Abstract
AbstractWitches’ broom of birch (Betula spp.) caused by Taphrina betulina is an understudied disease that causes the formation of woody tumours, from which ectopic axillary buds and branches grow to form a broom‐like structure. We have addressed two aspects of this disease using naturally infected mature trees in the field. Broom symptoms offer a convenient means of scoring susceptibility in the field. Variation in broom symptom presentation suggests possible variation in resistance against witches’ broom disease. We tracked the local distribution of susceptible individuals among 721 trees at 159 independent sites. The analysis supports the hypothesis that there was genetic resistance segregating in these birch populations. Anatomical changes in broom symptom bearing branches of European silver birch (Betula pendula) were also addressed by comparing sections of tissues from three locations in the same branch, which were normal, swollen in infected tissue adjacent to a tumour, and inside a tumour. Examination of tumours revealed disorganized and swollen xylem, expanded secondary phloem and expanded periderm. Swollen tissues newly infected from spreading disease adjacent to tumours exhibited enhanced growth only in secondary phloem and the periderm, which also exhibited distortions. This finding suggests that tumour formation and possibly pathogen colonization may initiate in these tissues.
Highlights
Witches' broom disease, caused by Taphrina betulina Rostrup, forms characteristic broom symptoms on the branches of birch (Betula species; Figure 1a; Fonseca & Rodrigues, 2011; Mix, 1949)
Witches’ broom disease can be seen by the appearance of many new buds, most of which develop into axillary shoots that grow in clumps around the initial infection site to form a broom-like structure, sometimes appearing similar to a bird's nest. (Figure 1a,b)
To strengthen the evidence of genetic resistance in birch against witches’ broom disease, we examined a collection of 159 infected birch trees (Table S1) identified in the greater Helsinki region (Figure 2a,b)
Summary
Witches' broom disease, caused by Taphrina betulina Rostrup, forms characteristic broom symptoms on the branches of birch (Betula species; Figure 1a; Fonseca & Rodrigues, 2011; Mix, 1949). KEYWORDS Betula pendula, birch, disease symptoms, genetic resistance, Taphrina betulina, tumour growth, tumours, witches' broom disease Taphrina species that cause witches’ brooms, including T. betulina, cause perennial infections by invading and overwintering in woody tissues (Fonseca & Rodrigues, 2011).
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