This article appears in the September 2020 issue of Potato Grower.
Growers face numerous storage diseases that can impact tuber quality in storage. Although symptoms of storage diseases begin in storage, control of them begins in the field. Sanitation practices are the first line of defense against disease development.
The primary purpose of sanitation practices is to reduce or eliminate sources of pathogen inoculum, which can initiate disease outbreaks. Field sanitation includes practices to reduce initial inoculum levels before planting, such as cleaning and disinfecting all handling equipment before the season begins. This includes seed cutters, planters, sprayers, trucks, belts and conveyors to reduce the risk of introducing and spreading pathogens.
During the growing season it is important to eliminate cull piles and to scout potato fields for diseases that cause problems in storage. Tuber diseases such as bacterial soft rot (Pectobacterium species) and bacterial ring rot (BRR) both display foliar symptoms. BRR is a vascular pathogen that is characterized by foliar wilt, chlorosis and stunting. Plants infected with Pectobacterium will develop black lesions on the stem, which will spread up (blackleg) or down the stem (aerial stem rot), eventually rotting the entire stem. The oomycete pathogen Phytophthora infestans (late blight) can cause both foliar and tuber blight, with spores washing off infected leaves and into the soil, infecting tubers.
Late in the growing season, tubers infected with pink rot (Phytophthora erythroseptica) and Pythium leak (Pythium ultimum) may become visible in the field before harvest. If these pathogens have been confirmed in a commercial field, it is recommended that the field be harvested last. Harvesting the field last will provide time for severely infected tubers to completely rot down and for other infected tubers to develop symptoms that are easier to detect and thus easier to remove before the potatoes are placed in storage.
Regardless of the disease, the methods for controlling them are generally the same. Late in the growing season is the time to minimize wounding (which allows pathogen entry), to reduce the amount of time that tuber surfaces are wet (which promotes a favorable environment for disease), and to optimize storage conditions. Storage facilities should be prepared well before harvest to ensure an optimal storage environment before any potatoes are loaded into the facility. Growers should check storage insulation to make sure it is intact to reduce problems with condensation and heat loss. Vapor barriers should be inspected to ensure they are continuous over all interior walls. Fans and humidifiers need to be working properly and proper maintenance be carried out. Finally, the storage should be thoroughly cleaned and disinfected.
Cleaning and disinfecting a potato storage is a two-stage process. Firstly, all storage walls and floors, duct tubing, bins and other equipment should be washed with hot soapy water using a high-pressure washer to remove decayed potatoes, soil and other organic debris. This step is critical since organic matter and soil will inhibit the action of most disinfectants, and pathogens such as BRR survive as biofilms that can be protected from disinfectants by dirt or plant material. Secondly, once surfaces have been cleaned, they must be sanitized with an effective disinfectant. Effective disinfectants include quaternary ammonium compounds, bleach, etc. In general, disinfectants must be wet and in contact with a surface for a minimum of 10 minutes to kill bacteria and other potato pathogens. For an in-depth guide to cleaning and disinfecting potato storages, refer to the University of Idaho Extension bulletin CIS 1180, “Cleaning and Disinfecting Potato Equipment and Storage Facilities.”
Before harvest, it is important to wash harvesting and handling equipment prior to use, especially if it is known to have handled diseased tubers, such as pink rot- or Pythium-infected tubers, which can contaminate healthy tubers and lead to disease in storage.
The key to healthy potatoes in storage is wounding—or lack of it. Therefore, every effort should be made at harvest to limit wounding and minimize mechanical damage whenever possible. The first way to minimize wounding is to ensure skin maturity. Potatoes need to be left a minimum of seven days after vines are totally dead to encourage skin set and reduce wounding and bruising. Harvesters should be modified to minimize mechanical damage to tubers. For example, equipment operators should ensure the blade positions tubers on the digger chain, rather than bumping tubers into it, minimize drop height from one conveyor to another, use padding devices where possible to minimize buffeting of tubers, and try to keep harvester conveyors at capacity to minimize potentially damaging tuber movement. Lastly, harvest should happen when pulp temperatures are between 45 and 60 degrees Fahrenheit. This will eliminate temperature gradients, which can promote condensation, which in turn enhances the chance of storage rot development. Furthermore, it has been shown that tubers with a pulp temperature above 60 degrees are at an increased risk of developing pink rot and Pythium leak in storage.
In summary, minimizing bruising, avoiding harvesting during wet conditions and placing potatoes into a disinfected storage are three easy, cheap and effective control practices to reduce loss in storage from storage rot diseases.