Agriculture Reference
In-Depth Information
Table 2.3. Descriptive key for assessment of late blight of potatoes caused by
Phytophthora
infestans (
Anon., 1947)
Blight (%)
Disease severity description
0
Not seen on field
0.1
Only a few plants affected here and there; up to 1 or 2 spots in
12 yards radius
1
Up to 10 spots per plant, or general light spotting
5
About 50 spots per plant or up to 1 leaflet in 10 attacked
25
Nearly every leaflet with lesions, plants still retaining normal
form: field may smell of blight, but looks green although
every plant is affected
50
Every plant affected and about half of leaf area destroyed by
blight; field looks green flecked with brown
About
3
/
4
of leaf area destroyed by blight: field looks neither
predominantly brown nor green. In some varieties the
youngest leaves escape infection so that green is more
conspicuous than in varieties like King Edward, which
commonly shows severe shoot infection
75
95
Only a few leaves left green, but stems green
100
All leaves dead, stems dead or dying
Notes on assessment
In the earlier stages of a blight epidemic parts of the field sometimes show more advanced decay
than the rest and this is often associated with the primary foci of the disease. Records may then
be made as, say 1
+
pf 25
,
where pf 25 means 25% in the area of the primary foci.
Make successive assessments at intervals of 7-14 days to record progress of blight. Begin in
good time as both nil and starting date records (0.1%) are important. Difficulties in judging
allowance to be made for stem blight on a particular date are overcome by making another
assessment later.
diagrams. Since the ultimate aim is to relate disease to yield loss, the plant units
assessed should ideally be important contributors to yield, for example, the top two
leaves of a cereal plant. Standard area diagrams were traditionally and painstakingly
prepared using graph paper outlines but the use of planimeters, electronic scanners
and image analyzers have improved and quickened their production.
Despite the above measures to standardize assessment keys and to eliminate as
far as possible operator error (subjectivity), the visual assessment of disease severity
suffers from fundamental errors. Standard area diagrams do not display the
variegated patterns of disease so commonly caused by a plant pathogen, especially
on a leaf. Thus an observer is compelled to visualize the total area that the various
lesion shapes would cover if they could be combined and then expressed as a
percentage of the total area of the leaf. A second problem relates to variation in leaf
