The 1995 season was not conductive to Botrytis infections until March. From March onwards, there have been many rain events, and also many Botrytis infection periods. This has created some problems, as not more than two to three dicarboximide fungicides can be used after véraison, and harvest is still 3-4 weeks away for some varieties. Thus other options need to be considered at this time, especially when potential Botrytis risks from more rain events are likely. One possibility is to open the canopy as much as possible, especially along the bunch line. The other option is to seek other chemicals which can be used post véraison. One such possibility is PMS.

In the past month, I have heard many growers in Marlborough talk about PMS and its use on grapes to control Botrytis.

What is PMS? This is the abbreviation for potassium metabisulphite (K₂S₂O₅), which will be referred to as PMS in the rest of this article. This chemical compound is commonly used as a preservative in cordials, dried fruit, for bleaching straw, and as an anti-oxidant agent in breweries and wineries.

Is PMS effective in controlling Botrytis? The short answer is that we are still uncertain. However, its use is providing noticeable indirect results. PMS seems to literally dry Botrytis infected berries. The actual mechanisms involved have not been closely studied. The advantage of infected berries drying is that they remain attached to the rachis, or else blow away during the mechanical harvesting process depending on the state of dryness at harvest. Therefore there are fewer Botrytis affected berries harvested with the non-infected grapes.

PMS in solution produces sulphur dioxide which acts as a fumigant. Therefore, it is thought that the fumigant action is instrumental in killing the Botrytis spores, and preventing infection of healthy berries. For the chemical buffs, when PMS (which is either a white powder or a crystalline solid) is dissolved in water, a mildly acidic solution is produced, with a strong sulphur dioxide odour (the smell of a burning match). This is caused by chemical reactions with water, which can be represented as:

Reaction B occurs to a great extent in more acid or low pH conditions. Because the product of reaction C is volatile, sulphur dioxide (SO₂) is not very soluble in water, it is emitted by the solution. This is more pronounced at higher temperatures, and under conditions of free air circulation through or over the solution.

Sulphur dioxide and related compounds are readily oxidised to sulphates by oxygen. The most likely fate of PMS applied to the outside of the fruit is that it will be washed off or oxidised on the skin to potassium sulphate (K₂SO₄). Other reactions, for example with the aldehyde groups that appear in sugars or in aroma compounds are theoretically possible if the treatment materials enter the fruit, but the extent to which they might occur is not known. If unoxidised PMS remains on the fruit, or if any enters the berry, it could theoretically raise the SO₂ content of the must.

It is expected that the fumigant action would be more pronounced in a closed environment compared to a field situation, e.g. using PMS pads in boxes of table grapes results in a stronger fumigant action compared to in the field. Unlike other fungicides, the effect of PMS will not be long-lasting once the sulphur dioxide is released into the atmosphere.

Research needs to be conducted to study the effect of timing and repeated use of PMS on sulphite levels in fruit. Pacific Rim Oenology Services Ltd (PROS) have produced a reference chart which outlines the relationship between pH and free and molecular SO₂. For further information, contact Lisa Van De Water.

Rates of use

Currently, various rates are being advocated by different people. In discussions with Australian researchers, rates used in their vineyards vary between 1-2 kg per 1000 l water. However, this rate has not provided encouraging results in controlling the Botrytis problem according to Peter Magarey, of SARDI, South Australia.

Preliminary use in Marlborough and Hawkes Bay has shown that a rate of 5 kg per 1000 l water is providing encouraging results with infected berries drying, and thus minimising subsequent sporulation.

Precautions with the use of PMS

Extreme caution need to be taken when using PMS. Some of the concerns are outlined below:

• Free SO₂ that is produced should not be inhaled, as it can cause breathing and respiratory problems. This is of particular concern to asthmatics. While mixing and spraying PMS, adequate precautions, such as wearing an "acid gas respirator" (SO₂) mask should be taken.
• Lower pH or acidic solutions may affect spray equipment such as nozzles, plumbing etc., depending on the material used. Discuss suitable materials with your spray equipment advisers.
• Sulphur dioxide is a major air pollutant in industrialised countries, either on its own, or by forming sulphurous acid on oxidation. If high amounts of sulphur dioxide are released into the air, acid rain can occur. Imagine the consequences of many vineyards using PMS on SO₂ production, and the subsequent effects!
• If PMS does get into the fruit, it has the potential to affect the amount of sulphites and SO₂ in must. Your winemakers would be the best people to talk to about these effects.
• Although reports are that PMS is not highly phytotoxic, the risk of damage to plants should be considered.

Please note that although it is known that PMS is being used in Marlborough this season, the authors are not liable for the outcomes of using this product.