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A Perspective from the Eastern USA

Wayne F. Wilcox
Department of Plant Pathology
Cornell University, NY State Agricultural Experiment Station
Geneva, NY 14456 U.S.A.

I live and work in the cool-climate viticultural region of the Finger Lakes in New York State, about 500 km northwest of New York City and 160 km east of Niagara Falls. Our primary Vitis vinifera varieties are Chardonnay, Riesling, Pinot Noir, and Cabernet Franc. Temperatures during the growing season are moderate, with typical summer highs remaining in the 20’s (and a few days in the low 30’s). Humidity generally is high, and rainfall is regular enough that only a few vineyards are irrigated. Of course, these conditions are ideal for a number of different fungal diseases, which we must control every year.

Even though specific conditions may differ among viticultural regions around the world, there are certain principles that apply universally. Within this context, I’d like to present our experiences in controlling Botrytis, and will try to relate them to what I observed during my stay in Australia this past growing season. I’ll try to concentrate on:

• The fundamental biology of the diseases (weather effects, how they spread, etc.)
• Factors that influence the crop's susceptibility to infection (we concentrate heavily on identifying the critical periods for controlling infection during the season)
• The basics of the fungicides that are used in control programs.

Biology. The Botrytis fungus is a "weak" pathogen in that it seldom attacks robust plant parts. Rather, it attacks dead, injured (e.g., light brown apple moth damage), highly succulent, or senescent (expiring) tissues such as wilting blossom parts and ripening fruit. The fungus thrives in high humidity and still air (optimum temperature range is 15-25°C), hence the utility of cultural practices such as leaf pulling and canopy management that minimize these conditions around the fruit zone. Young fruit can become infected through attached blossom parts, with the infections remaining latent (dormant) until some resume activity and rot the berries as they start to ripen. Although latent infections can be common following a wet bloom period, the vast majority remain so through harvest (i.e., the fruit stay healthy). Some studies (e.g., from New South Wales) show a strong relationship between the levels of fruit rot and early latent infections, whereas others (e.g., from France and NY) show little. Factors that cause latent infections to activate or not are poorly understood. High humidity and tissues with high nitrogen content (how high, specifically, is still not defined) appear to be two factors that promote this process. Botrytis is a disease that is governed by complex interactions between the grapes, the weather, and the fungus itself, many of which are poorly understood.

Susceptibility. There is a long-running debate about when berry infection is most likely to occur, and thus, when protective sprays are most beneficial. One school of thought is that most fruit rot is simply due to the preharvest activation of early latent infections, i.e., disease symptoms at harvest are merely the expression of infections that were initiated back during bloom. By this thinking, sprays through late bloom (e.g., 80% cap fall) should provide all the benefit that any fungicide program can, and later sprays would be unnecessary. This has NOT been our experience in New York, nor of colleagues in France.

A second school of thought is that early, latent infections do indeed account for some proportion of the rotten berries seen at harvest, but that berries are actually most susceptible to acquiring infections from veraison onwards. By this model, activated latent infections are perhaps most important as a source of Botrytis spores within the clusters, which can then spread the disease to other berries as they become highly susceptible during the postveraison period. Thus, sprays at or after veraison should be very important since they protect berries when the fruit are most prone to infection, although early sprays also can provide significant benefits by reducing the opportunity for the fungus to establish itself within the clusters. Our research and experience in New York supports this second model.

Varieties and clones with tight clusters also appear to be at increased risk of developing Botrytis. In order to examine the effects of both cluster architecture and berry age on disease development, we inoculated Botrytis spores onto berries of a tight- and loose-clustered clone of Pinot Noir (PN29 and Mariafeld, respectively) at late bloom, pea-sized berry stage, bunch closure, and veraison. Also inoculated were PN29 clusters that had been thinned after fruit set to approximate the looseness of Mariafeld. Two findings stood out in both years of the experiment:

(i) Although the incidence of latent infection was similar among all three clonal treatments, significantly more berries developed gray mold in the tight PN29 clusters than in the Mariafeld or unthinned PN29 clusters. Thus, cultural or chemical practices that loosen clusters should aid in control of Botrytis. Unfortunately, there are no easy recommendations, although various options are being tried.

(ii) Inoculations at veraison produced significantly more disease than did inoculations at any other time. For instance, inoculations from bloom through bunch closure produced virtually no rot in Mariafeld or thinned PN29 clusters (despite the existence of latent infections), and only 4-15% of berries in the unthinned PN29 clusters became diseased. In contrast, inoculations at veraison resulted in 6, 16, and 41% berry rot within the Mariafeld, thinned-, and unthinned PN29 clusters, respectively.

In a different experiment on Chardonnay, we inoculated fruit to provide 0-5 moldy berries per cluster 2 weeks before harvest. Not surprisingly, there was a direct relationship between the number of initial infections and subsequent disease spread: 1, 10, 24, and 40% of all berries eventually became diseased following inoculation of 0, 1, 3, and 5 berries per cluster, respectively. Thus, the establishment of only a few early infections led to significant spread as the berries ripened.

Despite these theoretical issues, the real question is,What kind of spray program works? To examine this, we've conducted spray-timing trials for the last 6 years in a Finger Lakes vineyard of the susceptible hybrid variety ‘Aurore’. Conclusions from the 4 years in which significant disease developed are:

(i) in two of the years, equivalent control was provided by applying either two early sprays (bloom, bunch closing) or two late sprays (veraison, 2 weeks later), with no additional benefit from applying all four;

(ii) in the two other years, sprays at veraison plus 2 weeks later provided good control but was improved by adding applications at bloom and bunch closure; and

(iii) in one of those years, the two early sprays provided little control by themselves, although they did improve the activity of the two late sprays when all four were applied. Thus, it appears that the late sprays always provided benefit (about 50-90% control relative to the unsprayed vines), that this control was sometimes improved when the early sprays were applied first, and that early sprays by themselves provided inconsistent results.

Local conditions have a profound influence on Botrytis development, and it is risky to assume that what happens in one place will happen the same way somewhere else. Nevertheless, it is clear that berries are highly susceptible to infection as they begin to ripen, and that fungicidal protection during that time can be very beneficial if conditions favor infection then. Australian fungicide recommendations tend to emphasize sprays at 80% cap fall rather than veraison or preharvest. In part, this probably reflects the greater probability of rain during the earlier period, but I suspect that it also reflects a desire to minimize residues of fungicides that, ironically, are widely used during the later period in both the United States and Europe. In any case, the issue of fungicide application timing is one that probably deserves additional attention under Australian conditions.

Fungicides. Unlike powdery mildew, a good fungicide program is no guarantee of Botrytis control when conditions favor its development. Cultural practices and site selection to maximize air movement are at least as important as fungicide sprays, but such sprays can be an important component of an integrated control program. Within this context, it is worth remembering that most general fungicides are ineffective against Botrytis. Also be aware that all Botrytis-specific fungicides are prone to resistance development, and should be used in rotation with each other to maintain their utility.

• Rovral is a moderately systemic fungicide in the dicarboximide group, which also includes Sumisclex (the latter is not registered in the U.S.). Although primarily a protectant fungicide, it has some limited postinfection activity and is a good antisporulant material. Activity is improved by mixing it with an agent that improves uptake into the fruit, such as an oil or a nonionic surfactant. Until recently, Rovral has been the only Botrytis fungicide recommended for use in New York. Unfortunately, resistance to Rovral and other dicarboximides has reduced their efficacy in many parts of the world, including New York. The occurrence of dicarboximide-resistant isolates also has been confirmed in multiple Australian vineyards. The good news is that, unlike other fungicides, resistance to the dicarboximides declines over time if they are withdrawn from the spray program. Therefore, in NY we recommend that where Rovral has been used intensively, it should be "rested" for a year or two, then used no more than once a year afterwards. In Australia, Rovral appears to be the only fungicide that exporters currently allow at veraison, when fruit are at high risk if the weather turns wet. Thus, where fungicides are used to control Botrytis, it would seem prudent to avoid dicarboximides earlier in the season and save Rovral in case it’s needed at veraison, thereby maximizing the probability that it will be effective if so applied.

• The anilinopyrimidines include Scala and a related compound (cyprodinil) that is one of the components of the new fungicide, Switch. These are systemic materials with excellent activity, but they are highly prone to resistance development and ideally should be used no more than once per season. Neither Scala nor Switch are yet registered for use in the United States, although we do have a formulation of cyprodinil. It has provided very good control in both experimental trials and commercial usage, equivalent to that provided by Rovral in its prime.

• Teldor is a new surface-protectant fungicide in the hydroxyanilid group (new chemistry). It has been effective in our trials, although commercial experience has been limited. I have found it a bit less consistent than cyprodinil (the "Scala-like" compound), although other researchers have found these two materials to be equivalent. I believe that it has a useful place in rotational strategies. The 2001/02 season was the first for registration on grapes in Australia.

• The strobilurins are not traditionally considered Botrytis fungicides, but they do have significant activity, especially under moderate pressure. In NY, we have evidence to show that they can provide supplemental control of this disease when used to control powdery mildew during the bloom through bunch closure period. To what extent this may or may not be able to replace Botrytis-specific compounds during the same period is under investigation.

• Although the benzimidazole fungicides (Benlate, Spin Flo, etc.) can be excellent Botrytis fungicides, resistance to them is so widespread in NY that they are no longer recommended.

• Captan is the only traditional protectant fungicide labeled for use on grapes in the United States (chlorothalonil—e.g., Bravo—is not registered for this crop). Our repeated experience is that it only works when we don’t need it. It may provide sufficient activity under light disease pressure, but we haven’t seen much under even moderate pressure.

Take-home messages.

• Latent infections at flowering can be common under wet conditions; some, but not most, can develop into berry rot by harvest. The relationship between latent infection levels and disease severity at harvest appears to be inconsistent.

• Under wet conditions, fruit rot is much more likely to develop when berries are exposed to the fungus at veraison rather than during earlier stages of development (including bloom).

• A few active infections during the preharvest period can spread rapidly throughout the cluster. Thus, control of latent infections at bloom may reduce this source of the fungus, although introduction and spread from external sources is still possible.

• Reducing cluster compactness reduces Botrytis. The only question is how to do so.

• Cultural practices (e.g., leaf pulling, canopy management) are important for Botrytis control. Fungicides can provide significant additional control.

• Several new, effective Botrytis fungicides have become available recently. These should be used in rotation with one another and existing materials in order to reduce the possibility of resistance development.

• Currently, Rovral is the only Botrytis fungicide available to many growers when fruit become highly susceptible at veraison. Those who wish to maximize the likelihood of its effectiveness at this time should not apply dicarboximides earlier in the year.