April 2021 - 26

PLANT HEALTH
Powdery mildew
on a perennial
verbena showing
individual
colonies and
coated leaves.
powdery mildew fungi themselves are most favored by high humidity
rather than outright wetness. Because of their sensitivity to environmental
conditions, powdery mildew problems fluctuate in seriousness from one
year to the next. They tend to be problematic primarily in late summer
and fall.
White colonies of powdery mildew are usually easy to identify. But be
forewarned that sometimes the fungus will be only on the undersurface of
leaves. If you are growing a plant you know is prone to powdery mildew,
turn leaves over if you see any spotting, just to make sure. Chlorotic
(yellow) spots or purplish
areas sometimes show
on the top of a leaf
when white colonies
are growing on the
undersurface.
Tan rust pustules and powdery mildew colonies
together on the undersurface of a monarda leaf.
The powdery mildew
disease most often
misidentified is the one
on sedum, which forms
brown scabby spots on
the leaves. The fungus
in this case is injuring
the tissue more than it
does ordinarily, so you
will need to look closely
with a hand lens or a dissecting microscope to see the whitish strands of
fungus growing across the scabby spots. Sometimes the fungus forms
water-soaked spots on the back of sedum leaves, looking more bacterial
than fungal.
The crops most inclined to powdery mildew include delphinium,
helianthemum, monarda, peony, phlox, sedum and scabiosa. As a first
line of defense, investigate whether you can find cultivars to grow that
are less susceptible. Popular Phlox paniculata used to be extremely prone
to mildew but shifts in growers' cultivar choices appear to have much
reduced the problem. Among whites, for example, phlox 'David' is far less
susceptible than phlox 'Mount Fuji'.
For powdery mildew control, look to potassium bicarbonate,
Rust
pustules on
veronica
leaves.
chlorothalonil and biocontrols containing Bacillus or Streptomyces as
surface-acting protectants that will keep minor problems in check. If a
serious problem with a powdery mildew arises, look to fungicides in FRAC
Group 11 (strobilurin) and FRAC Group 3 (DMI) categories for the benefit
of systemic action.
Fungicides in FRAC Group 11, 3, 11 + 3 or 11 +7 can be rotated with
contact-action protectants for a strong management program. See labels
for details on whether materials are labeled for greenhouse, nursery or
landscape use, which crops can be treated, and how many times a material
can be used before rotating to another FRAC Group.
RUST
Rust diseases look quite different from powdery mildews, but they also
are caused by fungi, and they are similar enough that generally the same
fungicides work on both diseases. As with powdery mildew, rust fungi can
be seen on the surface of the plant - at least their spores are external,
poised for dissemination by wind or water. Rust spores are held in clumps
referred to as rust " pustules " that are found mostly on the undersurface
of leaves. They often come in eye-catching colors, in a range of hues from
white to yellow to orange to rusty brown to chocolate brown or black.
Rusts are not as common as powdery mildews, but you probably grow
a few perennials that are susceptible. Heuchera has a rust with tan spore
pustules, Puccinia heucherae, which can affect other members of the
24 APRIL 2021 GPNMAG.COM
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April 2021

Table of Contents for the Digital Edition of April 2021
