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Summary.-ln the middle of the nineteenth century, Miles Joseph Berkeley and Camille Montagne contributed actively to the knowledge of plant diseases. The history of the powdery mildew of the vine allows one to outline the ir role and the importance of international relationships in the struggle against parasitic fungi. 2 -Ce nouveau charfl>ignon est-il responsable de la maladie observée ? 3 · Quel est le meilleur traitement à employer ? Si la réponse à la première question révèle les obstacles à la compréhension de ces formes fongiques imparfaites, la façon de répondre à la deuxième est représentée par deux théories : les uns, à la

Recent changes in seaweed communities due to rapid shifts of their environment have been largely documented in temperate waters; however, long-term studies are much scarcer. In this study, we investigated the potential of Dinard Herbarium to inform us on temporal changes in seaweed communities of Brittany since the second half of the 19 th century. From nearly 3000 specimens collected in France, we identified and, when possible, corrected taxonomic and localisation errors usually encountered in unchecked natural history collections therefore minimising data inaccuracies. This original study aimed to describe the temporal dynamics of this collection and discuss the relevance of Dinard Herbarium in assessing temporal changes in seaweed communities of Brittany. The use of herbaria to assess spatio-temporal changes of seaweed biodiversity is limited, and to our knowledge, this is the first study trying to investigate this potential in the European Atlantic. This work presents new perspectives for inferring temporal changes of seaweed biodiversity.

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