At the NZW Grape Days Peter Wood from Plant and Food described powdery mildew as “arguably the most important fungal pathogen” in grape vines. Why? Because it reduces grape yields and quality and can go on to reduce vine vigour and productivity. Anyone who suffered from this year, will agree. Once it affects the berries and reaches epidemic proportions, it is fatal to fruit quality.
Now that the sexual phase of this pathogen has been confirmed by Wood as being present in New Zealand, management techniques will have to be carefully controlled. Especially when it comes down to use of fungicides.
What is powdery mildew?
It is a biotrophic pathogen, which means it doesn’t kill the host, unlike botrytis which will kill the berries it is growing on, Wood said. It lives on the outside of the berry and only penetrates one cell deep.
Genetically speaking there are two types of powdery mildew. Group A is an asexual type, which over winters in a dormant bud and emerges from flag shoots, spreading infection steadily.
Group B is the sexual stage, which has been discovered in Gisborne, Hawke’s Bay, Kapiti and Marlborough during the past season. This group can over winter in the bark of the vine as chasmothecia (cleisotothecia), and doesn’t require a bud to survive during these months. Wood said this type is active during the entire epidemic and is a more genetically diverse population of powdery mildew.
Both groups thrive in low UV light, in other words as the canopy becomes dense, the pathogen can build up more rapidly. It is damaged by rain or cold conditions, but loves humidity. The disease cycle continues to go round and round, building up. It affects all green parts of the vine, including shoots, leaves and berries. If measures are taken in the early part of the season, it can be controlled. But this requires regular spraying at timely intervals.
The crop is very susceptible preflowering to five weeks after capfall, a period when the canopy has covered the fruit zone. As it is vital to ensure your spray is hitting the target and protecting the crop, use spray papers to ensure good coverage.
The spray programme should begin when shoots hit the 5cm mark and the interval between sprays should be every 10 – 14 days, with emphasis on shorter intervals when the crop is susceptible.
Fungicide resistance
This is the greatest issue facing growers now that the sexual stage of powdery mildew is present. The reason why is explained in the following extract from a paper released by Margaret Tuttle McGrath of Cornell University, Riverhead, NY.
“Systemic and translaminar fungicides are generally more at risk for resistance development than contact fungicides because they typically have specific, single-site modes of action, which means they are active against only one point in one metabolic pathway in a pathogen. When resistance results from modification of a single major gene, pathogens are either resistant or sensitive to the pesticide and disruptive selection occurs. Resistance in this case is seen as complete loss of disease control that cannot be regained by using higher rates or more frequent fungicide applications.”
What you use, when you use it, how often and how well applied, will become the most important decisions for growers trying to control powdery mildew in the future. If resistance occurs, they will lose one of the most important weapons in their arsenal.
NZW have released a Powdery Mildew fact sheet, which provides in-depth information on how to manage this pathogen. One of the key points, is the description of the two broad groups of fungicides which can be used for control, and how best to use them.
Multi-site fungicides (including sulphur, oils, bicarbonates and fatty acids):
These feature multiple modes of action and are not at risk of resistance development. They form the basis of protectant/eradicant programmes. Apart from oils, these fungicides may be recommended for use alone or in mixtures or for alternating with fungicides at risk of developing resistance.
Single-site fungicides (including DMIs, strobilurins and azonaphthalenes):
These are useful for protecting the foliage and bunches at any time but may have more strategic value when spray coverage is harder to achieve – that is, from the onset of flowering to pre-bunch closure. Note: It is important that applications are managed to reduce resistance. Meaning you should restrict the number of applications of these products per season. Use in mixtures or alternate with other produces and do not use when attempting to control high levels of disease. ν
For a full copy of this fact sheet, visit the NZW members website.
