Downy mildew of grapes refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to Peronosporaceae. In commercial agriculture, they are a particular problem for growers of crucifers, grapes and vegetables that grow on vines. slide contains vivid descrition of the plant pathogen.

1. Downy mildew of g
Saugat Bhattacharjee,
Abbs
Department of Microbiology

2. Occurrence
•The disease is known to occur under humid conditions.
• In Europe and in eastern half of U.S.A , Asia, Australia, South
America.
• In India the disease is common in Maharashtra and Karnataka.
• Downy mildew is a highly destructive disease of
grapevines in all grape-growing areas of the world where
there is spring and summer rainfall at temperatures above
10ºC (50ºF).

3. • Downy
mildew affects the leaves, fruits and vine.
• Losses trough necrosis of leaf tissue and defoliation.
Fruits are low quality or are destroyed. Dwarfing and killing
of young shoots will occur.
• All green parts of the grapevine are susceptible, the
first
symptoms of downy mildew of grapes, caused
by Plasmopara viticola, are usually seen on the leaves as
soon as 5 to 7 days after infection.

4. SYMPTOMS
Leaves: Infected leaves develop yellowish-green, and
translucent “oily” lesions on their upper surfaces. As
lesions expand, affected areas turn brown, necrotic or
mottled.
Downy mildew on upper side of grape leaf.

5. the corresponding underleaf surface sporulation of the
pathogen, – a delicate, dense, white to grayish, cottony
growth–occurs
Appearance of infection on underside of a
leaf.

6. The lesions may be so numerous on the upper leaf
surface that they merge to cover a large portion of the leaf.
Infected tissue gradually becomes reddish brown to dark
brown, irregular, and brittle
Affected leaves
eventually
turn brown, wither, curl,
and drop early
Symptoms on upper surface of a leaf

7. Shoots and Tendrils: Symptoms appear as watersoaked, shiny
depressions on which a dense growth of whitish mildew
appears. When young shoots, petioles, tendrils, or cluster stems
are infected, they often become distorted, thickened, or curled.
Downy mildew symptoms on a shoot

8. Fruit:
Two periods of fruit
infection may occur during a
growing season.
The first is when the berries are
about the size of small peas. If
infected at this stage, the young
fruit turns light brown and soft,
shatters easily, and in damp
weather is frequently covered
with the white downy growth of
the pathogen

9. When the nights become
cooler in the late summer and
early
autumn,
a
second
infection period may develop.
Berries infected at this time
generally do not soften or form
the downy mildew growth

10. Causal Organism
Plasmopara viticola.
Kingdom:
Fungi
Class:
Oomycetes
Order:
Peronosporales
Family:
Peronosporaceae
Genus:
Plasmospara

11. Disease cycle
The fungus overwinters in infected leaves on the ground and
possibly in diseased shoots.
•
The overwintering spore (oospore) germinates in the spring
and produces a different type of spore (sporangium).
•
These sporangia are spread by wind and splashing rain.
•
When plant parts are covered with a film of moisture, the
sporangia release small swimming spores, called zoospores.
•
Zoospores, which also are spread by splashing rain,
germinate by producing a germ tube that enters the leaf through
stomates (tiny pores) on the lower leaf surface
•
The optimum temperature for disease development is 64 to
76 degrees F (18 to 25 degrees C).

12. The disease can tolerate a minimum temperature of 54 to 58
degrees F (12 C to 13 degrees C), and a maximum temperature of
about 86 degrees F (30 degrees C).
•
Once inside the plant, the fungus grows and spreads through
tissues .
•
Infections are usually visible as lesions in about 7–12 days.
•
At night during periods of high humidity and temperatures
above 55 degrees F (13 degrees C), the fungus grows out through
the stomates of infected tissue and produces microscopic, branched,
tree-like structures (sporangiophores) on the lower leaf surface.
•
More spores (sporangia) are produced on the tips of these
tree-like structures.
•
The small sporangiophores and sporangia make up the
cottony, downy mildew growth. Sporangia cause secondary
infections and are spread by rain.

14. Plasmopara viticola, the grape
downy mildew pathogen, seen
under a highpower microscope.
Thick-walled oospore
(lower left).
Remainder,branched
sporangiophores
bearing terminal, lemon-shaped
sporangia (L. Gray).

15. CONTROL
•
Downy mildew is comparatively easy to control - foliage and
fruit are kept protected by fungicide sprays
•
Properly space vines, and choose a planting site where the
vines will be exposed to all-day sun and good air circulation. Keep the
vines off the ground and properly tied.
•
Space vines properly in the row, if possible, orient the rows
to maximize air movement down the row.
•
To improve air circulation, control weeds and tall grasses
in the vineyard and surrounding areas.
•
Sanitation is important. Remove dead leaves and berries
from vines and the ground after leaf drop.

16. •
When pruning, select only strong, healthy, well-colored canes
of the previous year’s growth.
•
Grape varieties vary greatly in their susceptibility to
downy mildew. In general, vinifera (Vitis vinifera) varieties are
much more susceptible than American types, and the French
hybrids are somewhat intermediate in susceptibility.
•
A good
important.
fungicide
spray
program
is
extremely