Plasmopara viticola

Downy mildew is one of the most destructive vine diseases known. It occurs especially in regions that are warm and wet during the vegetative growth stage of the vine. When control is poor and/or weather conditions are favourable, the disease may cause crop loss due to total or partial destruction of grape bunches, and also due to the secondary influence of foliage loss. While crop losses may range from 10 to 20% if poor control is exercised, favourable weather conditions, especially during flowering, may even cause total crop loss (Magarey, Wachtel & Emmett, 1994).
Due to recent epidemics in the Western Cape, Orange River and other summer rainfall regions, downy mildew has become one of the most feared diseases in vineyards. The purpose of this article is to reintroduce producers to this disease and to concentrate on control strategies under extraordinary weather conditions.
Symptoms
The fungus responsible for the disease attacks only the green tissue of the plant. Symptoms are seen mainly on the leaves, but also on bunches and sometimes on shoots.
Leaves. Depending on the incubation period and age of the leaf, initial lesions on younger leaves are either round, yellow and greasy (oil spots)
, or more angular, yellow to reddish brown, adjacent to leaf veins on older, adult leaves. The characteristic white, downy fungal growth is noticeable on the undersides of infected leaves following warm, wet nights . During warm weather conditions the leaf lesions dry out and become reddish brown, surrounded by yellow discolouration. Defoliation may occur if the infection is heavy.
Bunches. Infected young bunches initially turn brown and oily. White, downy fungal growth develops on the affected parts of the bunch after warm, wet nights. Berries are susceptible until the pea berry stage is reached. Infected berries become brownish purple, shrink and fall off. Although berries become resistant after the pea berry stage, bunch stems (rachi and laterals) are still susceptible. The latter parts turn brown and the berries attached to them become shrivelled and brown (raisin-like), dry out and may even fall off . These symptoms are sometimes confused with sunburn.Young, green shoots may also be attacked. The shoots turn brown and oily, whereafter fungal growth may appear on the infected parts. Infected shoots later become necrotic and die.
Causal organism
The organism that causes this destructive disease is Plasmopara viticola. Like familiar pathogens, Phytophthora and Pythium, this fungus also belongs to the Oömycete group of fungi. Plasmopara viticola is an obligate parasite, which means that the fungus can only survive on living tissue and can therefore not survive as a saprophyte. It forms intercellular hyphae with haustoria in the plant cells for the absorption of nutrients. Asexual reproduction occurs through the formation of sporangia on branched sporangiophores. Each sporangium gives rise to 10 zoospores, each containing two flagelli. Sexual reproduction involves the coupling of a male anteridium with a female oogonium, which leads to the formation of thick-walled oospores (Lafon & Clerjeau, 1994).
Disease cycle
Downy mildew occurs all over the world and the disease cycle has been well documented by researchers from inter alia Australia (Magarey et al., 1994), France (Lafon & Clerjeau, 1994) and South Africa (Marais, 1981).
Overwintering. Plasmopara viticola overwinters as oospores, mainly in infected residues (leaves, bunches or shoots). These structures can survive for 3-5 years. In regions with mild winters, fungal strands can also hibernate in buds or remaining leaves.
Primary infection. A very specific set of environmental parameters is essential for the oospores to germinate. At least 10mm rain should fall and the temperature should be at least 10°C for a period of 24h [10:10:24]. Hereafter zoospores are released in the soil. The next requirement is rain or water splash in order to distribute the spores to the green, susceptible parts of the vine. Using their flagelli, zoospores swim to the vicinity of the stomata where they encyst. These cysts germinate and the germ tube penetrates the stoma. The downy mildew fungus can only infect through stomata and consequently only those sections of the plant with functional stomata are susceptible. For infection to be successful, susceptible plant tissue has to remain wet for 2-3h. Oil spots or lesions occur 5-15 days after infection, depending on the weather conditions (20-25°C) and the age of the tissue. Primary infection levels are usually very low and only a few leaves in a vineyard row will have oil spot symptoms.
Secondary infection. Sporulation only occurs after a warm, wet or very humid night. Sporangiophores with sporangia grow from the stomata below the oil spots or lesions. The sporangia are distributed by rain and/or wind and once again require 2-3h of free water for germination and penetration. The potential of the sporangia to form zoospores decreases with age, especially when conditions are unfavourable. However, this is not the case with the fungal strands inside the plant tissue and old lesions or oil spots may again produce sporangia.
During favourable conditions (repeated rainfall or heavy dew in humid areas) the disease can spread extremely rapidly. According to Australian sources, 20-50 oil spots in a vineyard may increase to more than 100 000 oil spots in one night and in certain instances total crop loss has been caused in one night (Magarey et al., 1994).
Chemical control
Successful control of downy mildew depends on controlling the primary infection. Since the disease spreads incredibly fast during the secondary cycle, it is very difficult to control the disease during this explosive phase. The disease usually becomes epidemic during wet or rainy conditions, when control measures are taken too late.
The critical control period is early in the season, which is when active vegetative growth occurs. Infection before pea berry stage may cause total crop loss. From the time when the first shoots reach 10cm in length until the pea berry stage, vines must be monitored for the occurrence of oil spots. Epidemics during this period may result in total crop loss, therefore an attempt should be made to prevent or limit the primary infection. Two strategies for chemical control may be followed (Magarey et al., 1994):
Post-infection strategy. This strategy entails no chemical control until an infection period [10:10:24] occurs. A systemic product (Table 1) must be sprayed immediately afterwards, before the oil spots occur. If oil spots do occur, a systemic product is again sprayed after a warm, wet night in order to reduce sporulation and also to protect the plant against secondary infection. This strategy involves a high measure of risk, since repeated rainshowers, soil that is too wet, insufficient spraying equipment or other factors may prevent the producer from spraying his vineyards before the disease is established. In this scenario the disease may progress to the epidemic secondary cycle, which is very difficult to control. This strategy is only recommended for regions with a scant history of downy mildew and predominantly unfavourable conditions for the development of the disease.
Preventive strategy. The preventive strategy is the more conservative and less risky choice. All green parts of the vine must be protected with registered contact fungicides (Table 1). If wet conditions are forecast or experienced, vineyards should be sprayed with systemic products before, or otherwise directly after the primary infection period. During wet conditions vineyards must be sprayed at 14 day intervals and weekly during the critical period around flowering. Later in the season, when active growth is less, intervals may be extended to 3 weeks.
Important considerations are that new growth is not protected since the previous application of a contact fungicide. Rain and overhead irrigation also reduce the residue levels on the plant and spraying should be repeated if heavy precipitation is experienced shortly after applications. A good coverage of all the green parts of the plant is essential.
Cultivation practices
Successfol control of downy mildew may depend on the integration of both these chemical strategies. Additional viticultural practices may reduce the intensity of infection and facilitate disease control. Vineyard location, row direction, canopy management, weed control, irrigation and fertilisation should be properly managed to avoid excessive vigour. Humidity in the vineyard should be kept as low as possible to assist rapid evaporation of rain, dew or irrigation drops. The downy mildew fungus can only infect in free water and these actions will therefore limit the number of successful infections.