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A Technical Guide for Wine Producers
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Reflections On The 2001/2002 Downy Mildew Epidemic
Introduction
This past season wine farmers suffered one of the worst downy mildew epidemics ever. At a recent post-harvest workshop, presented by the South African Society for Enology and Viticulture at ARC Infruitec-Nietvoorbij, representatives and viticulturists from various regions gave a summary of the harvests. Crop losses of between 10% and 75% were reported for the Stellenbosch area, while an average crop loss of 21% was mentioned for Paarl, Wellington and Franschhoek. Various reasons for these crop losses were given, but the main cause was the downy mildew epidemic.
Downy mildew epidemic
The occurrence and rate of development of a plant disease depend on three factors (disease triangle) : 1. Disease causing organism or pathogen; 2. Disease conducive weather conditions; and 3. Susceptible plant. A fourth factor, control measures, which also influence the extent to which the disease occurs and develops, then completes the disease pyramid. This season, downy mildew, one of the most dreaded grapevine diseases, once again caught farmers unawares, despite expensive lessons learnt in the past. What follows is a summary of reasons, presented according to the disease pyramid, that could possibly have contributed to insufficient control of downy mildew:
1. Pathogen
The fungus that causes downy mildew on vines, Plasmopara viticola, occurs practically everywhere where grapevines are grown and may survive in the soil as overwintering oospores for several years. The oospores germinate when weather conditions are favourable for the development of the disease.
2. Weather conditions
Primary infection period. Early spring rains and mild temperatures in the spring of 2001 meant that the primary infection period (at least 10 mm rain, while the temperature is warmer than 10øC for a period of 24 h) was already noted by the middle of September. Infection of vine tissue with active stomata (shoot growth of 10 cm and longer) has therefore occurred at this early stage already.
Long incubation period. Depending on prevailing weather conditions (optimally 20 to 25øC), the first symptoms ("oil spots") appear from 5 up to 15 days after primary infection. The occurrence of primary infection symptoms could be very limited; as little as 1 oil spot may occur in a 50 m vineyard row. Producers are inclined not to monitor thoroughly enough for these symptoms and supposing their vineyards to be "clean", they fail to implement proper control measures.
3. Susceptible plant
Vigorous vine growth. Sufficient winter rain, early spring rain, thunder and possibly excessive nitrogen fertilising resulted in very vigorous growth early in the season. Not only are vigorous vines more susceptible to downy mildew, but sufficient cover of all the susceptible parts of the plant with fungicides is also hardly possible. In such instances spraying volumes must be increased and spraying intervals shortened.
4. Control measures
Warning or prediction services. Since the primary infection and subsequent disease development of downy mildew depend on very specific climatic conditions, it is possible to modulate and/or predict the disease and its course, provided the climatic conditions are carefully measured and/or forecast. Various prediction or warning systems are available at present and these could be valuable at the time of decision making in order to control the disease more effectively. A free downy mildew warning service for the Robertson, Worcester and Paarl valley is supported by various role players in the vine industry (ARC Infruitec-Nietvoorbij, ARC ICW, VinPro SA, Paarl Vintners, Robertson Wine Valley and Syngenta). This information is available on the internet at http://www.vinfruitec.co.za.
Choice of fungicide. There is much confusion about which fungicide to spray when. And which fungicide is best? All the registered downy mildew fungicides inhibit the fungus sufficiently to control the disease. Producers have a choice of fungicides from two groups : 1. broad spectrum contact sprays that are cheaper, but do not offer the same degree of effectiveness, protection and rain fastness than the systemic sprays, and 2. systemic sprays. However, most systemic sprays are only locally systemic (taken up in the green plant tissue and distributed in the tissue to a certain extent), a characteristic that results in better rain fastness and improved protection. These fungicides are also more effective and some also contain a certain measure of curative or healing (read more about this under Misconceptions) and anti-sporulating action. Fungi have the ability to develop resistance to most of the systemic sprays due to the specific way in which the sprays work. To ensure the effectiveness and preservation of the systemic sprays, they should be used with great precision in an anti-resistance programme (by mixing and alternating sprays).
Preventive control. Successful control of downy mildew necessitates preventive control measures before the primary infection period. In other words, spraying before rain and/or wet weather and therefore before the fungus has infected the vine and long before any symptoms are visible. Numerous producers were reluctant to apply these sprays for various reasons, amongst which logistic reasons (vineyards too wet, tractors getting stuck), but also due to the fact that they underestimate the importance of these preventive sprays. Producers are advised to consult multi-day weather forecasts in order to facilitate decision making as far as control measures are concerned. Various multi-day weather forecasts are available on the internet: www.weathersa.co.za (7-day forecast), www.iafrica.com/weather (5-day forecast) or www.netfor.co.za (3-day forecast).
Curative control. Most producers started to apply downy mildew control once the symptoms appeared. Most of the systemic sprays have some curative action, but this "kick-back action" is only effective 1 to 3 days after infection. Curative control of downy mildew is therefore effective only if the sprays are applied directly after an infection period, in other words even before the oil spots appear and any spores are formed. Curative control during epidemic conditions will not eradicate downy mildew, it will merely delay the rate of the epidemic. Producers should take care not to rely solely on curative spraying since the risk is extremely high.
Spraying intervals. Recommended spraying intervals during conditions that are conducive to the disease are 7 to 10 days for contact sprays and 10 to 14 days for systemic sprays. If conditions that are conducive to the disease occur in the pre-flowering to pea-berry stages, and especially when the disease takes on epidemic proportions, systemic sprays should preferably be applied every 7 days until the disease is under control or until weather conditions are no longer conducive to the disease.
Coverage. Good coverage of all susceptible parts of the plant is absolutely essential. Parts that are not covered and new growth since the previous application of a contact spray are not protected. Rain and overhead irrigation also reduce the levels of fungicide residue on the plant and sprays must be repeated if heavy precipitation was experienced after applications. Poor coverage must be avoided since even systemic sprays do not protect all uncovered parts. The highly susceptible flower bunches, often hidden behind dense foliage, in particular are not within easy reach of the fungicide.
Concentrate sprays. Numerous producers relied on the application of concentrate sprays (2x or even 4x) at reduced volumes per hectare. No downy mildew fungicide is registered for use as a concentrate spray. The risk of poor coverage is thereby increased and the effectiveness of control sacrificed. Use fungicides at registered dosages and at high volumes per hectare to ensure sufficient coverage.
Spraying equipment. For the above-mentioned reasons producers are compelled to have sufficient spraying equipment so that they are able to spray all their vineyards in less than 7 days. Spraying equipment should also be calibrated correctly to apply the necessary volume of fungicide on the target area. Pressure that is either too high or too low will result in insufficient coverage. According to Marius Ras (Rovic and Leers) the best coverage that is possible under ideal conditions, given a spraying pump that is correctly calibrated, is at most 64%. Tractor speed must be 3 to 3.5 km/h. Any increase in tractor speed will result in less coverage; sprayi volume/ha must be doubled if tractor speed increases to 4.5 km/h.
Weather conditions during spraying. Too warm and/or windy conditions during spraying will reduce coverage even more. In times of crisis, producers should consider spraying applications on windstill nights, rather than sacrificing coverage by spraying on days that are too windy.
Block sprayings. Block sprayings, in other words 2 to 3 subsequent sprays, of certain products are recommended by the respective chemical companies. The reason is that sufficient quantities of the active ingredient of a fungicide must be applied to and taken up by the plant. However, some producers have been chopping and changing between products to try and find the "wonder cure". This may impede successful control.
Resistance to fungicides. During the epidemic secondary cycle the downy mildew fungus forms millions of spores and reproductive cycles can be repeated in a very short period. Consequently the fungus can build up resistance to certain fungicides fairly quickly. In such cases the sprays will be less effective and control may be impaired. Fungicides with a resistance-risk must be applied strictly in accordance with anti-resistance strategies to prevent or delay resistance build up.
General viticultural practices. Viticultural practices such as weed control, row direction, trellis system, canopy management, irrigation and fertilisation must be aimed at reducing humidity and facilitating quicker drying of foliage.
Misconceptions
The terms "systemic" and "curative" result in misconceptions among producers that may prompt them to rely on the "eradicative action" of these sprays, thereby sacrificing the extremely important preventive sprays.
"Systemic" sprays. Most of the fungicides that are being marketed as "systemic", only have a local systemic action. In other words, spraying the fungicide on one leaf will not ensure its translocation to another leaf and its concomitant protection. The systemicity of most of these sprays is purely trans-laminar, in other words it will also protect the reverse side of the leaf if it is sprayed on one side. However, new growth is protected to a certain extent. Contact sprays protect superficially only, i.e. where the spray lands. Systemic sprays on the other hand should be seen as more rain fast, in other words their systemicity ensures that they are not washed away altogether by subsequent rain showers. Substances in the phosphonate group are systemic in the true sense of the word and will spread throughout the entire plant.
"Curative" sprays. Certain systemic sprays have a limited curative action when sprayed shortly (1 to 3 days) after infection (in other words before symptoms appear). However, these sprays are not "wonder cures" that will eradicate the disease, especially not if coverage is insufficient. During the recent epidemic, under conditions of high disease pressure, no substance proved itself to be either curative or eradicative.
"Dead downy mildew ". Various producers and consultants were under the impression that they killed the downy mildew by spraying. Although it may sometimes look as though the lesions are dead, the fungus still survives in the tissue surrounding the necrotic lesion. If warm, wet and humid conditions prevail, spores will form again and the disease will be present until autumn, when the leaves drop.
Research requirements
In an attempt to prevent future epidemics of downy mildew, various research requirements must be addressed as soon as possible. Here spraying surely deserves the highest priority. Notwithstanding the fact that producers were armed with an arsenal of highly effective fungicides, coverage of vineyards, and therefore control, was disappointingly insufficient. Other aspects that should be addressed urgently are the streamlining and updating of warning and/or prediction systems and the occurrence of fungicide resistance against systemic sprays. Organic alternatives to chemical downy mildew control should also be researched, since large crop losses were experienced in the majority of organically cultivated vineyards in the Western Cape.
Lessons learnt?
The past season all the registered downy mildew fungicides, "recipes" and means of successful downy mildew control were tried to the utmost, with varying success. Visits to vineyard blocks where control failed and others where control was successful, clearly underlined the importance of preventive spray applications and thorough coverage. Downy mildew in vineyards is truly a disease where the English proverb "Prevention is better than cure" holds true.
Further queries may be addressed to Dr Paul Fourie (tel. 021-809 3104; e-mail paul@infruit.agric.za) or Abraham Vermeulen (tel 021-809 3161; e-mail abraham@infruit.agric.za)
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Paul Fourie (pictured) and Abraham Vermeulen, ARC Infruitec-Nietvoorbij, Stellenbosch
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