NC State Watermelon Disease Handbook
Downy Mildew (Pseudoperonospora cubensis)
Disease:
Downy mildew.
Cause (pathogen):
Pseudoperonospora cubensis (Berk. & M. A. Curtis)
Rostovzev.
Symtoms:
Initial symptoms appear small, angular, yellow lesions on the
upper side of the leaf. A white to purplish mildew may be observed
on the lower side of the leaf during humid weather. As the lesions
expand, their centers turn brown. Often the margins of diseased
leaves curl upward. During favorable weather, leaf lesions coalesce
and kill large areas of the leaf surface. This results in a stunting
of the plant and a failure of the fruit to mature properly. Even
those fruit which reach maturity may have an off-flavor. In severe
cases, the entire plant will die.
Source of primary inoculum:
Active colonies.
Source of Secondary inoculum:
Spores produced on infected plants.
Spread:
Wind dispersal.
Disease cycle:
The fungus does not produce an overwintering spore and cannot
survive in winter. The purple fungal layer on the abaxial leaf
surface consists of sporangia which function as conidia. These
may be spread by winds which transport the organism long distances
from overwintering sites in warm areas. The infection process
is favored by temperatures between 16 and 22 C and by a thin layer
of water on the leaf surface.Fungus enters the plant through stomates.
Control:
Control of downy mildew is dependent on cultural practices, early
detection and timely applications of fungicides.Avoid using infected
transplants.
Microscopic Description:
Mycelium: Coenocytic and intercellular in host tissue; haustoria
small ovate and intracellular. Conidiophores arise in groups of
one to five through the stomata; either dichotomous or intermediate
between dichotomous and monopodial branching habit. Spores: Sporangia
produced singly on tips of sporangiophore branches; grayish to
olivaceous purple; avoid to ellipsoidal (14-23 x 21-39 microns);
thin walled, with a papilla at the distal end. Sporangia germinate
by formation of flagellate zoospores and asexual conidia. Oospores
reported from USSR, China and Japan.
Source:
P.H. Williams; Department of Plant Pathology; University of Wisconsin;
1630 Linden Drive; Madison, WI 53706.
Relative Stability:
Biotypes of biological races are known. Pathogenic stability
in nature not studied.
Variants:
No information.
Storage and Retrieval:
The fungus can be maintained at 16°C in infested cucumber seedlings
for 3 to 4 weeks after inoculation. Sporulating cotyledons, frozen
in plastic bags, remain pathogenic at least 10 weeks, but viability
is decreased (may result in loss of pathogen). See inoculum increase
section for inducing sporulation on cotyledons.
Inoculum Increase:
Fungus maintained in susceptible watermelon seedlings. To increase
inoculum, inoculate cotyledons of susceptible
plants grown in plastic freezer
cups (9.5 x 9.5 x 7.5 cm), 12 plants/cup, as for routine inoculation
(see inoculum preparation, quantification, and inoculum distribution
and delivery sections). Incubate in the dark for 48 hr at 20°C
and 100% relative humidity (RH). Subsequently grow plants at 16°C.
Fertilize with Hoagland's solution 3 times/wk.
Inoculum preparation:
Induce sporulation on infected seedlings 2-3 weeks after inoculation
by misting and placing in the dark for 24 hr at 20°C and 100%
RH. Necrotic tissue does not produce sporangia. Remove cotyledons
and dip in distilled water to wash off plant exudates. Place cotyledons
in a beaker of distilled water and rub cotyledons with the fingers
to dislodge the sporangia.
Quantification:
Count sporangia with hemacytometer. Adjust spore concentration
to 12 x 104 sporangia/ml with distilled water.
Inoculum Distribution and Delivery:
Using a Pasteur pipette place a .01-.03 ml droplet of inoculum
on the upper surface of the cotyledon.
Host:
Citrullus lanatus, watermelon.
Source of Resistance:
Not reported.
Growth of Host:
Watermelon seeds are sown in steam sterilized coarse grade vermiculite
in wooden flats (52 x 36 x 7 cm). Each flat contains 10 rows,
25 seeds/row. Resistant and susceptible checks are sown in row
6. The flats are placed on a heated germination bench. Vermiculite
temperatures of 32°C insure rapid and uniform germination. Newspaper
on top of the flat prevents cooling by evaporation. Newspaper
is removed when seeds germinate.
Tissue age:
Inoculate cotyledons 3-4 days after emergence when cotyledons
are expanded.
Postinoculation Environment:
Incubate plants in the dark for 48 hr at 20°C and 100% RH. Subsequently
place plants in greenhouse 24-28°C.
Disease Response:
Plants are rated as susceptible or resistant 7-8 days after inoculation.
Susceptible plants have strong chlorosis, while resistant plants
have no chlorosis or very faint localized chlorosis.
Multiple Inoculation:
Simultaneous inoculation with scab, anthracnose,
angular leaf spot, or bacterial wilt. Subsequent inoculation with
cucumber mosaic virus (CMV) and powdery mildew.
Saving Host:
Both resistant and susceptible plants can be transplanted to
steam sterilized soil.
Paul H. Williams
Mary J. Palmer
Department of Plant Pathology
University of Wisconsin
10-11-82
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