Recently I was informed of there being quite a lot of Powdery Mildew on grape vines in Marlborough. This was in spite of all the Powdery Mildew sprays having been applied as per recommendation. This situation may be as a result of poor early season control of the disease. As Sulphur is commonly used during the early part of the season, this article outlines the factors to consider when using Sulphur for Powdery Mildew control to obtain maximum effect.

Disease management requires a good understanding of the interactions between pathogen, host and the environmental factors.

Understanding the Pathogen

Powdery Mildew is caused by the fungal pathogen Uncinulla necator, and has been a consistent problem in most grape growing regions of the world. Unlike most other diseases, Powdery Mildew requires dry, overcast conditions rather than free moisture for its active growth. Germination, infection and colony development by Powdery Mildew is more closely related to temperature than to moisture. What we often consider as good weather for vine growth can also be very favourable for the development of Powdery Mildew epidemics. Powdery Mildew can actively grow between the temperatures of 10-27°C and ceases growth above 35°C. At certain times the rate of development of Powdery Mildew can be very much faster than the rate growth of leaf tissue, whereby the disease tends to cover a larger surface leaf area. These relationships between temperature and germination, infection and colony development of Powdery Mildew in conjunction with vine development have been used as a guideline for Sulphur applications in certain areas of the world.

Use of Sulphur for Powdery Mildew Control

Environmental conditions, in particular temperature, is also closely related to the effectiveness of Sulphur as a fungicide. However, temperature is also an important factor in the rate of vine growth as well. Therefore, when using Sulphur, two factors need to be considered for getting maximum effect from this particular fungicide. These include ambient temperature and spray coverage. The radius of effectiveness of Sulphur particles declines rapidly below 20°C. Therefore, at low temperatures, Sulphur coverage on leaves and bunches must be very dense to obtain any form of Powdery Mildew control. Furthermore, with active growth, leaf tissues expand thereby reducing the radius of Sulphur particle coverage and activity. Under active growth conditions the frequency of Sulphur application needs to be increased and the interval between the applications reduced from 14 to 7-9 days. After a rain event it may be necessary to apply a further Sulphur spray. Under Marlborough’s climatic conditions, from budburst up to 6-8 weeks after budburst, the temperatures are often below 16°C. These temperatures are often unsuitable for obtaining maximum effect from the use of Sulphur. There are two options one may follow in this situation.

1. To choose warm days where the temperatures are well above 17°C and , preferably, above 20°C.

2. To increase the coverage and penetration for deposition of Sulphur particles on both the upper and lower surfaces of the leaves.

I understand that many of the current sprayers in use do not provide the level of coverage required on the under surface of leaves. Often, when one sprays, it is possible to see spray deposition on the upper surface. However, since Powdery Mildew develops mainly on the under surface of leaves it is important to provide adequate coverage of chemicals on this surface as well.

Disease Monitoring

It is important to monitor the existence and progress of any disease. This is a fundamental process in any disease management system. Monitoring is a two step process - namely, identification and documentation. It is important for vineyard staff to be able to identify the various diseases. The use of a disease identification handbook will be helpful and attending disease identification workshops will provide the practical knowledge as well. The occurrence of disease is recorded as % incidence or severity. Incidence is defined as the frequency of occurrence of disease; proportion of leaves or bunches affected in a sampling population expressed as a percentage.

Disease monitoring enables us to track the progress of disease over time and to determine the effectiveness of any disease control strategy.

Summary

To summarise - when using Sulphur for Powdery Mildew control, adopt the following:

1. Preferably apply Sulphur at temperatures above 20°C.
2. If temperatures are below 20°C then provide complete coverage of leaf on both upper and lower surfaces, and bunches.
3. Reduce the interval of Sulphur sprays to 7-9 days when vine is in active growth (rapid leaf expansion).
4. Use suitable spray equipment which provides good spray penetration and deposition and minimises losses.