|Scientific Name||Plasmopara viticola (Berk. & M.A. Curtis) Berl. & de Toni|
Botrytis viticola Berk. & M.A. Curtis
Peronospora viticola (Berk. & M.A. Curtis) de Bary
Plasmopara amurensis Prots.
Rhysotheca viticola (Berk. & M.A. Curtis) G.W. Wilson
|Common Names||English: Downy mildew of grapevine, grapevine downy mildew, brown rot, grey rot, gray rot; German: Rebenperonospora, Lederbeerenkrankheit; French: mildiou de la grappe; Spanish: Peronospora della vite|
|Description||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.
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 swelling.
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.
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.
Additional Crop Information
Beside Vitis vinifera L. Vitis labrusca is a major host, Ampelopsis, Cissus (grape), Vitis arizonica (canyon grape) and Vitis rupestris (sand-grape) are minor host plants.
At the 'brown rot' phase potential yield losses range from 50 to 100% under favorable conditions. Indirect damage occurs when severe foliar infections cause early defoliation, exposing the fruit to sunburn and reducing winter hardiness.
Integrated Crop Management
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
- - dithiocarbamates
- propineb and mancozeb, metiram
- - sulfamides
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 above.
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.
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