Potato Early Blight. Identification and Life Cycle. Plant Response and Damage. Management Approaches. Biological Control

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1 Potato Early Blight Howard F. Schwartz, David H. Gent, and Gary D. Franc Identification and Life Cycle 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. Plant Response and Damage 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. Management Approaches Biological Control No biological control practices have been developed for potato early blight.

2 Cultural Control 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. Chemical Control 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 (days) Remarks Boscalid Endura oz 7-14 days Max of 4 applications (20.5 oz) per a different mode of action; 30 day Chlorothalonil and Chlorothalonil Mixtures not all formulations listed Agronil /8 pt 7-10 days May Agronil pt 7-10 days May Bravo /8 pt 7-10 days Maximum of 23 pints per acre; May

3 Bravo pt 7-10 days Maximum of 16 pints per acre; May Bravo S 3-4 pt 7-10 days Bravo Ultrex lb 7-10 days Maximum of 14.5 pounds per acre Bravo Weather Stik 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 pt 5-10 days Maximum of 15 pints per acre; May 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 Ridomil Gold Bravo 2 lb 14 days Maximum of 3 applications, protectant fungicide; 14 day Ridomil Gold Bravo Liquid 1 pack per 10 acres 14 days Maximum of 3 applications, protectant fungicide; 14 day Terranil 6L pt 7-10 days Maximum of 16 pints per acre; 7 day Terranil Cu pt 7-10 days Maximum of 36 pints per acre; 7 day 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 Copper Fungicides Copper Flowable 2/3-4 pt 3-10 days Will also suppress Colorado Potato Beetle C-O-C-S Copodust 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 lb 3-10 days Kocide 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.

4 Kocide DF 1-4 lb 3-10 days ph of spray solutions should not be below 6.5 as phytotoxicity will result. KOP-Hydroxide 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 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 lb 7-10 days 14 Manex qt 7-10 days Maximum of 11.2 quarts per acre Ridomil Gold MZ 2.5 lb 14 days Maximum of 3 applications, protectant fungicide; 14 day Polyram 80 DF 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 Rovral 4 Flowable 1-2 pt Do not irrigate within 24 hours of application; Maximum of 4 applications per season; 14 day 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 Headline 12 fl. oz 7-14 days Maximum of six applications per Quadris FL fl oz 7-14 days Maximum of six applications per Quadris Opti 1.6 pt 5-7 days Maximum of six applications per

5 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 Triphenyltin Hydroxide Super Tin 80WP oz Maximum of 15 ounces per season; tank mixes with Polyram or mancozeb are recommended; will suppress Colorado potato beetle; 21 day 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