Introduction

In most grape growing regions of the world, powdery mildew is a recurring disease. Traditionally, fungicides and elemental sulphur have been used as a conventional means of control. In many areas of the world, including New Zealand, multiple applications of fungicides for the control of this disease has resulted in resistance to the benzimidazole and DMI groups of fungicides. Thus traditional disease control is becoming less effective. The over use of fungicides and their effect on food and beverages is also raising public concerns. Hence, all production systems are now in a transitional phase. This provides us with the opportunity to seek alternative methods for controlling disease which reduces fungicide usage, and also becomes more acceptable to the public.

Various fruit production industries are looking towards the use of soft pesticides while the search continues for more reliable methods of biological control and other systems approaches. Soft pesticides are those commonly referred to as oils, soaps, plant extracts, and some of the common kitchen substances such as baking soda and dishwashing liquid. Research conducted in California, Canada, Australia, and preliminary studies in New Zealand, has indicated that some plant-based oils and petroleum-based oils are effective in controlling powdery mildew in grape vines. This article discusses the options for the use of petroleum-based and plant-based oils. Knowledge of the mode of action of soft fungicides permits us to use these compounds to their greatest advantage in relation to the time of inoculation by the fungus, infection, symptom expressions as lesions, and subsequently sporulation. When using soft fungicides, it is important to understand whether they are most efficient when used as protective, pre-lesion, pre-symptomatic, post-lesion, or post-symptomatic curatives. The reduction of spore production is yet another mode of action that needs to be considered - this anti-sporulant activity may result from the curative properties of the compounds, either before or after lesion appearance.

Overseas Research

Some of the petroleum-based oils are Stylet Oil, Sunspray UFO, Safe-T-Side, and light mineral oils, eg Johnson’s baby oil. The plant-based oils include canola, safflower and soybean. Overseas studies suggest that petroleum-based oils are more effective than plant-based oils for powdery mildew control. Petroleum-based oils are more effective when used as a protectant between 1-4 days prior to inoculation, and this can provide protection up to four days. This means that oils need to be applied well before an infection occurs. The Sunspray UFO and Stylet Oils also provided suppression of surface mildew when applied as pre-lesion curatives between 1-5 days after infection. When petroleum based oils were used after symptom expression, the established lesions were smothered, and the area covered by mycelium was reduced.

When petroleum oils and plant oils were applied as post lesion applications, they smothered the powdery mildew colonies, but also turned grape foliage bright green with pale mottled areas. The smothering effect prevented further growth of the mycelium and the production of new conidia until the effect of the oil was lost after seven days.

Mineral oils are not essentially fungicidal, but when used with some surfactants, they may have fungicidal properties. However, there is a risk that these surfactants may be phytotoxic. Mineral oils are not effective in preventing spore germination nor infection. These oils have a fungistatic, ie. not preventing any further growth, rather than a fungicidal effect. Reports also suggest that these oils decrease photosynthesis and transpiration, but increase respiration. This suggests that these oils do not have a direct effect on the pathogen, but may indirectly influence the physiology of the grape vine, and have a plant-induced effect. The effect of reduced photosynthesis on yield has not been investigated so far.

Use of Soft Fungicides in a Conventional System

If one chooses to solely rely on the use of soft fungicides, then it would be important to increase the frequency of application between 5-7 days. However, this would create additional application costs in terms of machinery usage, which may lead to soil compaction and other related issues. Thus there is a down side to some of the uses of soft fungicides.

Use of Soft fungicides in an Integrated Disease Management System

The high degree of curative and anti-sporulant activity of petroleum oils make it a very viable option for use in disease management systems. In these systems, the monitoring of disease levels enable the timing according to disease risk and/or severity of the problem. For example, the first application could be made after initial infection has taken place, or after a threshold of disease incidence is reached. However, if infection pressures increase, and disease incidence increases beyond the set threshold, a soft fungicide can always be placed with a DMI fungicide. It is possible to reduce the amount of DMI fungicides used in a given season by adopting such a strategy.

Although traditional methods remain easy to use, a commitment needs to be made to change, and to select suitable alternatives. More immediate is the need to consider factors in relation to efficacy and cost, the two main criteria that determine the use of soft fungicides. However, when considered in conjunction with the use of conventional pesticides in an integrated disease management programme, it provides a more acceptable and cost viable option.