant (P < 0.05), with disease incidence high at the edge of the orchard and few trees infected at 10 m south of the orchard. Disease gradients were shallower and the magnitude of the area under the disease progress curve (AUDPC) greater in 0.1-ha mini-orchards of highly susceptible cv. Ennis than in mini-orchards of moderately susceptible cvs. Barcelona or Casina. Lower AUDPC values were observed in mini-orchards of Barcelona interplanted with a moderately resistant pollenizer Hall's Giant compared with the highly susceptible pollenizer Daviana. Fungicides applied biweekly starting at bud break reduced AUDPC values in Ennis mini-orchards to values observed in Barcelona and Casina mini-orchards. Data from aspirated spore samplers placed on towers adjacent to severely diseased hazelnut orchards indicated that spores of A. anomala dispersed horizontally and vertically away from the canopy during periods of extended branch wetness and, thus, show potential to be transported long distances in wind currents. Weather patterns in the Pacific Northwest may account for the relatively slow, southward spread of eastern filbert blight within Oregon's Willamette Valley. Of 196 precipitation events greater than 10 h in duration recorded from 1974 to 1995, conditions most favorable for ascospores discharge, periods with wind from the north were rare, representing <6% of total hours.     

Selection and orchard testing of antagonists suppressing conidial production by the apple scab pathogen <Emphasis Type="Italic">Venturia inaequalis</Emphasis>

Apple scab caused by Venturia inaequalis is a major disease in apple production. Epidemics in spring are initiated by ascospores produced on overwintering leaves whereas epidemics during summer are driven by conidia produced on apple leaves by biotrophic mycelium. Fungal colonisers of sporulating colonies of V. inaequalis were isolated and their potential to reduce the production of conidia of V. inaequalis was evaluated on apple seedlings under controlled conditions. The four most effective isolates of the 63 screened isolates were tested subsequently under Dutch orchard conditions in 2006. Repeated applications of conidial suspensions of Cladosporium cladosporioides H39 resulted in an average reduction of conidial production by V. inaequalis of approximately 40%. In 2007, applications of conidial suspensions of C. cladosporioides H39 reduced conidial production by V. inaequalis by 69% on August 6 and by 51% on August 16, but no effect was found on August 20. However, viability of available conidia of C. cladosporioides H39 was low at the end of the experiment. Epiphytic and endophytic colonisation by Cladosporium spp. of leaves treated during the experiment with C. cladosporioides H39 was significantly higher than on control leaves sampled 6 weeks after the last application. It is concluded that C. cladosporioides H39 has promising potential as a biological control agent for apple scab control. More information is needed on the effect of C. cladosporioides H39 on apple scab epidemics as well as on mass production, formulation and shelf life of conidia of the antagonist.     

A method to monitor airborne Venturia inaequalis ascospores using volumetric spore traps and quantitative PCR

Apple scab caused by the fungus Venturia inaequalis can result in significant crop losses if not managed effectively. Sanitation as part of an integrated management strategy aims to significantly reduce primary inoculum to lower the disease pressure. This study evaluates the possibility of molecular detection and quantification of ascospore discharge and the use of this method to test for efficacy of orchard sanitation treatments. A method to detect and quantify airborne ascospores was developed using volumetric spore traps (VSTs). V. inaequalis specific primers were tested on daily VST samples from two orchard sections (leaf litter removed compared to leaf litter left) during spring. A molecular method to detect and quantify ascospores was tested by amplifying genomic regions of the mitochondrial CYP51A1 gene, and the ITS region using SYBR® green. Timing of ascospore discharge was compared to predicted infection risk and a degree day model using weather data. The average spore detection limit was estimated to be at levels of 1 pg μl^?1 DNA (approximately 37 ascospores) per daily spore trap reading using CYP51A1 primers. Using the CYP51A1 primer pair, primary inoculum was estimated to be 51 % lower in the orchard sections where leaves had been removed, indicating that this method could be used to evaluate the efficacy of alternative control strategies such as leaf removal to reduce potential ascospore dose. This is the first report of combining VSTs and quantitative PCR to monitor airborne V. inaequalis ascospores.     

Heterogeneity of the aerial concentration and deposition of ascospores of <Emphasis Type="Italic">Venturia inaequalis</Emphasis> within a tree canopy during the rain

Scab is an important disease of apple and its control depends almost exclusively on frequent use of fungicides. Primary scab infection in the spring assumes several steps: ascospore maturation, liberation of ascospores that become airborne, deposition on susceptible tissues, and infection. However, the spatial heterogeneity of ascospores within the tree canopy is unknown. Aerial concentration of ascospore (ACA), ascospore concentration in rain water (ACR) and ascospore deposition (AD) were therefore measured at six heights (20–257 cm from the ground) with rotating-arm air samplers, funnels, and greased glass slides, respectively, during five rain events in 2001 and in 2002. In addition, ACR and AD were measured at eight locations within tree canopy at 196 cm height. Apple scab was assessed at the end of the primary infection period in each sampling location within the apple tree. A similar experimental design was used in 2003 to study the spatial heterogeneity of both AD and primary scab lesions. ACA and AD decreased with increasing height, while ACR increased with increasing height. Based on both variance to mean ratio and the power law relationship in both years, the ACR was heterogeneous, while AD was heterogeneous only during the peaks of ascospore release. The ACR was significantly higher at the centre of the trees and the AD was significantly higher at the centre and at the western edge of the trees. Only the cumulative AD was significantly correlated with apple scab lesions at the same location (r = 0.83). In 2003, a similar pattern of spatial heterogeneity within the tree canopy was observed for AD and primary scab lesion counts and there was a linear relationship (R ² = 0.84) between these two variables. It was concluded that ACR and AD within the tree canopy are not randomly distributed at least during peaks of ascospore release and that AD is a good estimate of primary scab lesion development. This spatial heterogeneity should be considered when estimating ascospore deposition using mathematical models or when quantifying ascosporic inoculum using spore samplers.     

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.     

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.