1. Downy mildew of cucurbits is caused by the oomycete Pseudoperonospora cubensis. It affects crops like cucumber, squash, melon, and watermelon. 2. Symptoms include angular leaf lesions that become water-soaked during periods of leaf wetness and can progress to complete leaf death. 3. The disease is managed through cultural practices to improve airflow and reduce leaf wetness, as well as fungicide applications following resistance management strategies.

1. CC
DOWNY MILDEW OF
CUCURBITS
COLLEGE OF AGRICULTURAL TECHNOLOGY
(Affiliated to Tamil Nadu Agricultural University, Coimbatore-3)
Kullapuram (Po),ViaVaigai Dam, Theni-625 562
Course teacher: Student:
Dr. Parthasarathy. S Miss. C. Pon Alagammai.
Asst.Professor (Plant Pathology) 2015021095

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3. NAME OF THE DISEASE :
Downy mildew of cucurbits.
CASUAL ORGANISM :
Pseudoperonospora cubensis
(Berkeley & Curtis) Rostovtsev.

4. HISTORY:
 Pseudoperonospora cubensis (Berkeley & Curtis)
Rostovtsev is the causal agent of downy mildew of
plants in the Cucurbitaceous (gourd family),
including cucumbers, melons, squashes, gourds and
watermelons.
 Berkeley and Curtis first reported the disease in
Cuba in 1868.

5. DISTRIBUTION:
Downy mildew of cucurbits can be found in temperate
areas, such as the Americas, Europe, Japan, Australia
and South Africa, tropical regions and some semi-arid
regions, such as the Middle East.
The disease affects cucurbit crops in the field and those
grown in passive or traditional greenhouses.

6. HOSTS:
Members of Cucurbitaceae, the gourd family,
including,
1.Cucumber (Cucumis sativus),
2.Squash (Cucurbita spp.),
3.Melon (Cucumis melo) and
4.Watermelon (Citrullus lanatus).

7. SIGNIFICANCE:
In 1991, 21% of expenses for chemical disease control
were allocated to the downy mildew and late blight
pathogens, 30% of this was devoted to vegetable crops.
The largest percentage of downy mildew control was for
grape downy mildew caused by Plasmopara viticola
(54%) followed by the cucurbit downy mildew caused
by Pseudoperonospora cubensis (10%).

8. • These values fluctuate based on weather conditions,
acreage and market conditions.
• Downy mildew is an annual late-season problem for
squash and pumpkin growers in the eastern United
States.
• Successful breeding of resistant cucumber cultivars in
the mid-20th century provided adequate control of
downy mildew without the use of fungicides.

9. • The presence of a more virulent strain of the
pathogen on cucumber appeared in the eastern
United States in 2004 and has resulted in major
economic losses.
• The disease is a major concern for cucumber
growers throughout the eastern half of the United
States resulting in substantial economic losses.

10. SIGNS:
 The key to identifying downy mildew is observing
the signs (sporangia and sporangiophores) of the
pathogen.
 Sporangia and sporangiophores are most
noticeable during humid conditions (e.g., early
morning hours before natural humidity dissipates
or immediately following rainfall) on the
underside of the leaf.

11. SYMPTOMS:
 Symptoms on cucumber and squash are angular
lesions that are limited by the leaf veins.
 During periods of leaf wetness from dew,
irrigation or rainfall, incipient lesions can become
conspicuously water-soaked.
 This is the earliest symptom produced by the
disease, but will disappear as moisture dissipates.

12. • Severe infection results in leaves that are completely
dead and curled up.
• This symptom has been described as “wildfire” as the
leaves appear to be burned.
• Symptoms on watermelon and cantaloupe are typically
irregular shaped lesions on the foliage that turn brown
rapidly.

13.  Symptoms on watermelon and cantaloupe are not as
distinctive as on cucumber and squash and are more
easily mistaken for other diseases such as
• Anthracnose (causal agent: Colletotrichum orbiculare),
• Target spot (causal agent: Corynespora cassiicola),
• Alternaria leaf spot (causal agent: Alternaria alternata f.
sp. cucurbitae),
• Gummy stem blight (causal agent: Didymella bryoniae).

14.  Early lesions are light green in appearance and
become chlorotic and finally necrotic as host plant
cells die.
 Infected leaves may experience an upward leaf curl.

15. SYSTEMIC POSITION:
Kingdom : Fungi
Phylum : Heterokonta
Class : Oomycetes
Order : Peronosporales
Family : Peronosporaceae
Genus : Pseudoperonospora
Species : Pseudoperonospora cubensis

16. PATHOGEN BIOLOGY:
1. Like other downy mildew organisms, P. cubensis,
is a biotroph or obligate parasite, meaning that the
organism requires living host tissue to grow.
2. The organism cannot be propagated on artificial
media. P. cubensis overwinters on infected
cucurbits, either wild or propagated, in areas that
do not experience a hard frost.

17. SPORANGIA AND SPORANGIOPHORES:
Pseudoperonospora cubensis forms large (20-40 x 14-
25 mm in diameter), lemon-shaped sporangia with a
conspicuous papilla.
Sporangia appear smooth under the compound
microscope but with the scanning electron microscope
the finely decorated surface of the sporangium is
evident .

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19. • The sporulating capacity of purely necrotic lesions
is low and that of yellow-necrotic lesions.
• Warming and drying of the atmosphere, typical of
early morning hours, causes twisting of the
sporangiophores, which may be of importance for
the detachment of sporangia.

20. MANAGEMENT:
CULTURAL PRACTICES.
 Downy mildew severity can be decreased by taking
actions that encourage airflow and reduce leaf
wetness.
 However, such actions are often insufficient during
prolonged, favorable environmental conditions and in
the presence of high inoculum levels.
 Growing cucurbits in environments where humidity
levels can be manipulated can help to manage downy
mildew.

21. CHEMICAL PRACTICES
 Efficacious fungicides include fluopicolide,
famoxadone + cymoxanil, cyazofamid, zoxamide and
propamocarb hydrochloride.
 Pseudoperonospora cubensis is known to develop
resistance to fungicides very rapidly.

22.  Reduced efficacy of mefenoxam, metalaxyl and the
strobilurin fungicides has been reported.
 As a result, the fungicides listed previously should be
applied under strict resistance management strategies
that include tank-mixing with protectant fungicides,
such as chlorothalonil or mancozeb, and rotating with
fungicides of different modes of action.