Apple Scab Disease Index

Approximate number of hours of leaf wetness necessary to produce primary apple scab infections and approximate number of days required for lesions to appear.

Average Temperature		Wetness Period (hours)*			Appearance of Lesions (days)**
(°F)	(°C)	Light Infection	Moderate Infection	Heavy Infection	Appearance of Lesions (days)**
33-36	0.5-2.2	48	72	96	---
37	2.7	41	55	68	---
38	3.3	37	50	64	---
39	3.9	33	45	60	---
40	4.4	29	41	56	---
41	5.0	26	37	53	---
42	5.5	23	33	50	---
43	6.1	21	30	47	---
44	6.6	19	28	43	---
45	7.2	17	26	40	---
46	7.8	16	24	37	---
47	8.3	15	23	35	---
48	8.9	15	20	30	17
49	9.4	14.5	20	30	17
50	10.0	14	19	29	16
51	10.6	13	18	27	16
52	11.1	12	18	26	15
53	11.7	12	17	25	15
54	12.2	11.5	16	24	14
55	12.8	11	16	24	14
56	13.3	11	15	22	13
57	13.9	10	14	22	13
58	14.4	10	14	21	12
59	15.0	10	13	21	12
60	15.6	9.5	13	20	11
61	16.1	9	13	20	10
62	16.7	9	12	19	10
63-75	17.2-23.9	9	12	18	9
76	24.4	9.5	12	19	---
77	25.0	11	14	21	---
78	25.6	13	17	26	---

*Hours of wetness from the beginning of rain (data of W. D. Mills as modified by A. J. Jones). If secondary spores (conidia) are already present, wetting periods required to produce secondary infections are about three hours shorter than listed in the table for primary infection.

**Number of days required for lesions to appear after infection period has begun. Additional days may be required if temperatures are above 80°F or weather is dry for long periods of time.