Shoot-feeding Aphids Promote Development of Gremmeniella abietina, the Fungal Pathogen Causing Scleroderris Canker Disease in Conifers

We tested experimentally whether shoot feeding aphids Cinara pinea (Mordv.) can promote the development of Gremmeniella abietina (Lagerb.) Morelet, a fungus which causes Scleroderris canker disease in conifers. Pine seedlings were infested with aphids at two different times, and subsequently inoculated with conidia of G. abietina at two different times. The degree of infestation was classified into three groups based on the number of aphids/seedling: none, one (low level), and three (high level). Because of parthenogenetic reproduction, the number of aphids increased during the summer, so we also used aphid numbers to explain the development of the symptoms. Necrosis caused by G. abietina was more prevalent in seedlings infested by aphids in June, and inoculated with fungus in July. Expressed as percents of the total length of the shoots, the necrosis had advanced 50%, 70%, and 95% with no, low, and high aphid levels, respectively. Canker formation was significantly greater when G. abietina conidia were inoculated in July, compared to August. In August, there were no statistically significant differences in the intensity of disease between aphid infestation levels, but the mean number of aphids was related to disease intensity. In the spring following infection, seedlings with aphids had more dead and fewer healthy terminal buds compared to seedlings without aphids.