Dark septate hyphomycetes in swiss conifer forest soils surveyed using Norway-spruce seedlings as bait

Soil samples were collected under Norway-spruce trees (Picea abies (L.) Karst.) on 72 sites throughout Switzerland, passed through a disinfected sieve and put into autoclaved clay pots. Surface-sterilized Norway-spruce seeds were sown into these pots and the seedlings were reared in a growth cabinet for 4 months. The root systems of nine seedlings per site were surface-sterilized and one 2–3 mm long segment of each of two root-diameter classes (class 1: dia <1 mm, class 2: dia 1–2 mm) was aseptically excised from each root system and plated out on 2% (w/v) malt-extract agar to isolate fungi. Dark septate hyphomycetes (DSH) could be isolated from the soil samples of all except two sites. The frequency of seedling roots colonized by DSH ranged from 11–100%, the average being 76%. Colonization of root diameter class 2 was significantly higher than the one of class 1. No site-dependent pattern of colonization could be recognized except for the soil pH value. The frequency of DSH was correlated with the pH value of the collection sites and reached the maximum between pH values 3.5 and 4.5. The health status of the seedlings as measured by needle color, presence or absence of needle tip chlorosis, the number of living seedlings per pot and the dry weight was not correlated with the colonization of the roots by DSH. About 49% of the DSH sporulating after 1 y of incubation at 4°C in the dark were identified as Phialocephala fortinii. Advantages and disadvantages of the baiting technique are discussed.