Botany  

Algae

• Algaebase: Database of algae that includes terrestrial, marine and freshwater organisms

Bryology

• Bryology (Missouri Botanical Garden): Contains links to a wide range of bryological specimen databases

Fungi

• Dictionary of Fungi 10th edition (CAB International)
• Index Fungorum (CAB International): Contains over 370,000 names of fungi

Plants

• Index Nominum Genericorum (Smithsonian Institution): Compilation of generic names published for all organisms covered by the International Code of Botanical Nomenclature.
• Integrated Taxonomic Information System (United States Government): Information on plants, animals, fungi, and microbes of North America and the world (United States Dept. of Agriculture)
• International Plant Names Index: Database containing the names and associated basic bibliographical details of all seed plants.
• Kew Database (Royal Botanic Gardens): The Kew Record database contains references to all publications relating to the taxonomy of flowering plants, gymnosperms and ferns.
• National Plant Database (United States Dept. of Agriculture): The PLANTS Database provides standardized information about the vascular plants, mosses, liverworts, hornworts, and lichens of the U.S. and its territories
• Index of Botanical Specimens (Harvard University): Specimen records, primarily type specimens from the Harvard University Herbaria and New England Botanical Club.
• Tropicos (Missouri Botanical Garden): Largest database of plant information contains fully web-searchable records for over 900,000 plant names and nearly 2 million specimens.

Plant Diseases

• Common Names of Plant Diseases (The American Phytopathological Society)