Establishment of a strong Powdery Mildew Population – Experimental approaches to Remediation

In recent years, a high population of powdery mildew has built up in many areas in Württemberg and other winegrowing regions in Germany. Due to the biology of the powdery mildew fungus, which overwinters as a dormant mycelium in the buds and grows as a pointer shoot in the following year, triggering new infections, a previous year's infestation means higher infestation pressure in the following year (Kast and Bleyer, K, 2011). Many areas are so heavily infested that it is difficult for winegrowers to control the disease in subsequent years. This was the reason why we carried out trials in an area where extremely high Oidium pressure had built up over several years, so as to see if it is possible to rehabilitate this area. Until 2017, powdery mildew and downy mildew trials were conducted in Weinsberg at the locations Schemelsberg and Glückenhälde. In the Powdery mildew trials on the grape variety Cabernet Dorsa in the Glückenhälde site, after increasing infestation a change was possible within the plot. 2018 the trials have been taking place at the location Grabenäcker. The plots at this site were specifically established for plant protection trials: 11 ares with the grape varieties Pinot meunier; 30 ares with Blauer Trollinger (Vernatsch) and 58 ares with Müller-Thurgau. The powdery mildew trials were to be carried out mainly on the sensitive grape variety Blauer Trollinger. After a low infestation of powdery mildew in the first trial year 2018, an infestation intensity of 20% at the grapes of some variants we would recommend to the winegrowers, was already recorded in the second trial in 2019. In the third trial year 2020, all variants, including many "practice standard variants", had a total infestation of more than 80% intensity (Figure 1). The reason for this increase may have been the high infection pressure on the plants due to the weather and location, the heavy infection in the intermediate rows by inoculation with plants infested with powdery mildew and resistant fungal strains against two groups of active ingredients. Figure 1: Establishment of the powdery mildew potential – powdery mildew experients 2018 to 2020.


______________________________________________________________________________ 1 Background for the implementation of the trial
In recent years, a high population of powdery mildew has built up in many areas in Württemberg and other winegrowing regions in Germany. Due to the biology of the powdery mildew fungus, which overwinters as a dormant mycelium in the buds and grows as a pointer shoot in the following year, triggering new infections, a previous year's infestation means higher infestation pressure in the following year . Many areas are so heavily infested that it is difficult for winegrowers to control the disease in subsequent years. This was the reason why we carried out trials in an area where extremely high Oidium pressure had built up over several years, so as to see if it is possible to rehabilitate this area. Until 2017, powdery mildew and downy mildew trials were conducted in Weinsberg at the locations Schemelsberg and Glückenhälde. In the Powdery mildew trials on the grape variety Cabernet Dorsa in the Glückenhälde site, after increasing infestation a change was possible within the plot. 2018 the trials have been taking place at the location Grabenäcker. The plots at this site were specifically established for plant protection trials: 11 ares with the grape varieties Pinot meunier; 30 ares with Blauer Trollinger (Vernatsch) and 58 ares with Müller-Thurgau. The powdery mildew trials were to be carried out mainly on the sensitive grape variety Blauer Trollinger. After a low infestation of powdery mildew in the first trial year 2018, an infestation intensity of 20% at the grapes of some variants we would recommend to the winegrowers, was already recorded in the second trial in 2019. In the third trial year 2020, all variants, including many "practice standard variants", had a total infestation of more than 80% intensity ( Figure 1). The reason for this increase may have been the high infection pressure on the plants due to the weather and location, the heavy infection in the intermediate rows by inoculation with plants infested with powdery mildew and resistant fungal strains against two groups of active ingredients. Figure 1: Establishment of the powdery mildew potentialpowdery mildew experients 2018 to 2020.

Procedure for the rehabilitation trial
As in 2020 the Trollinger area could not be used for an accurate trial after the heavy powdery mildew infestation, in 2021 the powdery mildew demonstration and strategy trial was carried out on Müller-Thurgau. In order to not further increase the powdery mildew pressure due to the heavy infestation of the previous year and the humid location, a block trial was set up for the grape variety Blauer Trollinger. It was carried out with two variants in two repetitions without untreated control and without infected intermediate rows. In addition, the experiment was divided into 4 blocks so that data could be collected from a total of 16 replicates ( Figure 2). An establishment of the resistant strains was to be prevented. One variant was treated only with sulfur and partial addition of Wetcit, during the critical period for infections of the clusters (Kast and Bleyer,K, 2011). Wetcit is an oil-based wetting agent and additive. On the second variant, only agents with organic active ingredients were used, excluding the two groups of active ingredients to which resistance was detected ( Figure 3). The sprayings were carried out as far as possible following the given intervals of the effect duration table according to OiDiag-3.0 ( Figure 4) which is integrated in vitimeteo as "Vitimeteo -Oidium" (Kast, W.K., Bleyer,G., Bleyer,K., 2013). In "Vitimeteo -Oidium" an index value is calculated. It indicates the current danger and thus the risk of infestation on the grapes for powdery mildew.   In the parallel running powdery mildew demonstration and strategy trial with the grape variety Müller-Thurgau, intermediate rows were established. In 2021, these were not inoculated with powdery mildew. The treatment of the separation rows was started at stage BBCH 68. A total of four treatments with sulfur, potassium hydrogen carbonates and Wetcit were carried out in these rows ( Figure 5). Though the grapes of these intermediate rows remained surprisingly healthy compared to the untreated control and the standard treatment, we took data from the untreated control, the standard variant and the intermediate rows.

Results and discussion
The assessment took place two weeks after the last treatment. In order to obtain a very good average of the results, in each replication of the experimental variants, in eight places data has been collected. In total 16 replicates per variant. Variant11: grape infestation frequency of 99.4% and infestation intensity of 36.7%. Variant 2: grape infestation frequency of 98.9% and infestation intensity of 21.6% ( Figure  6).

Date in 2021
Var 1 -Sulfur

Conclusions
This combination of agents (sulfur, potassium bicarbonates and wetcit) could thus be tested as another possibility for rehabilitation. However, the lighter infestation of the previous year and the sensitivity of grape variety to powdery mildew could be responsible for this positive result. In this trial, this could play a decisive role. The result is in any case worth pursuing further.