Agriculture Reference
In-Depth Information
size and how this affects the observer's assessment of severity. The key for the
assessment of barley leaf blotch disease caused by
Rhynchosporium secalis,
devised
by James
et al.
(1968) (Fig. 2.6), usefully attempted to relate comparable percentage
areas of disease on four standard area leaf diagrams of barley of differing size
classes divided into 10% divisions.
Figure 2.6. Pictorial assessment key for leaf blotch of barley caused by
Rhynchosporium secalis
(from James
et al
.
,
1968).
Parker
et al.
(1995) compared visual estimates of wheat disease severity with
actual severities using image analyses of tracings of diseased leaves infected by
Septoria tritici
and
Blumeria (Erysiphe) graminis;
results showed that observer
estimates were imprecise, inaccurate and varied considerably over short time-
scales, but that relative bias decreased with increasing disease severity, so that
