Every year, the same finding. In September, a grower looks at a block and thinks “we had a bit more downy mildew than last year”. But how much? On which organs? At which phenological stage exactly? Impossible to say with certainty. Scores exist — in a notebook, in a photo, in a half-filled spreadsheet — but they are not comparable from one season to the next.
The problem is not a lack of scores. It is the absence of a stable protocol.
Here is how to structure a downy–powdery mildew protocol that will produce data you can actually use at the end of the season.
1. Choose the organ first
Downy mildew and powdery mildew do not express themselves the same way depending on the organ observed. Before you even talk about a scale, fix your target:
- Leaf: for general pressure monitoring, from budburst onward.
- Bunch: for harvest impact, from bunch closure onward.
- Inflorescence: for the flowering peak, where powdery mildew expresses strongly.
A good protocol focuses on one main organ per parameter. Mixing leaf and bunch in the same score makes analysis impossible. If you want both, create two distinct parameters in the protocol.
2. Fix a repeatable counting method
The “100 leaves, 100 bunches” method is a standard for good reasons:
- Sample large enough to be representative.
- Achievable in under 20 minutes per block.
- Enables the classic severity–incidence calculation.
For each organ observed, you record two values:
- Incidence (F): percentage of organs affected (at least one lesion).
- Severity (I): average surface area affected on diseased organs.
The synthetic indicator is then calculated automatically. If you record these two values carefully on every outing, you have exactly what you need to compare blocks, dates, and vintages.
3. Standardize the scoring scale
This is the point where estates lose the most comparability.
A good scale:
- Fits in 5 or 6 classes maximum — beyond that, the field can no longer differentiate reliably.
- Stays the same all season and the following year.
- Is used by every observer on the estate without exception.
Example of a proven severity scale (0–5 scale, downy mildew on leaf):
- 0 — healthy
- 1 — a few diffuse spots, < 5% of leaf area affected
- 2 — clear lesions, 5–25%
- 3 — 25–50%
- 4 — 50–75%
- 5 — > 75%, defoliation possible
What matters is not the exact scale you choose. It is that it is stable.
4. Align scoring frequency with phenology
Rather than scoring “every 15 days” mechanically, align your scores with BBCH phenological stages. That is what will make your vintages comparable — because a BBCH 75 in 2024 and a BBCH 75 in 2025 are at the same point in the cycle, even if calendar dates differ.
Recommended scores across the season:
- BBCH 53 (inflorescences clearly separated) — first reference point.
- BBCH 65 (full flowering) — peak powdery mildew risk.
- BBCH 75 (berries pea-sized) — start of downy mildew pressure on the bunch.
- BBCH 81 (beginning of veraison) — mid-season assessment point.
- BBCH 89 (berries ripe for harvest) — pre-harvest review.
Five well-structured scores are worth more than ten partial ones.
5. Standardize across blocks
If you manage several blocks, use the same protocol on all of them. That is what lets you answer: “Which block suffered most this year?” with a figure, not an impression.
Exception: if a block is managed very differently (rootstock trial, particular cover-crop modality), create a dedicated experimental design with its blocks and modalities. But the observation protocol stays the same.
6. The end-of-season review: what you gain
With a stable protocol repeated 5 times over the season, across 10 blocks:
- You have 50 structured measurement points per parameter.
- You can plot an evolution curve per block.
- You can compare blocks with each other at the same BBCH stage.
- You can compare 2025 with 2024 on identical bases.
- You have enough to feed an HVE report, an internal summary, or a technical meeting.
Without a protocol, you have 50 lines in a notebook. With a protocol, you have an observation system.
In practice
The hardest part is not understanding the method. It is maintaining it season after season without it fraying. Paper notebooks and spreadsheets are not designed for that.
A structured scoring tool lets you set the protocol once, then apply it in a few minutes in the vineyard. Data accumulate automatically, in the right format, ready to analyze.
Create your protocol in VitiObs in 5 minutes. Your next scores go straight into a comparable database, with no extra work on your side.