Sporulation and wet decay on begonias attacked by Botrytis
Photo courtesy of Margery Daughtrey
Sporulation on a leaf spot indicates that high humidity is encouraging disease spread.
Photo courtesy of Margery Daughtrey
Botrytis leaf spots that started from petal tissue landing on vinca leaves
Photo courtesy of Margery Daughtrey

Botrytis leaf spot is one of the aspects of a wide-ranging disease often called Botrytis blight. The ubiquitous fungus Botrytis cinerea is most famous for what it can do to flowers, but on some species, under favorable environmental conditions, it can cause spotting on leaves as well. Plants that are especially prone to Botrytis attack include begonia, geranium, vinca, snapdragon, lisianthus, osteospermum, poinsettia, New Guinea impatiens, exacum and zinnia. Spots may be round or oval if they begin in the middle of the leaf blade, or form wedges if they develop at the leaf edge. The galloping leaf spots continue to enlarge at every opportunity, sometimes showing several zones that indicate a series of humid periods. If the environment is favorable to disease development, the spots may progress down the petiole or via a cutting wound into the stem. Low light will favor Botrytis, so it is often seen in the greenhouse during winter. The fungus will attack shaded lower leaves, but may also colonize leaves killed by other causes because it can grow on dead plant tissue. Wounds made during cutting transit are a common site of entry. Botrytis leaf spots are easily recognized by the grayish brown sporulation that covers the surface of leaf lesions when humidity is high. Management is centered around manipulation of the environment to reduce relative humidity and minimize the length of time that leaf surfaces are wet.

Petals falling onto geranium leaves can give Botrytis leaf spots their start.
Photo courtesy of Margery Daughtrey
Zonate Botrytis leaf spots on geranium leaf
Photo courtesy of Margery Daughtrey

Margery is a plant pathologist specializing in ornamentals at Cornell University’s Long Island Horticultural Research & Extension Center. mld9@cornell.edu