Effect of temperature, soil type and matric potential on proliferation and survival of Fusarium oxysporum f sp erythroxyli from Erythroxylum coca

Natural epidemics of Fusarium wilt on coca have stimulated interest in the causal agent Fusarium oxysporum f. sp. erythroxyli. Effects of constant and fluctuating temperatures, soil matric potential, and soil type on the proliferation of F. oxysporum f. sp. erythroxyli isolate EN4-FT from an alginate prill formulation were studied. Three or four soils were used: a Galestown gravelly loamy sand (GGLS), Hatboro loamy sand (HLS), and red clay subsoil (RC) were collected in Beltsville, MD. A clay loam soil from a planting of coca in Hawaii (HCL) was substituted for RC soil in some experiments. Frill containing F. oxysporum f. sp. erythroxyli were placed on the surface of GGLS, HLS, or HCL soils and maintained at -10, -100, or -500 kPa for 1 week. Matric potential and soil type significantly affected proliferation of the pathogen into the soils. Proliferation was greatest in HCL soil and least in HLS soil. The population density of F. oxysporum f. sp. erythroxyli at -10 and -100 kPa was approximately 10(4) to 10(5) CFU/g, whereas the population density at -500 kPa was approximately 10(3) CFU/g. During 17 weeks of sampling GGLS, HLS, and RC soils maintained at -50 kPa, F. oxysporum f. sp. erythroxyli proliferated from prill into soil when constant temperatures ranged from 10 to 32 degrees C, resulting in 10(3) to 10(5) CFU/g of soil after 1 wk of incubation. Optimum proliferation was at 25 degrees C in all soils. Compared to other temperatures, lower numbers of propagules were recovered from soils stored at 40 degrees C. Two regimes of fluctuating temperature (cycles of 12 h at 15 degrees C followed by 12 h at 25 degrees C or cycles of 12 h at 25 degrees C followed by 12 h at 35 degrees C) also were tested for their effect on proliferation of F. oxysporum f. sp. erythroxyli from prill into three soils maintained at -50 kPa. Population densities were initially 10(3) to 10(6) CFU/g of soil and declined over the 17-week test period. After 17 weeks, population densities in GGLS soil were greater than in HLS or RC soils in both temperature regimes. In HLS and RC soils, the number of CFU of F. oxysporum f. sp, erythroxyli per g was significantly higher in autoclaved than in nonautoclaved soil, indicating that these two soils were fungistatic. No differences were observed between autoclaved and nonautoclaved soils for HCL and GGLS soils, and these two soils were considered conducive to proliferation of F. oxysporum f. sp. erythroxyli.