ELECTRONIC SUPPLEMENTARY MATERIAL V.
Are changes in pathogen DNA load correlated with changes in crop management?
Plotting the ratio of pathogen DNA in leaf and pathogen DNA in seed against the leaf and
stem pathogen-DNA content, i.e. epidemic severity, (Fig 3c) shows two clearly distinct groups of
data points, each grouped around a straight line. The data grouped around the lower line are all from
a single historical period and have a lower ratio of horizontal to vertical transmission than the data
grouped around the upper line.
An obvious hypothesis to explain these groupings is that they represent some change in crop
management at the Broadbalk experiment. Changes in the ratio between seed and vegetative
pathogen load could be directly caused by a range of factors other than evolutionary changes in
transmission characteristics. The extensive archives on the Broadbalk experiment (e.g. Anon. 1969)
allowed us to work systematically through a range of possible factors that might explain the
changes we observed during the 160 year experiment. We considered (i) the effect of harvest and
storage protocols as they may affect the amount of pathogen DNA in the material archived, (ii) seed
treatments as they might change seed pathogen DNA content, (iii) the effect of cultivars grown in
different periods in the experiment, (iv) yield, and (v) changes in agronomic variables such as
fallowing, liming, herbicide, fungicide and pesticide use and the introduction of semi-dwarf
varieties (Figure V.1)
wheat grain yield (t/ha)
12
fallowing
10
herbicides
liming
8
semi-dwarfs
Fig. 1C
6
4
fungicides
2
0
1850
1875
1900
1925
1950
* *
Old Red Lammas
Old Red Cluster
Red Rostock
Red Club
2000
Red Standard
Little Joss Capelle Desprez
*Squarehead’s Master
crop + seed dried on field
hand harvest
1975
*
Hereward
Apollo
Brimstone
Flanders
dried indoors
machine and combine harvest
Seed treatment
mercurials various others
Figure ESMV.1: Changes in crop management at the winter wheat experiment on Broadbalk
(Harpenden, UK) between 1844 and 2003. Solid line: yield on the plot from which most samples
were analysed. Arrows and names below the x-axis are the dates on which wheat cultivars were
grown. The filled triangles, ▼, represent isolated years in which a different cultivar was used.
Time-lines below the x-axis show when harvest was dried outdoors or indoors; when hand and
machine harvested; and when seed was treated with a fungicide to reduce infection in the early
growing season. Arrows and words in upper right half of the graph show dates of introduction of
specific methods. The inset is a copy of Fig. 1C, each data point given a colour that corresponds to
the coloured bars superimposed on the graph. Red Line: 1844 till 1910. Blue line: 1910 till 1960.
Green line: 1960 till 2003.
Note: Pre-1900 cultivars were unlikely to be completely genetically homogeneous and were
landraces. Red Standard and Squarehead’s master are very similar in morphology and some authors
consider these names as synonyms (Zeven 1990).
Clearly if agronomic changes would be the reason for the changes described in Figure 3 it would be
very surprising that the data points from the earliest part of the experiment, between 1843 and 1910,
and the data points from the most recent part of the experiment, between 1960 and present, both
group around the upper line (Fig 2c), with the intermediate period between 1910 and 1960 grouping
around the lower line. Moreover, none of the factors considered (Anon. 1969) (Fig. 3) coincide with
the observed group of data around the lower line in the 1910-1960 time window, nor do any of
these factors explain why the changes in crop management would cause both the oldest and the
most recent data to group around the upper line. We conclude that it is unlikely that changes in crop
management caused the existence of two distinct groups in the data, though it can never be ruled out
entirely.
Additional reference:
Zeven, A.C. 1990 Classification of landraces and improved cultivars of rivet wheat (Triticum turgidum) and
bread wheat (T. aestivum) from Great Britain and described in 1934. Euphytica 47, 249-258.