An Introduction to Water Molds
Spring rains can create growing conditions that are
devastating to most landscape plants. For
example, excess water is responsible for a disorder called “wet feet,” which results
from the suffocation of plant roots as waterlogged soil loses oxygen. However, suffocation is not the only injury to
nursery or landscape plants during rainy spring weather. Wet soils are favored by a group of pathogens
called water molds, or oomycetes, which cause a range of root and stem diseases.
Water molds are found in most soils, but plant stress and
high pathogen numbers can lead to severe disease. Common water molds such as Phytophthora and Pythium cause roots
rots, stem rots, collar rots, and damping off diseases in both woody and
herbaceous plants. They are also
responsible for downy mildews and some foliar blights in upper plant parts.
Symptoms
Symptoms differ according to plant type and infection
site. Root rot symptoms begin, not
surprisingly, at the roots. However,
because roots are concealed, disease often goes undetected until plants begin
to decline or upper plant parts wilt as a result root reduction. Disease often begins during rainy spring
weather, but it is typically not noticed until hot dry weather initiates
wilting.
Water molds can also cause above-ground infections. These symptoms can range from yellow mottling
of leaves to water-soaked lesions on leaves and succulent stems. Woody stems and trunks may develop cankers
just above the soil line, often at a wound site. Cankers are usually dark-colored and may
exude sap or “bleed.”
Uniqueness of water molds
Water mold pathogens are very different from true fungi;
they require free water to complete their life cycles. Initial infections often occur during rainy
spring weather as temperatures begin to warm.
After infection, pathogens release large numbers of “swimming” spores
that move on films of water. This is the
repeating stage that leads to disease epidemics if wet conditions continue. Spores are spread by splashing water and
movement of contaminated soil particles.
Once established, water molds can produce survival
structures that allow them to lie dormant during hot dry seasons. Available water can reinvigorate these structures
and the disease cycle can begin again. Many
water molds occur naturally in soils, and proliferation under wet conditions
can be devastating to plants.
Disease Prevention Using Cultural Practices
Most water mold diseases can be prevented or managed using
cultural practices. Consider the
management tips below to prevent infections or to help manage infected nursery
or landscape plants.
·
Improve drainage
o
Manage surface water
o
Plant in raised beds
o
Divert downspouts
o
Use organic matter to improve internal drainage
o
Limit irrigation
o
Manage nursery runoff from infested areas
·
Disinfest tools, containers, and greenhouses to eliminate spread
o
Commercial sanitizers are available
o
10% Lysol® concentrate and 10% bleach are also
effective. Bleach is corrosive on metals,
so rinse tools well before storage.
·
Dispose of infested potting media
o
Do not reuse contaminated soils
·
Destroy infected nursery and greenhouse plants
as soon as possible
o
Do not compost infected plants
·
Remove plant debris and other sources of
inoculum before spring
o
Rake and destroy leaves and debris
·
Reduce splash
o
Use drip irrigation
o
Mulch exposed soils
·
Use resistant cultivars whenever possible
Management using fungicides
Water molds are not true fungi, so not all fungicides will
be effective against these pathogens. Fungicides
must be specifically labeled for oomycetes.
Select fungicides that contain one of the active ingredients listed
below. Efficacy of these fungicides is
dependent on plant and pathogen type; read labels carefully.
Fungicide active ingredients effective against water molds:
·
Azoxystrobin (Heritage)
·
Cyazofamid (Segway)
·
Etridiazole (Terrazole, Banrot)
·
Mefenoxam (Subdue)
·
Propamocarb (Banol*)
·
Phosphorus acid (Alude, Agri-Fos**)
* Not for use in residential landscapes, for commercial use
only.
** Available to homeowners.
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