The influence of available water, expressed as osmotic potential (psi;s), on germination of microsclerotia (MS) and expansion of colonies ofVerticillium dahliaewas studied in osmotically adjusted aqueous and agar media. In distilled water (psi;sensp;=ensp;0 bar) at 25 deg;C, 64ndash;70 of MS germinated within 36ensp;h and the mean number of germ tubes per germinated MS (germination vigour) was 4.7. In aqueous solutions of KCl or NaCl percentage germination and germination vigour were increasingly inhibited as psi;s was lowered, reaching 50 inhibition at minus;11 to minus;14 bar and 100 inhibition at ensp;minus;98 bar. In aqueous sucrose solutions germination percentage and vigour were significantly higher (Pensp;=ensp;0.05) than in salt solutions; maximum activity occurred at minus;4 bar, 50 inhibition occurred at minus;40 bar, and 100 inhibition occurred at ensp;minus;114 bar. On corn meal dextrose agar (CMDA) amended with KCl, percentage germination at 25 deg;C was maximal at minus;9 bar, was reduced 50 at minus;40 bar, and was 0 at minus;90 bar. For a given psi;s, germination percentages at 25 and 30 deg;C were significantly greater than at 20 and 15 deg;C. No germination occurred at 4 or 35 deg;C. Where psi;s was adjusted with KCl, agar media were less inhibitory to germination than aqueous media between 0 and minus;50 bar. On CMDA media amended with KCl, colony expansion was progressively faster as psi;s was lowered from minus;1.1 to minus;13 bar, then progressively slower as psi;s was lowered further. Rates of colony expansion at 20 and 25 deg;C were significantly greater than at 4, 15 and 30 deg;C. At 25 deg;C growth was reduced 50 at minus;49 bar and was completely suppressed at minus;119 bar.
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