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Department of Plant Pathology, University of California at Davis, Salinas, California 93905
Y.-L Peng 1
The State Key Laboratory for AgroBiotechnology, China Agricultural University, Beijing 10094, China
Q.-M Qin
K.V. Subbarao 2
Department of Plant Pathology, University of California at Davis, Salinas, California 93905
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Synchronized maturation of ascospores of Sclerotinia sclerotiorum is desirable for establishing a transformation system, conducting genetic analyses of the pathogen, defining the precise epidemiological roles of ascospores and screening plant germplasm for resistance. In general, fresh apothecia collected from germinated sclerotia contained primarily immature or discharged asci. This study was undertaken to investigate whether maturation of asci and ascospores could be enhanced by incubation of excised apothecia and to determine the effects of factors such as temperature, excision time, light and ventilation on maturation of asci and ascospores in excised apothecia. Maturation of asci was compared between intact and excised apothecia that were incubated under similar conditions. Results demonstrated that temperature was an important factor affecting ascus maturation of S. sclerotiorum during incubation of excised apothecia, and the optimum temperature was around 21 C. After incubation at 21 C for 30 h, the percentage of undischarged mature asci in excised apothecia increased up to 70–80%. This increase was accompanied by a significant increase in ascospore production of up to 5 x 105 ascospores per apothecium. Detailed time course studies indicated that mature asci peaked at 30–36 h of postexcision incubation. Mature asci and the number of ascospores were higher in open incubation than in closed incubation, suggesting that accumulation of volatile substances was not required for ascus/ascospore maturation during postexcision incubation and ventilation could enhance the maturation process. Light also did not affect the maturation of asci during the incubation of excised apothecia. Germination rates for ascospores from excised apothecia under various treatments were similar to those from untreated apothecia but declined slightly with time postexcision. The incubation of excised apothecia promoted ascus maturation compared with intact apothecia.
Key words: ascospore delineation, ascospore germination, epidemiology, fungal biology, light, temperature, ventilation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, Boland and Hall 1994). The fungus survives as sclerotia in soil, which germinate carpogenically to produce apothecia that release ascospores. Ascospores are the major primary inoculum source (Newton and Sequeira 1972; Abawi and Grogan 1975, 1979; Adams and Ayers 1979) although the pathogen also occasionally infects plants by myceliogenic germination of sclerotia. Concentration of airborne ascospores is correlated with infection and therefore is a useful predictor of disease incidence (McCartney and Lacey 1991, 1999; Twengstrom et al 1998). Carpogenic germination of S. sclerotiorum sclerotia has been studied widely (Schwartz and Steadman 1978; Phillips 1986, 1987; Huang and Kozub 1991, 1993, 1994; Dillard et al 1995, Sun and Yang 2000, Thaning and Nilsson 2000, Ekins et al 2002, Hao et al 2003, Clarkson et al 2004). The production of stipes (initials of apothecia) requires continuous moisture at the optimum temperature, which is dependent on the origin and conditions during production of sclerotia (Phillips 1987; Huang and Kozub 1991, 1993). Although light is not required for forming stipe initials, light of < 390 nm is necessary for complete expansion of apothecia (Thaning and Nilsson 2000, Hao et al 2003). Apothecia puff-off ascospore (up to 2 x 106 ascospores per apothecium) (Schwartz and Steadman 1978) clouds during a sudden decrease in atmospheric humidity or pressure (Dickson and Fisher 1923, Harthill and Underhill 1976) and their release continues for at least 72–84 h (Clarkson et al 2003). In nature, apothecia release spores during day and night, with a major peak between 9 AM and 1 PM, with air turbulence playing an important role (Bourdot et al 2001, Wu and Subbarao unpublished data).
Although extensive studies have been conducted on carpogenic germination and ascospore release, it is unclear how the asci and ascospores of S. sclerotiorum mature. How events such as a short-term drop in environmental humidity, soil moisture and cultivation influence this process is unknown. Knowledge of ascus/ascospore maturation process not only will increase our ability to predict the airborne inoculum levels in agricultural systems but also improve techniques to produce large quantities of ascospores of S. sclerotiorum of similar age and genetic background. Although both ascospores and mycelium of S. sclerotiorum often are used as a source of inoculum in artificial inoculations (Whipps et al 2002, Zhao and Meng 2003) using ascospore inoculum is preferable for resistance screening because ascospore infection mimics infections in field (Whipps et al 2002). Because of the difficulties in preparing a large number of uniformly aged ascospores of S. sclerotiorum, sporulating apothecia usually are used as inoculum sources in small scale experiments (Hudyncia et al 2000) while sclerotia often are buried and promoted to produce apothecia and release ascospores as inoculum source for large scale experiments. Because the majority of ascospores of S. sclerotiorum remain viable and can infect plants for up to 2 y when stored dry at low temperatures (Hunter et al 1982) researchers often preserve ascospores of S. sclerotiorum collected at different times for use in inoculation (Zhao and Meng 2003). Even careful planning to collect large number of ascospores does not ensure the uniformity of the age of ascospores.
Molecular studies on S. sclerotiorum so far have relied on studying the mutants or transformants of the mycelial phase for this genus (Godoy et al 1990; Boland 1992; Melzer and Boland 1996; Zhou and Boland 1997; Rollins and Dickman 1998, 2001; Deng et al 2002; Rollins 2003; Girard et al 2004; Jurick et al 2004) or analyzing gene functions by placing target genes into other fungi (Vautard-Mey et al 1999, Vacher et al 2003). Mutagenesis (including transformation) and screening for mutants at the mycelial stage are not efficient in this group of fungi because of the multinucleate nature of their hyphal cells (up to hundreds of nuclei per cell). Uniform ascospores are desirable for transformation because of their relatively identical and simple genetic background.
Our objectives therefore were to study the factors that affect maturation of asci and ascospores of S. sclerotiorum and S. minor and to develop a technique to produce ascospores of similar age from a limited number of sclerotia.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Evaluating maturation of asci.— –
Apothecia were macerated in a 1 mL centrifuge tube with 1–2 drops of water, and at least 150 asci per sample were examined microscopically to determine their maturity. An ascus of S. sclerotiorum or S. minor was considered immature if individual ascospores were not distinguishable and the cytoplasm remained visible in the ascus. An ascus was considered mature when the individual ascospores were distinguishable and remained in the ascus; it was considered empty if the ascospores had been released before or during the macerating process and neither ascospore nor cytoplasm remained in the ascus (FIG. 1). The numbers of immature, mature and empty asci were counted separately, and percentage of each group was calculated.
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Experiment 1: Temporal progress of ascus maturation in excised apothecia.— – Two experiments were conducted to investigate the temporal progress of ascus maturation during incubation of excised apothecia. In the first experiment apothecial heads of isolates BS014 and BS001 were excised from sclerotia at 8 PM. Three randomly selected heads were placed in 1 mL centrifuge tubes and incubated at room temperature (21 ± 2 C). At 0, 12, 24, 36 and 48 h postincubation, tubes were retrieved and the number of immature, mature and empty asci on the apothecia, number of ascospores released from each apothecium and percentage of viable ascospores were evaluated as described above. Because there was no significant difference in the responses of the two isolates, only apothecia of BS014 were used in the second experiment. Apothecia were excised at 2 AM and evaluated after 0, 30, 36 and 42 h of postexcision incubation at room temperature for immature, mature and empty asci. The experiments were repeated once with three replications each.
Experiment 2: Effects of excision time on ascus maturation.— – At 8 AM, 12 PM and 4 PM, apothecia of S. sclerotiorum (BS014) and S. minor (Sm1) were excised from sclerotia. One randomly chosen apothecium was placed in autoclaved centrifuge tubes. For each excision time and isolate combination a tube was examined immediately and another tube was examined after incubating in closed centrifuge tubes at room temperature for 30 h. Maturation data were collected as described above and the experiment was repeated three times.
Experiment 3: Effects of incubation temperature on ascus maturation in excised apothecia.— – Maturation of excised asci after incubation at 13, 21 and 26 C was compared with an unincubated control. Apothecia of BS014 were excised at 8 AM, randomly placed in centrifuge tubes and incubated at 13, 21 and 26 C. After incubation for 30 h, the maturation of asci and the number of ascospores were evaluated as described above. For the unincubated control, excised apothecia were macerated and examined immediately after excision. The experiment was repeated five times.
Experiment 4: Effects of ventilation on ascus maturation in excised apothecia.— – Apothecia of BS014 were excised at 8 AM and 8 PM, and one apothecium from each time was placed randomly in separate centrifuge tubes. Four replicate tubes each were incubated in open and closed centrifuge tubes at room temperature for 30 h, and maturation of asci and viability of ascospores were determined as described above. For controls, apothecia were macerated immediately after excision for determining maturation of asci and viability of ascospores.
Experiment 5: Effects of light on ascus maturation in excised apothecia.— – Apothecia of BS014 with similar cup size and stage of expansion were identified and marked. Nine of the apothecia were excised at 8 AM and randomly placed into centrifuge tubes with three apothecia in each tube. The tubes were assigned to one of three treatments and examined immediately or after incubation at room temperature for 36 h (in darkness or 12 h/12 h light and dark cycle under fluorescent light) with the centrifuge tube lids open. Data similar to those described above were collected. The experiments were repeated once, with three replications per experiment.
Experiment 6: Effects of excision on ascus maturation.— – Apothecia of BS001 and BS014 with similar cup size and stage of expansion were identified and marked. The apothecia were randomly assigned to different treatments, (i) excised and examined immediately; (ii) excised immediately and examined after incubation at 20 C in the dark for 36 h with the lids open in a moist chamber; and (iii) excised and examined after incubation in the same moist chamber for 36 with apothecia intact on the sclerotia. There were five replications (each containing three apothecia) in the first experiment with BS001 and four in the second experiment with BS014. Data similar to those described above were collected.
Data analysis.— – Data from experiment 1 were analyzed by repeated measures analysis of variance (ANOVA) using general linear model procedure (GLM, SAS v9.1, SAS Institute Inc., Cary, North Carolina 27513) to determine isolate differences and the effects of postexcision incubation. These analyses revealed significant effects of incubation duration, but the response of the two S. sclerotiorum isolates was similar. Hence data from the two isolates were pooled for further analyses.
The maturation of asci during postexcision incubation can be defined as conversion of the asci that were immature at excision into mature or empty asci. The conversion ratio was calculated as: Pt = 1 – Pret/Pre0, where Pret was the percentage of immature asci at time t (h) after postexcision incubation and Pre0 was the percentage of immature asci at excision for the corresponding isolates and treatments. A nonlinear regression model (NLIN, SAS v9.1) was fitted to the progress of postexcision maturation Pt over time. Similar analyses were conducted on the number of ascospores per apothecium released over time during postexcision treatments. A linear regression analysis was conducted on the percent mature asci, the total ascospores released from each apothecium, the percent that germinated and temporal changes in ascospore germination.
For data (S. sclerotiorum and S. minor) from experiment 2, analyses of variance were performed on the arcsine-transformed percentage of immature, mature, and empty asci with the GLM procedure in SAS. The same transformation and analyses were performed for the other experiments on the percentage of immature, mature and empty asci, and the number of ascospores (no transformation) released from each apothecium and percentage of ascospores that germinated. In addition to multiple comparison among different treatments, orthogonal contrasts were constructed between pre- and postexcision incubation for experiments 2–5, between open- and closed-lid incubation for experiment 4, between continuous dark incubation and incubation under a dark-light regime for experiment 5 and between excised and intact apothecia for experiment 6.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Similarly, the number of ascospores (x105) released from each apothecium also fitted a logistic model: Spt = 3.6128e0.0913t – 1.6720/(1 + e0.0913t – 1.6720) (N = 7, R2 = 0.86, P < 0.001, FIG. 2B). The percentage of viable ascospores that germinated in water declined marginally during postexcision incubation and was estimated with a linear model, Gt = 78.56 – 0.3536t (N = 5, R2 = 0.82, P < 0.05, FIG. 2C). The total number of germinated ascospores released from an apothecium was positively related to the percent mature asci, but negatively related to percent immature asci.
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). The percentage of immature asci was lowest and the percent mature asci highest at 8 AM. Those excised at 12–4 PM had no significant (P > 0.05) difference, but the percentage of empty asci was highest at 4 PM and lowest at 12 PM (FIG. 3A). ANOVA also demonstrated that before postexcision incubation, there were no significant differences (P > 0.05) in the percent immature, mature or empty asci between the two species, S. sclerotiorum and S. minor. Regardless of the species and excision time, the percent mature asci increased significantly (P < 0.05) from 2.0–23.1 to 43.7–72.9, while the percent immature asci decreased from 69.5–89.7 to 13.4–35.1 during postexcision incubation (FIG. 3A and B), representing significant effects of postexcision incubation (P < 0.0001 in ANOVA). After postexcision incubation, the percentage of immature asci was significantly lower (P < 0.05) for apothecia excised at 8 AM than for those excised at other times (FIG. 3B). S. sclerotiorum exhibited significantly lower (P < 0.05) percent immature asci and higher mature asci than S. minor (FIG. 3B) after postexcision incubation, suggesting a differential response of the two species (significant interaction between incubation treatment and species in ANOVA).
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). Temperature however did not affect the percent empty asci. The percentage of immature asci was reduced significantly at all temperatures indicating significant postexcision maturation, and this reduction was more evident at 21 C than at either 13 or 26 C (TABLE I). The percent mature asci increased significantly after postexcision incubation at 21 and 13 C (P < 0.05), and it was significantly higher at 21 C than at either 13 or 26 C (TABLE I). Generally the number of ascospores released per apothecium was highly correlated to the percent mature asci.
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). The changes in the percent empty asci were similar for both treatments (TABLE II). Neither the percentage of immature nor the percentage of empty asci was significantly different between open and closed tubes (TABLE II). However the percentage of mature asci was slightly, but significantly (P < 0.05) higher in open tubes than in closed ones (TABLE II). As in previous experiments, ascospores released from each apothecium followed a trend similar to the percent mature asci.
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). No significant difference between incubation in continuous dark and that in dark/light regime was observed. The percent empty asci and the number of ascospores increased during the 36 h incubation under light or dark conditions (TABLE III). Again the number of ascospores that germinated in water was closely related to the percent mature asci (FIG. 4).
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). The percent immature asci decreased and the mature asci increased in both excised and intact apothecia after incubation at 20 C in the dark for 36 h compared with apothecia before incubation (FIG. 5A). However the decrease of immature asci and the increase of mature asci were significantly greater (P < 0.05) in the excised than in intact apothecia (FIG. 5A). This difference was considerably large when conversion ratios of immature to mature asci were compared (19% for intact apothecia vs. 42% for excised apothecia). The number of viable ascospores per apothecium also was significantly different (P < 0.05) between excised (9.2 x 105 ascospores per apothecium) and intact apothecia (6.5 x 10 5 spores per apothecium) (FIG. 5B).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Based on an exhaustive literature review, this appears to be the first study to document that incubation of excised apothecia promotes the maturation of asci. Because ascospores of S. sclerotiorum play a critical role in the disease cycle and apothecia produced by S. minor have rarely been observed in nature, the primary focus of this study was on S. sclerotiorum with a single isolate of S. minor included for comparison. Incubation of excised apothecia significantly increased the mature asci and reduced the immature asci, and this response was consistent in both isolates of S. sclerotiorum and in both species.
The increase in the mature asci theoretically could be attributed to a combination of reduced formation of new asci (immature asci), enhanced maturation of existing asci and limited release of ascospores by some asci. In this study however the percentage of empty asci did not vary significantly between intact apothecia and those subjected to postexcision incubation, and the total viable ascospores were significantly greater in excised apothecia than in intact apothecia. This increase in all likelihood was caused by the enhanced ascus/ascospore maturation.
This enhanced maturation during postexcision incubation was caused neither by the accumulation of volatile compounds nor by light, which is critical for expansion of stipes into apothecia (Thaning and Nilsson 2000). Of all the factors studied, temperature influenced the process of ascus maturation significantly. The optimum temperature for the ascus maturation of S. sclerotiorum was 21 C, which is close to the optimum for the fungus to produce apothecia in nature (Huang and Kozub 1991, 1993; Clarkson et al 2004). High (26 C) or low (13 C) temperatures retarded the process of maturation during postexcision incubation. These findings are entirely novel to the study of the biology of fungi related to Sclerotinia spp. and bring up many questions regarding ascus maturation. For example, what processes drive post-excision maturation of asci and delineation and release of ascospores? And how do they do it? When the conversion of immature to mature asci was used as a measure of ascus maturation, during postexcision incubation, the process reached a peak about 30–36 h after excision at 21 C. This was much more rapid than the normal ascus maturation in intact apothecia, which can continue for 72–84 h (Clarkson et al 2003). Ascospores of this group of fungi can survive long under many environmental stresses such as drought, wetness, high temperature, UV, etc. (Caesar and Pearson 1983, Hudyncia et al 2000, Clarkson et al 2003). It is unclear whether the enhanced maturation of asci is a response to dehydration of apothecia caused by excision. More studies on ascus (and ascospore) maturation under drought stress undoubtedly will improve our understanding of the biology and ecology of this group of pathogens as well as fungi in general. Because ascospores are the primary inoculum source for many economically important diseases caused by the genus of Sclerotinia, a better understanding of the maturation process of asci will also improve our prediction of the availability of inoculum (Newton and Sequeira 1972; Abawi and Grogan 1975, 1979; McCartney and Lacey 1991, 1999).
The percentage of mature asci in pretreatment apothecia was highest at 8 AM and that of empty asci was highest at 4 PM. This is consistent with the major spore release peak observed 9 AM–1 PM in S. sclerotiorum (Bourdot et al 2001, Wu and Subbarao unpublished data). It is possible that the maturation process occurs throughout the day, while the major release of ascospores occurs 9 AM–1 PM, with some small peaks at other times of the day. This would explain why the lowest percentage of immature asci and the highest percentage of mature asci were observed at 8 AM, and why the highest percentage of empty asci was observed around 4 PM.
Thirty hours after postexcision incubation of fully expanded apothecia, up to 5 x 105 ascospores per apothecium of S. sclerotiorum were regularly produced. Assuming three to four apothecia are produced on each sclerotium, the number of ascospores per sclerotium will be around 1–2 x 106. This technique allows the production of a large number of uniformly aged ascospores from a few sclerotia of Sclerotinia spp., facilitating not only a variety of studies on the biology of the ascospores, pathogenicity of the fungus and epidemics caused by the fungus but also the use of ascospores instead of mycelia or protoplasts derived from mycelia for genetic transformation and other studies.
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FOOTNOTES |
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1 First and second authors contributed equally to this work. 
2 Corresponding author. E-mail: kvsubbarao{at}ucdavis.edu
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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———, ———. 1979. Epidemiology of disease caused by Sclerotinia species. Phytopathology 69:899–904.
Adams PB, Ayers WA. 1979. Ecology of Sclerotinia species. Phytopathology 69:896–899.
Boland GJ. 1992. Hypovirulence and double-stranded-RNA in Sclerotinia sclerotiorum. Can J Plant Path 14:10–17.
———, Hall R. 1994. Index of plant hosts of Sclerotinia sclerotiorum. Can J Plant Path 16:93–108.
Bourdot GW, Hurrell GA, Saville DJ, De Jong MD. 2001. Risk analysis of Sclerotinia sclerotiorum for biological control of Cirsium arvense in pasture: ascospore dispersal. Biocontrol Sci Technol 11:119–139.[CrossRef]
Caesar AJ, Pearson RC. 1983. Environmental-factors affecting survival of ascospores of Sclerotinia sclerotiorum. Phytopathology 73:1024–1030.
Clarkson JP, Staveley J, Phelps K, Young CS, Whipps JM. 2003. Ascospores release and survival in Sclerotinia sclerotiorum. Mycol Res 107:213–222.[CrossRef][Medline]
———, Phelps K, Whipps JM, Young CS, Smith JA, Watling M. 2004. Forecasting Sclerotinia disease on lettuce: toward developing a prediction model for carpogenic germination of sclerotia. Phytopathology 94:268–279.[Medline]
Deng F, Melzer MS, Boland GJ. 2002. Vegetative compatibility and transmission of hypovirulence-associated dsRNA in Sclerotinia homoeocarpa. Can J Plant Pathol 24:481–488.
Dickson LF, Fisher WR. 1923. A method of photographing spore dispersal from apothecia. Phytopathology 13:30–32.
Dillard HR, Ludwig JW, Hunter JE. 1995. Conditioning sclerotia of Sclerotinia sclerotiorum for carpogenic germination. Plant Dis 79:411–415.
Ekins MG, Aitken EAB, Goulter KC. 2002. Carpogenic germination of Sclerotinia minor and potential distribution in Australia. Austr Plant Pathol 31:259–265.[CrossRef]
Girard V, Fevre M, Bruel C. 2004. Involvement of cyclic AMP in the production of the acid protease Acp1 by Sclerotinia sclerotiorum. FEMS Microbiol Lett 237:227–233.[Medline]
Godoy G, Steadman JR, Dickman MB, Dam R. 1990. Use of mutants to demonstrate the role of oxalic acid in pathogenicity of Sclerotinia sclerotiorum on Phaseolus vulgaris. Physiol Mol Plant Pathol 37:179–191.[CrossRef]
Hao JJ, Subbarao KV, Duniway JM. 2003. Germination of Sclerotinia minor and S. sclerotiorum sclerotia under various soil moisture and temperature combinations. Phytopathology 93:443–450.[Medline]
Harthill WFT, Underhill AP. 1976. Puffing in Sclerotinia sclerotiorum and S. minor. NZ J Bot 14:355–358.
Huang HC, Kozub GC. 1991. Temperature requirements for carpogenic germination of sclerotia of Sclerotinia sclerotiorum isolates of different geographic origin. Bot Bull Acad Sinica 32:279–286.
———, ———. 1993. Influence of inoculum production temperature on carpogenic germination of sclerotia of Sclerotinia sclerotiorum. Can J Microbiol 39:548–550.
———, ———. 1994. Germination of immature and mature sclerotia of Sclerotinia sclerotiorum. Bot Bull Acad Sinica 35:243–247.
Hudyncia J, Shew HD, Cody BR, Cubeta MA. 2000. Evaluation of wounds as a factor to infection of cabbage by ascospores of Sclerotinia sclerotiorum. Plant Dis 84: 316–320.[CrossRef]
Hunter JE, Steadman JR, Cigna JA. 1982. Preservation of ascospores of Sclerotinia sclerotiorum on membrane filters. Phytopathology 72:650–652.
Jurick WM, Dickman MB, Rollins JA. 2004. Characterization and functional analysis of a cAMP-dependent protein kinase A catalytic subunit gene (pka1) in Sclerotinia sclerotiorum. Physiol Mol Plant Pathol 64:155–163.[CrossRef]
McCartney HA, Lacey ME. 1991. The relationship between the release of ascospores of Sclerotinia sclerotiorum, infection and disease in sunflower plots in the United Kingdom. Grana 30:486–492.
———, ———. 1999. Timing and infection of sunflowers by Sclerotinia sclerotiorum and disease development. Aspect Appl Biol 56:151–156.
Melzer MS, Boland GJ. 1996. Transmissible hypovirulence in Sclerotinia minor. Can J Plant Pathol 18:19–28.
Newton CH, Sequeira L. 1972. Ascospores as the primary infective propagule of Sclerotinia sclerotiorum in Wisconsin. Plant Dis Report 56:798–802.
Phillips AJL. 1986. Carpogenic germination of sclerotia of Sclerotinia sclerotiorum after periods of conditioning in soil. J Phytopathol 116:247–258.[CrossRef]
———. 1987. Carpogenic germination of sclerotia of Sclerotinia sclerotiorum: a review. Phytophylactica 19: 279–283.
Purdy LH. 1979. Sclerotinia sclerotiorum: history, disease and symptomatology, host range, geographic distribution and impact. Phytopathology 69:875–880.
Rollins JA. 2003. The Sclerotinia sclerotiorum pac1 gene is required for sclerotial development and virulence. Mol Plant Microbe Interact 16:785–795.[Medline]
———, Dickman MB. 1998. Increase in endogenous and exogenous cyclic AMP levels inhibits sclerotial development in Sclerotinia sclerotiorum. Appl Environ Microbiol 64:2539–2544.
———, ———. 2001. pH signaling in Sclerotinia sclerotiorum: identification of a pacC/RIM1 homolog. Appl Environ Microbiol 67:75–81.
Schwartz HF, Steadman JR. 1978. Factors affecting sclerotium populations of and apothecium production by Sclerotinia sclerotiorum. Phytopathology 68:383–388.
Sun P, Yang XB. 2000. Light, temperature, and moisture effects on apothecium production of Sclerotinia sclerotiorum. Plant Dis 84:1287–1293.[CrossRef]
Thaning C, Nilsson HE. 2000. A narrow range of wavelengths active in regulating apothecial development in Sclerotinia sclerotiorum. J Phytopathol 148:627–631.[CrossRef]
Twengstrom E, Sigvald R, Svensson C, Yuen J. 1998. Forecasting Sclerotinia stem rot in spring sown oilseed rape. Crop Protect 17:405–411.[CrossRef]
Vacher S, Cotton P, Fevre M. 2003. Characterization of a SNF1 homologue from the phytopathogenic fungus Sclerotinia sclerotiorum. Gene 310:113–121.[CrossRef][Medline]
Vautard-Mey G, Cotton P, Fevre M. 1999. The glucose repressor CRE1 from Sclerotinia sclerotiorum is functionally related to CREA from Aspergillus nidulans but not to the Mig proteins from Saccharomyces cerevisiae. FEBS Lett 453:54–58.[CrossRef][Medline]
Whipps JM, Budge SP, McClement S, Pink DAC. 2002. A glasshouse cropping method for screening lettuce lines for resistance to Sclerotinia sclerotiorum. Eur J Plant Pathol 108:373–378.[CrossRef]
Zhao JW, Meng JL. 2003. Genetic analysis of loci associated with partial resistance to Sclerotinia sclerotiorum in rapeseed (Brassica napus L.). Theor Appl Genet 106: 759–764.[Medline]
Zhou T, Boland GJ. 1997. Hypovirulence and double-stranded RNA in Sclerotinia homoeocarpa. Phytopathology 87:147–153.[Medline]
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