Disease resistance: what the scales in variety lists mean
Is a "7 out of 9" score for Fusarium resistance good or bad? How official variety testing scales work, why they are not comparable across countries, and how to use them when selecting a variety.
"Fusarium resistance — 7" in a variety description looks like a score on a ten-point scale. It is not.
Nine-point scale
In variety descriptions following UPOV guidelines, traits are expressed on a scale from 1 to 9, where 9 is the maximum expression of the trait, not "excellent." For disease resistance, 9 indeed means the most resistant, but for plant height, 9 is the tallest, and there is no qualitative assessment involved.
Intermediate values — 2, 4, 6, 8 — are used when a variety falls between adjacent levels. A score of 5 is not the "middle of the scale" but a specific described state of the trait.
Resistance measured under specific conditions
The score is obtained at test sites in a specific country, during specific years, against specific pathogen races. A variety that showed a 7 in the Netherlands may behave differently in another zone due to a different race composition of the pathogen.
Therefore, scores from different national lists cannot be compared directly. In the directory, the source of the score—the list from which it was taken—is always indicated next to the value.
Missing data is not zero
If a trait was not evaluated, the field remains empty. This does not imply low resistance; the trial for this pathogen simply was not conducted. An empty field is more accurate than an assumed zero, which could lead someone to make a decision about sowing.
Discussion
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Growing experience and notes on the entry. Comments are not part of the reference data and are not verified against primary sources.