Variation and Heritability of Phenology in the Fungus Monilinia vaccinii-corymbosi on Blueberry

ABSTRACT The germination of field-collected pseudosclerotia and the development of apothecia from eight New Jersey populations of the mummy berry fungus Monilinia vaccinii-corymbosi were evaluated under controlled conditions in the greenhouse. Development data for apothecia were used to describe the timing of apothecium formation and to estimate broad- and narrow-sense heritabilities of fungal phenology. Mean development times for the formation of apothecia ranged from 35.4 to 54.7 days. The mean development times for populations collected from early-season cv. Weymouth ranged from 35.4 to 39.6 days and were significantly shorter than the development times for three of the four populations collected from late-season cv. Jersey (46.9 to 54.7 days) or for the population collected from mixed stands of cultivated blueberries (42.7 days). The development of populations from late cultivars planted in very close proximity to early cv. Weymouth was early (36.5 to 39.0 days) and not significantly different from the development of populations collected from cv. Weymouth. Phenotypic and genetic variances of apothecium development for individual populations ranged from 18.9 to 44.8 and 7.2 to 30.9, respectively. Broad-sense heritabilities of apothecia development for each fungal population, calculated by partitioning phenotypic variation into genetic and environmental components, ranged from 0.31 to 0.78. Narrow-sense heritabilities of apothecia development, based on parent-offspring regression, ranged from 0.58 to 0.78. These results indicate that populations of M. vaccinii-corymbosi differ in phenology and that a significant portion of the phenological variation within populations is genetic. Thus, it is plausible to propose that the phenology of apothecium development is a component of fungal fitness and that host phenology can influence the timing of pathogen development.     

Phenology of Apothecium Production in Populations of Monilinia vaccinii-corymbosi from Early- and Late-Maturing Blueberry Cultivars

ABSTRACT Pseudosclerotia were evaluated for differences in timing of apothecium development in four controlled experiments conducted over a 2-year period. In a separate experiment, conidia from 10 randomly selected isolates from both of the fungal populations were used to inoculate open flowers. Germination of pseudosclerotia produced from these artificial inoculations also was evaluated. The timing and rate of shoot elongation for cvs. Weymouth and Jersey were assessed in one greenhouse and two field experiments. Average development times for the fungal population from cv. Weymouth were 8 to 15 days earlier or 33 to 42% less than those for the population from cv. Jersey. The fungal population from Weymouth also exhibited less variation in development times for each developmental stage measured. Similarly, germination of pseudosclerotia produced in artificial inoculations differed between populations. On average, pseudosclerotia derived from the Weymouth population produced apothecia 16 days earlier. During spring 1995 and 1996, vegetative and truss buds on cv. Weymouth developed 4 to 16 days earlier than those on cv. Jersey. These results demonstrate that M. vaccinii-corymbosi exhibits variation in timing of pseudosclerotia germination and apothecium development within and between populations. We hypothesize that differences observed in the timing of apothecium development are related to the fitness of the populations on their original host cultivars and were selected by host phenology.     

Effects of soil temperature, moisture, and burial depths on carpogenic germination of Sclerotinia sclerotiorum and S. minor

Extensive studies have been conducted on the carpogenic germination of Sclerotinia sclerotiorum, but carpogenic germination in S. minor has not been studied adequately. It remains unclear why apothecia of this pathogen have seldom been observed in nature. In this study, a new method was developed to produce apothecia in the absence of soil or sand, and carpogenic germination without preconditioning was recorded for 95 of the 96 S. sclerotiorum isolates tested. Carpogenic germination of the two species was compared under a variety of temperature, soil moisture, burial depths, and short periods of high temperature and low soil moisture. The optimal temperatures for rapid germination and for maximum germination rates were both lower for S. minor than for S. sclerotiorum. The temperature range for carpogenic germination was also narrower for S. minor than for S. sclerotiorum. A 5-day period at 30 degrees C, either starting on the 10th or 20th day of incubation, did not significantly affect carpogenic germination of S. sclerotiorum. For both S. minor and S. sclerotiorum, the percentage of carpogenically germinated sclerotia increased as soil water potential increased from -0.3 to -0.01 MPa. In the greenhouse, a 10- or 20-day dry period completely arrested carpogenic germination of S. sclerotiorum, and new apothecia appeared after an interval of 35 days following rewetting, similar to the initial carpogenic germination regardless of when the dry period was imposed. In naturally infested fields, the number of sclerotia in 100 cc of soil decreased as depth increased from 0 to 10 cm before tillage, but became uniform between 0 and 10 cm after conventional tillage for both species. Most apothecia of S. minor were, however, produced from sclerotia located at a depth shallower than 0.5 cm while some apothecia of S. sclerotiorum were produced from sclerotia located as deep as 4 to 5 cm. These results provide the much needed information to assess the epidemiological roles of inoculum from sexual reproduction in diseases caused by the two Sclerotinia species in different geographical regions. However, more studies on effects of shorter and incompletely dry periods are still needed to predict production of apothecia of S. sclerotiorum in commercial fields under fluctuating soil temperature and moisture.     

Forecasting sclerotinia disease on lettuce: toward developing a prediction model for carpogenic germination of sclerotia

ABSTRACT The feasibility of developing a forecasting system for carpogenic germination of Sclerotinia sclerotiorum sclerotia was investigated in the laboratory by determining key relationships among temperature, soil water potential, and carpogenic germination for sclerotia of two S. sclerotiorum isolates. Germination of multiple burials of sclerotia to produce apothecia also was assessed in the field with concurrent recording of environmental data to examine patterns of germination under different fluctuating conditions. Carpogenic germination of sclerotia occurred between 5 and 25 degrees C but only for soil water potentials of >/=-100 kPa for both S. sclerotiorum isolates. Little or no germination occurred at 26 or 29 degrees C. At optimum temperatures of 15 to 20 degrees C, sclerotia buried in soil and placed in illuminated growth cabinets produced stipes after 20 to 27 days and apothecia after 27 to 34 days. Temperature, therefore, had a significant effect on both the rate of germination of sclerotia and the final number germinated. Rate of germination was correlated positively with temperature and final number of sclerotia germinated was related to temperature according to a probit model. Thermal time analysis of field data with constraints for temperature and water potential showed that the mean degree days to 10% germination of sclerotia in 2000 and 2001 was 285 and 279, respecttively, and generally was a good predictor of the observed appearance of apothecia. Neither thermal time nor relationships established in the laboratory could account for a decline in final percentage of germination for sclerotia buried from mid-May compared with earlier burials. Exposure to high temperatures may explain this effect. This, and other factors, require investigation before relationships derived in the laboratory or thermal time can be incorporated into a forecasting system for carpogenic germination.     

Forecasting Sclerotinia Disease on Lettuce: A Predictive Model for Carpogenic Germination of Sclerotinia sclerotiorum Sclerotia

ABSTRACT A predictive model for production of apothecia by carpogenic germination of sclerotia is presented for Sclerotinia sclerotiorum. The model is based on the assumption that a conditioning phase must be completed before a subsequent germination phase can occur. Experiments involving transfer of sclerotia from one temperature regime to another allowed temperature-dependent rates to be derived for conditioning and germination for two S. sclerotiorum isolates. Although the response of each isolate to temperature was slightly different, sclerotia were fully conditioned after 2 to 6 days at 5 degrees C in soil but took up to 80 days at 15 degrees C. Subsequent germination took more than 200 days at 5 degrees C and 33 to 52 days at 20 degrees C. Upper temperature thresholds for conditioning and germination were 20 and 25 degrees C, respectively. A predictive model for production of apothecia derived from these data was successful in simulating the germination of multiple burials of sclerotia in the field when a soil water potential threshold of between -4.0 and -12.25 kilopascals (kPa) was imposed. The use of a germination model as part of a disease forecasting system for Sclerotinia disease in lettuce is discussed.     

Distribution of Pseudosclerotia of Monilinia vaccinii-corymbosi and Risk of Apothecial Emergence Following Mechanical Cultivation

ABSTRACT Pseudosclerotia (infected, mummified fruit) of Monilinia vacciniicorymbosi overwinter on the orchard floor and germinate to produce apothecia in early spring, providing the only source of primary inoculum for mummy berry disease of blueberry. Three experiments were carried out to develop a model for the relative efficacy of mechanical cultivation in reducing the risk associated with primary inoculum. In the first experiment, apothecial emergence from pseudosclerotia buried 0, 1.5, 3, 6, and 10 cm below the soil surface was monitored to determine the critical depth necessary to inhibit emergence. No apothecia emerged from pseudosclerotia buried at depths of >/=3 cm, and the critical depth of burial was determined at 2.6 cm by regression analysis. In the second experiment, pseudosclerotia or plastic beads (used as surrogates for pseudosclerotia) were placed on the soil surface of experimental plots before cultivation with an in-row rotary cultivator, a disc harrow, or a rotary cultivator with each implement operated in a single pass. Vertical distribution profiles of pseudosclerotia or beads in the topsoil were characterized after excavation with a custom-built sampling device. The proportion of pseudosclerotia placed below the critical depth of 2.6 cm was 20.9, 52.6, and 78.6% for the in-row rotary cultivator, the disc harrow, and the rotary cultivator, respectively. For all three implements, vertical distribution profiles of pseudosclerotia and plastic beads were very similar, allowing the latter to be used in subsequent experiments in commercial fields. In the third experiment, two blueberry plantings were surveyed to determine the horizontal distribution of pseudosclerotia on the orchard floor with distance from the crowns of the plants. The greatest frequency of pseudosclerotia occurred between 30 and 40 cm from the plants. Based on measurements of the distance from plants within which different implements can operate, the proportion of pseudosclerotia accessible by cultivation ranged from 58.7% for the disc harrow to 87.2% for the in-row rotary cultivator. Taken together, results from the three experiments indicated that cultivation with a single implement can reduce risk of apothecial emergence by about 50%. More effective risk reductions may be obtained by combining implements that result in deep burial of pseudosclerotia with those that have access to pseudosclerotia near the plants. This was demonstrated by a commercial cultivation method that utilized three passes of different implements and resulted in extensive reshaping of plant beds, placing 88.2% of beads below the critical depth of 2.6 cm.     

Germination of Sclerotinia minor and S. sclerotiorum Sclerotia Under Various Soil Moisture and Temperature Combinations

ABSTRACT Sclerotial germination of three isolates each of Sclerotinia minor and S. sclerotiorum was compared under various soil moisture and temperature combinations in soils from Huron and Salinas, CA. Sclerotia from each isolate in soil disks equilibrated at 0, -0.03, -0.07, -0.1, -0.15, and -0.3 MPa were transferred into petri plates and incubated at 5, 10, 15, 20, 25, and 30 degrees C. Types and levels of germination in the two species were recorded. Petri plates in which apothecia were observed were transferred into a growth chamber at 15 degrees C with a 12-h light-dark regime. All retrievable sclerotia were recovered 3 months later and tested for viability. Soil type did not affect either the type or level of germination of sclerotia. Mycelial germination was the predominant mode in sclerotia of S. minor, and it occurred between -0.03 and -0.3 MPa and 5 and 25 degrees C, with an optimum at -0.1 MPa and 15 degrees C. No germination occurred at 30 degrees C or 0 MPa. Soil temperature, moisture, or soil type did not affect the viability of sclerotia of either species. Carpogenic germination of S. sclerotiorum sclerotia, measured as the number of sclerotia producing stipes and apothecia, was the predominant mode that was affected significantly by soil moisture and temperature. Myceliogenic germination in this species under the experimental conditions was infrequent. The optimum conditions for carpogenic germination were 15 degrees C and -0.03 or -0.07 MPa. To study the effect of sclerotial size on carpogenic germination in both S. minor and S. sclerotiorum, sclerotia of three distinct size classes for each species were placed in soil disks equilibrated at -0.03 MPa and incubated at 15 degrees C. After 6 weeks, number of stipes and apothecia produced by sclerotia were counted. Solitary S. minor sclerotia did not form apothecia, but aggregates of attached sclerotia readily formed apothecia. The number of stipes produced by both S. minor and S. sclerotiorum was highly correlated with sclerotial size. These results suggest there is a threshold of sclerotial size below which apothecia are not produced, and explains, in part, why production of apothecia in S. minor seldom occurs in nature.     

Effects of soil moisture, organic matter and grass mulching on the carpogenic germination of sclerotia and infection of bean by Sclerotinia sclerotiorum

Microplot experiments were carried out during the autumn and winter of 1995 to determine the effects of soil moisture and levels of organic matter on the carpogenic germination of Sclerotinia sclerotiorum and subsequent infection of Phaseolus vulgaris . Soil moisture treatments were established by irrigation to field capacity: (a) when soil had dried to − 25 KPa, (b) when soil had dried to − 54 KPa, and (c) eight days after the soil had dried to − 54 KPa. Three levels of organic matter were established by mixing compost and yellow-red latosol in the following proportions (by volume): (a) 0% compost and 100% soil, (b) 50% compost and 50% soil, and (c) 100% compost and 0% soil. More apothecia and diseased plants were observed in the plots with higher moisture levels. Drier treatments resulted in a reduction in number (in the winter experiment) or elimination (in the autumn experiment) of apothecia. Other experiments were designed to examine the effect of grass mulching and organic matter levels on the carpogenic germination of S. sclerotiorum . In one experiment, grass mulching was compared with no mulching, and in another, the effects of different depths of mulch on the soil surface (0, 1.5, 3, 6 and 9 cm) were examined. Fewer apothecia were formed in mulched soil and the effect was most pronounced with 6 and 9 cm of mulch. The highest level of mulch (9 cm) impaired the vigour and yield of bean plants. In these experiments, carpogenic germination was increased by high soil organic matter content. Control of soil moisture and grass mulching may reduce the production of apothecia of S. sclerotiorum even in soils rich in organic matter.     

Effects of cultivation, conditioning and isolate on sclerotium germination in Sclerotium cepivorum

Sclerotium germination in various isolates of S. cepivorum was studied following different cultivation and conditioning treatments. A simple and rapid laboratory test was developed to trigger sclerotial germination under unsterile conditions. In most isolates, sclerotia produced under sterile and unsterile conditions showed a constitutive dormancy immediately after maturing. In contrast to this, the sclerotia of some isolates germinated quite well as soon as they were mature. The dormancy of sclerotia from axenic cultures broke down after storage in soil for 12 weeks. In a few cases sclerotia produced under unsterile conditions germinated remarkably well. Freezing and thawing of sclerotia decreased the ability of most isolates to germinate.     

Ecological aspects of seed dormancy-break and germination in Heteranthera limosa (Pontederiaceae), a summer annual weed of rice fields

Summary Heteranthera limosa seeds were buried in flooded and in non-flooded soil and exposed to natural seasonal temperature changes in Lexington, Kentucky, USA. Seeds exhumed after various periods of burial ranging from 2 to 36 months were tested for germination under both flooded and non-flooded conditions. Seeds were dormant at maturity in September and became non-dormant during winter. Seeds buried in non-flooded soil during winter germinated to higher percentages and over a wider range of temperatures when tested under flooded conditions (in light) during spring and summer, than did those buried in flooded soil during winter. Thus, the water regime associated with rice culture (non-flooded in winter and flooded in summer) is optimal for dormancy-break and germination of H. limosa seeds. A portion of the buried seeds exhibited an annual dormancy/non-dormancy cycle, whereas others had a conditional dormancy/non-dormancy cycle. Regardless of the type of cycle, seeds buried in non-flooded soil retained the ability to germinate in light at high temperatures under flooded conditions throughout the summer. Thus, seeds potentially can germinate at any time during the growing season, whenever rice fields are flooded. Flooding fields during winter and/or sowing rice relatively early in the growing season may help in establishing rice before seeds of H. limosa germinate.     

Characterization of monilia disease caused by Monilinia linhartiana on quince in southern Spain

This study elucidates the aetiology and epidemiology of monilia disease of quince caused by the fungus Monilinia linhartiana in Spain. Disease incidence and the dynamics of apothecial development and ascospore discharge were quantified and the pathogen was characterized using morphological and molecular methods. The pathogen did not produce conidia or apothecia on agar media but produced conidia on leaves showing symptoms and apothecia on mummified young quince fruit. Monilinia linhartiana was not pathogenic on ripe quince fruit but was readily isolated from developing, mummified fruit (pseudosclerotia). Phylogenetic analysis based on 5·8S‐ITS region sequences placed M. linhartiana in the Disjuntoriae section of Monilinia species infecting rosaceous hosts. Studies during 2004–2008 in four commercial orchards in southern Spain determined two major infection periods for the disease. The first coincided with the unfolding of the first leaves and resulted in leaf blotch and shoot blight. The second coincided with flowering and led to mummification of developing young fruit. Foliar infection was apparently initiated by airborne ascospores produced on pseudosclerotia that overwintered on the soil surface, while flower infection was probably initiated by conidia produced on leaf lesions. Incidence of diseased shoots ranged from 1 to 91% and was correlated with calculated inoculum potential, based on the density and maturity of apothecia formed on pseudosclerotia. This epidemiological study has made it possible to characterize the life cycle of monilia disease on quince in southern Spain, which will help the development of new control strategies.     

Induction of myceliogenic germination of sclerotia of Sclerotinia sclerotiorum by exposure to sub-freezing temperatures

When sclerotia of a Canadian isolate of Sclerotinia sclerotiorum were exposed to temperatures of – 10 or– 20 C for 4 weeks, their germination behaviour changed from carpogenic to myceliogenic type. Mycelial growth from sclerotia exposed to– 20 C was more vigorous than that from sclerotia exposed to– 10 C. The mode of germination in sclerotia treated with above-freezing temperatures of 0 5, 10. 15. 25 or 30 C did not change: they retained the capacity for carpogenic germination. The possible epidemiological impact of this low-temperature induction of myceliogenic germination on the development of sclerotinia wilt of sunflower seedlings in western Canada is discussed.     

大豆菌核病的防治适期与防治指标

采用系统调查和统计分析的方法，研究了大豆菌核病的防治适期和防治指标。试验结果表明，子囊盘发生期与大豆开花期的重叠盛期是菌核病防治适期，一般以7月20－25日为宜。7月25日前子囊盘数量和病情指数与产量损失极相关，而且子囊盘数量作为产量损失估计和防治指标确定更为科学准确，其回归模型为Y＝－4．5499＋2．311X（r=0.8442)，防治指标为3－4个子囊盘／9．75m^2。     

STUDIES ON WEED SPECIES OF THE GENUS POLYGONUM L.

Summary. Seeds ni Polygonum lapathifolium from several sources or after certain treatments were subjected to various periods of moist chilling and then incubated for 3 weeks, during which time the numbers that germinated were recorded.
Populations from different localities differed in both seed weight and in the response of the seeds to chilling, but no relationships between the response and the environment of the locality of origin were found. Seeds from individual plants and from single inflorescences within populations also varied in both seed weight and germination with and without chilling. Application of dichlorprop to the parent plants was found to affect seed weight and % germination, the effect varying with both population and dose.
Etudes sur des espèces de mauvaises herbes du genre Polygonum L.
IV. Variations du poids et des modalités de germination des semences chez P. lapathifolium L.     

Effect of Environmental Conditions on Germination and Survival of Teliospores and Basidiospores of the Pear Rust Fungus (Gymnosporangium asiaticum)

Pear rust, caused by Gymnosporangium asiaticum, is an important disease of pear (Pyrus spp.) in China in areas where cypress (Juniperus spp. and Sabina spp.), the alternate host, is present. Controlled environment experiments were conducted to study the effects of temperature, relative humidity (RH) and duration of free water on germination of teliospores and basidiospores of G. asiaticum. Teliospores, sampled from Juniperus chinensis, germinated at temperatures ranging from 5 to 28 , with an optimum between 16 and 20 . For teliospores to germinate, telial horns required free water: soaking in water for 30 sec triggered teliospore germination. After initial wetting, RH had little effect on germination of teliospores except at extreme temperatures (5 and 30 ) where germination needed near-saturation moisture. The minimum time for telial horns to produce basidiospores was about 3 h at optimum temperatures. Teliospores in all telial horns germinated at 12--4 , but at 8 and 28 only those from a third of telial horns germinated. Basidiospores germinated at 5--0 with the optimum around 15 ; they needed free water or saturated moisture to germinate. Germination in free water was about eight times greater than at 100% RH. Logistic models described well the germination dynamics of basidiospores. Basidiospore survival declined exponentially with storage time, and linearly with increasing temperature and decreasing RH. Basidiospores survived for at least six days in dry conditions with RH as low as 45%.     

Differences Between Ascospores and Conidia of Didymella rabiei in Spore Germination and Infection of Chickpea

ABSTRACT Studies were performed to compare the germination and infection of ascospores and conidia of Didymella rabiei under different temperature and moisture conditions. Germination of ascospores and conidia on cover glasses coated with water agar began after 2 h, with maximum germination (>95%) occurring in 6 h at 20 degrees C. No germination occurred at 0 and 35 degrees C. Ascospores germinated more rapidly than conidia at all temperatures. Germination declined rapidly as the water potential varied from 0 to -4 MPa, although some germination occurred at -6 MPa at 20 and 25 degrees C. Ascospores germinated over a wider range of water potentials than conidia and their germ tubes were longer than those of conidia at most water potentials and temperatures. The optimum temperature for infection and disease development by both ascospores and conidia was around 20 degrees C. Disease severity was higher when ascospores were discharged directly onto plant surfaces from naturally infested chickpea debris compared with aqueous suspensions of ascospores and conidia sprayed onto plants Disease severity increased as the length of the wetness period increased. When dry periods of 6 to 48 h occurred immediately after inoculation, disease severity decreased, except for the shorter periods which had the opposite effect. Disease severity was higher with ascospore inoculum when no dry periods occurred after inoculation.     

Factors Affecting Germination of Puccinia jaceae var. solstitialis Teliospores from Yellow Starthistle

ABSTRACT The rust fungus Puccinia jaceae var. solstitialis is a candidate for biological control of yellow starthistle (YST). Part of the risk assessment includes determining if safflower seedlings are susceptible to infection by teliospores of P. jaceae. A protocol for germination of P. jaceae teliospores is needed to verify that teliospores used in comparative studies are viable. The protocol developed from this research has two steps: first, priming teliospores on water agar at 4 degrees C in the dark, and second, incubating them at warmer temperatures for 1 week in the presence of an exogenous stimulator. Priming longer than 4 weeks resulted in significantly greater germination than priming for shorter periods. Sources of effective stimulator included seeds and seedlings of YST or safflower. The greatest germination occurred during incubation at 18 degrees C in the dark. Teliospore germination was reduced after incubation with a 12- or 14-h photoperiod. A low percentage (<20%) of teliospores of two isolates germinated after 44 and 96 weeks of dry storage at room temperature; samples of each isolate tested after that did not germinate. Data indicate teliospores of several isolates of P. jaceae are viable, and the protocol will be used to prepare teliospores of P. jaceae for comparative studies with P. carthami on safflower seedlings.     

制约稻粒黑粉病菌冬孢子萌发的关键因素

在相同条件下，从稻田直接收集的病粒中的稻粒黑粉病菌厚垣孢子不能萌发，而取之于仓库中的孢子能萌发。为探明其制约因素，对该病菌进行了光照和浸水时间试验。研究结果表明：稻粒黑粉病菌冬孢子形成后，必须经光照射后，方可进入休眠期；度过5～6个月休眠期的冬孢子，必须浸水48h以上才开始复苏；复苏后的冬孢子必须在光照条件下才能萌发。未完成后熟作用的冬孢子和完成后熟但尚未复苏的冬孢子即使在光照条件下也不能萌发。即稻粒黑粉病菌冬孢子必须经历后熟、休眠、复苏三个阶段后，在适宜的光照与温湿条件下，方可萌发。光照对其冬孢子具有双重作用，促使后熟进入休眠和刺激萌发。     

Germination of Orobanche crenata Forsk. seeds in vitro

The effects of calcium hypochlorite [Ca(OCl)₂] and gibberellic acid (GA), both in combination and separately, were studied on the germination of Orobanche crenata seeds. Hypochlorite was applied either before or after the conditioning phase for periods up to 150 min. The seeds were exposed to GA during the conditioning phase. After conditioning the seeds were exposed to either the synthetic germination stimulant GR 7, flax root exudate (FRE) or distilled water. GR 7 exploited the germination potential of O. crenata better than FRE and this in turn better than water. In the presence of GR 7 or FRE the number of germinating seeds generally increased when the seeds were briefly (5–20 min) exposed to hypochlorite. In the absence of GR 7 and FRE, hypochlorite only increased germination if the seeds were treated during medium-long periods (20–60 min) after conditioning. If GR 7 or FRE were not applied the germination percentage was usually significantly higher when conditioning was done in the presence of GA.     

Influence of Nutrient and Environmental Factors on Conidial Germination of Potebniamyces pyri

ABSTRACT Potebniamyces pyri is the causal agent of Phacidiopycnis rot, a postharvest disease of pears. Infection of fruit occurs in the orchard, and symptoms develop during storage. Conidial germination of P. pyri in response to nutrient, temperature, wetness duration, relative humidity (RH), and pH was determined in vitro. Conidia germinated by either budding or developing germ tubes in various concentrations of pear juice solutions. The mode of conidial germination was nutrient-dependent. Low nutrient levels favored budding, whereas high nutrient levels favored germ tube development. Conidia germinated at 0 to 30 degrees C but not at 35 degrees C, with optimum temperature between 20 and 25 degrees C. Wetness durations of 4 to 5 h and 6 to 8 h at optimum temperature were required for budding and developing germ tubes, respectively, and 20 to 24 h of wetness was required to reach germination peaks. Regardless of temperature, conidia germinated primarily by budding in 10% pear juice. Secondary conidia, produced by budding of conidia, initially increased their dimensions and later germinated at 0 to 25 degrees C in the same manner as mother conidia. No germination of secondary conidia occurred at 30 degrees C. Germ tubes from conidia elongated at 0 to 25 degrees C but not at 30 degrees C. No germination occurred at 相似文献