April 2021 - 24

PLANT HEALTH
Perennial
Problems
with Mildew, Rust
and White Smut
Which pathogens should you expect,
and how can you counteract them?
BY MARGERY DAUGHTREY
H
erbaceous perennials are thought of as
rough and tough once out in the garden, but
when brought together in large groupings of
one species in a greenhouse or nursery, they can be
especially vulnerable to certain diseases. Fortunately,
many of the pathogens are host-specific, so that
problems are usually limited to one kind of plant. This
article will give you an idea of the powdery mildew,
rust and white smut diseases to expect, and how to
counteract them.
POWDERY MILDEW
Growing herbaceous perennials often means
growing powdery mildew as well. This group of fungal
plant parasites is irritatingly conspicuous, causing
white spots called " colonies " or coating plants with
a thin growth of fungus strands so they look gray
instead of green. Unlike other diseases, with powdery
mildew you can see the fungus because it grows
along the plant surface, inserting tiny haustoria
(structures for nutrient and moisture absorption).
The fungus spores (conidia) are in long chains
projecting out from the leaf, stem or petal surface.
Conidia are often produced in great abundance and
they are moved about by air currents, so a powdery
mildew can spread quickly within a production area
when the environmental conditions are just right.
The main drivers of powdery mildew are
temperature and moisture. Excess moisture favors
the parasites of the parasites - fungi such as
Ampelomyces quisqualis that prey upon powdery
mildews thrive under wet conditions, whereas the
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