Download Rhododendron and Azalea Diseases EPP 410 Diseases and

Rhododendron and Azalea Diseases
EPP 410 Diseases and Insects of Ornamental Plants
Iron Deficiency:
Symptoms: Iron deficiency is one of the most common causes of chlorosis in azalea and
rhododendron. Interveinal chlorosis develops in young foliage and young leaves become
yellow, cream colored, or white.
Disease cycle: Various conditions (including high soil pH, calcareous soils, water logging
- leads to poor aeration) can affect the availability of iron. Factors that raise soil pH
(including proximity to concrete foundations, sidewalks, driveways, etc.) cause soluble
iron to precipitate which results in iron deficiency and chlorosis.
Control: Apply dilute solution of ferric tartrate or another form of iron chelate. Test soil
for alkaline pH. These plants are acid loving, so reduce pH to below 6.0 by adding sulfur
if necessary.
Phytophthora Root Rot - Phytophthora cinnamomi
Symptoms and signs: Roots become necrotic and leaves become chlorotic and roll
downward and eventually wilt. In older plants, roots often regenerate in well-drained
upper layer of soil. As pathogen moves into stem, cambium tissue becomes dark brown.
Phloem and xylem becomes brown later and permanent wilt symptoms develop in
foliage.
Disease cycle: Fungus survives in soil as chlamydospores and mycelium in infected roots
and in host debris. Fungus can be introduced in infested soil or irrigation water. In
containers, fungus spreads most rapidly when pots are set on black plastic and least when
pots are set on a 5-7 cm layer of pine bark and gravel. Lack of soil moisture is a key
factor in root rot severity. Rapid buildup of zoospores is in free standing water or running
water on soil surface. This disease is most severe in June, July, and August. Peat moss,
when used as the only source of organic matter in potting mix, enhances disease
development. Tree bark media reduces threat of root rot.
Control: Place pots on gravel on slope to prevent puddling. Host debris (pruning waste,
abscised leaves, etc.) should be remove. Fungicides such as Subdue, Aliette, Banol, and
Truban are effective. Use tree bark as primary component in mixes. All tree bark has
inhibitors of P. cinnamomi. Hard wood bark releases inhibitors for almost 1 year. Most
rhododendron cultivars are susceptible, but resistance is available. In azaleas, cultivars
vary widely in resistance levels.
Phytophthora Die Back - Phytophthora cactorum, P. citricola, and P. parasitica
Symptoms and signs: More or less circular lesions form on young leaves within 1-2 days
after infection. Lesions become water soaked and turns dark brown. At 3-5 days after
infection, lesions expand and tissue dries out and becomes brittle and leaves curl inward.
Lesions often have characteristic V-shape along mid vein. From infected petioles,
infections can spread into stems where diamond shaped cankers are produced. Entire
shoot tips can become necrotic and die.
Disease cycle: Disease develops during hot, wet weather. A film of water is needed for
infection to occur. Periods of prolong cloudiness after rainfall is ideal for infection. Rain
or irrigation water may splash zoospores or sporangia onto foliage. Plants with high
nitrogen regimes are more susceptible. The pathogen survives in infested debris and
infected stems.
Control: Although covering naturally infested bases of plants with gravel is expensive, it
greatly reduces spread of the disease and can be very cost effective. Rogue out diseased
plants. Inspection of plants frequently is critical to good control. Irrigate when foliage
will dry rapidly. Avoid excessive nitrogen levels. A few cultivars are resistant.
Fungicides (Daconil 2787, Dithane, Fore, and Subdue) can be useful. Use good
sanitation practices.
Rhizoctonia Web Blight - Rhizoctonia spp.
Symptoms and signs: Disease develops as lesions just above soil line and in center of
plant canopy. Lesions may expand rapidly, killing entire leaves. During periods of high
RH, hyphae of fungus develop extensively and cause abscised leaves to remain attached
to plant shoots. Severely infected plants may die. Use good sanitation practices.
Disease cycle: Inoculum is sclerotia or hyphae associated with organic matter and debris
from previous crop.
Control: Increase plant spacing to reduce plant to plant spread and to reduce humidity
within plant canopies. Fungicides (Chipco 26019, Cleary s 3336, Daconil 2787, Domain,
Zyban) used at 7-10 day interval are useful.
Botryosphaeria Dieback - Botryosphaeria dothidea
Symptoms and signs: Branches die on otherwise healthy plants. Leaves on diseased
branches become dull green and turn brown. Leaves droop and roll downward. Dead
leaves remain on stem. In young plants, disease develops rapidly as stem dieback. In
older plants, disease appears as sunken cankers on large branches. Cankers enlarge until
stem is girdled.
Disease cycle: In young plants, dieback develops before fungus can sporulate. Infection
is through wounds. Heat stress and drought appear to increase incidence of the disease.
Control: The disease is hard to control in the landscape. Transplant plants in partially
shaded areas and water during drought. Prune out diseased tissue.
Ovulinia Petal Blight - Ovulinia azaleae
Symptoms and signs: Infected flowers exhibit small spots about 1 mm in diameter and
are water soaked. Spots occur before petals fully open. Entire petals become slimy and
limp. Infested areas become tan or light brown and entire flowers prematurely turn
brown. Infected flowers dry and cling to the plant. Small black sclerotia appear on
flowers 6-8 wk later.
Disease cycle: The disease develops during moist weather. The fungus overwinters as
sclerotia. At the beginning of bloom season, sclerotia germinate and give rise to
apothecia. Ascospores are released and carried in air currents to leaves. Temperature
range for disease development is 10-22 C and optimum temperature is 18 C. The
infection rate is high when warm, wet weather coincides with bloom time. Secondary
disease cycle is with conidia. This disease spreads rapidly in greenhouses.
Control: Fungicides (Bayleton, Cleary s 3336, Daconil 2787, Zyban) used at or before
flower break is best for disease control and fungicides should be used weekly until flower
bloom time ends. Rogue diseased plants. Keep foliage as dry as possible in greenhouses.
Leaf and Flower Gall - Exobasidium spp.
Symptoms and signs: Leaf and flower buds thicken and become fleshy. Galls may be
extensive, shapeless, or bladder-like. Galls retain greenish color or may turn red. Later
whitish fungal growth covers the gall's surface. In rhododendron, leaf infections can be
large yellow lesions that turn white when covered by fungal mycelium.
Disease cycle: The fungus overwinters as basidiospores in bud scales. Young leaves are
much more susceptible than older leaves. Infection is dependent on high RH or rain at
bud break. Plants in poorly aerated soils are more subject to infection than plants in well
aerated soils. Basidiospores are carried by wind or splashing rain to other buds or nearby
plants.
Control: Pick off and destroy newly formed galls before they turn white. Fungicides are
normally not needed for disease control.
Powdery Mildew - Microsphaera penicillata (in some area Sphaerotheca pannosa and
Erysiphe cruciferarum may be found on plants)
Symptoms and Signs: Signs of fungus include white fungal growth on young leaves. On
some cultivars, fungal mycelium may only be on lower leaf surface and faint chlorotic
spots may be present on upper leaf surface. In some cultivars the only symptoms are
necrotic ring spots or flecks.
Disease Cycle: Young plants grown in deep shade or most vulnerable to the disease.
Cleistothecia are found abundantly on some species of deciduous azaleas. Vegetative
mycelium invades dormant buds (where the fungus overwinters) and attacks newly
emerging buds in the following year. Although the fungus grows more rapidly at warmer
temperatures (25 C day, 15 C night) than at lower temps (20 C day and 10 C night) the
disease is more severe at lower temperatures.
Control: Control is easy to achieve with fungicides (Bayleton, Cleary s 3336, Domain,
Strike, and Zyban). Spray at first sign of disease.
Rhododendron Necrotic Ringspot Virus
Symptoms and signs: Symptoms do not occur in current season's leaves, but are
observed in 2 year old leaves. Necrotic rings become more numerous as season
progresses and leaves turn red and drop prematurely.
Disease cycle: How virus is transmitted and disseminated in nature is unknown.
Control: Rogue out diseased plants.