Agriculture Reference
In-Depth Information
Table 2.4. Calculation of a disease index for eyespot of wheat caused by
Pseudocercosporella
herpotrichoides
(Scott and Hollins, 1974)
Infection category
Disease severity description
0
Uninfected
1
Slight eyespot
(one or more small lesions occupying less
than half the circumference of the stem)
2
Moderate eyespot
(one or more lesions occupying at least
half the circumference of the stem)
3
Severe eyespot
(stem completely girdled with lesions;
tissue softened so that lodging would readily occur)
Notes on assessment
1.
Examine 20 tillers per 20 m
2
plot.
2.
Assign each tiller to one of the infection categories above.
3.
Write the number of tillers in each category on the record sheet.
4.
An index will be calculated from the data as follows:
(
)
+
(
)
+
(
)
+
(
)
0
×
a
1
×
b
2
×
c
3
×
d
100
3
Disease index =
×
(
)
a
+
b
+
c
+
d
where
a, b, c
and
d
are the number of tillers examined which fall into the categories
0, 1, 2, and 3, respectively.
2.5.2 Direct qualitative methods
Direct qualitative assessments of disease are used to differentiate host responses or
interactions, ideally under controlled conditions, where resistance or susceptibility is
determined by genetic systems in the host and pathogen. Thus, responses to
individual virulences (physiologic races), as required in breeding programmes or
race surveys, are measured using a qualitative method of assessment as shown in
Table 2.5 for cereal rusts and
Pyrenophora teres
(cause of barley net blotch disease).
Such qualitative keys clearly differentiate resistant from susceptible responses; in
the case of cereal rusts 0-, 1- or 2-type responses are resistant and 3- and 4-types are
susceptible, whereas for net blotch disease 0, 1 and 2 are resistant (no chlorosis), and
3 and 4 are susceptible (chlorosis present) (Khan and Boyd, 1969). Rosielle (1972)
developed a six-point qualitative assessment scale for
Septoria tritici
in which 0 =
an immune response - no pycnidia or leaf symptoms; 1 = highly resistant (HR) -
occasional isolated pycnidia with hypersensitive flecking; 2 = resistant (R) - very
light pycnidial formation with some lesion coalescence; 3 = intermediate (I) - light
pycnidial formation with lesion coalescence; 4 = susceptible (S) - moderate
pycnidial formation with considerable lesion coalescence; and 5 = very susceptible
(VS) - large abundant pycnidia with extensive lesion coalescence. Reddy
et al.
(1981) provided a qualitative assessment key for infection type in ascochyta blight
