Potato Harvest and Storage

Bacterial soft rot, Fusarium dry rot, pink rot, and Pythium leak are four seri­ous tuber rotting pathogens that cause the most significant losses in storage. These diseases can be brought in on infected tubers or survive on storage debris. Many of them take only a few weeks to destroy a tuber and then spread through the storage pile. In dry conditions, we might see lower than normal levels of soft rot and pink rot. However, early maturity, high temperatures and dry conditions create other issues. Growers may be aiming for short term or long term storage and sales, or some of each, and attention to the harvest, curing, and handling issues can help maintain quality.

Two of the main management practices that will reduce losses to these diseases after harvest are allowing tuber skins to mature in the field before harvesting, and eliminating free moisture in storage areas. However, Rhizoctonia black scurf and silver scurf may be at high levels on the tubers and will increase in severity the longer the tubers remain in the soil. Therefore to avoid these diseases, as soon as skins are set, harvest should begin. If the weather remains wet during the harvest, soil may adhere to the tubers during harvest. This soil will promote conditions for soft rot.

Potatoes should be harvested at pulp temperatures that allow for successful storage. Allowable pulp temperatures will vary based on storage ventilation systems, varieties, availability of cooling air, and timeliness. If potatoes are harvested during hot weather (above 80F) and cool off slowly the likelihood of storage rot is increased. Potatoes destined for storages with refrigeration could be harvested warmer to a maximum of 62 to 65°F pulp temperature. Storages with no refrigeration should not be loaded with potatoes with a pulp temperature above 60°F. Potatoes newly loaded into storage will require fresh air, humidity, and temperature goals near 55°F during preconditioning. If pulp temperatures are higher than recom­mended it is more difficult to manage critical environmental conditions once in storage. Time your harvest when cooling air is available to promote open outside doors and 3 to 6 hours of fresh air per day. Questionable potato lots should be harvested closer to 55° F if they must be stored. For later harvests, avoid harvesting at temperatures lower than 45 degrees as this increases the occurrence of bruising.

Below is a list of guidelines that can be used during harvesting and storage to help prevent the spread of the diseases mentioned above and to maintain high quality potatoes:

Vine killing

• Vine kill stops tuber growth at the desired maturity, stabilizes the tuber solids, and promotes skin set.
• Mechanical or chemical methods or a combination of the two can be used to kill potato vines.
• More than one application of a chemical desiccant may be required.
• Vine killing permits easier digging and harvesting operations.

Disease management

• Foliar diseases, especially late blight, are still a threat as vines begin to die or vine killing methods are implemented. These pathogens can spread to tubers and cause problems in storage if they are not controlled prior to harvest.
• Application of a desiccant followed by a fungicide application a few days later is recommended instead of apply­ing the desiccant and fungicide at the same time. This way thorough coverage of the remaining plant material can be achieved.

Skin set

• Most tuber diseases require a wound to get into the potato. Good skin set greatly reduces the amount of wounding at harvest and increases the storage ability of the tuber.

• Allow for skin set on the tubers in the field for at least 10-14 days before harvesting.

Wounding and bruising prevention

• Check harvesting and transporting equipment to make sure it is working properly and that it causes minimal damage to tubers.
• Harvest when the soil is moist but not too wet. Tuber pulp temperatures are around 60-65°F will make the potatoes less susceptible to bruising and wounds.

Grading

• Grade out diseased tubers as quickly as possible. The longer they are mixed with healthy tubers, the higher the chance of disease spread.

Healing period

• The ‘curing’, ‘suberization’ or ‘wound healing’ period immediately after harvest is critical to successful storage.
• Store tubers at about 50-60°F at high relative humidity (95%) for 10-14 days to allow wounds to heal before placing potatoes into colder storage. Lower RH results in poor suberization.
• Airflow over and through the pile is important to supply oxygen and prevent condensation. However, do not overdry the potatoes during curing.

Storage

• Before storing potatoes, facilities should be cleaned thoroughly and inspected. Make sure to check the insulation, fans, humidifiers, and ventilation system. If any of these are in poor condition it could result in losses due to disease.

• After the curing period, cool potatoes gradually and steadily to the holding temperature suited to your goals: 38-40 F for tablestock, and seedpotatoes; 45-50F for chipping or 50-55 F French fry stock.

Diagnostics

• Don’t just guess, and don’t assume that every tuber rot that you see is late blight. Send samples to the Plant Disease Diagnostic lab to get an accurate diagnosis. Different tuber blights need different management, and even knowing what you need to do next year to prevent the problem is vitally important. Phone for UMass Diagnostics Lab: 413-545-3209.
• A good online resource on tuber diseases can be found at http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_Detection.htm
• However, finding a photo online that looks like your problem is not the same as having a plant pathologist confirm what is on YOUR tubers!

-- R Hazzard and C Cavanagh; compiled from the following sources: Vegetable Crop Update, U of WI, 8/13/10, edited by C. MacNeil, CCE, CVP for Veg Edge, Cornell CES; New England Vegetable Management Guide; Potato Production in the Northeast: A Guide to Integrated Pest Management;