Introduction

The 1995 vintage was one which many grapegrowers may wish to forget, while others would like to learn from it. A large proportion of grapes was affected by Botrytis bunch rot, with consequences for wine quality. Many discussions and publications have categorically stated that only those who did not vigorously spray suffered heavy crop losses. This and other mixed messages can be heard amongst those discussing the past season, confusing growers and preventing informed decisions being taken. It is important not to make generalisations, and to study each vineyard separately, based on timing of sprays in relation to various other factors. This article discusses some of the impacting factors on disease incidence and progress with a case study, and also considers aspects of Botrytis control.

Disease epidemics

The occurrence of Botrytis disease depends on the presence of the pathogen (in this case, Botrytis cinerea), suitable environmental conditions, and the presence of a suitable host, and their interaction. The amount of disease in a crop at any given time is dependant on the inoculum (spores produced by the pathogen) and the rate of disease development. The latter is often influenced by conducive weather and susceptibility of the host.

Inoculum-driven disease

Assuming that environmental conditions are always conducive to infection, and the rate of disease development is constant, disease will progress in a cultivilinear fashion, forming an S shaped curve. In this situation, the amount of primary inoculum often dictates the amount of disease at any given time, and is referred to as inoculum driven disease. Under these conditions, it is important to spray botryticides at most times to reduce the level of inoculum and to delay the onset of the epidemic. Note, however, that even when conducive conditions for infection prevail and Botrytis inoculum is present, the grapevine needs to be in a susceptible stage of growth - from flowering onwards. Areas where Botrytis is inoculum-driven include Hawke’s Bay, Poverty Bay and Auckland regions.

Weather-driven disease

In regions where weather conditions are not conducive to infection at most times, the disease becomes weather-driven, as opposed to inoculum-driven. In other words, the progress of disease is influenced by conducive weather. Marlborough, Canterbury and Central Otago regions fall within this category. Conducive weather is not normally prevalent in these regions from budburst to post-flowering. After the pre-bunch closure stage onwards, however, rain events can influence Botrytis disease epidemics. In some years, even the pre-harvest stages may escape unscathed if there are no significant rain events.

It is possible, however, for circumstances to change midway during the season and a weather-driven epidemic can become inoculum-driven. In this situation, a disease epidemic can rapidly develop to affect the entire crop. In such situations, Botryticides help to show down the build-up of inoculum and the spread of disease. Therefore, it is important to consider each season on its own merits, and also carefully monitor the progress within a season, and respond accordingly to changes that occur over time.

Case Study

The 1994/95 season in Marlborough was initially weather-driven. However, from the veraison period onwards, the rate of development of the disease epidemic was influenced by prolonged conducive weather conditions and hence became inoculum-driven. In this situation, it was important to control the epidemic by astute spray and fungicide resistance management.

As a case study, let us look at an example of two areas within the same block of Chardonnay grapes which were managed by two different people, and which ended up with different levels of disease severity. Three hundred bunches of grapes were assessed at random for incidence and severity of Botrytis in each of the two areas, one week prior to harvest. (Disease incidence in this case is the number of bunches showing disease expressed as a percentage, and disease severity is the area of a bunch affected by disease expressed as a percentage). The incidence of Botrytis bunch rot in areas 1 and 2 was 93.7% and 98.7%, - quite similar. However, there was a significant difference in Botrytis bunch rot severity between the two areas - 53.8% in area 1 and 86.3% in area 2. This means, that while only half of each bunch in area 1 was affected, more than three quarters in area 2 was affected. This difference may be explained by the spray management adopted in the two areas. Table 1 compares the routine management of these areas in chronological order and lists only the botryticides used. As no record of growth stages was maintained, I have used those recorded by Mr Warren Cairns of Montana, from the Renwick site (Warren’s meticulous diary maintenance is acknowledged and duly thanked).

There were two main differences between the two areas in their spray programme - the number of botryticides used and the timing of these sprays. The first two botryticides were applied at different times in the two areas, but this difference was of no major consequence to the incidence of Botrytis as they were applied immediately prior to/or after an infection period (Table 1). These sprays provided adequate protection and control from any possible infection. (A total of twelve Botrytis infection periods were identified by the Botrytis model used in the Disease Management Systems programme. Nine of these infection periods occurred from well after bunch closure, increasing the potential risk of an epidemic. However, the managers of these sites had no prior information of the occurrence of these infection periods).

Table 1. Management influences on bunch Botrytis in two areas of a Chardonnay block.

The difference in final disease severity can be attributed to one botryticide being omitted in area 2, especially when the infection risk was high from frequent rain events. It can be concluded that missing one spray after an infection period during the berry ripening phase had the greatest impact on the final severity of Botrytis at harvest.

It is important to also note that this situation may not apply to all vineyards, and the influence of variety, growth stage, canopy density, vine susceptibility, soil type etc. need to be considered in relation to infection conditions. Therefore, a Botrytis spray management programme for each vineyard needs to be developed to suit individual situations. In other words, tailor-make management practices to suit your own needs.

Lessons to be Learned

What is evident from the above case study is that even though up to seven Botrytis sprays were used, a high incidence and severity of Botrytis bunch rot could not be avoided. A rigorous spray programme could not have battled against the forces of nature in preventing the extent of disease encountered in the past season. However, the severity could have been reduced by astute spray management as seen in this case study. The following factors need to be considered in good disease management.

• Reduce primary inoculum.
• Manage the vine well to reduce canopy microclimates conducive to Botrytis development. An open canopy is desirable.
• Time the application of sprays to provide maximum protection, that is spray when the risk of infection is high.
• Use suitable spray equipment for maximum spray penetration and coverage.
• Monitor the level of fungicide resistant strains regularly each year, and adopt a suitable resistance management strategy.
• Develop and use a suitable postharvest sanitation and disease management practice.

Responsibility of Winemakers

The question one needs to ask is the consequence of the severity of bunch Botrytis on wine quality. Does a 50% bunch severity reduce wine quality as much as an 80% severity? Winemakers have a responsibility to study and inform growers on the threshold for bunch Botrytis in relation to wine quality. Standards need to be developed and established for grape quality in relation to the production of good quality wines. It is then the challenge of the growers to produce a crop to meet the grape quality standards, and be accordingly rewarded. We shouldn’t wait for discerning consumers to dictate, but rather provide them the best to maintain the industry’s reliability and quality.

Acknowledgments

I would like to thank Paula Kloosterman for data collection and analyses, and the two growers who supplied the information for the case study.