A Visual Rating Scale for Quantifying the Severity of Greasy Spot Disease on Grapefruit Leaves

Figures & data

FIGURE 1 Greasy spot disease severity for each grapefruit leaf expressed as percent of diseased area (color figure available online).

TABLE 1  Calculated Diseased Areas (cm²) of Non-Symptomatic (Control) Grapefruit Leaves Using WinFolia Imaging Software Assessing Greasy Spot Disease

Sample	Total area	Diseased area	Percent
1	35.824	0.0011	0.003
2	39.182	0.0026	0.007
3	26.120	0.0072	0.028
4	32.479	0.0017	0.005
5	43.860	0.0077	0.018

TABLE 2  The Range of Percentages for Each Disease Severity Class of Greasy Spot Infection and the Mean for the Respective Ranges

Severity class	No. of leaves	Range	Mean (±SE) (% leaf area infected)
1	5	0	0
2	41	0.002–8.605	4.388 (0.449)
3	43	10.034–19.336	15.214 (0.490)
4	20	20.900–29.400	25.604 (0.655)
5	10	31.300–37.100	34.336 (0.624)
6	29	40.700–80.700	55.72 (2.291)
Overall mean			23.115

FIGURE 3 Visual scale of greasy spot severity (percentage of diseased leaf area) in grapefruit leaves. Top lane: lower surface; Bottom lane: upper surface. Score 1 = 0%, 2 = >0–10%, 3 = 10–20%, 4 = 20–30%, 5 = 30–40%, 6 = >40%. Leaves on this scale display a percentage near the median of the score range for infection, i.e., leaf 2 has 4.85% diseased area (color figure available online).

FIGURE 2 The mean value of mutually exclusive classes with standard errors of 0.449, 0.490, 0.655, 0.624, and 2.291 for respective classes 1–6.