(continued from last month) The power of numbers: We don’t like it, but there is a lot of chance involved in scab control.
Eventually the number of spores that escape our management and successfully infect the tree determines the size of our scab problem.
Where there was a lot of scab last year, the potential ascospores dose (PAD) is high. This increases the chance that with each infection some spores will escape our control.
Therefore sanitation measures to get rid of the leaf litter to reduce PAD make every single infection easier to handle while there are less ascospores involved in the infection process.
Even with the best spraying technique there is a factor 15 in fungicide cover between the best and worst covered leaf. This uneven spray cover means that at moments of high spore release, there is a high chance that some spores germinate somewhere on the tree where the fungicide cover is not good.
A double treatment on key infections is not only necessary while you put on two fungicides, but also because the cover is better in two than in one spray round.
In most apple production areas in Europe relying on systemic curative chemistry has become a complete gamble. Growers have absolutely no guarantee that their local scab population is still susceptible to the systemic product.
How to do it
Each infection event is different. Weather, orchard history, fenological stage, RIM value, and previous treatments, create a unique situation every time.
RIMpro supports decisions by showing the infection event and how it will develop based on weather forecast. It also estimates what is left of the cover of the previous fungicide treatment.
Keeping in mind all written above, the following decision rules apply to practical scab management:
- Eight-hour reaction time. Equipment and availability of labour should be such that the whole farm can be treated within 8 hours.
- Build on contact fungicides. Use systemic fungicides only as last resource.
- No calendar sprays. These have no relation to the infection biology, and make it harder to decide on the real important moments.
- Pre-rain preventative treatment. When an infection is forecast, cover as shortly as possible before the rain. This can be an alternate row application if you are capable of treating the other rows during the germination window.
- Germination window treatment. As soon as an infection is likely to develop, and the remaining cover is less than 30% for a light, or less than 50% for a severe infection. If the previous treatment was alternate row, it should always be repeated now. A germination window treatment is highly effective even when applied during drizzle rain.
- Stop-curative treatment. If you could not spray during the germination window, or the infection becomes more severe than expected, a stop or curative treatment is necessary. Infections with a RIM-value over 600 should always be treated double! Organic fruit growers use lime sulfur or a combination of sulphur and potassium bicarbonate on wet leaves to stop the infection in the 300 DH post infection window.
Integrated fruit growers that still trust modern systemic chemistry would apply a combination of a contact and a systemic fungicide within 1000 DH. - Cleaning up during on-going infections. Infections developing from successive days of rain are more complicated to handle. Most spores are released during the first two rainy days.
This situation does not need a succession of curative sprays.Keep in mind that a contact fungicide will ‘clean the leaves ‘ from germinating spores, and ‘zeros’ the situation. Don’t hesitate to spray on wet leaves. This is always better than waiting for dry weather and rely on curative chemistry.
For more information, see Tree Fruit Sept 2014
