A Technical Guide for Wine Producers

RECENT ARTICLES   |   WYNBOER HOME DIEBACK DISEASE IN VINES CAUSED BY EUTYPA LATA J H S Ferreira, ARC-Fruit, Vine and Wine Research Institute, Nietvoorbij, Stellenbosch Eutypa lata, better known as dieback disease, occurs in all the wine-growing countries in the world. It has a broad host range and also occurs on various tree species. In South Africa it is a problem on vines in particular, as well as apricot and peach trees, where it causes considerable losses. The annual grape crop is harmed by downy and powdery mildew and botrytis, but dieback is the one which eventually costs the grape producer most. The disease affects production as well as the lifespan of wine grape vines in particular. Dieback is usually not a problem in vineyards younger than 10 years, but thereafter it can become one of the biggest limiting factors in the production and lifetime of a vineyard. The disease occurs when the vascular tissue of a vine is infected with the Eutypa lata fungus. The fungus releases a toxin in the vascular tissue which causes abnormalities in leaves, shoots and flowers/bunches. Once a vine displays the symptoms, they worsen each season until the vine dies altogether after 3 - 5 seasons. The fact that the symptoms only show up after 7 - 10 years does not mean that a vineyard is susceptible at this age only. After infection the fungus grows very slowly in the wood and infection can already occur when the vine is 4 - 5 years old, but due to the slow growth of the fungus noticeable symptoms only occur at 7 - 10 years of age. Lifecycle of the disease Infection occurs when an ascus spore lands on a fresh pruning wound and is washed by water into the xylem tissue where it germinates. The fungal mycelia grow through the xylem, spread in the wood and kill the infected parts. The toxin, eutypine, which is secreted by the fungus in the infected wood tissue, is responsible for symptoms in the shoots, leaves and bunches. The fungus grows into the wood at a rate of 10 - 20 cm per annum. After a few years, when the fungus has killed a large part of the wood, a dark mass of stroma is sometimes formed on the surface of the dead wood. These stroma are usually found on cancers surrounding old pruning wounds. Inside the stroma flask-shaped peritheciae of the fungus are formed, each containing a large number of fertile ascus spores. Usually stroma are only formed on wood which has been dead for a few years. It is possible for the fungus to cause symptoms on a vine for a few years without forming a stroma or perithecia. Where stroma have formed, new perithecia develop in the course of the winter and spores may be released throughout the season, after precipitation of 1 - 2 mm or more. Spores are released 1 - 2 hours after the beginning of a rainshower and if they remain wet, spores may be released for up to 36 hours. When it rains for more than 36 hours at a time, the inoculum is temporarily exhausted and new spores can only be formed in the asci after 8 - 12 days. Spores may be distributed by the wind or splashed onto neighbouring vines by raindrops. Cancers usually only occur in the 3rd or 4th season after infection, when they appear around infected wounds, accompanied by leaf and shoot symptoms. At this stage it may still take 2 - 3 seasons for an infected cordon arm or stem to die. The fungus grows further through the xylem tissue until the main stem eventually dies. Symptoms The toxin of Eutypa lata, which is formed by the fungus in the xylem tissue, is responsible for the leaf and cane symptoms. Symptoms are only displayed by the growing sections which are directly linked to the infected xylem tissue. Various initial symptoms, caused by low levels of the toxin during the early development of the infection, may occur. The following may be considered initial symptoms: Slight yellowing of leaves early in the season. The yellowing disappears later and leaves appear normal. Delayed budding, especially on the middle section of a shoot in the case of long bearers. Growth of a large number of sucker shoots around the crown of a vine. Vines on which the above symptoms occur, usually develop typical dieback disease symptoms after a few seasons. Leaves Affected leaves are smaller than normal, bowl-shaped and have a pale green colour (Fig. 1). Necrotic stains often develop on leaves and leaf edges turn brown. Necrotic stains become bigger with age and older leaves later have a scorched appearance (Fig. 2). Shoots The most characteristic symptom is dwarfing of the internodes (Fig. 3). Affected shoots are usually a quarter to half the length of healthy shoots and sometimes the leaves on such shoots do not display any symptoms. Affected shoots are easiest to observe during the first few months of the growing season due to the sharp contrast in growth compared to healthy shoots. Flower bunches and grape bunches In some affected bunches there is no fruit set. Bunches may form with small and large berries on the same bunch and they do not ripen at the same time (Fig. 4). After flowering bunches may die and fall off. Cordon arms and stems Cancers and loose bark usually form around an old pruning wound (Fig. 5). Infected parts of the wood have a flat or sunken appearance and elongated cracks may appear in the wood. The occurrence of affected shoots on infected arms will increase for a few years, but eventually no shoots will form on the infected parts and later they will die. The spore-carrying stroma formed by the fungus on old infected wood looks like a black crust on the surface of the dead wood and may be hidden by the bark. If this crust is dissected with a sharp knife, it has a beehive appearance and the dark, spheric fruiting bodies can easily be seen with a 10x magnifying glass (Fig. 6). These fruiting bodies have a shiny black appearance and if they are active, their appearance when moistened is jelly-like. A cross section will reveal a wedge-shaped necrotic section in the wood (Fig. 7). Symptoms of the disease differ from cultivar to cultivar. In some cultivars the typical symptoms are absent, but a gradual decay is noticeable in the cordon arms until they stop budding altogether after a few seasons. Economic implications The fungus destroys the wooden section of a vineyard, dwarfs and distorts new growth and destroys bunches so that the overall vitality and yield are reduced and the vines eventually die. The primary effect of Eutypa on yield results from the reduced number of eyes, bad budding and to a lesser extent the reduction of bunch size. The vegetative growth of the healthy parts on an infected vine are apparently not influenced. In a survey of Chenin blanc vineyards older than 15 years it was found that the occurrence of the disease ranged between 30 - 60 %. Between 12 - 20 % of the vines in these vineyards were taken out previously as a result of the disease. If a vineyard's potential yield is 10 tons per hectare, this would mean a loss of 1,2 - 4 tons per hectare in terms of vines being uprooted. Besides causing yield losses, the infected vines suffer as far as quality is concerned, with the grapes on affected vines ripening unevenly. The potential lifetime of a vine is about 40 years, but vineyards infected with Eutypa usually have to be uprooted after a period of 20 years as a result of reduced production. The economic cost of uprooting and re-establishing a vineyard is very high and besides it takes 3 - 4 years for a new vineyard to come into full production. Similar abnormalities There has been a lot of confusion about the symptoms of Eutypa (dieback disease) and Phomopsis (dead arm disease). Both diseases cause the cordons of vines to die. For someone who is not an expert, it is almost impossible to distinguish between the two diseases based on the dead cordon arm symptom in winter. It is easier, however, to distinguish between the two diseases on the grounds of spring symptoms. Symptoms caused by some herbicides, for example glyphosphate, can also be confused with the symptoms of Eutypa. Some symptoms of bacterial dieback disease may also be confused with Eutypa. Conditions conducive to infection The occurrence of the disease is influenced by precipitation. Reproduction of the fungal stroma and spore development are favoured if the rainfall is higher than 500 mm per annum. It is unlikely that the fungus will occur in areas where the rainfall is less than 250 mm and very common where the rainfall is higher than 600 mm per annum. Once a spore has landed on a pruning wound, it has to be washed into xylem tissue to germinate. Spores germinate at 2°C - 30°C, but at 2°C it may take 100 hours to germinate. The optimum temperature for germination is 20°C - 25°C and under these conditions it usually takes 12 hours. In time, as the wounds heal, infection of pruning wounds decreases. Low temperatures can also cause the natural wound healing to be delayed and consequently the wounds remain susceptible for longer periods. Usually pruning wounds should be resistant against infection after 2 weeks. There is a possibility that other non-pathogenic organisms which infect the wound may prevent the germination of ascus spores. Factors such as environmental conditions, the number of spores present, the size of the wound and the age of the wood will also play a role in infection of pruning wounds. The proximity of other crops which act as hosts to the fungus will increase the chances of infection; however, spores can spread over considerable distances. There is proof that spores of the fungus can travel a distance of 100 km. The occurrence of the disease may be influenced by various factors: Young vines are more resistant than vines 3 - 4 years and older. The number and size of pruning wounds. The more pruning wounds per vine and the bigger the pruning wounds, the better the chances of a spore or spores landing on the wounds. Environmental conditions which promote spore formation and the release of the fungus. Stress conditions increase the occurrence of the disease. Cultivar susceptibility Currently no cultivar is known to be resistant to Eutypa. Well-known cultivars which are the most susceptible, are the following: Chenin blanc Cabernet Sauvignon Sauvignon blanc Chardonnay Shiraz Colombard Muscadel Pinotage Riesling Palomino Gewürztraminer Bukettraube Control of the disease Prevention of infection is the best way to control Eutypa. Control of the disease by means of chemical practices has not been successful. Management practices to reduce the occurrence of Eutypa are mostly very labour intensive. The only "control measures" for prevention are: Prevent any stress conditions in the vineyard as this causes vines to be more susceptible. Prevent infection of pruning wounds. Using a household spray can, pruning wounds may be sprayed with Benomil (10 g/L) directly after pruning, or be painted with an elastic wound sealer. Do not prune in rainy conditions, when the spore release of the fungus is very high. Renewal of infected vines. During the season, infected vines can be marked and in winter they can be cut off 20 cm above the graft union. A cane which develops in the following growing season can be used to renew the vine. The wound where the vine was cut must be covered with an elastic wound sealer. Removal and burning of infected wood. Dead wood must be cut out during pruning and burnt to reduce the inoculum in a vineyard. Eutypa may be considered one of the most important diseases in wine grapes, but the right management practices and preventative treatment of pruning wounds can largely reduce the effect of the disease.

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