Discharge of Venturia inaequalis ascospores during daytime and nighttime wetting periods in Ontario and Nova Scotia

This article was downloaded by: [University North Carolina - Chapel Hill] On: 04 November 2014, At: 07:03 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Canadian Journal of Plant Pathology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tcjp20 Discharge of Venturia inaequalis ascospores during daytime and nighttime wetting periods in Ontario and Nova Scotia J. Warner a b & P. G. Braun a b a Research Station , Agriculture Canada , Smithfield Experimental Farm, Box 340, Trenton, Ontario, K8V 5R5 b Research Station , Agriculture Canada , Kentville, Nova Scotia Published online: 29 Dec 2009. To cite this article: J. Warner & P. G. Braun (1992) Discharge of Venturia inaequalis ascospores during daytime and nighttime wetting periods in Ontario and Nova Scotia, Canadian Journal of Plant Pathology, 14:4, 315-321, DOI: 10.1080/07060669209500870 To link to this article: http://dx.doi.org/10.1080/07060669209500870 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. 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Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions http://www.tandfonline.com/loi/tcjp20 http://www.tandfonline.com/action/showCitFormats?doi=10.1080/07060669209500870 http://dx.doi.org/10.1080/07060669209500870 http://www.tandfonline.com/page/terms-and-conditions CANADIAN JOURNAL OF PLANT PATHOLOGY 14:315-321, 1992 Discharge of Venturia inaequalis ascospores during daytime and nighttime wetting periods in Ontario and Nova Scotia J. Warner and P. G. Braun Research Station, Agriculture Canada, Smithfield Experimental Farm, Box 340, Trenton, Ontario K8V 5R5, and Research Station, Agriculture Canada, Kentville, Nova Scotia B4N 1J5. Contribution No. 133. Accepted for publication 1992 07 21 Venturia inaequalis ascospore discharge was examined during daytime and nighttime wetting periods in Ontario and Nova Scotia. In controlled environment studies in Nova Scotia in 1989 and 1990, average peak ascospore densities of 2529, 2268, and 2245 spores/m3 of air were recorded at 2300, 0300, and 0400 h Atlantic Standard Time (AST) when wet periods began at 2300, 0100, and 0400 h, respectively. In mist bed studies in Ontario in 1990, peak ascospore densities of 7495, 5366, and 4238 spores/m3 of air were recorded at 2100, 0200, and 0500 h Eastern Standard Time (EST) when wet periods were initiated at 2100, 0100, and 0400 h. A second peak in ascospore discharge occurred shortly after sunrise for most wet periods at both locations. In Ontario orchard studies in 1991, 87% and 13% of ascospores were captured in the daytime and nighttime, respectively, during 13 naturally occurring wet periods, eight of which began at night. Peak spore densities of 1302 and 960 ascospores/m3 of air were recorded during the dark period on 30 April and 27 May 1991, respectively. Additional peaks in ascospore release were also observed shortly after sunrise during most wet periods. Sixteen percent of the 172 apple scab lesions which developed on cv. Mclntosh seedlings placed in the orchard during wet periods resulted from ascospores released between 1800 and 0700 h (EST). Warner, J., and P. G. Braun. 1992. Discharge of Venturia inaequalis ascospores during daytime and nighttime wetting periods in Ontario and Nova Scotia. Can. J. Plant Pathol. 14:315-321. La libération d'ascospores de Venturia inaequalis a été étudiée en Ontario et en Nouvelle-Ecosse au cours de périodes humides survenant le jour et la nuit. Dans des études à environnement contrôlé réalisées en Nouvelle-Ecosse en 1989 et 1990, on a enregistré des densités maximales d'ascospores de 2529, 2268 et 2245 spores/m3 d'air à 23 h, 3 h et 4 h HNA (heure normale de l'Atlantique) lorsque les périodes humides ont débuté à 23 h, 1 h et 4 h, respectivement. Dans des études en lit nebulise réalisées en Ontario en 1990, on a enregistré des densités maximales d'ascospores de 7495, 5366 et 4238 spores/m3 d'air à 21 h, 2 h et 5 h HNE (heure normale de l'est) lorsque les périodes humides ont débuté à 21 h, 1 h et 4 h. La libération d'ascospores a connu un deuxième pic peu après le levé du soleil au cours de la plupart des périodes humides dans les deux provinces. Une étude réalisée dans un verger en Ontario en 1991 a révélé que 87 % des ascospores ont été capturées le jour et 13 % la nuit, à la suite de 13 périodes humides naturelles, dont huit ont commencé la nuit. On a enregistré des densités maximales d'ascospores de 1302 et 960 spores/m3 d'air durant la période sombre les 30 avril et 27 mai 1991, respectivement. Des pics additionnels ont également été observés peu après le levé du soleil durant la plupart des périodes humides. Seize pour cent des 172 lésions de la tavelure de la pomme qui sont apparues sur les semis du cultivar Mclntosh placés dans le verger durant les périodes humides ont été attribuées aux ascospores libérées entre 18 h et 7 h (HNE). Until recently, apple scab [Venturia inaequalis (Cke.) Wint.] infection periods in North America have been determined using the Mills (1944) criteria. The primary factors affecting ascospore release were considered to be leaf wetness and temperature. Brook (1966, 1969a) and MacHardy and Gadoury (1986) reported that light also affects ascospore discharge. Red light with wavelengths between 710 and 730 nm was responsible for stimulating ascospore release (Brook 1969b). In a New Hampshire study, from 1981 to 1984, 97.4% of the ascospores were trapped from 0700 to 1800 h eas te rn s tandard time (EST)(MacHardy & Gadoury 1986). Very little spore discharge occurred at night. MacHardy and Gadoury (1989) proposed a revised system for calculating primary apple scab infection periods. When rain begins during the dayt ime (0700 to 1800 h), a wetting period is calculated from the beginning of the rain. When rain begins at night (1800 to 0700), a wetting period is not started until 0700 h. MacHardy and Gadoury (1989) in a New Hampshire study reported that during four separate wet periods, which encompassed both day and night, 4% of the total number of ascospores (47 099) were trapped at night, and 3% of the total scab lesions (1009) that developed on Mclntosh seedlings were attributed to nighttime spore releases. These values for nighttime spore release and resulting scab lesions were considered insignificant. The objectives of this study were to estimate the density of ascospores in the air during daytime and nighttime wetting periods and to further assess the risk of apple scab infect ions from n igh t t ime ascospore releases. The potential risk of using the revised system in Nova Scotia and Ontar io to de termine infect ion per iods as proposed by MacHardy and Gadoury (1989) is discussed. Materials and methods Mist-bed studies, Kentville, 1989-90. Leaves heavily colonized with V. inaequalis were collected in October 1988 and 1989 beneath trees not sprayed with fungicide. The leaves were placed in nylon mesh bags and overwintered on the orchard floor at the Kentville 315 D ow nl oa de d by [ U ni ve rs ity N or th C ar ol in a - C ha pe l H ill ] at 0 7: 03 0 4 N ov em be r 20 14 316 CANADIAN JOURNAL OF PLANT PATHOLOGY, VOLUME 14, 1992 Research Station (K.R.S.). In May, when the leaves contained pseudothecia with mature ascospores, the leaves were placed in plastic bags and held at 5°C in a refrigerator. In 1989, colonized leaves were moved directly from the refrigerator to the sampling site several hours before the initiation of a wet period. In 1990, one week prior to use, a mesh bag of colonized leaves was transferred from the refrigerator to a clear polyethylene canopy type shelter at the sampling site. The canopy permitted the exposure of pseudothecia to ambient environmental conditions but not rain prior to an ascospore sampling period. Ascospores were collected with a Kramer-Collins volumetric spore trap (G. R. Manufacturing, Manhattan, KS, 66506) operated at 10 L/min. The mylar trapping surface, coated with a thin layer of silicone grease (Arthur H. Thomas Co., Philadelphia, PA.) and attached to a drum was rotated past the trapping orifice at 2 mm/h for 7 days. Exposed tapes were cut into 24 h segments and mounted on 75 X 25 mm microscope slides with Gelvatol® (Carleton Instrument Ltd., Ottawa, Ontario). Spores were counted in two-replicate, 0.9 mm wide bands, at 200 X magnification, along each 1 h interval, averaged and multiplied by 14 to give an estimate of the number of spores trapped per h ± 1 h. The number of spores trapped per h multiplied by 1.67 m3 air sampled per h gave an estimate of spore density in the air (spores/m3). The spore sampling orifice was placed 15 cm above six apple leaves infested with V. inaequalis. The apple leaves were placed on a double layer of cheese cloth on the bare ground and held down with two 4-mm- diameter galvanized-metal rods laid across the leaf tips and petioles. The rods were held firmly to the ground with inverted 'J'-shaped stakes made from the same metal rod material. The leaves were placed in the middle of a 1-m2 area of bare ground surrounded on three sides by low shrubs but fully exposed to the sky. No sources of artificial light were near the site. The colonized leaves were wetted by an air-aspirated mister (Adjustable Tip Atomizer 15, The DeVilbiss Co., Somerset, PA), with a 1-litre erlenmeyer flask reservoir, run by an electrical air pump (Cast Mfg. Corp, MI) activated by a Paragon Model 2300S-OOS timer (Paragon Electric Co. Inc., Two Rivers, WI). Wet periods were initiated at various times between 1700 and 0400 h. The mister operated for 5 min every 15 min from the time a wet period was initiated until 1200 h. This was sufficient to keep the leaves wet but not covered with a film of water. Leaves were replaced with new leaves after every wet period. The time of wet period initiation was repeated one to six times during 1989 and 1990 and the results averaged. Leaf wetness periods were monitored with a model 231 leaf wetness sensor purchased from Campbell Scientific (Logan, Utah 84321). The sensor was monitored at 10 min intervals and the data stored on magnetic tape. Mist-bed studies, Smithfield, 1990-91. Leaves heavily colonized by V. inaequalis were collected in the late autumn of 1989 or 1990. During 1989-90, the leaves were held overwinter at the Smithfield Experimental Farm (S.E.F.) in a cold frame covered with 100-mesh screen. Between mid April and early May 1990, the leaves were mixed occasionally to prevent heating and matting, and to provide alternate wet and dry periods which occur under field conditions. On 3 May 1990, the leaves were air dried, placed in black plastic garbage bags and put in cold storage at 3 ± 1°C. One to two weeks before the ascospore discharge studies began, the leaves were taken out of cold storage, placed in a cold frame and kept dry by covering them with glass frames only during wet periods. The mist-bed was approximately 1.4 X 2.4 X 1.2 m high, covered with clear polyethylene, and had a 430 cm2 vent at the top of each end. Three misting nozzles (type B8, Plant Products Co. Ltd., Brampton, Ont.) were mounted at a height of 66 cm, with 61 cm between each nozzle. In 1991, the polyethylene was removed from the top of the mist-bed. In 1990, ascospore discharge was monitored during six artificial wet periods, which ranged from 34 to 48 h in length and began at different times during the day or night. In 1991, ascospore discharge was monitored during 11 night/day wet periods, which began at 0100 h and continued for 12 h until 1300 h. The misting nozzles were operated intermittently using a Paragon 24 h electric time switch and mist timer (Plant Products Co. Ltd.) to provide approximately 2.5 mm of precipitation per h for the first 18 h and then 1.3 mm per h for the remainder of the wet period. This was sufficient to keep the colonized leaves continuously wet. V. inaequalis ascospores were released from apple leaves that were placed to a depth of three to four leaves in a 1-m2 area in front of the spore trap orifice. During 1990-91, infected leaves were held overwinter in the apple orchard between layers of nylon mesh. On each of 24 April, 7 May, and 27 May 1991, leaves with mature ascospores were moved to a covered storage area but subjected to ambient outside temperature and light conditions. The dry period preceding wetting in the mist bed ranged from 2 to 16 days. A new set of leaves was used for each wet period in both 1990 and 1991. The ascospores were trapped and counted as described for the mist-bed studies at the K.R.S. Orchard studies, Smithfield, 1990-91. The density of V. inaequalis ascospores was monitored in two apple (Malus domestica Borkh. cv. Mclntosh) orchards in 1990 and one in 1991 at the S.E.F. The D ow nl oa de d by [ U ni ve rs ity N or th C ar ol in a - C ha pe l H ill ] at 0 7: 03 0 4 N ov em be r 20 14 WARNER, BRAUN: VENTUR1A INAEQUALIS/ASCOSPORE RELEASE 317 spores were trapped and counted as described previously. V. inaequalis ascospore densities were low in both orchards in 1990. Inoculum potential was increased for the 1991 study by raking fallen leaves heavily colonized with V. inaequalis into a 2 X 3 m area in the late autumn of 1990. The leaves were covered with nylon mesh to prevent them from blowing away over winter. In the spring of 1991 the spore trap was placed in the center of the 2 X 3 m area. Six (1990) or 12 (1991) greenhouse-grown Mclntosh apple seedlings, approximately 30 cm in height, growing in 25-cm-deep by 6-cm-wide black plastic pots were placed within 1-2 m of a spore trap at the beginning of each wet period. The seedlings were replaced with another set at 0700 and 1800 h EST as long as the wet period continued. After removal from the field, seedlings were placed in a greenhouse mist-chamber for at least 24 h at 18 to 20°C to allow any deposited ascospores to infect the leaves. Seedlings were examined for V. inaequalis lesion development 21 and 28 days later. The experiment was discontinued when primary scab lesions were observed in the orchard. Leaf wetness was recorded by a Dewit leaf wetness recorder (Valley Stream Farm, Orono, Ont.) and verified by visual and tactile assessments. Rainfall was measured with a tipping bucket rain gauge and recorded on a CR21X micrologger (Campbell Scientific, Edmonton, Alberta). Results Mist-bed studies, Kentville. In 1989 and 1990, an estimated 206 906 ascospores were trapped during 36 U 16 18 20 22 24 2 4 6 8 10 12 14 16 18 20 22 24 2 Time (h) Time (h) Figure 1. Estimate of total ascospore capture (TAC), percentage and time (AST) of Venturia inaequalis ascospore captures at the Kentville Research Station in mist-bed studies where the wet periods were initiated at various times of the day or night by timer- controlled mister (A-J). Estimated total numbers of ascospores captured are the mean of two- to seven-replicate experiments conducted in 1989 and 1990. The solid bar and shaded area indicate the duration of the wet period and darkness, respectively. D ow nl oa de d by [ U ni ve rs ity N or th C ar ol in a - C ha pe l H ill ] at 0 7: 03 0 4 N ov em be r 20 14 318 CANADIAN JOURNAL OF PLANT PATHOLOGY, VOLUME 14, 1992 D O 1— H - O èR U Q) L- D -»— () D O V) Q) L- o a. (/) o u (/) < 50 40 30' 20 10 0 40 30 20 m 0 30i 2b 15' 10 fv 0 40 30 20 10 0 50 40 30 20 10 0 16 20 24 4 8 12 16 20 24 4 8 V / / A ''% WARNER, BRAUN: VENTURIA INAEQUALIS/ASCOSPORE RELEASE 319 Time (h) Figure 3. The mean ascospore capture (MAC) and time (EST) of Venturia inaequalis ascospore captures during 11 wet periods initiated at 0100 h and kept wet until 1300 h in mist-bed studies at the Smithfield Experimental Farm in 1991. The solid bar and shaded area indicate the duration of the wet period and darkness, respectively. orchard during seven wet periods in 1991 (Table 1). Lesions developed on leaves exposed in the orchard between 1800 and 0700 h EST during six of the seven wet periods. This accounted for 16% of the lesions that developed on all exposed plants. Lesions developed on seedlings exposed to orchard air that contained as few as 17 ascospores/m3. In 1990, ascospore captures in two orchards were 995 and 2105 over 14 and 11 wet periods, respectively (data not shown). Although the ascospore densities were low, similar trends to those observed in 1991 were recorded. Spores captured in the dark comprised 18 and 6% of the season totals in the two orchards. Lesions developed on seedlings exposed in the orchard between 1800 and 0700 h on only one occasion, when 61 of 68 lesions were recorded at night. The maximum spore density recorded on that night was 42 ascospores/m3. Discussion While this study agrees with previous reports (Brook 1966, 1969a, MacHardy & Gadoury 1986) that light stimulates ascospore release of V. inaequalis, it disagrees with MacHardy and Gadoury's observation that no spores are captured until morning when rainfall begins after 1800 h (MacHardy & Gadoury 1986). The numbers and density of ascospores trapped in the S.E.F. orchards in 1990 were relatively low but in the mist-bed studies and in the 1991 orchard studies, ascospore numbers and densities were similar to or higher than those reported by MacHardy and Gadoury (1986). The densities of peak ascospore captures in the dark were usually higher than the average daytime spore densities reported by MacHardy and Gadoury (1986). In mist-bed studies, where ascospores were mature (ready for release) and the wetting uniform and adequate for unimpeded spore release, peaks in spore capture were recorded shortly after the initiation of a wet period at various hours during the night. In the orchard study (1991), spores were trapped shortly after leaf wetness was recorded in all eight wetness events that began after dark (between 2000 and 0500 h). In three of the eight leaf wetness events that began in the dark, the number of ascospores captured was low. There are other factors besides light and moisture, such as temperature and maturity of ascospores, that could have reduced or delayed ascospore captures (Brook 1969a, MacHardy & Gadoury 1986, Miller & Waggoner 1958). The data in this study are also consistent with Brook's (1969a, 1969b) observation that light stimulates spore release. Peaks in ascospore capture were recorded shortly after sunrise for all wet periods initiated in the mist- bed experiments and on all days in the orchard except on two occasions. On those two days the wet period ended shortly after sunrise and the drying conditions may have inhibited spore release. Conditions that trigger nighttime ascospore release, such as leaf wetting, mature ascospores and favourable temperatures, appear to be the same conditions which stimulate daytime release, except that light is a further stimulus favouring daytime release. Based on extensive ascospore trapping data from New Hampshire, MacHardy and Gadoury (1986) designated 0700 to 1800 h as day during which greater than 95% of all ascospores were captured. At the S.E.F. in 1991, only 69% of the ascospores were trapped between 0700 and 1800 h and 87% between 0500 and 2000 h, the average time of sunrise and sunset, respectively, at the S.E.F. during the ascospore season. In 1990, when spore captures in the two orchards were low, 66 and 86% of all ascospores were captured between 0700 and 1800 h and 82 and 94% between 0500 and 2000 h. Peaks in ascospore capture were observed between 1800 and 2000 h in mist-bed trials at the K.R.S. and at the S.E.F. Delaying the start of an infection period until morning if rain begins between 1800 and 2000 h could cause a failure to recognize an infection period Table 1. Number of foliar lesions caused by ascospores of Venturia inaequalis on Mclntosh seedlings exposed to wet periods in the orchard between 0700 and 1800 h (daytime) and 1800 and 0700 h (nighttime)(EST) in 1991 at the Smithfield Experimental Farm, Ontario Date of wet period Number (%) Day Night Apr 19-21 Apr 21-22 Apr 30 May 1-2 May 14 May 16-18 May 26-28 14 (70) 25 (78) 0 (0) 29 (97) 0 (0) 43 (98) 34(100) 6 (30) 7 (22) 8 (100) 1 (3) 4 (100) 1 (2) 0 (0) D ow nl oa de d by [ U ni ve rs ity N or th C ar ol in a - C ha pe l H ill ] at 0 7: 03 0 4 N ov em be r 20 14 320 CANADIAN JOURNAL OF PLANT PATHOLOGY, VOLUME 14, 1992 D ~o I— *+- O & CD Q_ D U (/) CD i_ O Q_ C/) O u (/) < * * - • C M - T T 5 70 6CH 50 40 30 20 10 10 12 14 16 18 20 22 24 2 4 6 8 10 12 14 252 18 20 22 24 2 4 6 8 10 12 14 16 18 20 22 24 ■» KJ \0 •- CM CM O CM O O * ■» » r» ■» "> c; b — CM — — ° o T—l—r~ f A n SS» il "lljw, n WARNER, BRAUN: VENTURIA INAEQUALIS/ASCOSPORE RELEASE 321 In 1991, 16% of the ascospore infections on seedlings placed in the orchard during wet periods occurred between 1800 and 0700 h. This level of infection is greater than the 3% in New Hampshire reported by MacHardy and Gadoury (1986). This may be the result of greater numbers and densities of ascospores in the air in Ontario and Nova Scotia during nighttime wet periods. Infections were recorded on nights when the maximum spore density was as low as 17 ascospores/m3 of air. Peak ascospore densities at night in the mist-bed trial were more than 50 times greater than in this orchard study. Based on their data, MacHardy and Gadoury (1986, 1989) considered the disease impact of nighttime ascospore release and infection to be insignificant. Aylor and Sutton (1992), however, expressed concern over excluding nighttime rains in model calculations after having observed a significant ascospore release at night during one of the 20 wetness events they examined in North Carolina. It is our opinion that under some circumstances in Nova Scotia and Ontario, nighttime ascospore release and infection may be great enough to cause a significant amount of disease, particularly since apple scab is a polycyclic disease. Recurring disease cycles could increase an initially small amount of disease to an economically significant amount of disease by harvest time. We agree with Brook (1969a) that "the influence of light is second in importance only to that of leaf wetness in triggering ascospore release." While ascospore captures at night may be low on average over the entire ascospore season, peaks of ascospore release when rain begins after dark and when large numbers of mature ascospores are present could cause significant disease in regions where numerous infection cycles occur. A plausible explanation for different ascospore release patterns between New Hampshire and Ontario and Nova Scotia has not yet been found. Aylor, D.E., and T.B. Sutton. 1992. Release of Venturia inaequalis ascospores during unsteady rain: Relationship to spore transport and deposition. Phytopathology 82:532-540. Brook, P.J. 1966. The ascospore production season of Venturia inaequalis (Cke.) Wint., the apple black spot fungus. N. Z. J. Agric. Res. 9:1064-1069. Brook, P.J. 1969a. Effects of light, temperature and moisture on release of ascospores by Venturia inaequalis (Cke.) Wint. N. Z. J. Agric. Res. 12:214-227. Brook, P.J. 1969b. Stimulation of ascospore release in Venturia inaequalis by far red light. Nature 222:390-392. MacHardy, W.E., and D.M. Gadoury. 1986. Patterns of ascospore discharge by Venturia inaequalis, Phytopathology 76:985-990. MacHardy, W.E., and D.M. Gadoury. 1989. A revision of Mill's criteria for predicting apple scab infection periods. Phytopathology 79:304-310. Miller, P.M., and P.E. Waggoner. 1958. Dissemination of Venturia inaequalis ascospores. Phytopathology 48:416-419. Mills, W.D. 1944. Efficient use of sulfur dusts and sprays during rain to control apple scab. N. Y. Agric. Exp. Stn. (Ithaca) Ext. Bull. 630 pp. D ow nl oa de d by [ U ni ve rs ity N or th C ar ol in a - C ha pe l H ill ] at 0 7: 03 0 4 N ov em be r 20 14