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How to Manage Pests
UC Pest Management Guidelines
Carrot
Alternaria Leaf Blight
Pathogen: Alternaria dauci
(Reviewed 10/05,
updated 10/05)
In this Guideline:
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Alternaria leaf blight symptoms appear as dark brown to black
irregularly shaped lesions on leaf blades and petioles. Spots are initially
surrounded by a yellow margin and often begin on the older leaves. Leaves can
be killed when spots grow together. Lesions that develop on petioles may kill
entire leaves. Leaves weakened by blight may break off when gripped by
mechanical harvesters, resulting in the roots being left in the ground. The
pathogen also causes damping-off of carrot seedlings.
The disease is favored by rainy weather and/or overhead irrigation.
The pathogen, Alternaria dauci, is seedborne and survives on and is spread on
carrot seed. It can also survive in carrot debris and on volunteer carrots.
Spores are dispersed in air and splashing water. The optimum temperature for
growth and infection is 82°F
with some infection occurring at temperatures as low as 57°F and
as high as 95°F.
Although the fungus survives on carrot debris left in the field after harvest,
once the crop residue decomposes, the fungus dies.
Cultural Control
Planting Alternaria-indexed seed or treating seed in a hot water bath is
very important. Turn under carrot residue by tillage or plowing to hasten
decomposition of debris. Practice 2-year rotations: avoid continuous carrot
culture. Do not plant new fields near existing fields with blight symptoms.
Organically Acceptable Methods
Cultural control, hot water dips, and foliar sprays of Serenade ASO and
MAX are acceptable for use on organically grown produce.
Monitoring and Treatment Decisions
Seed treatments may reduce Alternaria dauci. Assay the seed and if pathogen is found, treat. If
seed are not treated, apply fungicides when the first blight symptoms appear,
at biweekly intervals, and/or when conditions are favorable for disease
development.
| Common name |
Amount/Acre |
P.H.I.+ |
| (trade name) |
|
(days) |
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| SEED TREATMENT |
| A. |
IPRODIONE |
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(Rovral) 4F |
0.5
lb/6 gal water
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MODE OF ACTION: A dicarboximide (Group 2)1
fungicide.
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COMMENTS: Soak seeds for 24 hours at 86°F
(30°C) in a solution of iprodione and water. Use 6 gal of solution for 3 lb
of carrot seed. Use of this material allowed under a 24(c) registration.
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| B. |
HOT WATER DIP# |
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COMMENTS: Soak seed in hot water (122°F;
50°C) for 25 minutes. Do
not soak longer or seed could be damaged.
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| FOLIAR TREATMENT |
| A. |
AZOXYSTROBIN |
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(Quadris) |
9.2–15.4 fl oz |
0 |
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MODE OF ACTION: A Qol (Group 11)1
fungicide.
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COMMENTS: Do not make more than one
application before rotating to a fungicide with a different mode of action.
Do not make more than four foliar applications of strobilurin fungicides per
crop.
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| B. |
BOSCALID |
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(Endura) |
4.5 oz |
0 |
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MODE OF ACTION: A carboxamide (Group 7)1 fungicide. |
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COMMENTS: To limit potential for resistance development, do
not make more than 5 applications per season or more than two sequential
applications before alternating to a fungicide with a different mode of
action. |
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| C. |
CHLOROTHALONIL |
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(Bravo Weatherstik) |
1.5–2 pt |
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MODE OF ACTION: A multi-site contact (Group M)1 chloronitrile
fungicide. |
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COMMENTS: Repeat applications at 7- to 10-day intervals if
necessary to maintain control. |
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| D. |
IPRODIONE |
|
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(Rovral) 4F |
1–2 pt |
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MODE OF ACTION: A dicarboximide (Group 2)1 fungicide. |
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COMMENTS: Do not make more than 4 applications of iprodione/season. |
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| E. |
PYRACLOSTROBIN |
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(Cabrio) EG |
8–12 oz |
0 |
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MODE OF ACTION: A Qol (Group 11)1 fungicide. |
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COMMENTS: Do not make more than two sequential applications
before rotating to a fungicide with a different mode of action. Do not make
more than three applications of strobilurin fungicides per crop. |
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| F. |
TRIFLOXYSTROBIN |
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(Flint) |
2–3 oz |
7 |
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MODE OF ACTION: A Qol (Group 11)1 fungicide. |
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COMMENTS: To limit potential for resistance development do
not apply more than 3 sequential applications or a total of 4 applications
of all strobilurins per season. Do not apply more than 12 oz/acre/year. |
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| G. |
BACILLUS SUBTILIS |
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(Serenade MAX)# |
1–3 lb |
0 |
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(Serenade ASO)# |
2–4 qt |
0 |
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MODE OF ACTION: Unknown. |
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COMMENTS: Serenade ASO has a special local needs registration
for organically grown carrots. Apply Serenade MAX as a foliar spray; thorough
coverage is essential. Serenade ASO is applied with chemigation equipment. |
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UC IPM Pest Management Guidelines: Carrot
UC ANR Publication 3438
Diseases
R. M. Davis, Plant
Pathology, UC Davis
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara Co.
J. Nunez, UC Cooperative Extension, Kern Co.
T. A. Turini, UC Cooperative Extension, Imperial Co.
Acknowledgment for contributions to the diseases section:
B. W. Falk, Plant Pathology, UC Davis
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