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.
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) [Fig. 1], 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 [Fig. 2]. 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 [Fig. 3]. These symptoms are sometimes confused with sunburn.
Shoots. 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.
The organism that causes this destructive disease is Plasmopara viticola. Like familiar pathogens, Phytophthora and Pythium, this fungus also belongs to the Omycete 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).
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 10C 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 (20-25C) 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).
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.
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.
Since the disease is dependent on specific weather conditions, it occurs only sporadically which makes research all the more difficult. However, valuable information is obtained when natural epidemics occur. The recent epidemic in the Orange River and other summer rainfall regions therefore gave researchers new insight into the downy mildew disease:
The previous downy mildew epidemic in the Orange River region was in 1976. Since then, isolated cases with low disease intensity have been noted. Any occurrence of the disease may contribute to the survival of the fungus, since new oospores are formed each time. A unique situation in the Orange River region is the large number of new plantings, some in new areas where grapes were never cultivated before. These vines were also infected, revealing the remarkable hibernation and distribution abilities of the downy mildew fungus. Australian researchers are of the opinion that the oospores can only survive 3-5 years and that the spores can be distributed downwind for a few hundred metres at most. This also forces us to look at other means of distribution and hibernation. Most of the plant material comes from nurseries in the Western Cape. Irrigation in these nurseries is overhead and together with the high plant density, it creates ideal conditions for the development of downy mildew. Nurseries regularly spray for this disease and resistance against certain fungicides has already built up in the downy mildew populations. Therefore downy mildew occurs fairly regularly in these nurseries. The oospores that are formed may be distributed to new areas on roots of dormant vines or in the soil, while fungal strands could possibly survive in the buds and spread from there, thus explaining the occurrence of downy mildew in “new” areas.
- Although berries are susceptible until the pea berry stage, the bunch stems remain susceptible to downy mildew. Total crop loss was noted in some low-lying vineyards, where the rachis was infected after veraison. This infection causes ring-barking of the rachis and consequently die-back of laterals as well as whole bunches. Control during wet weather conditions is therefore essential in all vegetative stages.
- The Orange River epidemic continued, despite warm, rain-free weather conditions. However, it seems that heavy dew, experienced in low-lying vineyards in particular, is sufficient for the epidemic to continue. Heavy dew at night provides sufficient humidity or moisture for sporulation, while the free water on the susceptible plant tissue is sufficient for new infection. This “dew downy mildew” was also worse in excessively vigorous vineyards and/or vineyards with poor weed control.
- Heavy dew, combined with irrigation, may possibly comply with the environmental parameters required for primary infection (10:10:24). Producers are warned to be on the lookout for oil spots, especially after periods of heavy dew.
- Defoliation in heavily affected vineyards has an additional detrimental effect on vines. Valuable reserves were used by the vines to form new foliage. Excessive vegetative growth during the post-harvest stage should not be stimulated. In future seasons, insufficient reserves in vines may result in delayed and weak budding, poor growth and even partial or total die-back.
Since the epidemiological development of downy mildew is strictly governed by climatic conditions, it is important to control the disease at the critical moments. The organism causing the disease infects, grows and spreads optimally under a specific set of environmental conditions. This information is well-known and if weather conditions are monitored, it is fairly simple to develop a prediction model for the disease.
Computer programme. During the 1995/96 season a downy mildew warning programme was developed for the MC-Systems weather stations by ARC Infruitec-Nietvoorbij (Disease Management) in conjunction with ARC Institute for Soil, Climate and Water (Agromet). The computer programme uses hourly weather data to send out warnings as soon as environmental conditions are favourable for infection.
Reliability and use. The model has been intensively evaluated over the past four years and good control, together with a reduction in the number of fungicide applications, was already obtained in the first season of evaluation. An average saving of two to three applications was obtained in the trials, when spraying only took place within two days of receiving a warning and where an effective protection period of 14 days was followed.
Producers are advised to follow a preventive spraying programme early in the season. These applications could be made with contact fungicides at 10 to 14 day intervals, because the vines are growing rapidly at that stage. Systemic products should be used before rainshowers or directly after infection periods. If producers make use of the prediction service, it is very important to apply a preventive control programme and then to spray a chemical product with systemic properties as soon as possible after an infection period. This application must then be followed by an additional application with the same product. Hereafter contact fungicides may again be used, provided there are no further infection periods.
By using the warning service, producers are able to spray much less fungicide and doing it more cost-effectively. The disease prediction model is therefore another tool in the producer’s hand to help combat downy mildew more effectively. Should there be any doubts about using the disease warning information, do not hesitate to contact the experts for advice.
Availability. The downy mildew prediction service was first introduced in the Robertson valley and then extended to the Worcester and Paarl valleys. This service is maintained by various role players in the viticultural industry (ARC Infruitec-Nietvoorbij, ARC ISCW, VinPro SA, Paarl Vintners, Robertson Wine Valley and Novartis). The information is currently available free of charge on the internet at http://www.vinfruitec.co.za.
Lafon, R. & Clerjeau, M., 1994. Downy mildew. p. 11-13 in: Compendium of Grape Diseases. R.C. Pearson & A.C. Goheen (eds.). APS Press, St. Paul. 93 p.
Magarey, P.A., Wachtel, M.F. & Emmett, R.W., 1994. Downy mildew. p. 5-11 in: Grape Production Series Number 1: Diseases and Pests. P.R. Nicholas, P.R. Magarey & M.F. Wachtel (eds.). Winetitles, Adelaide. 106 p.
Marais, P.G., 1981. Donsige skimmel. p. 391-395 in: Wingerdbou in Suid-Afrika. J. Burger & J. Deist (eds.) CTP Boekdrukkers, Kaapstad. 552 p.
Nel, A., Krause, M., Ramautar, N. & van Zyl, K., 1999. A Guide for the Control of Plant Diseases. Directorate: Agricultural Production Input, National Department of Agriculture, Republic of South Africa.
Vermeulen, A.K., 1999. A Guide to the Use of Registered Fungicides and Pesticides against Grapevine Diseases and Pests: Wine grapes. ARC – Fruit, Vine and Wine Research Institute, Private Bag X5026, Stellenbosch, 7599, South Africa.