Potato early blight is caused by the fungus Alternaria solani and occurs wherever potatoes are grown, and is the disease most targeted by potato growers with fungicides in the U.S. The pathogen can infect both foliage and tubers. Infection occurs when spores (conidia) land on leaves, germinate in the presence of free moisture or high relative humidity, and infect the plant by direct penetration. As lesions develop and produce spores they become a source of secondary spread of the fungus to other leaves and plants. Sporulation can occur over a range of temperatures (41 to 86ºF), but most spores are produced near 68ºF when alternating wetting and drying of foliage occurs. Spores are readily moved by wind. Tuber infections occur when contaminated soil and spores come into contact with wounds made during harvest. The fungus can survive between potato crops in and on other hosts such as tomato, volunteer and cull potato, soil, and crop debris.
Initially potato foliage appears as dark spots with concentric rings of alternating raised and depressed dead (necrotic) tissue. Lesions are initially circular with or without a yellow border, but later become angular and irregularly shaped as they encounter veins. As the disease progresses, necrotic lesions form on stems and petioles. Severely infected leaves may become entirely necrotic, but remain attached to plants. Lesions first appear on lower, older leaves of older plants.
Tuber lesions appear as circular to irregularly-shaped dark, sunken areas often with a raised margin. Lesions have a leathery or corky appearance with yellow to greenish yellow water-soaked margins.
Yield losses associated with early blight typically approach 20% if left untreated in the High Plains, but are generally less than 5% if treated with fungicides. Tuber infections reduce seed quality, marketability, and processing value and are often as important as foliar infections in the High Plains.
No biological control practices have been developed for potato early blight.
Early blight is most severe when plants are stressed by other diseases, insects, or environmental conditions. Plant only high quality seed free from other pathogens. Provide adequate but not excessive irrigation and fertility for optimal crop development. Varieties differ widely in their susceptibility to early blight; highly susceptible early maturing varieties such as Russet Norkotah should be avoided if possible. Practice a 2 to 3 year crop rotation with non-susceptible hosts. Harvest at least 14 days after vine kill to ensure tubers have complete skin set which will reduce skinning injury and storage losses. Adjust equipment to reduce tuber bruising and injury during harvest and handling. Store in an environment that promotes rapid suberization and wound healing immediately after harvest.
Fungicides are necessary for effective early blight management in most potato production regions. The first spray should be applied when airborne spores appear, which coincides with the first appearance of lesions. Fungicides applied before the first lesion appearance do not contribute to disease suppression and are not necessary. Several disease forecasting systems have been developed and validated in the High Plains region, and can significantly reduce the number of fungicide applications necessary for effective early blight control. Fungicide chemistries should be rotated or tank-mixed to delay resistance development in the fungus. Copper-based fungicides are not highly effective against early blight. Resistance to strobilurin fungicides such as azoxystrobin (Quadris) have been document in several potato production regions in the U.S., and these fungicides may provide little or no control if resistant strains of the pathogen exist.
Common/Trade Name |
Product per Acre |
Application Frequency
|
Remarks |
Boscalid |
|||
|
Endura |
2.5-4.5 oz |
7-14 days |
Max of 4 applications (20.5 oz) per season; Rotate with fungicides with a different mode of action; 30 day PHI |
Chlorothalonil and Chlorothalonil Mixtures—not all formulations listed |
|||
|
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 |
|
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 |
|
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 |
|||
|
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 |
|
|
C-O-C-S WDG |
1 ½-4 lb |
3-10 days |
|
|
C-O-C-S Wettable |
1 ½-4 lb |
3-10 days |
|
|
Kocide 101 |
1-4 lb |
3-10 days |
|
|
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 not be below 6.5 as phytotoxicity will result. |
|
KOP-Hydroxide 50 |
1-4 lb |
3-10 days |
|
|
Manicure T/O Flowable |
1 ½ tsp/gal to cover 200 sq ft of crop |
7-10 days |
|
|
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. |
EBDC and EBDC Mixtures—not all formulations listed |
|||
|
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 |
|
Ridomil Gold MZ |
2.5 lb |
14 days |
Maximum of 3 applications, alternating with a full rate of a protectant fungicide; 14 day PHI |
|
Polyram 80 DF |
1.5-2 lb |
|
Include a nonionic surfactant to improve performance |
Iprodione—not all formulations listed |
|||
|
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 |
Neem |
|
|
|
|
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 Strobilurin Mixtures |
|||
|
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 |
5-10 days |
Maximum of six applications per season; rotate and tank mix with fungicides with a different mode of action; 14 day PHI |
Triphenyltin Hydroxide |
|||
|
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. |
|||
Categories: Potato, Disease, Early Blight
Date: 03/06/07