Andrew Plant— Plant Pathologist, Maine Potato Board
Written in Consultation with: Gerry Saddler, Plant Pathologist, Science and Advice for Scottish Agriculture (SASA); Amy Charkowski, Plant Pathologist, Colorado State University; and James Dwyer, Crops Specialist, UMaine Extension
Blackleg is a bacterial disease of potatoes. During the disease development, an inky dark colored and sometimes slimy lesion will develop on the potato stalk, which is where the name is derived. Other symptoms that blackleg bacteria may be associated with are seed-piece rot, yield loss, and tuber soft rot (2). This disease can be caused by several different species of bacteria including multiple Pectobacterium species and multiple Dickeya species. The field expression of the disease appears to be very weather dependent. Under dry conditions, symptom development may be quite limited, however, when sufficient moisture and high temperatures are present, the disease may progress rapidly and the entire plant can wilt and die. Below is a checklist (1) of actions growers can utilize to help minimize the risk and impact of potato blackleg on their farm.
- Utilize the North American Health Certificate or other information such as dormant tuber testing results to help assess the potential risk of blackleg in particular seed lots. Depending on results of field inspections and dormant tuber testing one may decide the risk is too high, or may use the available information to make decisions such as what fields to put higher risk seed into. In general, higher risk seed for blackleg would likely do better in well-draining sites as opposed to sites with heavy, poorly draining soils.
- The following labs are following recommended protocols for dormant tuber testing:
- Maine Seed Certification Testing Laboratory
Attn: Andrew Plant
744 Main Street
Presque Isle, Maine 04769
- University of Maine
Attn: Jay Hao
School of Food and Agriculture
5735 Hitchner Hall
Orono, Maine 04469
- Plant Diagnostic Lab
North Dakota State University
Attn: Jesse Ostrander
206 Walster Hall
Fargo, North Dakota 58102
- Agdia Inc.
Attn: Elizabeth Kmieciak
Elisa Laboratory Supervisor
52642 County Road 1
Eikhart, Indiana 46514
- Maine Seed Certification Testing Laboratory
- Plant whole seed tubers if possible. Suberize cut seed before planting. Allow to suberize for at least two days prior to planting; two weeks would be preferable.
- Clean and disinfect tools and equipment used for cutting and planting seed with quaternary ammonium products. This should be done between seed lots, or at least once daily.
- Avoid poor seed storage conditions that may lead to condensation build-up on tubers. Seed should be stored in manner that it can be ventilated and its temperature controlled.
- Plant seed tubers during conditions that favor fast emergence. Plant seed into well- drained soil with temperatures > 50F.
- Avoid wounding during seed cutting, planting and harvest.
- Fungicidal seed treatment of potatoes to prevent seed piece decay may indirectly prevent seed contamination, especially during the cutting operation. Consider using dry formulations of seed treatments as opposed to liquids.
- Consider using sanitary precautions such as disinfectants (quaternary ammonium products) on footwear and equipment prior to entering different fields or lots during the growing season.
- Utilize crop rotation of two or more years with a non-host crops such as small grains or legumes.
- Avoid over-irrigation.
- Avoid excessive fertilization, which may impact plant and tuber maturity.
- Consider copper fungicides in conjunction with mancozeb, or famoxadone in conjunction with cymoxinil and mancozeb. These products have shown partial effectiveness against disease spread and dry out existing lesions. Use of pesticides to aid in controlling blackleg is likely limited in efficacy. It won’t prevent infections resulting from infected seed pieces but may help in preventing further spread once external symptoms appear. Typically, these symptoms appear just prior to or after row closure. To penetrate the canopy with an application of pesticides, growers should consider increasing spray volumes and pressures, or consider chemigating the products if irrigating (3).
- For seed production, try to promote rapid vine desiccation as best you can. To try to target spray deposition into the middle and lower portions of the canopy, consider the following: increasing droplet size, increasing volume per acre, decreasing sprayer speed, angled nozzles, air-induction nozzles, and using spray formulations that minimize drift (4).
- Delay harvest until skin set is complete (up to 21 days after top-kill).
- Avoid wet conditions during harvest to prevent soil from sticking to tuber skins.
- Disinfect harvesting equipment between lots or fields to help prevent spread of bacteria during harvest operations.
- If using post-harvest applications of fungicides for control of pink rot, late blight, or Pythium leak, consider use only on suspected problem lots or fields.
- Store possibly contaminated potato lots separately.
- Provide adequate ventilation in storage.
- Check storages regularly for temperature increase and odors. If problems are detected, hot-spot fans can be used to cool the pile.
- Dry potatoes before storage or shipping.
1. Adapted from: Al-Mughrabi, K. 2016. Dickeya: A new threat to potato production in North America. Spud Smart. Online: http://spudsmart.com/dickeya-new-threat-potato-production-north-america/
2. De Boer, S. H. 2004. Blackleg of potato. The Plant Health Instructor. DOI:10.1094/PHI-I- 2004-0712-01
3. Johnson, D. A., Dung, J. K. S., Cummings, T. F., and Schroeder, B. K. 2011. Development and suppression of aerial stem rot in commercial potato fields. Plant Disease, 95:285- 291.
4. de la Pasture, L. 2016. Blackleg in potatoes- rapid haulm destruction key to blackleg control. Crop Production Management. Online: http://www.cpm-magazine.co.uk/2016/12/06/blackleg-potatoes-rapid-haulm-destruction-key-blackleg-control/