Late blight, caused by the fungus-like organism Phytophthora infestans, is a potentially devastating disease of potato, and may be the most important disease of potato worldwide. The pathogen can infect several Solanaceous plants, including tomato, eggplant, petunia, and hairy nightshade. P. infestans can infect both potato foliage and tubers. Infection begins when sporangia or zoospores form germ tubes and infect foliage when temperatures are cool to moderate (46 to 75ºF) and free moisture is available. Lesions can develop in as little as three days after infection, producing additional sporangia that are responsible for secondary spread of the disease in a field. Tuber infections are thought to occur when sporangia are washed from leaves and contact buried tubers through cracks in the soil. Tuber infections may also occur during harvest and handling. In the High Plains, the pathogen requires a living host to survive. Therefore, cull potatoes, volunteers, and contaminated seed are the primary means of pathogen survival and dissemination. Survival in soil for months or possibly years occurs by oospores in certain areas of the world where P. infestans sexual mating occurs.
Late blight symptoms first appear on foliage as small, irregularly-shaped necrotic spots. As lesions develop, they become more circular with a necrotic center surrounded by a yellow or pale-green margin. Under humid conditions, white downy mycelium is visible surrounding necrotic lesions. Petioles and stems also become infected and produce water-soaked, dark green to black lesions.
Tuber infections appear initially as reddish brown, dry lesions that can extend an inch or more into tubers. The boundary between healthy and infected tissue is discontinous.
Yield losses of 100% in the field and continuing into storage are not uncommon with susceptible varieties if not regularly treated with fungicides. The disease reduces yield, seed quality, marketability, and processing value.
No biological control practices have been developed for potato late blight.
Plant high quality seed free from the late blight pathogen. Varieties less susceptible to late blight can slow epidemics and reduce the need for fungicides, and should be planted if available. Promoting air movement and leaf wetness in the crop canopy can slow secondary disease spread. Since the pathogen must have a living host to survive in the High Plains, destruction of volunteers, alternate hosts, and culls will reduce initial inoculum sources.
Fungicides are essential to suppressing potato late blight when disease occurs. Seed treated mancozeb-containing fungicides can reduce the contamination of healthy seed pieces during cutting and handling.
Protectant fungicides should be applied prior to infection. Disease forecasting systems have been developed that can improve scouting and fungicide timing in the High Plains, and should be used, if available, to reduce unnecessary sprays. Fungicides banded over seedlings at emergence can reduce secondary spread of disease early in the growing season, if forecasts call for treatments to be applied. Strains of the pathogen resistant to metalaxyl are found in many areas of the High Plains. Fungicides with a different mode of action should be used where resistant strains are common.
Common/Trade Name |
Product per Acre |
Application Frequency
|
Remarks |
Chlorothalonil |
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Agronil 500 |
1-2 1/8 pt |
7-10 days |
May be applied through sprinkler irrigation with 10-day application interval. |
|
Agronil 720 |
0.75- 1.5 pt |
7-10 days |
May be applied through sprinkler irrigation with 10-day application interval. |
|
Bravo 500 |
1 - 2 1/8 pt |
7-10 days |
Maximum of 23 pints per acre; May be applied through sprinkler irrigation with 10 day application interval. |
|
Bravo 720 |
0.75-1.5 pt |
7-10 days |
Maximum of 16 pints per acre; May be applied through sprinkler irrigation with 10 day application interval. |
|
Bravo S |
3-4 pt |
7-10 days |
|
|
Bravo Ultrex |
0.7 –1.4 lb |
7-10 days |
Maximum of 14.5 pounds per acre |
|
Bravo Weather Stik |
0.75-1.5 pt |
7-10 days |
Maximum of 16 pints per acre |
|
Bravo Zn |
1 –2 1/8 pt |
7-10 days |
Maximum of 23 pints per acre |
|
Chlorothalonil 4L |
1 -2 1/8 pt |
7-10 days |
|
|
Equus 720 |
0.75-1.5 pt |
5-10 days |
Maximum of 15 pints per acre; May be applied through sprinkler irrigation at no more than 10-day interval; other formulations of Equus are available |
|
Quadris Opti |
1.6 pt |
5-7 days |
Maximum of six applications per season; Rotate with fungicides with a different mode of action; 14 day PHI |
|
Ridomil Gold Bravo |
2 lb |
14 days |
Maximum of 3 applications, alternating with a full rate of a protectant fungicide; 14 day PHI |
|
Terranil 6L |
0.75-1.5 pt |
7-10 days |
Maximum of 16 pints per acre; 7 day PHI |
|
Terranil Cu |
1.7-3.4 pt |
7-10 days |
Maximum of 36 pints per acre; 7 day PHI |
|
Terranil S |
3-4 pt |
7-10 days |
|
|
Terranil Zn |
1-2 1/8 pt |
7-10 days |
Maximum of 21 pints per acre; 7 day PHI |
Copper Fungicides |
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Copper Flowable |
2/3-4 pt |
3-10 days |
Will also suppress Colorado Potato Beetle |
|
C-O-C-S Copodust |
25-35 lb |
3-10 days |
|
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C-O-C-S WDG |
1 ½-4 lb |
3-10 days |
|
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C-O-C-S Wettable |
1 ½-4 lb |
3-10 days |
|
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Kocide 101 |
1-4 lb |
3-10 days |
|
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Kocide 2000 |
0.75 to 3 lb |
3-10 days |
|
|
Kocide 4.5 LF |
2/3- 2 2/3 pt |
3-10 days |
Efficacy will be improved by tank mixing with other compatible fungicides registered for use on potatoes. |
|
Kocide DF |
1-4 lb |
3-10 days |
pH of spray solutions should no be below 6.5 as phytotoxicity will result. |
|
KOP-Hydroxide 50 |
1-4 lb |
3-10 days |
|
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Manicure T/O Flowable |
1 ½ tsp/gal to cover 200 sq ft of crop |
7-10 days |
|
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Nu-Cop 3L |
0.5-4 pt |
3-10 days |
Will also suppress Colorado Potato Beetle |
|
Nu-Cop 50DF |
1-4 lb |
3-10 days |
If late blight is a problem, apply prior to digging or in vine kill spray. |
Cymoxanil |
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Curzate 60DF |
3.2 oz |
5-7 days |
Maximum of 7 applications per season; Must be tank-mixed with another late blight fungicide such as Bravo Weatherstik; 14 day PHI |
Dimethomorph |
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Acrobat 50WP |
4-6.4 oz |
5-10 days |
Maximum of 32 ounces per season; 4 day PHI |
|
Acrobat MZ |
1.25-2.25 lb |
5-10 |
Maximum of 11.25 pounds per season; 14 day PHI |
|
Forum |
4-6 oz |
5-10 days |
Maximum of 30 ounces per acre; minimum of 5 gal of water/acre aerially and 20 gal/acre by ground; rotate with other fungicide chemistry; 4 days PHI |
EBDC |
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ManKocide |
1.5-5 lb |
7-10 days |
14 PHI |
|
Manex |
0.8-1.6 qt |
7-10 days |
Maximum of 11.2 quarts per acre |
|
Polyram 80 DF |
1.5-2 lb |
|
Include a nonionic surfactant to improve performance |
Fluazinam |
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Omega |
5 fl oz |
7-10 days |
Maximum of 3.5 pints per season; 14 day PHI |
Iprodione |
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Rovral |
1-2 lb |
|
Do not irrigate within 24 hours of application; Maximum of 4 applications per season; 14 day PHI |
|
Rovral 4 Flowable |
1-2 pt |
|
Do not irrigate within 24 hours of application; Maximum of 4 applications per season; 14 day PHI |
Mefenoxam Mixtures |
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Ridomil Gold Bravo |
2 lb |
14 days |
Maximum of 3 applications, alternating with a full rate of a protectant fungicide; 14 day PHI |
|
Ridomil Gold MZ |
2.5 lb |
14 days |
Maximum of 3 applications, alternating with a full rate of a protectant fungicide; 14 day PHI |
|
Ridomil Gold Bravo Liquid |
1 pack per 10 acres |
14 days |
Maximum of 3 applications, alternating with a full rate of a protectant fungicide; 14 day PHI |
Neem |
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Trilogy |
1.0% in 25 to 100 gal per acre |
|
Maximum of 2 gallons per acre; cannot be tank-mixed with sulfur, Bravo, or other similar fungicides |
Strobilurin and Stobilurin Mixtures |
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Gem |
6-8 oz |
7- 14 days |
Maximum of six applications per season; Rotate with fungicides with a different mode of action; 14 day PHI |
|
Headline |
12 fl. oz |
7- 14 days |
Maximum of six applications per season; Rotate with fungicides with a different mode of action; 14 day PHI |
|
Quadris FL |
6.2 - 15.4 fl oz |
7- 14 days |
Maximum of six applications per season; Rotate with fungicides with a different mode of action; 14 day PHI |
|
Quadris Opti |
1.6 pt |
5-7 days |
Maximum of six applications per season; Rotate with fungicides with a different mode of action; 14 day PHI |
|
Tanos |
6-8 oz pt |
5-10 days |
Maximum of six applications per season; Rotate andt tank mix with fungicides with a different mode of action; 14 day PHI |
Triphenyltin Hydroxide |
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Super Tin 80WP |
2.5-3.75 oz |
|
Maximum of 15 ounces per season; tank mixes with Polyram or mancozeb are recommended; will suppress Colorado potato beetle; 21 day PHI tank mixes |
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: Potato, Disease, Late Blight
Date: 03/03/2007