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Population Dynamics of Fythium Aphanidermatum In Population dynamics of Pythium aphanidermatum in field soil Item Type text; Thesis-Reproduction (electronic) Authors Burr, Thomas James, 1949- Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 30/09/2021 02:59:16 Link to Item http://hdl.handle.net/10150/566316 POPULATION DYNAMICS OF FYTHIUM APHANIDERMATUM IN FIELD SOIL b y Thomas James Burr A Thesis Submitted to the Faculty of the DEPARTMENT OF PLANT PATHOLOGY In Partial Fulfillm ent of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 19 7 3 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillm ent of re­ quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Deem of the Graduate College when in his judg­ ment the proposed use of the material is in the interests of scholar­ ship. In a ll other instances, however, permission must be obtained from the author. Ty APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: ZMICHAEL E . STAgGHELLINI Assoc. Professor of Plant Pathology ACKNOWLEDGMENTS The author expresses deep appreciation for the concern and guid­ ance of Dr. Michael E. Stanghellini, Associate Professor of Plant Pathology, who made this study a rewarding educational experience. Appreciation is also expressed to Dr. Richard B. Hine and Dr. Homer E. Bloss who contributed constructive suggestions to this study. The author is also grateful to Judith A. Bonsall for her unself­ ish contributions toward manuscript preparation. i l l TABLE OF CONTENTS Page LIST OF TABLES.............................................................................................................. v LIST OF ILLUSTRATIONS.................................................................................................v i ABSTRACT................................................................................................................................v i i 1 . INTRODUCTION...................... 1 2 . LITERATURE R E V IE W ..................................................................................................... 3 3 . MATERIALS AND M ETHODS........................................................................................... 6 4 . R E S U L T S ............................................................................................................................. 8 5 . DISCUSSION AND CONCLUSIONS....................................................................................... 22 LITERATURE C IT E D ............................................................................................................... 29 i v LIST OF TABLES T ab le Page 1. Origin of colonies of Pythium aphanidermatum from naturally infested soils on a species specific i s o l a t i o n m e d iu m ............................................................................................... 11 2. Effect of cyclic wetting and drying, asparagine amendments and supersaturation on populations of Pythium aphanidermatum in a naturally in­ f e s t e d f i e l d s o i l ....................................................................................... lU 3. Variability of oospore size and growth rate of 25 Pythium aphanidermatum isolates obtained from v a rio u s n a t u r a l l y in f e s t e d f i e l d s o i l s ........................................... 21 v LIST OF ILLUSTRATIONS Figure Page 1. Relationship between Pythium aphaniderroatum popula­ tion and soil dilution ................................................................................ 9 2. Photographs of A) colony morphology and B) originat­ ing propagule (oospore) of Pythium aphanidermatum on a species specific isolation medium after 72 h o u rs in c u b a tio n a t 35 C ............................................................................. 10 3* The verticle distribution of oospores of Pythium aphanidermatum in soil cropped to alfalfa for th r e e c o n s e c u tiv e y e a rs ........................................................................... 15 4. Sequential stages of germinating zoospore cysts of P . aphaniderm atum in f i e l d s o i l ...............................................................18 v i ABSTRACT Oospores were the sole survival structure of Pythium aphanider- matum in naturally infested soil. Population counts determined on a species-specific isolation medium from naturally infested soils of var­ ious types ranged from 10- 2$0 oospores/g soil. The highest concentra­ tion of oospores were found in the top 15 cm of soil "but could be detect­ ed t o 30 cm in depth. No evidence was obtained for the presence of a constitutively dormant oospore population in naturally infested soils. Enzymatic deg­ radation of colonized organic matter apparently resulted in liberation of oospores which are exogenously dormant. In naturally infested soils, the minimum inoculum density of P. aphanidermatum necessary for host colonization was two oospores/g soil. Among 25 soil isolates, little or no variation was observed with respect to oospore size, growth rate, colony morphology, spore-producing ability or pathogenicity. All isolates caused 100$ post emergent damping off of sugar-beet seedlings after 48 hour incubation at 35 C. v i i CHAPTER 1 INTRODUCTION The genus Pythium vas first described by Pringsheim in 1858 ( 4 3 ) . Since that time over 66 species have been identified (35)• The genus is worldwide in distribution, parasitizing aquatic as well as terrestrial plants (20). One member of the genus, Pythium aphanidennatum (Edson) Fitzpatrick, which was first described causing a seedling disease on sugar beets in 1915 (ll), has subsequently been reported as a parasite of over 100 different genera of higher plants ( 63). Although the organ­ ism is usually reported as a pathogen of seedlings (6, 21, 24, 4$, 46, 53> 6l, 66), it is also known to cause rots of mature roots (l6, 23, 31* 32, 40, 4l, 44), blights (l8, 28), stem rots (12, 34, 42, 62), a n d s o f t r o t s (4 , 7 , 8, 30, $6). Disease incited by this fungus is associated with high soil moisture conditions and high soil temperatures, 27 C or g r e a t e r . In Arizona P. aphanidennatum was found to be indigenous on na­ tive plant species and responsible for extensive crop losses during the summer months (23, $ 2 ). The objectives of this thesis were to determine l) the nature of the persisting propagule of P. aphanidennatum in naturally infested soils, 2) the verticle distribution of the fungus in soil, 3 ) th e m in­ imum inoculum density necessary for colonization of a host, 4) environ­ mental and physical conditions which tend to alter natural soil populations 1 and, 5) pathogenic variation among soil isolates. Attainment of these objectives could provide knowledge for application of techniques for biological control of P. aphanidermatum. CHAPTER 2 LITERATURE REVIEW Selective media allow for the isolation of a specific microflora from a complex environment such as soil. Before the advent of media con­ taining the polyene antibiotics (pimaricin, endomycin, or nystatin), se­ lective isolation of most Pythiacious fungi was virtually impossible. The first selective medium for the isolation of Phytophthora and Pythlum from plant roots was developed by Eckert and Tsao in i 960 with the aid of the polyene antibiotic, pimaricin (10). Singh and M itchell in 1961 (47) were the first to report a highly selective pimaricin containing me­ dium specifically for the isolation of Pythium spp from soil. Pimaricin inhibits the growth of nearly a ll common soil fungi except a small group that includes the members of the Pythiaceae (10, 17, 47). L ittle empha­ sis has been previously placed on development of media for isolating in­ dividual species within the genus Pythium (60). A wide range of natural populations of Pythium spp in soil have been reported (19, 25, 51, 65). Where quantitative attempts have been made for determining populations of a single species up to 38OO p ro p a - gules/g of naturally infested soil have been reported (51)• Hine and In n a in 1963 developed the first baiting technique for isolation of P. aphanidermatum from soil (22). However, quantitative data and the na­ ture of the persisting propagule of P. aphanidermatum could not be de­ termined with this method. Indirect methods have implicated oospores as 3 4 the primary survival structure of P. aphanidermatum in field soil (2, 22, 52, 59)• Direct observation has shown that sporangia of P. aphaniderma­ tum are incapable of persisting in air dried soil for more than three days (49). Zoospore survival capabilities of P. aphanidermatum have not been fully elucidated, although zoospores have been recovered from soil up to seven days after artificial infestation (59)• The verticle distribution of P. aphanidermatum in soil has been previously determined using semi-quantitative techniques (57, 39)•
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