Ericaceous plants (rhododendrons, azaleas, and andromeda),
lilacs, Potentilla and yews experience serious disease problems caused
by various species of the fungus Phytophthora. Disease problems can develop
as either shoot dieback or root rot, depending on the fungus species and
when it entered the plant. Eventually, these fungal diseases can kill
landscape and nursery plants, but they are a more serious concern in container-grown
nursery stock. There are a number of species that cause both diseases,
but the two most common are Phytophthora cactorum fungus (dieback) and
Phytophthora cinnamomi fungus (root rot).
Life Cycle
Before discussing these two diseases and their management, a brief introduction
to the life cycle of this fungus would be helpful. There are four possible
forms of infective structures produced by Phytophthora-sporangia, zoospores,
oospores and chlamydospores. Water is required for germination of all
spore types.
The sporangia are vegetative structures that can germinate by a germ tube
that produces new mycelium. However, they usually convert their structures
into motile spores (zoospores) that exit through the tip of the sporangium.
These spores have a "tail" that enables them to move in water.
They are attracted to amino acids and other chemicals given off by roots
or succulent plant parts. After a zoospore contacts one of these plant
parts, it sheds its "tail" and produces a germ tube that penetrates
the plant and forms mycelium in the plant tissue.
Oospores develop from the sexual combination of two fungi and serve as
a fungus resting stage. Though few in numbers, they are important because
they combine the genetic material from two different fungi. Chlamydospores
are vegetative cells, found within hyphae that develop thick walls and
also serve as resting structures. Both of these stages germinate through
a germ tube that can develop into either mycelium or sporangia depending
on the species. They can also withstand adverse environmental conditions.
Disease Development
Plants that contract dieback (P. cactorum) become infected when zoospores
are splashed by water from the soil to plant leaves. In order for this
type of mobile spore to enter the plant, the leaves must be wet. Also,
initial infection primarily occurs on young, tender leaves. Lesions begin
to appear on the leaves in two to three days, particularly during hot
and humid weather. These leaves will turn brittle and curl inward. The
disease can then progress into the petiole and on into the stem, gradually
working down the stem and into mature leaves. Infected leaves containing
fungal spores fall to the ground or open to release spores, which allows
the cycle to repeat. Also at this time, the spores of Phytophthora species,
that can cause both dieback and root rot, can be moved into the soil by
water. Eventually, these spores may contact and enter the fine roots of
surrounding plants and cause root rot.
Phytophthora root rot disease (P. cinnamoni) begins with an invasion
of the fine roots causing them to turn brown and die. The pathogen spreads
into larger roots and moves towards the root crown. The plant can be girdled
as the fungus moves up the stem. The stem cambium turns brown first followed
by the phloem and xylem. Since root and stem tissues are being destroyed,
the leaves will become chlorotic, roll downward toward the midrib and
gradually wilt. One-to two-year-old container-grown rhododendrons that
are highly susceptible to the disease may die within 14 days. Older landscape
plants can show symptoms for a year before dying, or they may show no
above ground symptoms until various stress factors cause the weakened
plants to die.
Prevention and Management
Disease prevention is easier in landscape locations than in nurseries.
Since Phytophthora dieback affects the young upper foliage, low to moderate
nitrogen fertilization prevents the plant from continually putting out
new, vulnerable leaf flushes. Watering via drip irrigation or using a
water wand to keep water off the leaves will keep the fungus from splashing
onto the plant. Phytophthora root rot can be prevented in landscapes by
locating the plants in areas that have good soil drainage and do not suffer
through extended wet periods, and by not overwatering. If excess moisture
is a problem planting in raised bed with the addition of sand and composted
pine bark will help to control Phytophthora root rot. Also, there are
cultivars and hybrids of rhododendron and azalea that are resistant to
Phytophthora root rot that can be planted to reduce the risk of infection.
If a landscape plants is diagnosed with the disease, they should be removed
from the site. Near by plants should be carefully monitored to see if
they start to show symptoms.
Prevention and management in the nursery requires a variety of techniques
due to the increased number of plants and certain nursery practices. Since
container plants must be produced quickly, they receive high levels of
fertilization to encourage new growth. Frequent overhead irrigation splashes
spores into the air and onto young leaves. These two cultural practices
make container-grown plants susceptible to Phytophthora dieback for the
entire growing season. Also, the runoff from heavy and frequent irrigation
of container-grown ericaceous plants moves the root rot fungus spores
from infected plants to the roots of neighboring plants. The following
lists provide a brief overview for Phytophthora prevention in a nursery.
In the propagation area:
- Clear away all debris and old medium which may be contaminated with
Phytophthora spores.
- Wash the area with a sanitizing agent to remove and destroy any disease
agents.
- Maintain good general sanitation (eliminate debris, weeds and puddles,
keep hoses off the floor).
- Fill raised benches with fresh propagation medium containing 25 to
35 percent air-filled pore space. This will enable root initiation to
begin quickly and make the plant less susceptible to disease.
- Harvest cuttings from soil-free plant parts so that spores in the
soil are not transferred to new plants.
- Periodically dip harvesting tools in 70% alcohol to prevent contamination.
- Apply appropriate fungicidal drenches and sprays after cuttings are
potted or lined out
In the Nursery
- Select a root-rot-suppressive growing medium containing tree bark
that provides good aeration and drainage. Tree bark also releases chemicals
as it decomposes which kill Phytophthora spores.
- Remove plant debris, which may contain pathogens, from - the container
area.
- Place containers on well-drained & puddle-free beds (not plastic
which allows for easy pathogen movement and accumulation of water around
pot bases).
- Manage the water system so that irrigation water does not spread
pathogens or cause salinity problems.
Fungicides will not cure a plant if it is already infected but
will provide preventative protection. Follow label recommendations.
Consider growing root-rot-resistant species and cultivars (see table below).
For pesticide recommendations call the UConn Home
and Garden Education Center at 877-486-6271. |
