Call it Cupid's disease, indiscretion, or perhaps a case of flying a wee bit too close to the sun, but my rubeckia (Black-Eyed Susans) have contracted Septoria Leaf Spot: small, dark brown or purplish lesions peppered across its leaves.
Most likely caused by the fungus Septoria rudbeckiae, the damage is primarily cosmetic. The fungus overwinters on dead leaves and can remain viable for two or three years in the soil. By late spring or early summer -- right about now -- the fungus emerges and works its way up the plant from bottom to top. Moisture is essential to spread the creeping crud, so overhead watering is strongly discouraged. (Mutter-Natur take notice. Enough with the overhead watering already.)
Prevention is the only real treatment option. Remove infected leaves at the end of a growing season, consider spraying a fungicide early in the season, and have a candid and frank dialog about the dangers of running with a fast crowd.
UPDATE -- (6/24/07) A week ago, all infected leaves were removed and each plant was sprayed with broad spectrum fungicide containing chlorothanil. So far, so good. There are only a very small number of spots on the leaves and each plant appears to be doing well despite the loss of foliage. All other things being equal, the weather has turned much warmer and drier, so it's tough to pinpoint just what caused the spot to go away. At the least, removal of infected leaves was likely a major component of any turnaround.
Sources & More Information: U. of Minn., Plant & Pest Digital Library, Purdue U., New England Greenhouse, Walters Gardens, U. of Ill.
It's been seasonably warm, but not too hot. Rudbeckia, echinacea, and astilbe are nearly in full bloom. The beans and peas keep producing while the squash plant continues growing by leaps and bounds.
