Alternaria leaf spot is caused by the fungus Alternaria dauci. Infection can be initiated by seedborne or windborne spores (conidia), which germinate in the presence of free moisture and penetrate leaves through wounds or natural openings. The fungus sporulates profusely on dead and dying plant tissue, especially during moderate to warm weather (60 to 77ºF) with prolonged periods (8 to 12 hours) of leaf wetness. Spores (conidia) are readily disseminated by wind and splashing water in and among adjacent fields; large numbers of conidia become airborne during harvesting. The pathogen can be moved among fields on contaminated equipment. A. dauci survives between carrot crops as a pathogen of wild and volunteer carrot, in infested crop debris, in the soil for up to one year, and in and on contaminated seed.
Alternaria leaf spot symptoms first appear as greenish brown, water-soaked lesions. Lesions quickly become dark brown to black with or without yellow halos. Under disease-favorable conditions, lesions coalesce and cause entire leaves to become yellow, collapse, and die. Older leaves are most susceptible to infection and often the first to develop symptoms, but all leaves can be infected. Petiole lesions appear similar to leaf lesions, but are more elongated; petiole lesions quickly kill entire leaves. Disease symptoms are often confused with Cercospora leaf spot and bacterial blight, and microscopic examination is often necessary for accurate diagnosis.
Carrot seedlings can also be attacked by A. dauci within 2 to 3 weeks after emergence. Infection occurs at or just below the soil line. Infected hypocotyls and upper roots become girdled and shrivel, turning a gray or black color. Seedling infection appears similar to Pythium damping-off, but A. dauci infection and decay is drier than that of Pythium.
No biological control practices have been developed for Alternaria leaf spot.
Plant high quality seed free of the Alternaria leaf spot pathogen. Hot water treatment can reduce seed contamination, but may reduce seed storability and germination. Select varieties resistant or tolerant to Alternaria leaf spot, such as ‘Orlando Gold,’ if available. Practice a three-year or longer crop rotation to nonhosts such as small grains. Eliminate potential sources of the pathogen by deeply burying crop debris after harvest and controlling wild and volunteer carrot. Avoid prolonged periods of leaf wetness by avoiding dense planting, orientating rows parallel to the prevailing wind direction, and timing irrigations to end before dusk. Avoid overhead irrigation if possible.
Fungicides are rarely necessary for Alternaria leaf spot control in many High Plains carrot production regions, but are essential in warm, humid environments. Disease forecast models have been developed that can improve the timing and efficiency of sprays. Chemical controls are most effective when combined with as many cultural control strategies as possible.
Product List for Alternaria Leaf Spot:
Pesticide |
Product per Acre |
Application Frequency
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Remarks |
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Boscalid |
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Endura |
4.5 oz |
7-14 days |
Max of 5 times (22.5 oz) per season; 0 day PHI |
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Chlorothalonil and Mixes |
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Bravo Ultrex |
1.4-1.8 lb |
7-10 days |
Maximum of 16.5 pounds per season; 0 day PHI |
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Bravo Weather Stik |
1.5-2.0 pt |
7-10 days |
Maximum of 20 pints per season; 0 day PHI |
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Echo 720 |
1.5-2.0 pt |
7-10 days |
Maximum of 2.5 gallons per season; 0 day PHI |
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Echo 90DF |
1.2-1.6 lb |
7-10 days |
Maximum of 16.67 pounds per season; 0 day PHI |
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Echo Zn |
2.2 to 2.8 pt |
7-10 days |
Maximum of 3.6 gallons per season; 0 day PHI |
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Ensign |
1.5-2 pt |
7-10 days |
7 day PHI |
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Ridomil Gold/Bravo |
1.5-2.0 lb |
7-14 days |
Make 2 to 4 total sprays; 7 day PHI |
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Copper Fungicides |
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Basicop |
3-4 lb |
7-14 days |
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Champ Dry Prill |
1.33 lb |
7-14 days |
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Champ Formula 2 |
1.33 pt |
7-14 days |
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Copper-Count-N |
4-6 pt |
7-10 days |
Maximum of 5 applications/season |
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Cuprofix Disperss |
2.5 lb |
7-14 days |
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Kocide 101 |
2 lbs (197 oz) |
7-14 days |
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Kocide DF |
2 lbs (157 oz) |
7-10 days |
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Kocide 4.5LF |
1.33 pts (8 fl oz) |
7-14 days |
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Kocide 3000 |
0.75 lb |
7-14 days |
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Ridomil Gold/Copper |
1 pack/per 2.5 acres |
14 days |
Make 2 to 4 total sprays; 7 day PHI |
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Top Cop + S |
4 pt |
7-10 days |
1 day PHI |
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Cyprodinil / Fludioxonil |
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Switch 62.4WG |
11-14 oz |
7-10 days |
Maximum or 56 oz/acre; after 2 applications, alternate with other modes of action,;7 day PHI |
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Iprodione—generic formulations also available |
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Rovral |
1-2 lb |
7-14 days |
Maximum of 4 applications per season; 10 application may be made if tank-mixed with another fungicide; 0 day PHI |
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Rovral 4F |
2 pt |
7-14 days |
Maximum of 4 applications per season; 10 application may be made if tank-mixed with another fungicide; 0 day PHI |
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Strobilurin and Strobilurin/Boscalid |
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Amistar |
3 to 5 oz |
7-14 days |
Maximum of 4 applications per season; Do not make more than 1 application before rotating to a fungicide with a different mode of action; 0 day PHI |
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Cabrio |
8 to 12oz |
7-14 days |
Maximum of 3 applications of 48 oz per seaon; Do not make more than 2 sequential applications of Cabrio before rotating to a fungicide with a different mode of action; 0 day PHI |
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Pristine |
8-10.5 oz |
7-14 days |
Maximum of 6 applications or 63 oz per season; 0 day PHI |
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Quadris |
9.2-15.4 fl oz |
7-14 days |
Maximum of 4 applications or 3.75 quarts per season; Do not make more than 1 application of Quadris before rotating to a fungicide with a different mode of action; 0 day PHI |
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The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality. |
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Categories: Carrot, Disease, Alternaria Leaf Spot
Date: 04/01/2007