NO. 10 DOWNY MILDEW: 1. Development and Spread

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1 NO. 10 DOWNY MILDEW: 1. Development and Spread By Adam Pietsch and Andrew Weeks, Viticultural Officers, CCW Downy mildew (Plasmopara viticola) is a contagious disease of grapevines that can cause serious economic loss in commercial vineyards. It is now known as an algae and not a fungus. It is native to North America where it causes minor damage to the tolerant indigenous vine species. However, when suitable weather conditions prevail, grapevines of European origin (Vitis vinifera) are particularly susceptible to the disease. It has spread to all major grape-growing regions of the world and was first recorded in Australia in When suitable wet weather prevails in spring and early summer, and the disease is left uncontrolled, it spreads rapidly and can cause total crop loss. In the Riverland downy mildew epidemics have caused serious grape production losses in , , , and in SYMPTOMS The disease symptoms are: Oilspots (Figures 1-6) on the upper side of leaves. This is often the first sign of the disease that growers notice in their vineyards. The spots may appear yellow at first and take on an oily golden yellow appearance as they grow (hence the term oilspot ). A chocolate halo may be visible on the outer edge of newly emerged oilspots. In some red grape varieties, the oilspots may turn pink, red or burgundy as they enlarge. Tapestry symptoms on mature leaves (Figures 7-8). These small, angular yellow spots are caused when downy mildew infection is confined by the finest veinlets as they gain tolerance to the disease. Defoliation of unprotected vineyards. Leaves with multiple infections can turn yellow and fall prematurely (Figure 9). Vines suffering widespread leaf loss will have limited ability to ripen a crop and harvested grapes may be tainted by mouldy flavours. Young bunches turn brown and die. Part and/or all of individual inflorescences (flower clusters) and young bunches may be infected and subsequently die (Figures 10-11). Young bunches are highly susceptible to the disease and are easily killed. Berries become immune to infection at pea size (i.e. E-L 31) and the remaining parts of the bunch slowly become less susceptible as they mature. Berries that are infected just before they become immune, later turn bluish-purple (Figure 12) and harden. White down on the underside of oilspots. Fresh, white down (sporangia) will develop on any infected green tissue, but only during suitably warm ( 13 C), humid ( 98% RH) nights. Figures show the down on the undersides of oilspots and on infected flowers. Under magnification, the down looks like microscopic trees (Figure 17). It can sometimes be confused with the woolly threads that are produced by grapevine blister mite (Erinose mite). Oily brown-black lesions on shoots. The disease may infect and kill young (green) shoots during favourable conditions, but this is rare. CCW Co-operative Limited Page 1

2 Figures 1-6: Downy mildew infection of leaves show as oily yellow oilspots. Figures 7 and 8: Tapestry symptoms of downy mildew infections on mature leaves develops when infection is confined by the finest veinlets. Figure 9: Severe infection of leaves can cause leaves to fall early and vines to defoliate. 9 CCW Co-operative Limited Page 2

3 Figures 10 & 11: Downy mildew infection of young bunches. Figure 12: Purple berries develop when downy mildew infects berries just before pea-size. Figures 13-15: White down on the underside of oilspots following a sporulation event. The dead centre of the oilspot in Figure 15 had previously sporulated and subsequently died. Figure 16: White down on flowers before capfall. Figure 17: Magnified white down showing the tree-like shape of the sporulation CCW Co-operative Limited Page 3

4 LIFE CYCLE Downy mildew has three distinct phases in its life cycle. They are the: 1. Dormant phase: Oospores are dormant survival spores that are formed during autumn within infected leaf and bunch tissue. They fall to the vineyard floor as the infected foliage material falls and the oospores are dispersed in the soil where they can survive for many years. 2. Primary infection phase infection arising from dormant spores: During this phase the dormant oospores are activated if suitable weather occurs. They germinate to produce macro-sporangia, which then release swimming spores called zoospores. The zoospores must then be splashed by rain into the vine canopy where they infect susceptible vine tissue through the vine s breathing pores (stomates). The rule of thumb 10:10:24 is a guide to the minimum weather conditions needed for primary infection to occur: 10 mm or more rainfall (or a mixture of rainfall and sprinkler irrigation); 10º C or warmer; during a period of 24 hrs. More specific description of the conditions and a timeline of primary infection follows: Further rainfall Zoospores splashed onto lower surface of leaf tissue Infection Zoospores germinate and infect leaves through stomates Rainfall or irrigation to wet soil: 3 5 mm if the soil is dry 2 3 mm if the soil is already wet 16 hours of soil wetness 45 C-hours of leaf wetness T 0hr T 16hr T 24hr Dormant oospores in the soil Oospores germinate in warm, wet soil Zoospores released after 16 hours Incubation The infection will incubate with no visible symptoms until the oilspot appears 5-17 days later (temperature dependant most rapid at C) Adapted from GWRDC Fact Sheet Managing Downy Mildew, 2010, CCW Co-operative Limited Page 4

5 Further to this process, the primary infection period requires: The temperature to be 8 C for the entire period. The optimum temperature is C, which will promote a greater number of oospores to activate. The 45 C-hours of leaf wetness is a measure of degree hours (temperature x time) of leaf surface wetness needed for infection. For example: at 20 C, if the leaves are wet for 2.25 hrs, (20 x 2.25 hrs,) 45 C-hours will have accumulated. Downy mildew zoospores need free-moisture to survive. As a guide, the relative humidity (RH) should not fall below 98% for more than 3 hr during the 24 hr period. If all of these conditions are met, the risk of primary infection is very high. During the infection period, if any one of these requirements is not met, the likelihood of primary infection occurring can be severely reduced or even stopped. Primary infection events are likely to produce sparsely spread oilspots (approx. 3-4 oilspots / metres of vine canopy). The low numbers of oilspots produced by a primary infection event will not cause economic damage themselves and can be hard to find in the vine canopy. However, the oilspots from a primary infection can trigger significant risk of spread of downy mildew infection can now increase exponentially in the secondary infection phase. Carefully consider control options at this point in the disease cycle, as warnings of a primary infection signify that a critical stage of disease development has been reached. 3. Secondary infection phase infection arising from a sporulating oilspot: Secondary infection first involves sporulation. The underside of visible infections (oilspots) can produce 1000 s of sporangia, each of which can release zoospores that may infect unprotected, susceptible (green) vine tissue elsewhere in the canopy. The conditions necessary for sporulation are much less stringent than those for primary infection, requiring only the following four conditions to occur simultaneously: An oilspot (or part thereof) that has NOT yet produced white down on its underside; 98% or more relative humidity; at 13ºC or more; for 4 or more hours of darkness. The resulting zoospores require 45 C-hours of leaf wetness for infection to occur (as in the primary infection cycle). Similarly, the new infections need to incubate for 5-17 days (temperature dependent) before the new generation of oilspots appear. Once visible, if favourable conditions recur, these oilspots will themselves sporulate, produce fresh white down and cause another secondary infection. Each secondary infection event can multiply the number of oilspots up to 200 or more times, triggering massive disease spread and causing extensive crop loss. The disease pressure is greatly increased when optimum conditions occur. For example, warmer temperatures, higher humidity and longer periods of wetness, will promote stronger sporulation and improved survival and infection of the zoospores. Other points to note i. After the oilspot appears it continues to expand and it may merge with adjacent oilspots. The leaves and bunches may be completely destroyed by this growth causing defoliation and crop loss. ii. The leading edge of a growing oilspot cannot be seen with a naked eye. It is similar to an oilspot that has not yet appeared and is still in the incubation stage. CCW Co-operative Limited Page 5

6 iii. Any part of an oilspot that sporulates (produces white down) will subsequently die and hence not sporulate again. This is very useful to note when planning a postinfection spray programme in vineyards with established downy mildew. iv. The zoospores produced during primary and secondary infection phases dehydrate and die easily. They cannot live without free water for more than four hours. This means that four hours after the foliage has dried out all newly produced zoospores are dead. Any subsequent infection will require the release of new zoospores, which will require another suitable weather event. v. An aged oilspot may have up to five distinct regions; Zones 4 & 5 are the only areas of an oilspot that can be reliably killed by postinfection fungicides. The visible active zone 3 cannot be reliably killed using these fungicides but it will die once it sporulates. The correct timing of sprays after sporulation and secondary infection is essential when managing outbreaks of downy mildew. SUSCEPTIBILITY AND DAMAGE All grapevine varieties are susceptible to downy mildew but those with native North American grapevine heritage are less susceptible (e.g. Rubired and Chambourcin). Young rapidly growing shoots, leaves and bunches are very susceptible to downy mildew infection. In the Riverland, disease outbreaks from budburst (E-L Stage 4) through to mid December (E-L stage 30) can be particularly destructive. Total crop failure can result when young bunches, which are particularly susceptible, become heavily infected at or near flowering. As the young vine tissue grows and matures it becomes less susceptible to the disease. By E-L Stage 30 the berries have become resistant to infection and the shoots become less susceptible as they harden off (mature). All other unprotected green parts of the vine toughen to infection as they age, but they never attain total resistance. Infection of bunch stems can cause further crop loss right up to harvest. In this was particularly observed in Cabernet Sauvignon. Infection of bunches and berries can provide a wound site for bunch rot organisms. Downy mildew can defoliate vineyards. This can lead to major difficulties in meeting minimum winery grape maturity standards; for example, sunburn and ripening to a required baume. It can also cause crop reduction in the following season by reducing the number and size of bunches in the buds that will form next season s crop. Infected berries and foliage in grape deliveries can impart a mouldy taint to grape juice. The resultant wine is often unsaleable. CCW Co-operative Limited Page 6

7 MONITORING Downy mildew outbreaks can cause severe unexpected crop loss. For this reason it is important to: 1. Monitor the weather forecasts carefully during the growing season for conditions likely to favour primary and secondary infection. This is particularly relevant during spring when new growth should be protected with a suitable pre-infection fungicide as close as possible before the predicted infection periods. 2. Monitor past weather events for risk of infection. If it seems likely infection occurred in an unprotected block of vines, seek advice from a CCW viticultural officer on the options for a post-infection spray. 3. Look in the vineyard for signs of downy mildew infection, i.e. oilspots, especially after infection events and the predicted incubation period. Areas of the vineyard that are low lying, sheltered, subject to water pooling, or with water-shoots near the ground, are a good place to start looking. If canopies are large or tall, make the effort to look at the extremities of the canopy where spray coverage may have been limited. 4. To confirm that an oilspot is active and that it is not being confused with other possible yellow spots (e.g. leaf hopper damage or Spray Seed damage) conduct an overnight bag test (see below). If the spot is active downy mildew, it will sporulate and produce fresh new spores seen as the distinctive white down on the underside of oilspots. Bag test identification Add about one tablespoon of water to a sealable plastic bag (not too much water). Close and then vigorously shake the bag to wet it evenly inside. Open the bag and drain off the free water (enough moisture will be retained for the needed humidity). Place the infected leaf in the bag with the upper leaf surface facing down. Partially inflate the bag and then seal it. Place the bag in a dark cupboard overnight (>13 C). Next morning carefully examine the undersides of oilspots for the diagnostic fresh white down. Sometimes the down will be faint. Note: Down will not form if the spot is covered by water. 18 Figure 18: The bag test produces fresh white down on the underside of viable downy mildew oilspots. CROPWATCH CropWatch is a monitoring service that, amongst other things, warns grape growers of possible downy infection events. The warnings are based on forecast weather conditions or follow potentially suitable weather events. The latter are assessed using data from a network of automatic weather stations across the Riverland. Due to the very specific requirements of this disease, the specialised staff interpreting the weather data sometimes have difficulty deciding if an infection event has actually occurred in a specific locality. In the Riverland, CropWatch is funded by the Riverland Wine Industry Development Council (RWIDC) and overseen by the Riverland Viticultural Technical Group. Since its commencement, this service has saved the Riverland s grape growers many millions of dollars by giving timely advice. Growers are urged to ensure that they are registered with CropWatch to receive free, either the fax or messages. To register, either telephone RWIDC on (08) or wine@riverlandwine.org.au. CCW Co-operative Limited Page 7

8 DOWNY MILDEW MANAGEMENT For information about managing downy mildew, see CCW s Fact Sheet 13. FURTHER INFORMATION Downy Mildew, pp 1-11, in Diseases and Pests, Grape Production Series No. 1. Nicholas PR, Magarey PA, and Wachtel MF, (1994) (Editors) Winetitles Adelaide. 106 pp. The Australian and New Zealand Guide to Diseases, Pests and Disorders of Grapes. Magarey, PA, MacGregor, AM, Wachtel, MF and Kelly, MC, (1999). (Editors) 108 pp. A companion to Diseases and Pests Grape Production Series No.1 for information on the symptoms and management of the mildews and other diseases of vines. GWRDC Innovators Network Resources Factsheets: Managing Downy Mildew and Downy Mildew Questions and Answers. ACKNOWLEDGEMENTS Peter Magarey, Magarey Plant Pathology, Loxton South Australia, for editing this fact sheet. DISCLAIMER The information supplied in the Fact Sheet was the best available at the time of publication. However, the understanding and control of pests and diseases is constantly evolving and recommendations regularly change. The reader should seek a professional opinion before acting upon information in this Fact Sheet and should always comply with the winery s requirements and recommendations, food safety legislation and the information on chemical product labels. Published by CCW Co-operative Limited. October Box 238, Berri, South Australia 5343 Telephone (08) Facsimile (08) CCW Co-operative Limited Page 8