BACTERIAL AND FUNGAL DISEASES OF POTATO AND THEIR MANAGEMENT Jessica Rupp, Extension Plant Pathology Specialist, Department of Plant Sciences and Plant Pathology Barry Jacobsen, Associate Director and Head, Montana Ag Experiment Stations, Department of Research Centers EB 0225 new January 2017 BACTERIAL DISEASES THE SEED POTATO INDUSTRY IN MONTANA Blackleg 1 Seed potatoes are an important crop in Montana and Aeiral Stem Rot 1 are a crucial quality seed source for potato production Soft Rot 2 across the United States. The cooperation of commercial producers and home gardeners to control diseases of great Ring Rot 2 concern, such as late blight, is essential. Brown Rot 5 Dickeya Blackleg 5 Montana is one of the top five seed-potato producing states. According to the Montana Department of Common Scab 6 Agriculture, the state’s seed potatoes are prized because FUNGAL DISEASES growing areas are somewhat isolated from airborne spores of diseases such as late blight. To protect this industry, Alternaria Brown Spot 6 Montana only allows potatoes that originate in Montana to Early Blight 7 be grown as certified seed, and requires all seed potatoes Late Blight 8 to be inspected at their shipping point. Businesses can Powdery Mildew 9 sell garden seed potatoes from outside Montana, but need to be inspected at the point of shipping and have an Glossary 9 accompanying health certificate. © 2017 The U.S. Department of Agriculture (USDA), Montana State University and Montana State University Extension prohibit discrimination in all of their programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital and family status. Issued in furtherance of cooperative extension work in agriculture and home economics, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Jeff Bader, Director, Montana State University Extension, Bozeman, MT 59717. Information in this document is provided for educational purposes only. Reference to commercial products or trade names does not imply an endorsement of them by MSU Extension. Common chemical and trade names are used in this publication for clarity for the reader. Inclusion of a common chemical or trade name does not imply endorsement of that particular product or brand of herbicide and exclusion does not imply non-approval. This publication is not intended to replace the product label. 1 BACTERIAL DISEASES OF POTATO AND THEIR MANAGEMENT IN MONTANA BLACKLEG, AERIAL STEM ROT and SOFT ROT are all caused by bacteria, that via their production of pectolytic enzymes cause a wet, mushy rot of tissues they infect. Pectolytic enzymes dissolve the middle lamella, composed of pectin, which holds plant cells together. BLACKLEG AERIAL STEM ROT General Information General Information Currently in Montana, blackleg is most commonly caused Aerial stem rot, also known as aerial blackleg, aerial soft by Pectobacterium spp. In the United Kingdom and Europe rot, or bacterial stem rot can be found anywhere potatoes another genus of soft rot bacteria, Dickeya spp. can cause are grown. This disease can be caused by several different blackleg and this will be discussed separately. bacteria including: Pectobacterium carotovorum subsp. As infected tubers are the source of this disease and carotovorum (syn. Erwinia carotovora subsp. carotovora), the pathogen is easily spread during seed cutting, blackleg Pectobacterium atrosepticum and Dickeya dianthicola is of great concern to certified seed growers. This disease (syn. Erwinia chrysanthemi). In Montana, aerial stem rot is has been minimized by the tissue culture-based, flush-out typically caused by Pectobacterium carotovorum subsp. certification system used by Montana seed growers. carotovorum, the causal agent of soft rot. Symptoms and Spread Symptoms and Spread Blackleg symptoms include soft rot of seed pieces, black to Aerial stem rot often begins as a water-soaked lesion brown discoloration of the stem extending from the seed on the stem that becomes a soft, rotted area. Under dry piece to above ground portions of the stem, and stunting conditions, stems appear shriveled and dark, while in wet, and wilting of affected stems (Figure 1). In some cases only humid conditions appear soft and slightly water soaked. the interior of the stem may show discoloration. Blackleg Internal stem pith is often discolored outside the boundary infected tubers can show soft rot symptoms in storage, but of lesion edges. When present, external discoloration symptom development is favored by wet soils at planting due to decay ranges from dark green to dark brown, and and temperatures between 50 and 60ºF before emergence black. The infection is not tuber borne, and often remains and >68°F after emergence. Under unfavorable conditions confined to the aboveground portion of the plant. Stem for disease development, plants may be asymptomatic. infections occur through wounds or natural openings, such as a leaf scar. Affected stems become wilted and Management often die. Aerial stem rot is differentiated from blackleg by The key to management of Blackleg is disease-free seed. occurring higher up the stem and spreading downward, Cleaning seed handling, planting, and cutting equipment whereas blackleg occurs from the ground up. The soft rot is important and this will be discussed in the Soft Rot/Ring bacterium is a common soil inhabitant and is spread by Rot disease sections. FIGURE 1. Typical blackleg infection FIGURE 2. Aerial stem rot following hail event Photo: Payton Strawser, The Ohio State University Photo: Barry Jacobsen, Montana State University 2 BACTERIAL & FUNGAL DISEASES OF POTATO AND THEIR MANAGEMENT IN MONTANA irrigation, splashing water or insects. This bacterium must favored by 70-80ºF temperatures, but can be active at any infect through wounds with hail injury (Figure 2, page 1) temperature between 32 and 90ºF with decay and bacteria and mechanical damage (equipment, blowing sand) being reproduction being sharply retarded below 50ºF. Soft rot the most common routes for infection. Where European bacteria spread from infected to non-infected tubers via corn borer is found, their oviposition and feeding sites are water films in storage. another common infection site. Under moist conditions Symptoms and Spread decay is soft and slimy. Factors that lead to prolonged Symptoms of soft rot are characterized by soft, mushy canopy wetness such as rain, dense canopies, excessive decay of tuber tissue. Rotted tissues are typically cream nitrogen fertilization, etc. are predisposing factors. Under to tan in color. When infected tubers are cut, the margin dry conditions infected tissues rapidly shrivel, and dry of the rotted area will turn brown to black with a clear up. Warm to hot temperatures are favorable for bacterial boundary between the rotted and sound tissue. Initial replication. Simple molecular laboratory tests can easily decay is relatively odorless, but as other bacteria begin to differentiate black leg from soft rot bacteria. grow within the rotted tissue, the odor will become foul. Management When tubers are harvested from wet soils or washed before Management of the disease can prove difficult. The storage, lenticel infection is common. Lenticel infections are suggestions for integrated control follow. typically ¼-½ inch in diameter and while still moist, may 1. Plant only clean, certified seed potatoes. be raised and slightly darker than the potato skin. Flesh up 2. Use whole tubers, or allow cut seed pieces to to ½ inch deep under the infected lenticel is water-soaked suberize, or cork over, before planting. and yellow to cream in color. When these infections dry 3. Use recommended plant spacing. out under dry or cold conditions, they are typically slightly 4. Avoid over irrigation, use less frequent irrigation, with sunken. Un-suberized bruises or wounds are common longer durations. Early day irrigation allows for drying entry points and seed decay is common when seed pieces to occur later in the day. have not been properly suberized before planting. Typical symptoms of soft rot are shown in Figures 3. 5. Avoid over fertilization (especially nitrogen) to prevent excessive vine growth. Management 6. Avoid stem damage during hilling and use optimal Management of soft rot requires addressing the ever- sanitation practices present pathogen and its requirement for wounds or The use of copper-based fungicides with or without damaged tissues for infection. The following are important mancozeb is recommended after hail events. Spray suggestions. programs with famadone +cymoxanil+ mancozeb and 1. Avoid harvest when temperatures are >65-75ºF, copper hydroxide + mancozeb have been shown to reduce particularly when conditions are wet. disease incidence and severity. 2. Have good skin set before harvest. Generally this requires 10-14 days post vine kill. SOFT ROT General Information This disease can be caused by any one of several FIGURE 3. Typical cross section of tuber affected by soft rot pectolytic bacteria including: Pectobacterium carotovorum subsp. carotovorum, Pectobacterium carotovorum subsp. odoriferum, Pectobacterium atrosepticum, Dickeya dianthicola (syn. Erwinia chrysanthemi), and certain strains of bacteria in the genus Pseudomonas, Bacillus and Clostridium. Decay by Clostridium species usually only occurs under anaerobic conditions. Soft rot decay of seed pieces and potatoes in storage is most commonly caused by Pectobacterium carotovorum subsp. carotovorum,