Timothy J. Smith, WSU Cooperative Extension, 303 Palouse, Wenatchee, WA 98801
e-mail smithtj@wsu.edu Further information on model use:

Fire blight danger varies from orchard to another, and over time within each orchard. To assess the risk of fire blight blossom infection, the model user must consider the factors below throughout the Spring and early Summer:

Tree Condition: The potential number of strikes is greatly affected by the number of blossoms in the orchard, but the model user should not discount the relatively light late blossoms produced on certain apple/rootstock combinations and on many pear varieties. Start counting degree hours as the first blossoms open, and continue until few remain. Younger trees, those growing rapidly, and certain highly susceptible varieties or rootstocks are at higher risk, as infection may cause extensive tree damage or death.

Recent Blight History: Flowers must be first contaminated by Erwinia amylovora bacteria before they can be at risk of infection. Many orchards do not experience fire blight even when blight infection conditions occur. The risk of blossom contamination leading to blight infection greatly increases if blight has occurred recently in the area near the orchard, even when the cankers have been (apparently) removed. Bacterial contamination of blossoms occurs much more rapidly if there is a near-by active canker. Bacteria also tend to infest an increasing percentage of the flowers as the primary bloom period advances toward petal fall.

The model user is asked to take in to account the recent history of blight in the area around the orchard, observe the stage and number of bloom, and set appropriate situation-specific degree hour thresholds.

Severe blight outbreaks may occur without apparent prior-season infection in the region when risk of infection is "Extreme." Never assume that E. amylovora is not present.

Temperatures: As the blight bacteria can thrive only about four days on each individual flower, the bacteria must develop to dangerous numbers during the immediate three or four days leading up to blossom wetting. Warmer temperatures allow rapid bacterial growth in flowers. If bacteria numbers exceed a certain minimum while the flower is in good condition, then the flower is lightly wetted, infection is possible.

Infection can occur on a "cool" day if temperatures during the three days leading up to the cool, wet day were warm. Watch temperatures over time.

Blossom Wetting: Blossom wetting alone does not cause fire blight. Rain during cold or cool weather does not lead to infection, or blight would be common everywhere, every year. However, if recent temperatures have allowed rapid bacterial development, blossom wetting is the infection "trigger." Moisture can be from rain, dew or light irrigation wetting. The water moves the bacteria from the stigma tip into the flower's nectary. The bacteria may then grow rapidly, attack the fruitlet, then move into the tree, attacking the living portion between the wood and the bark. The visible strike appears at this time.

Spray controls are often necessary while risk of infection is near or above the "High " threshold and blossom wetting is likely, or has happened within the past few hours.

Sprays are most effective if applied prior to blossom wetting, especially during "extreme risk" temperature conditions. Antibiotic sprays are less effective when applied after an infection period, but are very beneficial for about the first 12 hours post infection, beneficial if applied within 12-24 hours after infection, and may be of some effect during the 24-48 hour post infection period.

The best version of this model is automated, and totals the hourly fire blight degree hour value each hour of the 96 hours leading up to the present, or a wettness period. Most growers do not have this sort of data available. The look-up chart method described below has served the users well, despite the fact that the values are estimates, and can vary by 10 percent. If you use the look-up table, take this 10 percent uncertainty into account when deciding risk levels.

Use the degree hour look-up chart to assign a degree hour value to each day. The total of the degree hours for the four full days prior to blossom wetting helps you assess risk of blossom infection. If blossoms are wetted during the day or evening, total the number of degree days that have accumulated over the past three days, plus the number predicted for the current days high temperature to equal the "four-day degree hour total." If blossoms are wetted in the early morning hours, use the degree hour total from the past four days.

Example:

.	HIGH TEMP	LOW TEMP	DAILY DEGREE HRS.	.
3 DAYS AGO	76	45	130	.
2 DAYS AGO	80	51	230	.
YESTERDAY	80	45	195	.
TODAY (PREDICTED)	70	42	52	.
.	.	4 DAY TOTAL:	607	WET BLOSSOMS?

EXAMPLE: Three days ago, 76/45 = 130 degree hours. ( See table below for DHr. values) Two days ago, 80/51 = 230 degree hours. Yesterday, 80/45 = 195 degree hours. Today's predicted temperature, 70/42 = 52 degree hours. The sum of these four days degree hours equals 607. If contaminated blossoms are wetted, fire blight infection risk is "High."

You should also use temperature forecasts to watch for future risk levels. This is the most useful way to use this model, as you may need to make control decisions two or three days in advance of potential infections.

To accurately evaluate potential infection periods, you must have a way to monitor blossom wetting. A leaf wetness sensor is probably the best method. If blossoms are wetted, usually by rain, but sometimes by heavy dew (3+ hours) or by light irrigation wetting, the following table may help you evaluate infection risk.

TABLE FOR INFECTION RISK RELATIVE TO 4-DAY DEGREE HOUR TOTAL:

Potential Pathogen Presence (Fire Blight History)	LOW RISK	MARGINAL RISK	HIGH RISK	EXTREME RISK
No fire blight in the area during the past two seasons.	0 - 400	400 - 500	500 - 800	800+
Blight present in the region, but not near your orchard, last year.	0 - 350	350 - 400	400 - 650	650+
Blight in your or neighboring orchard last year.	0 - 150	150 - 300	300 - 500	500+
Active Blight strikes or cankers are presently in your or neighboring orchard.	0 - 100	100 - 200	200 - 350	350+

Note: "Marginal" infection conditions do not often lead to fire blight outbreaks- this is a precautionary level, nearing conditions that may more likely lead to blight. Watch forecasts carefully if risk is "Marginal". The threshold numbers on the table above are not Absolute. If your Degree hour total is near a threshold, use your judgment, taking into account the total tree and orchard risk factors discussed above.

DAILY DEGREE HOUR ESTIMATION CHART:

Daytime High Temperature	Degree Hours per day if Night low is 49.9F or Lower	Degree Hours per day if Night low is 50F or Higher	Daytime High Temperature	Degree Hours per day if Night low is 49.9F or Lower	Degree Hours per day if Night low is 50F or Higher
60	0	0	83	243	280
62	2	5	84	257	292
63	5	12	85	266	302
64	10	22	86	274	310
65	14	29	87	280	315
66	20	35	88	285	320
67	26	42	89	288	325
68	33	50	90	290	330
69	42	60	92	287	335
70	52	70	93	284	333
71	62	80	94	280	330
72	74	92	95	274	325
73	87	105	96	267	317
74	100	120	97	260	309
75	115	134	98	254	302
76	130	151	99	246	293
77	146	169	100	238	285
78	162	189	101	230	275
79	178	209	102	222	268
80	195	230	103	216	259
81	212	250	104	208	250
82	228	265	105	200	240

More Fire Blight Information: CLICK HERE

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