Variations in growth and virulence of Leptographium wingfieldii Morelet, a fungus associated with the bark beetle Tomicus piniperda L.

Growth and virulence of various isolates of Leptographium wingfieldii Morelet, isolated from Tomicus piniperda L. and galleries initiated by this insect on Scots pine (Pinus sylvestris L.) at different parts of the forest of Orleans, were compared. Six isolates were tested in 1994 and 15 in 1995. The tested parameters included the growth on malt agar at different temperatures, the ability of the fungus to stimulate the hypersensitive reaction in the phloem and to grow into the reaction zone after isolated inoculations of Scots pine trees, as well as fungal virulence to Scots pine after mass inoculations. Significant differences between isolates were observed for all parameters. These differences mainly refer to gradual variations among isolates, even though some isolates were constantly opposed to each other for all or most parameters. No obvious effect of the locality, from where the isolates had been obtained, was detected. The results suggest that the population of L. wingfieldii in the forest of Orleans consists of isolates that possess a high individual variability in growth characteristics and virulence to their host tree. Fungal growth both on malt agar and in the phloem was positively correlated with the length of the reaction zone in the phloem after isolated inoculations and with parameters related to fungal virulence after mass inoculations. Likewise, length of the reaction zone in the phloem was positively correlated to the number of killed trees and the percentage of blue stained sapwood area after mass inoculations. The results are discussed in relation to the possible origin of the variability in fungal growth and virulence, and to practical use of fungal growth on malt agar and in the phloem for screening and comparing the virulence of various isolates within the same blue stain fungal species.