Agriculture Reference
In-Depth Information
Figure 2.8. Saari-Prescott (0-9) scale (Saari and Prescott, 1975) for appraising the intensity
of foliar diseases in wheat and barley. (See text for explanation).
Other direct quantitative methods of measuring disease involve computing
coefficients and indices, and measuring components of partial disease resistance
(PDR). Eyal and Ziv (1974) used a Septoria Progress Coefficient (SPC) in which
plant and disease height were determined, where SPC = disease height (cm)/plant
height (cm). SPC indicates the position of pycnidia relative to plant height regardless
of pycnidial coverage and allows a comparison of infection placement on cultivars
with different plant stature. A disease index for measuring eyespot infection on wheat
caused by
Pseudocercosporella herpotrichoides
was produced by Scott and Hollins
(1974). Here, tillers taken at random from the field are assigned to one of the
infection categories and an index calculated from the formula (Table 2.4). Oyarzun
(1993) computed a root disease index (RDI) - a measure of soil inoculum potential -
for root rot of pea in which disease of the epicotyl and root was weighted more
heavily than that of the cotyledons and xylem. Chaube and Singh (1991) defined an
index number as a 'specialized type of average' used to measure the changes in some
quantity that we cannot observe directly. The authors concluded that weighted indices
may be preferable to unweighted indices and reviewed the following four methods for
computing a disease index: as a simple arithmetic average; as a weighted arithmetic
average; as a logarithm of the geometric mean of the percentage assessment; and as
severity estimates for larger areas. Diamond and Cooke (1999) developed a novel
in
vitro
detached leaf assay for early screening of
Fusarium
ear blight resistance in wheat
using
Microdochium nivale
, a member of the
Fusarium
ear blight complex capable of
forming discrete leaf lesions. PDR components measured on the leaf segments were
incubation period (time to first appearance of symptoms), latent period (time to
sporulation) and lesion size; correlations were obtained between these components
and the development of
Fusarium
ear blight on whole wheat plants. The bioassay was
further developed and evaluated by Browne and Cooke (2004), and later extended to
barley and oats (Browne and Cooke, 2005).
