Eradication of endophytes from needles, their inoculation with Lophodermium piceae and persistence of this fungus in needles of Norway spruce

Abstract

Lophodermium piceae is the most common endophyte of Norway spruce (Picea abies) needles, and it probably occurs in the total distribution area of Norway spruce. Its significance to the host tree is still largely unknown mainly because no methodology is known for generating endophyte-free saplings nor for inoculation of intact needles with this fungus. We report here how a daily heat treatment at 40 °C for 8 h eradicates endophytes of Norway spruce needles within a few days without causing visible harm to the plants. We compared various methods for inoculation of endophyte-free saplings. In order to obtain effective inocula, we investigated what factors activate dormant L. piceae infections in the needle tissue resulting in ascomata formation. Best inoculation success was achieved by suspending naturally shed, highly infected needles above the saplings. Infection frequencies of up to 70% of needles were achieved with repeated inoculations in the course of 3 years. Once established in needles, individual infections by L. piceae can persist for at least 5 years. Individual infections remain small in young needles, but at a needle age of 6 years, they start to grow and spread resulting in presence all over the needle from base to tip at a needle age of 7 years, but still without causing visible symptoms on the needles. The presented methodology for generating both endophyte-free and endophyte-infected saplings of Norway spruce provides a new approach for testing the significance of infections by L. piceae to insect herbivory and diseases of Norway spruce needles.

Palatability of Norway spruce needles infected with Lophodermium piceae to larvae of two sawfly species

“…Eighteen Norway spruce saplings, six of each of three clones (T4658, T4748 and T5134 derived from rooted cuttings and generated at Luke from trees in Central Finland in 2006) were heat treated for eradication of needle endophytes (Müller and Hamberg 2021). Then the saplings were transplanted to a shady garden site outdoors in Helsinki in October 2008 in groups of three seedlings per group including one sapling of each clone.…”

“…Thereafter half of the sapling groups were inoculated with L. piceae and the rest were used as controls. Inoculation was achieved by placing naturally shed, highly infected needles on a metal sieve (mesh size 5 mm) that was suspended above each group of three saplings (Müller and Hamberg 2021).…”

“…This restricted infection habit means that wound-inoculation of twigs with this fungus would probably not result in colonization of nearby needles similarly as with P. scopiformis mentioned above. The infections by L. piceae in needles are persistent and can exist in a dormant mode for at least six years within the needles (Müller et al 2021).…”

“…We recently reported a method for eradicating endophytic fungi from needles of Norway spruce saplings (Müller and Hamberg 2021). In that publication we also described how endophytefree intact needles can be inoculated with L. piceae in an outdoor experiment where the saplings were confined within plastic bags to prevent infections by ambient airborne spores.…”

Evidence to support phytosanitary policies–the minimum effective heat treatment parameters for pathogens associated with forest products

“…Horner et al (2019) found soil should be heated to 50°C for 72 h, while plants could be treated at 40°C for 1-2 days or 35°C for 7 or more days to deactivate P. agathidicida. Furthermore, repeated daily heat treatment of 40°C for 8 h eradicated culturable foliar endophytes of Norway spruce (Picea abies) after 9 days of treatment (Müller and Hamberg, 2021). Lesions on live Citrus paradisi trees were heat treated with a blowtorch resulting in complete mortality of P. parasitica above 60°C with full recovery of the tree (Hough et al, 1979).…”