following question was sent to the P&PDL diagnosticians here
at Purdue University:
Question: The leaves on our black-eyed Susans are covered
with small, dark brown spots. This happens every summer. It happens
to plants in full sun as well as those in partial shade. What causes
Answer: Almost every Rudbeckia 'Goldsturm'
(a cultivar of black-eyed Susan) I see this time of year is infected
with Septoria leaf spot, caused by the fungus Septoria rudbeckiae.
Symptoms begin as small, dark brown lesions, which enlarge to one-eighth
to one-fourth inch in diameter.
The fungus overwinters in infected
plant residue. Spores are produced in late spring and early
summer, causing leaf spots on the lower leaves. As the season progresses,
lesions develop on upper leaves as well. The spores of the
fungus are dispersed by splashing water (either irrigation or rainfall),
and can cause lesions throughout the growing season. Like most
fungal leaf spot diseases, the spores require moisture to germinate
and cause infection.
It is important to remove the infected leaves
at the end of the growing season to reduce the amount of spores
available the following year. Proper plant spacing will increase
air circulation around the foliage and allow leaves to dry off
quickly after dew or rainfall events. Since Rudbeckia plants
spread quickly, this will involve pulling volunteer plants. Avoid
overhead irrigation, which will promote leaf wetness and also splash
spores from plant to plant.
While Septoria leaf spot is unsightly, the damage
is primarily cosmetic, and infected plants will bloom. Infected
leaves may die a little earlier in the fall than uninfected leaves.
A general-purpose garden fungicide may help reduce the spread of
the disease, but these chemicals are protectants and do not cure
infected leaves. Application in early to mid June may help reduce
initial infection, and result in a slower onset of disease symptoms.
For maximum control, application of a protectant fungicide should
be made periodically throughout the growing season (check label
for instructions on spray interval and rate). This is impractical
for most gardeners, however. I try to plant something in front
of them to hide the unsightly leaves, instead.
-- Beverly Shaw, Advanced Master Gardener, Purdue University
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