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A South African perspective of powdery mildew in grapevines
INTRODUCTION
It is accepted that the fungus causing powdery mildew has been occurring on South African vines since approximately 1860. Since then the extent of the disease has increased and today powdery mildew is considered the most important fungal vine disease (Halleen, 1999). Powdery mildew, also known to grapevine producers as oidium, is caused by the fungus Uncinula necator. Without necessarily causing obvious disease symptoms, the fungus may have a harmful effect on all facets of the vine and its products. Unlike certain other diseases, powdery mildew now occurs annually, and precautions to control the disease have to be taken on an ongoing basis. The fact that grapevine producers spend approximately R30 million per annum to control powdery mildew (half the total fungicide cost), is an indication of the importance of the disease to the vine industry.
The fungus follows a specific pattern in each part of the world to create an epidemic. This pattern is determined by biological characteristics of the fungus, climatic factors, cultivation practices and cultivar choices. Effective disease control, a priority to the grapevine producer, therefore requires a thorough knowledge of the biology of the fungus and the epidemiological principles of the disease. Knowledge of these aspects is available for various parts of the world, but little is known about its relevance to South African vineyards. In this article a South African perspective of the fungus and its control is outlined, based on recent local findings (Halleen, 1999; Halleen & Holz, 2000a; Halleen et al., 2000b), and considered in the light of knowledge available in other parts of the world.
THE CYCLE OF THE DISEASE
The oidium disease occurs in two forms on the vine, namely as a so-called asexual phase, and as a sexual phase. The asexual phase is characterised by the powdery white growth which characteristically occurs on the leaves. The white structure consists of thin hyphae growing superficially on the plant surface, penetrating the epidermal cells only. On the hyphae small (approximately 0.03 mm in circumference) barrel-shaped spores are formed. The spores, also known as oidia, are transported, by the wind mainly, to other parts of the plant, where they are able to germinate and continue the cycle. At the end of the season, when the leaves fall off and the shoots must be pruned, the hyphae hibernate inside dormant buds. In the new season the hyphae grow with the new shoots, infect them as well as the new leaves on the shoots. The infected shoots look stunted due to loss of vigour (dwarfed), and are known as flag shoots. The infection is considered to be a primary infection. Primary infections on parts of the stunted shoots form new generation oidia, acting as secondary inoculum. The extent of the secondary inoculum and the cyclical recurrence of its formation play a cardinal role in the severity of the occurrence of the disease epidemic in a specific season.
The oidia do not require free water for germination and infection, and can develop and infect under a broad range of climatic conditions. Even a dry climate is advantageous to the development of the disease. Stunted shoots are generally not noticeable in vineyards. Consequently the disease will develop as points of focus around the stunted shoots and therefore occur at intervals in the vineyard.
An interesting characteristic of the fungus is that it consists of two pairing types. This means that there are "male" ([+]pairing type) and "female" ([-]pairing type) hyphae. Thus, when oidia deriving from both the [+]pairing type and the [-]pairing type arrive on the leaf, grow on it and the hyphae make contact, pairing occurs and from this develops the sexual stage. This stage is characterised by the formation of cleistothecia in the fungal colony after pairing. A cleistothecium is a round, tough structure with appendices, inside of which are being carried ascuses with ascospores. The ascospores differ from the oidia in that they possess the characteristics of both the [+]pairing types and the [-]pairing types. Pairing occurs later in summer or autumn, on any heavily infected part of the plant. Autumnal rain rinses Cleistothecia onto the bark, where they anchor themselves using their appendices, and then hibernate. In spring, rain or overhead irrigation of more than 2.5 mm induce the cleistothecia to burst open, and the ascospores are released. Other than infection by oidia, rainfall or moisture is essential for infection by ascospores to take place. While the occurrence of cleistothecia is scattered throughout the vineyard, in the case of ascospore infection the disease will occur in the same widespread pattern. Since ascospore release is synchronised with the occurrence of the first leaves, ascospore infection will also cause disease symptoms earlier in the season, as infection by oidia from stunted shoots.
From the above short discussion it is clear that the form in which the fungus survives on the plant, determines its future pattern of development. What is important, moreover, is that both forms of the fungus initially establish themselves on young shoots and young leaves. The inoculum that develops in both these cases affects the young bunches, and causes the biggest headache for the grapevine producer. However, the ability of the secondary inoculum to infect the bunches is mainly during the flowering until shortly after pea bud stage. This is due to the fact that the bunches are susceptible only until they reach 8øBrix. It is also of cardinal importance to note that the first bunch infections are not noticeable and that the fungus reveals its presence only later on parts of the bunch.
LOCAL BEHAVIOUR OF THE FUNGUS
Our research indicated that the powdery mildew fungus completes both its survival stadia on vineyards in the Western Cape. Locally cleistothecia were discovered for the first time in April and May 1996, notably in the vicinity of Stellenbosch. Typical flag shoots were found in September 1997 for the first time, shortly after budding in a vineyard close to Somerset West. No conclusion could be reached about cleistothecia's ability to survive. The characteristics of the symptoms that developed on leaves, namely separate, individual blemishes that occur on the first leaves in the vicinity of the bark, prove, however, that locally cleistothecia do indeed spread to the bark, where they survive and cause primary infections. Weather conditions conducive to the release of ascospores from hibernated cleistothecia, occurred regularly between budding and flowering.
The fact that so few cleistothecia have been found on leaves, plus the fact that they are formed so late in the season, lead one to suspect that a suitable opposing pairing type may have entered South Africa only recently. Since the occurrence of the pairing type is not yet widespread in vineyards, therefore, the cleistothecia too will not occur freely in all vineyards.
DISEASE CONTROL
Fungicides
At present fungicides provide the only solution for efficient control of the powdery mildew fungus. Sulphur was the first fungicide used to control the fungus. In the 1970s, however, sulphur was largely replaced by the so-called sterol biosynthesis inhibitors (SBI). These fungicides are extremely effective, but have one drawback. They act on a few main localities within the fungal cell to toxify the fungus. The result is that within a natural population, there may be a few members who have undergone a change in the main locality and then prove resistant to the fungicide. Repeated use of the same substance, or chemically related substances, put selection pressure on the resistant individuals in the population. With each further application of the fungicide the resistant subpopulation can therefore multiply and indeed to such an extent that practical resistance occurs from time to time in the vineyard. In such a case the fungicide, at the recommended concentration, will have no effect on the fungus. In practice this means that the producer suffers a double loss. He pays for a substance that is ineffective, and the disease cannot be controlled despite all his efforts and the costs incurred to prevent it.
The SBI fungicides consist of various chemical groups, namely the triazoles, the pyrimidenes and the pyridines. In South Africa the following fungicides are registered (Nel et al., 1999) for use against powdery mildew in grapevines: triadimefon, triadimenol, penconazole, flusilazole, hexaconazole, myclobutanil, tebuconazole and fluquinconazole (all triazoles); nuarimol and fenarimol (pyrimidines); and pyrifenox (pyridine).
Resistance against triazole fungicides
In the Western Cape, grapes are cultivated mainly in five regions, namely the Hex River valley, Tulbagh-Worcester-Robertson valley, Paarl-Franschhoek valley, Stellenbosch region and Riebeeck-Kasteel region. The Hex River valley is separated from the Tulbagh-Worcester-Robertson valley by a mountain range. The same applies to the Paarl-Franschhoek, Stellenbosch and Riebeeck-Kasteel regions. Fungus populations in the regions were all exposed to triadimefon or triadimenol before 1989, whereafter these fungicides were phased out and other SBI substances applied instead. A recent study of the distribution of U. necator variants resistant to triadimenol, penconazole and flusilazole in vineyards of these regions where practical resistance was suspected, indicated that all the fungal populations display a measure of resistance to triadimenol. The findings indicate that a change in fungicide resistance occurred at an earlier stage in the populations, and that the resistant variants of the fungus are well adjusted and highly competitive. Since grapevines are intensively cultivated in these regions, wind may play an important role in the distribution of spores of resistant variants, and the establishment thereof in vineyards. This assumption is confirmed by preliminary data indicating that isolates from vineyards where the fungicides were not previously applied, or where sulphur only was used, are also more resistant to triadimenol.
The occurrence of resistant variants in these populations was also compared to that of a vineyard in the Ceres-Karoo area, which is isolated from other grapevine producing areas by two mountain ranges. Triadimefon was used in the vineyard before 1989, whereafter no SBI fungicide was applied. The extent to which resistant variants were identified in the populations, clearly indicated that cross-resistance occurred among the triazoles. At the basic level the Ceres-Karoo population was sensitive to penconazole and flusilazole. On the other hand the four populations (De Doorns, Franschhoek, Riebeeck Kasteel and Stellenbosch) which displayed the highest shift against triadimenol, indicated a high level of reduced sensitivity to flusilazole. This occurred despite the fact that the Stellenbosch population was the only one that had been regularly treated with flusilazole. The other three populations were exposed mainly to penconazole. Furthermore, reduced sensitivity was more obvious in the Paarl, Riebeeck-Kasteel and De Doorns populations. Of these populations the Paarl population received mainly penconazole, while the other two populations were treated with a series of SBIs.
The findings indicate that the fungus differs in sensitivity to triadimenol, penconazole and flusilazole. Resistance to SBIs is therefore a multigenic characteristic in U. necator, a factor which should be carefully considered when selecting fungicides for a spraying programme.
Practical resistance
Estimates of the effectiveness of penconazole and flusilazole against resistant populations relative to the Ceres-Karoo population, which was sensitive to the two fungicides at basis level, indicate without any doubt that there was practical resistance in the pathogen in the vineyards in question. For example, the concentration of penconazole required to bring about a given mortality in the De Doorns population (ED50 value of 1.942 æg/ml), will have to be increased 39.57 times to cause the same mortality than in the Ceres-Karoo population (ED50 value of 0.049 æg/ml).
CONCLUSION
Throughout the world there is increased pressure to cultivate disease-free grapes that are minimally exposed to fungicides. Grapevine producers can adjust to this by taking careful note of locally acquired knowledge about the occurrence of powdery mildew. Since cleistothecia are indeed formed in local vineyards, ascospore infections may already occur in the vineyard at the time of leaf formation. At the same time the fungus can also grow out with and infect the new shoots. It is therefore essential for effective control measures to be implemented at an early stage, namely during the early susceptible period between the budding and pea bud stages. By so doing the primary infections on young shoots and leaves, and the establishment of the first bunch infections, may be arrested.
The recommendation of five to seven SBI sprayings per season resulted in U. necator developing resistance to triadimenol, penconazole, and flusilazole in South African vineyards. In order to ensure the efficient use of the fungicides, the recommendation is adjusted, and a maximum of three SBI sprayings recommended. The sprayings should occur 2, 4 and 6 weeks after budding. These window periods are of critical importance, since this is the period when the fungus establishes itself in the vineyard. Fungicides in the SBI group should be carefully selected to prevent a succession of triazole fungicides, for example. The new strobilurin fungicides, azoxystrobin, kresoxim-methyl and trifloxystrobin may be used to alternate SBI substances. In such a case the manufacturers' recommendation should be heeded, namely that strobilurin fungicides should be given in "block sprayings" of two or three successive applications. Spiroxamine, a new fungicide that differs from the SBI fungicides in its modus operandi, may also be used for this purpose. This fungicide may be applied at 2-5 cm shoot length, but should not be used after the beginning of the flowering stage.
In conclusion, it is extremely important for producers to heed manufacturers' recommendations. Fungicides should be applied at the maximum recommended dosis. Good coverage of the plant surface is ensured by spraying the vines on both sides. These practices reduce the selection pressure for fungicide resistance in the natural fungus population.
ACKNOWLEDGMENT
Appreciation is due to the following organisations for financially supporting the research: Winetech, Deciduous Fruit Producers' Trust, National Research Foundation, University of Stellenbosch, and THRIP.
LITERATURE
Halleen, F., 1999. Resistance in Oidium tuckeri to triazole fungicides. M.Sc.Agric tesis. Universiteit van Stellenbosch. Stellenbosch, Suid-Afrika.
Halleen, F. & Holz, G., 2000a. Cleistothecia and flag shoots: sources of primary inoculum for grapevine powdery mildew in the Western Cape province, South Africa. South African Journal of Enology and Viticulture 21: 66-70.
Halleen, F. Holz, G. & Pringle, K.L., 2000b. Resistance in Uncinula necator to triazole fungicides in South African grapevines. South African Journal of Enology and Viticulture 21: 71-80.
Nel, A., Krause, M., Ramautar, N. & Van Zyl, K., 1999. A guide for the control of plant diseases. First edition. National Department of Agriculture, Pretoria.
* Current address: ARC Infruitec-Nietvoorbij, Stellenbosch.
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