Reports on Plant Diseases |
RPD No. 657 - Downy Mildew of Snapdragons
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December 1988
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[ Symptoms ] [ Disease Cycle
] [ Control ]
| Downy mildew is a common but sporadic disease of greenhouse-
and nursery-grown snapdragons (Antirrhinum majus) in the United
States, Canada, Australia, New Zealand, and most of Europe. The disease,
caused by the fungus Peronospora antirrhini, is most common
on seedlings and young plants growing in very humid, poorly ventilated greenhouses
where low temperatures exist. The disease can cause local infections (leaf
spots) or more destructive systemic infections that can cause stunting,
yellowing, and distortion of leaves as well as an overall stunting of plants.
Systemically infected seedling rarely survive |
Figure 1. Downy
mildew of snapdragon; stunted, bunchy, yellowish seedlings growing in
a greenhouse.
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Symptoms
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Local, nonsystemic infections commonly appear as pale, rounded spots
(lesions) on the leaves up to 15 millimeters in diameter with smooth,
diffuse margins that are rarely destructive. The more important systemic
infections result in a downward marginal curling of the leaves and a significant
reduction in growth, which causes a rosette of dwarfed, yellowish leaves
at the tips of shortened shoots that give affected seedlings a stunted,
bunchy appearance (Figure 1). Infected leaves are a pale dull green with
a lower surface covered by a mealy, grayish white to bluish purple downy
fungus growth in overcast, cool, damp weather. The seedlings often wilt
and die from the top of the plant down to the soil surface. Symptoms may
appear from the time when the plant has two cotyledons, and continue to
appear until the plants are about six inches tall. Usually, after the
tips die, affected plants begin to produce secondary shoots from their
base.
On older plants (1 to 4 feet tall) the terminal shoot growth is checked
and the leaves become pale green. On plants about to bloom, a rosetting
of the growing point occurs. Flowering is reduced.
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Figure 2.
Close-up of an affected seedling.
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Disease Cycle
The Peronospora fungus overseasons as thick-walled, rounded oospores
within the cortex of petioles, stems, and roots of systemically infected plants.
When temperatures are cool and the foliage is wet, spores (conidia or sporangia)
are produced on newly infected leaves and disseminated by air currents. Conidia
that land on leaves may germinate within 24 hours and infect a leaf by producing
a germ tube which penetrates the leaf through a stoma on the lower leaf surface.
Mycelium then spreads through the intercellular spaces of the leaf, obtaining
food through branched, fingerlike haustoria (modified feeding structures) which
it sends into adjacent cells. The mycelium continues to spread and eventually
forms a fungal cushion in the substomatal cavities within the leaf from which
the conidiophores (sporangiophores) arise and emerge through the stomatal openings.
At the tips of the dichotomously branched condiophores are produced the oval
conidia, that in mass form a mealy, grayish white to bluish purple downy growth
in the lower leaf surface of infected plants (Figure 2). Downy mildew is favored
by high humidity and low temperatures. Conidia are produced at temperatures
between 46 and 72 F (8 to 22 C), with an optimum of 55 F (13 C). Infection and
disease development are favored by temperatures of 40 to 60 F (4.4 to 15.5 C).
In a suitable environment, symptoms may appear in as short a period as four
days after inoculation.
Control
- Greenhouses provide heat and ventilation to keep the humidity below 80
to 85 percent and thus prevent water condensing on the plants. Space plants
for good air circulation. Keep night temperatures above 52 F (11 C).
- Avoid overwatering. Practice surface watering to keep the foliage as dry
as possible. Water in the morning on a rising temperature to promote rapid
drying.
- Buy seed that has been properly cleaned and seedlings grown in disease-free
nurseries. Plant in soil steamed at 160 F (71 C) for one hour or 180 F (82
C) for 30 minutes at the coolest spot. Also treat all tools, seed flats, and
other containers.
- Remove and burn dead and dying plants when first seen. Carefully destroy
all plant debris to eliminate possible sources of inoculum.
- When the above cultural practices fail to control downy mildew, spray the
plants with a suggested fungicide at 5- to 10-day intervals in overcast, damp
or rainy weather. It is critical to cover the lower leaf surfaces with each
application. Start the spray program when disease first appears. Use one of
the fungicides suggested in Illinois Commercial Landscape and
Turfgrass Pest Management Handbook or the Illinois Home,
Yard and Garden Pest Guide.
The publications mentioned above should be available at your nearest Extension
office.
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For further information concerning
diseases of crucifers and other vegetables, contact Mohammad Babadoost, Extension
Specialist in Fruit and Vegetable Diseases, Department of Crop Sciences, University
of Illinois at Urbana-Champaign. University
of Illinois Extension provides equal opportunities in programs and employment.
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