How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS AND SIGNS

Bacterial leaf blight is often first noticed in fields as brown areas about 3 to 4 feet in diameter. Leaf symptoms appear as irregular brown spots, often beginning on the leaf margins. Lesions initially have an irregular yellow halo and may appear watersoaked. Spots coalesce and cause a leaf blight and dark brown streaks develop on leaf petioles. Floral parts may also be blighted. A sticky amber-colored bacterial exudate, which is a diagnostic sign of the disease, may be present on leaves or observed flowing downward on petioles and flower stalks.

COMMENTS ON THE DISEASE

Xanthomonas campestris pv. carotae is seedborne, and survives on and is spread with carrot seed. The bacteria also survive in carrot debris but cannot survive in the soil in the absence of debris. Rain or sprinkler irrigation is required for optimum disease development. Warm weather favors infection and disease development. Optimum temperatures are between 77° and 86°F; infection does not occur below 65°F. The pathogen is dispersed in splashing water. Plant-to-plant spread may occur under heavy dew conditions.

In most carrot-growing areas, bacterial blight does not warrant control. In a few areas, such as the Antelope Valley, severe outbreaks may occur.

MANAGEMENT

Cultural Control

Plant Xanthomonas-indexed seed or treat seed in a hot water dip. Use furrow or drip irrigation rather than sprinklers. Turn under carrot residue to hasten decomposition. Avoid continuous carrot culture by using a 2- to 3-year crop rotation scheme.

Organically Acceptable Methods

Cultural controls, hot water dips, and sprays of certain copper sulfate formulations are acceptable for use on organically grown produce.

Monitoring and Treatment Decisions

Assay seed and treat if pathogen is found. Foliar treatments occasionally necessary if rainy weather persists.

Common name		Amount/Acre	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 environmental impact. Not all registered pesticides are listed. Always read the label of the product being used.
SEED TREATMENT
A.	HOT WATER DIP#		NA	NA
	COMMENTS: Soak seed in hot water (126°F; 52°C) for 25 minutes.
FOLIAR TREATMENT
A.	COPPER SULFATE#	Label rates	12	0
	MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
	COMMENTS: Repeat applications at 7- to 14-day intervals, if necessary, to maintain control. Not all copper compounds are approved for use in organic production so be sure to check individual products.
B.	COPPER HYDROXIDE
	(Champ, Kocide 2000, etc.)	Label rates	24	0
	MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
+	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 to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
#	Acceptable for use on organically grown produce.
1	Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.
NA	Not applicable.

PUBLICATION

UC IPM Pest Management Guidelines: Carrot
UC ANR Publication 3438

Diseases

J. Nunez, UC Cooperative Extension, Kern County
R. M. Davis, Plant Pathology, UC Davis
T. A. Turini, UC Cooperative Extension, Fresno County
Acknowledgment for contributions to Diseases:
B. W. Falk, Plant Pathology, UC Davis
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara County

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