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Black spot management – experiences with ‘popcorn weather’ in Europe

Pest and Disease Management

Ongoing frequent rainfall and wet conditions across many regions in Australia this spring have made control of black spot (scab) in both apples and pears challenging.

Frequent and erratic rainfall events have become more common in Europe too. Dr Peter Triloff from Marktgemeinschaft Bodenseeobst in Germany shares his experiences in managing black spot in these conditions.

Key points from the European experience

  • Protectants are more effective than curative fungicides.
  • Rainfastness varies – choose a fungicide with higher rainfastness where possible.
  • Spray a protectant as close to an upcoming rainfall event as possible.
  • Prioritise blocks with the most severe symptoms.
  • Don’t do nothing.

Over the last decade regular rainfall events resulting from low pressure systems moving in from the Atlantic Ocean have been replaced with irregular showers of up to 100mm of rain in one hour, with thousands of these localised rain events occurring over central Europe – a phenomenon that has been called ‘popcorn weather’ in Europe. This highly unpredictable situation with frequent rain events means that sprays are washed off quickly. It also makes knowing when to spray, and being able to spray and maintain cover, very challenging.

We see increasing rainy periods where growers spray the entire farm two or three times a week because of heavy rain washing off fungicides overnight. Unfortunately, there are not many opportunities to control black spot infections under conditions like this.

However, there are a few recommendations made in these situations with either heavy rains in a short period (e.g. overnight) or within a rainy period where leaf growth is not the limiting factor of the residual activity of protectant fungicides.

In Europe, even if curative fungicides were an option, they would not be recommended for use during the secondary infection season. There are a few reasons for this:

  • The selection pressure towards resistance increases as the number of lesions
  • These curative fungicides also have no protective activity because they do not prevent spore germination, but only interrupt the establishment of the primary stroma that develops in the first approximately 1000 DH (degree hours) > 0°C after an infection.
  • They do not have good activity on fruit because the active ingredient moves into the deeper cell layers, reducing the concentration underneath the cuticle, leading to insufficient efficacy on fruit.
  • They do not sufficiently control infections on transporting tissues like petioles and fruit stems, because the active ingredient is removed by the moving sap, also leading to an insufficient concentration underneath the cuticle.

This means that the most suitable fungicide group for this situation is the protectant fungicides. They can be sprayed more frequently with less risk of resistance – but they also may have insufficient rainfastness.

The best rainfastness has been found in the QoI-group (strobilurins) but they have a high risk for resistance development and therefore must not be sprayed in orchards with visible lesions.

The second best fungicide, Delan® (Dithianon) has a rainfastness of >50mm. In a trial where we compared an old (liquid) formulation of Dithianon with the new WG formulation we found both formulations being equal in rainfastness with about 5 per cent less efficacy after 50mm of rain compared to the efficacy on just wet leaves.

Any other protectant fungicides available for use in Europe have a lower to significantly lower rainfastness. Captan formulations showed a rainfastness of no more than 20–25mm. The lowest rainfastness was seen on wettable sulphur, which was washed off by approximately 10–15mm.

Generally, the highest risk of successful infection is at the begin of the rainfall, especially after a few dry days when lesions have produced a lot of new conidia. Also, young lesions produce a lot more conidia compared to older ones later in the season. As a rainfall event continues, the conidia release declines and finally reaches the maturation rate.

The strategy in Europe for controlling black spot during periods with heavy rains starts with a Dithianon spray as close as possible before the onset of the rain. If the fruitlets are still sensitive to blossom end rot or have an open calyx for core rot and still stand upright with the calyx facing the sky, we add a regular dose of Captan to the Delan to get at least some activity, especially after the onset of rain on these fungi not controlled by Dithianon.

If the rainfastness of Delan is going to be surpassed during the rainfall and the rain is stopping on the same day, we try to spray the blocks with the highest scab attack with a mixture of 5kg potassium bicarbonate and 3kg of wettable sulphur as a stop spray onto the wet leaves. This spray is not used across the entire farm, because this period of wet leaves after a rain event is often not longer than around 3 hours during daylight, limiting the area that can be sprayed in that time window. Therefore, this spray is often only for the blocks or orchards with the highest scab attack, because once the leaves have dried off, the efficacy of this spray fades away. In such situations, this spray has an efficacy of approximately 85–90 per cent. This is not as good as with regular fungicides, but helps quite a lot. The sulphur is necessary because only the mixture has this high efficacy; bicarbonate on its own is less efficient.

As a final example, let’s look at a situation where almost all of the rainfastness is used up when the rainfall ends and the next rainfall event is predicted soon after the previous one (e.g. at night or the next morning). Even if leaf growth has not yet produced too much new tissue prone to infection, we use the first dry hours to renew the spray deposit, starting with the orchards that show the highest attack and spraying the others in descending order. Whatever you do in such a situation, we want growers to start a spray in the orchards with the highest attack.

The worst option is to do nothing, because these extended rain periods always produce lots of new infections on leaves and fruit, which increase as the lesions are still young, producing huge amounts of conidia.

Further information

Information is general in nature. Please see professional advice for further information on how to manage black spot in your orchard. Always read and follow the label on crop protection products.

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