P. viticola is an obligately biotrophic oomycete placed into the kingdom Chromista.
All green parts with stomata of the host plant are infected. On young leaves, lesions appear as translucent 'oil spots'. Oil spots become
dry and necrotic, first in the centre and later throughout the entire lesion. Lesions are restricted by veins to form angular, yellow to
reddish-brown spots which combine to form a patchwork. Sporulation only occurs on the lower leaf surface, where the sporangia on
sporangiophores appear as a white growth. On older leaves sporulation occurs primarily on the margins of the lesion.
Infected inflorescences and young berries become yellow or gray and may be covered with cottony spores under favorable conditions.
Berries infected later in the season become discolored and shrivel but do not support sporulation. This stage is sometimes referred to
as the 'brown rot' phase. Potential yield losses remain high, ranging from 50 to 100% under favorable conditions.
P. viticola is heterothallic with two mating types. An antheridium fertilizes an oogonium to form the sexual oospore in fallen
leaves infected in the previous season. In mild climates the pathogen overwinters as mycelium in buds and canes of wild grape species.
Sporangial dispersion was observed only in wind-blown rain, not in the air. However, there is some evidence of long-distance (500-600 km)
spore dispersal in regional air currents.
Oospores are spherical, 28-40 µm in diameter, covered by two inner oospore membranes and an outer wrinkled oospore wall. They germinate in
spring when temperatures reach 10°C and vineyard soils are wet. The germ tube terminates in a macrosporangium which releases an average of 8-20
and up to 60 zoospores. Zoospores require surface wetness to infect the host and infection takes place only through the stomata. Zoospores swim
on the tissue surface, encyst near stomata, and each spore forms a single germ tube which penetrates the stomata. In the substomatal cavity the
germ tube swells, forming a substomatal vesicle from which a single hypha arises growing intercellularly. In as little as 3.5 hours the first
haustorium forms where the pathogen contacts the host cells. Later additional haustoria form parasitizing the mesophyll cells. The incubation
time, the period between infection and the first appearance of symptoms, depends on temperature and ranges from 4 to 21 days, with an average
of 7-10 days. The pathogen sporulates through stomata during warm, humid nights.
The sporangiophores are hypophyllous, arborescent, 130-250(-700) x 11-14 µm, branching monopodially in the upper third at right angles to
the main axis, and with a base tapering to a conical point; branches in a whorl of 4-5, 35-45 µm long, often with two opposite secondary
branches 15-20 µm long, all having 3-4 conical tips 10 µm long, 6 µm wide at base, diverging at right angles and tapering to a terminal
The sporangia are ovoid, colorless, 20 x 14µm, each producing 1-6 zoospores.
For sporulation, P. viticola requires at least 95-98% RH, temperatures between 10 and 30°C and at least 4 hours of darkness. Individual
lesions resporulate a number of times under favorable conditions, and can retain the potential to sporulate for several months. Secondary
cycles of infection occur repeatedly throughout the growing season if weather conditions are favorable.
Strategies to prevent the spread of P. viticola on plant material include heat treatment of cuttings, maintaining disease-free tissue
culture plantlets, and avoiding the spread of soil and leaf debris which may bear oospores. Most cultivars of V. vinifera are highly
susceptible to downy mildew. Possible sources of resistance are wild American Vitis species and related genera.
Management must be rigorous in wet climates such as eastern North America and parts of Europe, and during unusually wet seasons in dry
locations such as California or Australia.
Forecasting models have been used in Europe since the early 1900s. Most downy mildew models incorporate temperature, rainfall, relative
humidity and leaf wetness, and more complex simulators incorporate information on host growth stage and varietal susceptibility. Models
can be integrated into pre- or post-infection treatment strategies.
Chemical control has been an important control measure since the late 1800s, after the classic discovery of Bordeaux mix (copper sulfate
plus lime) by Millardet in 1885.
Fungicides remain the most widely used management tool against P. viticola today. There are multiple pre- and post-infection chemicals
available. First applications are generally advised at 7.5-20 cm of shoot-growth, immediate pre-bloom, and post-bloom to protect the young
inflorescences and fruit. For the remainder of the season sprays may be based on a routine schedule (usually every 10-14 days) to maintain
continuous protection of the vines.
Preferably, spray schedules should be based on disease risk as determined by local weather conditions and / or by regional forecasting models.
Downy mildew fungicides are mostly classified upon their systemic mobility in the plant which determines if a fungicide can be used
preventively (for pre-infection control) or curatively (for post-infection control). Fungicides showing preventive activity have to be
applied before or during the infection by zoospores.
Beside copper-based products, there is a large choice of fungicides available including
- propineb and mancozeb, metiram
Grape downy mildew is a high risk pathogen in view of resistance development. Accordingly, in order to lower the resistance risk, curative
anti-Plasmopara compounds with a specific mode of action are mostly sold exclusively in mixture with one of the preventive partners listed
In most countries different mode of action classes are meanwhile available for the control of grape downy mildew. Accordingly, these
compounds should be used in the frame of an effective anti-resistance strategy.
The choice for specific control of downy mildew in grapes includes:
- CAA fungicides: Iprovalicarb, benthiavalicarb
- QoI fungicides: fenamidone
- phenylamides : metalaxyl
In the case of QoI fungicides and phenylamides widespread resistance has been detected in many regions. Accordingly, compounds belonging
to these mode of action classes should be used in accordance with local advisory services.