The following article summarises the outcomes of the 1994/95 seasons Botrytis management strategy tested in five different vineyards and on three varieties in Marlborough. The results are discussed in comparison with those of the 1993/94 season. The outcomes of the disease management strategy for powdery mildew in the 1994/95 season will be published in the next issue of Winepress.

The targeting of Botrytis sprays this season was based on infection periods and the phenological stage at which the crop was considered most susceptible to infection However, last season, the targeting of sprays was based solely on infection periods. The phenological stages at which the crop is considered most susceptible for Botrytis infection are GS 20-25 (flowering), GS 33 (pre-bunch closure), and GS 35 (véraison) onwards. Thus, if an infection period occurred at times when the crop was not at these stages, sprays were not recommended. In a commercial vineyard situation, there is often a time delay between informing growers that a spray is recommended, and the time when the sprays are actually applied. Sprays are also sometimes applied when they are not recommended, either inadvertently through poor communication between the manager and vineyard operator, or because of a lack of grower confidence in the target spray programme.

Overall, leaf Botrytis levels at all sites were low this season, with disease incidences ranging only 0 to 0.5% in both treatments. Last season, leaf Botrytis levels were also low. Although there were frequent wet conditions late in the season, leaf Botrytis levels did not increase. This can be attributed both to the reduction in susceptibility of leaf tissue over time and the change in preferred substrate by the pathogen (sugars in the developing berries). Cultural operations which damage leaves and hence increase their susceptibility to infection can occur with vine trimming, leaf plucking etc., and usually occur during the early part of the season.

The incidence of bunch Botrytis was low at all sites until GS 35 when it began to increase. Towards the later part of the season, there was an exponential increase in the disease epidemic, and by harvest, the incidence of bunch Botrytis ranged from 47.4% to 73% in the target treatments, and from 42.2% to 77.9% in the standard treatments. The exponential increase in the incidence of Botrytis prior to harvest was because of frequent wet conditions which are conducive for Botrytis infection. This increased the number of monocycles of the disease (from infection to sporulation to infection), thus contributing to the exponential increase in the epidemic. Although Botrytis is usually a weather-driven disease in Marlborough, weather conditions late in this season resulted in an inoculum-driven disease.

The severity of bunch Botrytis ranged from 3% to 21% at four sites, (site 4 was harvested without notice, and final assessments could not be made). Disease severities were low at sites 1, 2 and 5. Disease severity at site 3 was 16% and 21% in the target and standard treatments respectively. This was attributed to a highly dense canopy at this site, a later harvest, and possibly higher dicarbioximide resistance populations. Overall, disease severities were low, even though disease incidence were unacceptably high.

Disease incidence and disease severity provide useful information on the spatial and temporal distribution of Botrytis. These two measurements must be considered together, however, in order to provide an assessment of potential crop loss. A low disease severity and a high disease incidence, or vice versa, does not necessarily translate to a high potential crop loss. The potential crop loss from Botrytis was less than 5% in both treatments at sites 1, 2 and 5. At site 3, however, the potential crop loss was greater than 10%. The potential crop loss at this site was 65% greater in the standard treatment compared to the target treatment, because of greater disease severities in the standard treatment compared to the target treatment. At site 1, an assessment of potential crop loss was done in an untreated control area, and potential crop loss was 175% greater than that in the standard treatment. This clearly shows that fungicides are definitely needed to control Botrytis in a weather driven epidemic. The timing of their application is also important to provide a greater impact on disease control.

If environmental conditions are always conducive to infection, and the rate of disease development is constant, a disease epidemic will progress in a curvilinear fashion, forming an S shaped curve. In this situation, the amount of primary inoculum dictates the amount of disease at any given time, and is referred to as an inoculum driven epidemic. Under these conditions, it is important to spray 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 inoculum is present, the vine needs to be in a susceptible stage of growth).

In regions where conditions at most times are not conducive to infection, the disease becomes weather-driven, as opposed to inoculum-driven. In this situation, the progress of disease is influenced by conducive weather conditions. Botrytis epidemics in Marlborough are usually weather driven, where conducive weather is not normally present from GS 05 (bud burst) to GS 26 (post-flowering). In some years, even the pre-harvest stages may escape unscathed if there are no significant rain events. This season, however, circumstances changed later during the season, and a weather-driven epidemic became inoculum-driven. This occurred in Marlborough this season after GS 35. Prolonged conducive weather conditions resulted in several infection periods at close intervals, and a disease epidemic rapidly developed at each site.

There were many more infection periods detected this season (10-16 infection periods at various sites) compared to the previous season (5-6 infection periods). Most of these infection periods occurred after GS 35, when the crop was at a phenological stage conducive to Botrytis infection. Hence, from GS 36 onwards (post véraison), the epidemic became inoculum driven rather than weather driven, and the high infection pressures at the end of this season resulted in a very high disease incidence at all sites. The high number of infection periods during the preharvest period did not result in sprays being applied at each infection period. This was because only a maximum of 2-3 dicarboximide sprays can be used during the season. With the expectation of more infection periods occurring close to harvest, and the concern about resistance to dicarboximide fungicides increasing, these fungicides were used with caution. Furthermore, any application of fungicides after rain could have been washed off by a rain event shortly after. The above concerns, and the potential for large crop losses this season, resulted in a spray of potassium metabisulphite to dry the diseased berries and prevent any sporulation. This was used at sites 3, 4, and 5. Potassium metabisulphite is a cheap chemical, and does not have any residual properties. However, the benefits of its use could not be determined as it was applied in both treatments. There is a need to determine the potential benefits of potassium metabisulphite, the optimal application rate, and the pH of solution at which this chemical should be applied.

The effect of the inoculum-driven epidemic could not have been counter-acted even with a most rigorous spray programme late in the season, and as disease levels were similar between treatments, it can be concluded that the targeting of sprays was again very effective, and resulted in substantial cost savings and fungicide savings. Overall, there was a 22% reduction in the number of sprays used in the target treatments compared to the standard treatments (4-6 sprays were applied in the target treatment compared to 3-10 in the standard treatments). It can be concluded that for Botrytis epidemics in Marlborough, targeting sprays based on infection periods identified by the model and phenological stage at which the vine is susceptible (not solely infection periods alone) was effective in managing disease levels.