Tulip Fields, Keukenhof, The Netherlands

Diseases
Botrytis tulipae is a major fungal disease affecting tulips, causing cell death and eventually the rotting of the plant. Other pathogens include anthracnose, bacterial soft rot, blight caused by Sclerotium rolfsii, bulb nematodes, other rots including blue molds, black molds and mushy rot.
Variegated varieties admired during the Dutch tulipomania gained their delicately feathered patterns from an infection with the tulip breaking virus, a mosaic virus that was carried by the green peach aphid, Myzus persicae. These aphids were common in European gardens of the seventeenth century. While the virus produces fantastically colourful flowers, it also causes weakened plants that die slowly.
Today the virus is almost eradicated from tulip growers' fields. Tulips that are affected by mosaic virus are called "broken tulips"; while such tulips can occasionally revert to a plain or solid colouring, they will remain infected with the virus. While some modern varieties also display multicoloured patterns, the patterns result from intentional breeding. In these tulips, natural differences in the upper and lower layers of pigment in the flower are responsible for the patterns.