Agriculture Reference
In-Depth Information
levels on Bintje, Eigenheimer and Voran were surveyed over time in potato fields in
The Netherlands. The progress of disease, which had first been noticeable on all
varieties at the beginning of July, was much slower in Varan than in Bintje, with the
result that all of the foliage of the latter variety was destroyed by the start of August
whilst that of Voran survived, at least in part, into September.
Sorghum, a tropical cereal grown as a subsistence crop in many drier parts of the
world, is affected by a wide range of foliar diseases. Among them, anthracnose
(
Colletotrichum graminearum
) is of major importance, especially in the tropics.
Rate-reducing resistance in sorghum to anthracnose has been identified (Casela
et al.,
1993) using isolated plots in the United States and Brazil and calculating the
area under disease progress curves (AUDPC). More recently, Peacocke (1995) made
an extensive experimental study of the reactions of sorghum varieties to anthracnose
infection in southern Africa. Using three cultivars over two growing seasons at two
locations, clear differences in the rate of epidemic progress were determined; mean
estimates of the rate parameter were significantly greater for the varieties MMSH
413 and Kuyuma than for Sima (Table 5.4). Estimates ranged from 0.09 for Sima to
0.15 for Kuyuma; in all cases, estimates of the intercept were low, corresponding to
moderate levels of initial infection, and final disease severities were high (Table
5.4).
Table 5.4. Logistic parameter estimates for the progress of anthracnose on three varieties of
sorghum with different levels of horizontal resistance ( from Peacocke, 1995)
Year
Site
Parameter
Sorghum variety
Kuyuma
MMSH 413
Sima
1992/93
Mansa
Intercept
-13.10
-14.26
-12.12
Point of inflection
89.67
100.10
143.27
Rate
0.15
0.14
0.09
Golden
Valley
Intercept
-10.51
-10.62
-10.73
Point of inflection
87.73
87.55
113.77
Rate
0.12
0.12
0.09
1993/94 Mansa
Intercept
-10.04
-11.46
-10.28
Point of inflection
112.82
115.25
110.80
Rate
0.09
0.10
0.09
Mean
Intercept
-11.21
-12.11
-11.04
Point of inflection
96.74
100.97
122.61
Rate
0.12
0.12
0.09
Such rapid rates of epidemic progress as those reported by Peacocke (1995) have
important ramifications for the determination of effective control strategies. Van der
Plank (1963) considered that 'in the long run, a high rate of interest is more impor-
tant than a large balance in the bank today'. In other words, whereas a modest
decrease in the rate of epidemic development may prevent significant yield losses,
considerable efforts expended in sanitation practices which reduce the initial amount
of inoculum may have little effect on disease control. Empirically, this is confirmed
