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United States Department of Agriculture

Agricultural Research Service

Title: Effect of Temperature, Soil Type and Matric Potential on Proliferation and Survival of Fusarium Oxysporum F. Sp. Erythroxyli from Erythroxylum Coca

Authors
item Fravel, Deborah
item Stosz, Sarah - UNIV MARYLAND
item Larkin, Robert

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 7, 1995
Publication Date: N/A

Interpretive Summary: Coca is an economically important plant both for legal use and for illicit production of cocaine. A soil-inhabiting fungus, Fusarium oxysporum f. sp. erythroxyli, causes a wilt disease coca but not other plants. This research was undertaken to understand how temperature, soil moisture and soil type affect the growth and survival of F. oxysporum. Soil moisture significantly affected populations of this pathogen. Growth occurred in moist ( > -100 KPa) but not dry soil (- 500 KPa). Growth of the fungus occurred at temperatures ranging from 15 to 40 C with optimum growth at 25 C. Survival over a 4 month period was significantly less at 40 C compared to other temperatures. To simulate day / night temperatures, the fungus was stored with cycles of either 15 C for 12 hours then 25 C for 12 hours, or 25 C then 35 C. With these fluctuating temperatures, populations of the fungus declined slowly over the 4 month test period. Populations of F. oxysporum were consistently higher in two of the four soils tested. This difference was lost when the soil was sterilized, indicating that soil microbes inhibited growth of F. oxysporum in the two soils that had lower populations. This information will be used by scientists to understand where coca wilt might become established.

Technical Abstract: A natural epidemic of fusarium wilt on coca, caused by Fusarium oxysporum f. sp. erythroxyli, stimulated interest in this pathogen because of the economic importance of coca for both legal use and for illegal production of cocaine. Research was undertaken to determine the effects of constant and fluctuating temperature, soil matric potential, and soil type on the proliferation of F. oxysporum. Three to four soils were used in these tests. Prill containing F. oxysporum were placed on the surface of soils and maintained at -10, -100 or -500 KPa for one week. Matric potential and soil type significantly affected proliferation of the pathogen into the soils. Populations of F. oxysporum at -10 and -100 KPa were 10**4 to 10**5 CFU/g, while populations at -500 KPa were 10**3 CFU/g. F. oxysporum proliferated from prill into soil when constant temperatures ranged from 10 to 32 C resulting in 10**3 to 10**5 CFU/g soil after 1 wk incubation. Optimum proliferation was at 25 C in all soils and significantly lower populations were recovered from soils stored at 40 C. Two regimes of fluctuating temperature (cycles of 12 h at 15 C then 12 h at 25 C, or cycles of 12 h at 25 C then 12 h at 35 C) were tested for their effect on proliferation of F. oxysporum. Initial populations of 10**3 to 10**6 CFU/g soil declined over the 17 week test period. In the above tests, population were greater in two of the soils. Comparison of proliferation in sterile and nonsterile soils indicated that two soils were fungistatic and two were conducive to F. oxysporum.

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