This season (1994/95) has been a mixed bag as far as diseases are concerned. Although a good vintage has been predicted, nothing is certain until the grapes are in the crusher. It has been a good season for some diseases, while not so good for others ("good" meaning lower disease risk problems). The following article discusses the progress of a systems approach to disease management (DMS).

The DMS work is being conducted in five different sites on three different varieties, and three different canopy systems. The research trial is investigating the benefits of applying sprays in response to computer-simulated infection periods or infection thresholds (target treatment), and comparing them with the benefits of a calendar-based spray programme (standard treatment).

Botrytis

There was an excellent flowering period this season, with warm and dry weather. Fruit set was good, and there were no Botrytis infection risks during this period. The Botrytis infection simulation model identified a total of seven infection periods so far this season. Three of these infection periods occurred before flowering, while three occurred between flowering and bunch closure. The seventh infection period occurred most recently after the onset of véraison. In response to these infection periods, two botryticides were applied in the target treatment. These were prior to bunch closure and after the onset véraison. In contrast, four sprays were applied in the target treatments. The level of leaf and bunch Botrytis has been similar between the two treatments, except in the variety Müeller Thurgau. In this variety, 13% incidence of bunch Botrytis was recorded in the target treatment and 5% in the standard treatment. This is higher level of disease than was expected in the target treatment. Some factors which have contributed to this increased level in the target treatment include canopy density and poor penetration of sprays. Rain in the third week of February contributed to the rapid increase in Botrytis at a susceptible stage of berry growth. The effect was far less in the Sauvignon blanc and Cabernet sauvignon varieties because they were in the very early stages of véraison, and less susceptible to Botrytis at this stage. The level of disease in these two verieties was not high, and target treatments had similar levels of disease to the standard treatments.

The greatest risk for Botrytis is from véraison to harvest, and therefore, the overall effectiveness of targeting sprays can only be confirmed after harvest analyses.

Powdery Mildew

Powdery mildew pressures have been far greater, compared to other diseases. However, the standard sprays have managed to effectively contain the disease. Leaf powdery mildews have been effectively managed below the 5% incidence threshold. The threshold for bunch powdery mildew was reduced from 5% to 3% incidence this season. The targeting treatments have worked extremely well in all sites and varieties, except for Müeller Thurgau. This variety has presented a greater challenge with targeting sprays based on infection thresholds. Less sprays were applied in the target treatments in Sauvignon blanc and Cabernet sauvignon, and disease levels were similar to those in the standard treatment. This suggests that overall disease thresholds have worked well for controlling leaf and bunch powdery mildew this season.

In Müeller Thurgau, various factors contributed to disease levels increasing to an unacceptable level in the target treatment. This was mainly because of a high canopy density which, in combination with hydraulic sprayers, delayed the response when targeting sprays after the infection had established. It is evident that the DMS needs to account for these variations in canopy density through the use of suitable sprayers and, possibly altering the incidence threshold. However, it is encouraging to note that powdery mildew can be managed well through a reduced number of sprays when targeting and other techniques are combined well.

Downy Mildew

No downy mildew infections have been detected this season during weekly monitoring of the field sites. The computer model simulated from five to twelve infection periods, both primary and secondary. No other sprays were applied in response to the infection periods in the target treatment, except for the first three downy mildew sprays applied 2, 4 and 6 weeks after budburst. However, there were more downy fungicides applied in the standard treatment. The infection simulator has been a valuable tool for alerting us to possible downy mildew infections, but the model needs further fine-tuning for more accurate predictions.

The final outcomes of the DMS study will be published in the Winepress after the 1994/95 vintage.

The contributions of Paula Kloosterman, HortResearch; Corbans; Cloudy Bay; Montana; and Willi Crosse in their assistance with field monitoring work is acknowledged, and the financial assistance of AGMARDT, Marlborough Grapegrowers Association, and Marlborough Research Centre Trust is also acknowledged.