Impact of phytoplasma infection of common alder (Alnus glutinosa) depends on strain virulence

In nature, only a minority of alder trees infected by the alder yellows phytoplasma develop symptoms. To elucidate the reason for this phenomenon, healthy alder seedlings were inoculated with scion wood from five infected but differently affected trees. The symptoms developed by the inoculated plants largely corresponded to disease severity of the inoculum sources. Inoculation with tissue from three of four severely affected trees resulted in severe disease of the recipient trees as expressed by reduced vigour, sparse foliage and little leaves. Inoculum from one of the severe sources caused milder symptoms. No symptoms were developed when the inoculum was collected from a vigorous, non-symptomatic tree. From these results it can be concluded that the alder yellows phytoplasma is pathogenic to alder but that avirulent and low virulent strains do occur.