How to Manage Pests

UC Pest Management Guidelines

COMMENTS ON THE DISEASE

Erwinia amylovora bacteria overwinter in limb cankers and are spread by rain splash and insects. Infection occurs mostly through blossoms and less often through succulent shoots. Growth of the bacteria is favored by warm, humid weather in spring or fall. Droplets of a sticky, amber-colored ooze, containing millions of bacteria, exude from freshly blighted tissue. This can be used to distinguish fire blight from blossom blast infections.

DAMAGE

All parts of pear trees can be invaded by the fire blight pathogen. Tissue wilts, blackens, and dies. If infections are not removed, the entire tree may be killed as the disease spreads into the main scaffolds, trunk, and roots.

MANAGEMENT

Fire blight development is influenced primarily by seasonal weather. Warm spring weather, accompanied by intermittent rain and hail, is ideal for disease development. Other influences on disease development are the varieties and rootstocks used in the orchard, location of the orchard, application of too much nitrogen fertilizer, heavy pruning, or over-irrigation. Management relies on maintaining trees in the proper range of vigor, applying blossom sprays of antibiotics or copper that are supplemented with the A506 bacteria, and most importantly, promptly finding, removing, and destroying blight strikes.

Blossom applications of copper materials or the antibiotic streptomycin and terramycin are necessary in pear-growing areas to reduce the spread of fire blight bacteria. The timing of the first application is critical. In California, average daily temperatures or degree-hours are used to schedule fire blight sprays. For detailed information on these methods, see Integrated Pest Management for Apples and Pears, 2nd ed., UC ANR Publ. 3340.

Biological Control
The antagonistic bacteria Pseudomonas fluorescens (Blight Ban A506) is commercially available to prevent colonization of the blossoms by Erwinia amylovora during bloom. It is most effective when used in conjunction with antibiotic treatments such as streptomycin.

Cultural Control
One active overwintering canker located high in a tree can cause infection of surrounding trees, and a few such cankers per acre will render a preventive spring/summer spray program ineffective. Remove and destroy holdover cankers and diseased limbs by cutting at least 8 to 12 inches below signs of visible injury. This helps to stop disease movement in the tree and reduces the source for new infections. Be sure to sterilize pruning shears and saws whenever they come into direct contact with diseased tissues and periodically throughout pruning.

Organically Acceptable Methods
Organically acceptable methods include cultural and biological controls along with sprays of terramycin, streptomycin, some copper products, and Bordeaux.

Monitoring and Treatment Decisions
Several mean temperature and degree-hour models are available to assist in predicting infection periods and the need for control. All are based on the minimum and maximum temperature thresholds above and below which bacterial growth and subsequent infection ceases. These models are utilized in most commercial California pear districts to time antibiotic and copper treatments.

The UC model recommends the first spray when bloom and mean temperatures reach 62°F in March, 60°F in April, and 58°F in May. This model recommends treating every 3 to 5 days until the end of rattail bloom regardless of changes in weather that would inhibit bacterial growth and infection.

The newer degree-hour models are based on assessing actual conditions for bacterial growth and infection and also indicate when treatment is unnecessary. One such model is the degree-hour model, which takes into account early bloom and periods of continuous cool weather, allowing adjustments in treatment timings. Use of the degree-hour method requires a recording thermograph to obtain a continuous temperature reading in your orchard. One degree-hour equals 1 degree above 65°F for 1 hour. For example, a temperature of 70°F for 2 hours generates 10 degree-hours. Accumulate degree-hours each hour of the day unless 3 consecutive days below 66°F occur. In this case, the accumulation of degree-hours is then reduced to zero until temperatures again exceed 65°F. The accumulated degree-hour total is not reduced by continuous cool temperatures if the total has surpassed 400 degree-hours and has coincided with precipitation or simultaneous warm, humid infection periods of at least 57°F and 90% relative humidity. If the orchard is being irrigated, the humidity threshold is reduced to 80% relative humidity as measured outside the orchard. If possible, start the season with a full soil water profile so irrigation during bloom can be avoided.

In the Sacramento Valley, treat within 24 hours preceding rain if 1 to 150 degree-hours have accumulated. In the North Coast region, treat within 24 hours preceding rain when more than 150 degree-hours have accumulated. Treatment for both areas are recommended every 3 to 4 days when accumulation exceeds 150 degree-hours (Sacramento Valley) or 250 degree-hours (Lake County). Alternate day treatments are recommended in the Sacramento Valley whenever more than 500 degree-hours occur in conjunction with major bloom periods.

Two other models developed to predict fire blight infection periods are the Maryblyt model, used primarily in the mid-Atlantic states, and the Cougarblite model from the Pacific Northwest. These models have not yet been adequately validated under California conditions. For more information on these models, see the UC IPM "Disease Model Database").

Rain or hail may require immediate respray of the orchard if temperatures conducive to fire blight development exist. If conditions conducive to fire blight development have occurred and frost conditions develop that are severe enough to cause the pear skin to rupture, re-treat immediately. Varying degrees of bacterial resistance to streptomycin exist in California; terramycin resistance has never been detected.

Common name		Amount to use**	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy. When choosing a pesticide, also consider information relating to environmental quality. Not all registered pesticides are listed. Always read label of product being used.
A.	STREPTOMYCIN#
	(Agri-Mycin 17)	4.8–9.6 oz/acre	12	30
	MODE OF ACTION: A biological (Group 25)1 fungicide.
	COMMENTS: For dilute applications, do not use less than 4.8 oz/100 gal spray. May cause bleaching and burning on Asian pear fruit and foliage, especially dilute applications and under cold, wet conditions. Restricted use material only for fire blight control in an organically certified crop. Streptomycin is fully compatible with Blight Ban A506 either as a tank mix or applied anytime following an application of Blight Ban A506. Resistance has developed to streptomycin throughout California but especially in the San Joaquin and upper Sacramento valleys. Alternate or tank mix with terramycin to reduce or forestall resistance.
B.	OXYTETRACYCLINE#
	(Mycoshield) 17%	8 oz/50 gal water/acre, or	12	60
		1 lb/100 gal water/acre
	MODE OF ACTION: A biological (Group 41)1 fungicide.
	COMMENTS: Not registered for dilute application. Restricted use material only for fire blight control in an organically certified crop. Do not use tank mix with Blight Ban A506. Although terramycin is less effective than streptomycin, no resistance has yet been reported to terramycin. Alternate or tank mix with streptomycin to help reduce or forestall resistance to streptomycin.
C.	PSEUDOMONAS FLUORESCEN A506#
	(Blight Ban A506)	Label rate	4	0
	MODE OF ACTION: A biological fungicide.
	COMMENTS: Blight Ban is intended to complement an antibiotic program, allowing fewer antibiotic treatments especially during the less critical infection periods such as before the main bloom, after petal fall, and in between flushes of rat-tail bloom – particularly when weather conditions are marginal. The bacteria must colonize blossom tissue before fire blight bacteria. Blight Ban also inhibits growth of bacteria that cause russeting and frost. This material should not be used with copper-based materials or tank-mixed with terramycin or the scab fungicide mancozeb (Dithane). Compatibility with aluminum tris (Aliette) is still being researched, so use caution when using this material.
D.	COPPER DUST (6%)#	15–25 lb/acre	see label	see label
	MODE OF ACTION: A multi-site contact (Group M1)1 inorganic fungicide.
	COMMENTS: Copper materials can cause fruit scarring on some cultivars. To reduce the risk of russeting, make applications when fruit is dry and temperatures are not excessively cool or hot. Do not use in conjunction with Blight Ban A506. Do not use on Anjou or Comice varieties of pears. Not all copper compounds are organically acceptable; be sure to check the specific product.
E.	FIXED COPPER#	Label rates	see label	see label
	MODE OF ACTION: A multi-site contact (Group M1)1 inorganic fungicide.
	COMMENTS: Apply as a dilute spray only. Copper materials can cause fruit scarring on some cultivars. To reduce the risk of russeting, make applications when fruit is dry and temperatures are not excessively cool or hot. Do not use in conjunction with Blight Ban A506. Do not use on Anjou or Comice varieties of pear. Not all copper compounds are organically acceptable; be sure to check the specific product. Do not use within 5 days of an application of Blight Ban A506.
F.	BORDEAUX MIXTURE#
	0.5:0.5:100	1 lb	see labels	see labels
	MODE OF ACTION: A multi-site contact (Group M1)1 inorganic fungicide.
	COMMENTS: Apply as a dilute spray only. Copper materials can cause fruit scarring on some cultivars. To reduce the risk of russeting, make applications when fruit is dry and temperatures are not excessively cool or hot. Do not use Bordeaux mixture containing lime sulfur on Comice because it is very phytotoxic. Not all copper compounds are organically acceptable; be sure to check the specific product if you are mixing your own formulation. For information on making a Bordeaux mixture, see UC IPM Pest Note: Bordeaux Mixture, UC ANR Publication 7481.
G.	FOSETYL-AL
	(Aliette) WDG	2.5–5 lb	12	14
	MODE OF ACTION: A phosphonate (Group 33)1 fungicide.
	COMMENTS: Unlike antibiotics, fosetyl-al is a systemic material that increases the tree's resistance to infection.
H.	BACILLUS SUBTILLIS#
	(Serenade Max) WDG	2–4 lb	4	0
	MODE OF ACTION: A biological fungicide.
	COMMENTS: Use in a rotational program with antibiotics and apply in sufficient water to provide full coverage.
**  See label for dilution rate. + Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment until the harvest may take place. In some cases the R.E.I. exceeds the P.H.I. The longer of these two intervals is the minimum time that must elapse before harvest may take place. # Acceptable for organically grown produce. 1 Alternating pesticides with different modes of action is important in preventing the development of resistance. Rotate chemicals with those that have a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season. Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC). For more information, see http://www.frac.info/.

More information on pear bactericides

• Efficacy and timing of pear bactericides

PUBLICATION

UC IPM Pest Management Guidelines: Pear
UC ANR Publication 3455
Diseases
R. B. Elkins, UC Cooperative Extension Lake County
W. D. Gubler, Plant Pathology, UC Davis
L.G. Varela, UC IPM Program, UC Cooperative Extension Sonoma County

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