Our Asters are green and getting ready to bloom but the
lower 8″ are “rusty” or burnt looking. Did our bark mulch hurt them? Too much
fertilizer? This is their third year in the same spot.
There are quite a few potential causes of the rusty leaves you are
seeing. It might be entirely normal, as mature Asters (renamed Symphyotrichum) can start looking a bit ragged in late summer. It could be due to excessive heat,
overwatering (symptoms include yellowing and dropping of lower leaves),
or fertilizer burn (Asters are sensitive to soluble salts in chemical
fertilizers). It could be a fungal disease, rust, or Aster yellows, a
common disease caused by a microscopic organism (phytoplasma) and spread
from plant to plant by leafhoppers. With Aster yellows, you would notice
a loss of green in the leaf veins, and yellowing of new leaves.
Sometimes, infected outer leaves turn a rusty or reddish purple color. A
good general practice to keep your Asters looking full and less leggy is
to cut them back by one-half to two-thirds when they have reached 12 to
16 inches in late spring/early summer. (Source: The Well-Tended Perennial
Garden by Tracy DiSabato-Aust, Timber Press, 1998).) Here are
links to resources which describe other possible causes of the leaf
problem.
Connecticut Agricultural Experiment Station
To determine the exact cause, it might be worth bringing a sample to a
Master Gardener Clinic.