Agriculture Reference
In-Depth Information
Table 2.5. Examples of qualitative keys for disease assessment
An assessment key for cereal rust virulence response
Symbol Host: parasite interaction
Oi Immune; no visible signs of infection
Oc Highly resistant; minute chlorotic flecks
On Highly resistant; minute necrotic flecks
1 Resistant; small pustules with necrotic surrounding tissue
2 Moderately resistant; medium-sized pustules with necrotic
surrounding tissue
3 Moderately susceptible; medium-sized pustules with chlorotic
surrounding tissue
4 Susceptible; large pustules with little or no chlorosis
X Mesothetic reaction; mixed reaction types on one leaf
Reaction-type classes for
Pyrenophora teres
on barley (Khan and Boyd, 1969)
Class
Reaction
0
No observable infection.
1
Pin-point brown lesions, no chlorosis.
2
Small dark brown lesions, no chlorosis.
3
Restricted long brown streaks, slight associated chlorosis.
Brown elongated lesions with net-like cross variations, marked
chlorosis.
4
of chickpea, caused by
Ascochyta rabiei,
and Subba Rao
et al.
(1990) for infection
type in groundnuts
(Arachis hypogaea)
infected by rust
(Puccinia arachidis).
Edwards
et al.
(1997) developed an assessment key for lesion responses of celery
and related Umbelliferae to
Septoria apiicola
infection, where 0 = an extremely
resistant host response (no lesions) and 3 = very susceptible (grey lesions, leaf
chlorosis and numerous pycnidia). In the field, the assessment of reaction types such
as those described here is often more difficult than under controlled conditions, as
host-pathogen interactions can be modified by environmental variables such as
temperature and leaf surface wetness.
2.5.3 Indirect methods
Indirect methods of disease assessment have increased in number with the
development of new technologies. Traditional methods rely on monitoring pathogen
spore populations over infected crops or trapping insect vectors of a virus to
estimate the level of crop infection. Fox (1993a) identified two basic methods for
air-borne fungal spores: measuring the concentration of spores in a given volume of
air (concentration methods); and counting the number of spores deposited on a
