Kiran K. Shetty

Potatoes are stored successfully when the storage environment conditions are set to match the requirements of the crop and the purpose for which it is stored.  Storage disease problems arise due to several factors, namely: the crop condition, the harvest and handling conditions, the initial storage conditions, the holding conditions, and sometimes the external environmental conditions.  Very often corrective measures are taken after problems are identified.   With a careful understanding of the conditions that lead to problems and the necessary adjustments several potential problems can be avoided or reduced.

HARVEST CONDITION
One of the key factors that has a direct bearing on the storability of the potato crop is the physical condition of the potato brought into storage.  Ideally, potatoes are harvested when the following conditions are met: good skin set, availability of cool air during the night time (although this may not be a requirement if storages are refrigerated), appropriate soil moisture to move the harvester without clods, and pulp temperatures at or around 60°F.  In some cases the condition of the soils and pulp temperatures may be extremes, and therefore necessary adjustments in the initial storage settings may be required.  Some general guidelines to correct for the extreme conditions are as follows:

 For extremely warm and dry soil harvest condition
1. Run the fans and the humidifier continuously during filling the storage and for the first day or two.
2. Modulate the air entering the pile to no less than 5°F of the pulp temperatures.  Storages with air capacity over 17 cfm/ton can take advantage of the high air capacity using the outside cool night air, and decrease ventilation during the day if the warm temperatures persist.  Refrigerated storages can use the refrigeration to remove the field heat, however close monitoring of the pile temperature is required because gradual cool down is better than rapid cooling.
3. Once the pulp temperatures reach 50-55° F (50°F is more preferred if a condition of phythium leak or soft rot is suspected) the suberizing conditions can be set.

For extremely warm weather with wet soil harvest condition
1. Run fans continuously with the humidifier off until all free moisture on surface of the potatoes is removed.
2. Windrowing the potatoes under such conditions may be advantageous.
3. Modulate the incoming air to about 5°F less than the pulp temperatures.
4. Once pulp temperatures reach 50-55°F, regular suberizing conditions can be set.

Cool weather (50-60°F) with dry soil harvest condition
1. The potato pulp temperatures are already at or around curing temperatures.  In this case run fans intermittently, so that the pile temperature equilibrates and this will also help to provide the required oxygen for curing the potatoes.
2. If possible use the fresh air intake temperature closer to the existing pulp temperature.
3. If day time temperatures increase drastically, the fresh intake may be reduced or closed and internal air can be re-circulated to equilibrate the pile temperature.
4. Potatoes brought into the storage under these circumstances need 2 to 3 weeks to complete the wound healing at 50-55° F and 95 % relative humidity.  

Cool weather (50-60°F) with wet harvest condition
1.  It may be advisable to windrow potatoes that are being dug wet.  This will help to dry the surface of the tubers.
2.  Wet conditions on the surface of tubers will encourage diseases and also block air exchange through the lenticels.  If the pulp temperatures are near curing condition the fans need to operate continuously without any humidification.  This will dry the surface of the tubers.
3. The plenum air entering the pile should be slightly (1 to 2°F) lower than the pulp temperatures.
4.  Once the potatoes are dried, normal curing condition of 50 -55°F should resume with 95 % relative humidity.  These potatoes require a 2 to 3 week curing period because they came in cool.

Cold weather (40 to 50°F) with dry soil harvest condition
1. Under these conditions potatoes are very susceptible to bruises, therefore they have to be handled carefully.
2. There is no concern to remove the field heat in these potatoes, but instead the potatoes may need to be warmed up to 50-55°F.  To achieve this the fans can be run intermittently and this will help the potatoes to increase the temperature on their own accord due to the heat of respiration.  This heat may be required for a short time.
3. Humidification is required under this situation and can be timed along with the fans.
4. If this weather condition persists during filling the storage, continue the intermittent fan operation with humidity until the storage is filled and closed.  On the other hand if the weather warms up then the set points need to be adjusted accordingly.  It is advisable to pay extra attention to the potatoes brought in last.

Cold weather (40 to 50°F) with wet soil harvest condition
1. Windrowing the potatoes may be advantageous if there is no danger of frost.
2.  The first requirement as soon as the potatoes are brought into storage is to dry the surface of the potatoes and therefore continuous running of the fans may be required without any humidity added.
3. The cold wet condition may slow down the drying and therefore supplemental heat could be provided to warm the pile slightly.
4. When drying is complete, intermittent ventilation will provide the required oxygen and at the same time accumulate a little of the heat of respiration to warm the pile.
5. Once the potatoes reach curing temperatures of 50-55°F the potatoes can be cured for 2 to 3 weeks with 95 % relative humidity.

Ideal initial storage conditions and its implications
Ideally, potatoes are harvested when the pulp temperatures are around 60°F, and for this condition the initial storage temperature is set 5°F lower than the harvest pulp temperature.  A continuous supply of air, at a slow rate, will help equilibrate the pile temperature reducing the chances of temperature differential in different areas of the pile.  The relative humidity should be maintained at 90-95%.  Once the storage is filled to capacity the temperature of the pile is maintained at 50-55°F for two weeks to cure the potatoes.  This would include the time for bringing the pulp temperature to 50-55°F.  Once the curing is complete the pile temperature is cooled to the holding conditions. The holding temperature is determined by the end use of the potatoes.  Though this practice is the norm, there may be exceptions.  For example, if a bad infection of soft-rot or pythium is suspected the best curing temperature is below 50°F which means the actual set temperature is much lower when the storage is being filled.  In some cases initial storage temperature is set at 49-50°F during the entire filling period, running the fans only at the night time for 3 to 4 weeks. Subsequently, the pile  temperature is decreased gradually at the rate of about 1°F/week; or when return air gets within 2°F of set point, the set point is decreased by 1° F.  This procedure is continued until the potatoes are brought to holding temperature.  This practice is usually seen among fresh packers and has minimized the incidence of silver scurf disease on stored potatoes.  This colder curing condition may not apply for processing  potatoes because they are likely to synthesis sugars, and therefore affect the fry color.

In some situations the weather stays warm at harvest time.  If the weather is too warm during the day ( pulp temperatures at or above 80°F) the best time to harvest is in the morning hours or late evening hours.  It is important to note that if the warm weather persists the primary objective should be to remove the field heat first in storage.  These potatoes should be cooled as they are put in to storage using ventilating air at 50-55°F.  This temperature along with 90-95 % humidity is ideal for suberizing and wound healing for Russet Burbank potatoes.

At what temperatures should the potatoes be cured?
The recommended ideal temperature range for curing is 50-55°F.  The question whether to cure at 50 or at 55°F could depend on the inherent condition of the tubers, the end use of the potatoes, and giving consideration for potential diseases.  Russet Burbank potatoes, intended for processing, do best at 55°F curing temperature because there is less risk of sugar build-up.  However curing temperature of 50°F for Russet Burbank is not uncommon.  The growth and maturity of Russet Burbank potatoes determines weather there is a risk of sugar-build up if cured at 50°F.  If these potatoes are stressed or overmature then there is a risk of sugar build-up which will affect fry color.  This question is best judged with experience for a given area and for a given set of production practices.  If a threat of water rot problems are expected then a curing temperature of 45-50°F may be advisable.

Is there an advantage in rapidly cooling the potatoes?
It is generally believed that decreasing the initial storage temperatures quickly to near holding conditions may help in preventing storage disease problems.  However, this procedure is not without its risks for normal or over-mature potatoes.  In case of a normal crop, as discussed above, this question can be settled on the basis of the end-use of the crop. For fresh market potatoes, there may be an advantage in rapid cooling because it prevents or delays diseases such as silver scurf.  However, the disadvantage of rapid cooling is that there is a good chance the potatoes at the bottom of the pile may show pressure flattening and excessive shrinkage loss.  This is because as cool air enters the area of warm potatoes, the temperature of the air will increase thus reducing its vapor pressure.  So essentially, the air around the potato will have a defecit in vapor pressure compared to the internal water content of the potatoes.  This will force the internal water of the potato to move out to compensate for the defecit.  This moisture loss will affect the integrity of the internal cell structure of the potato.  In addition, the potatoes at the bottom of the pile are subjected to significant pressure due to the weight of the pile.  These conditions will increase the chances of pressure bruise and shrinkage loss of the stored potatoes.  The second disadvantage of rapid cooling is, if faced with a long spell of warm weather after the potatoes are cooled, the fresh intake will have to be closed for extended periods of time thus depriving the potatoes for oxygen, leading to build-up of carbon dioxide and trigger the build-up of sugars.  Over mature potatoes will be particularly sensitive to these situations. Over mature potatoes tend to slow down the wound healing process if temperatures are cooler.

COOL DOWN TO HOLDING CONDITION
Temperature of ventilating air is reduced at the rate of 0.5 ºF per day until holding conditions are reached. This is first done by measuring the return air temperature (although measuring the pulp temperatures at the top of the pile will be more accurate). If the return air temperature is within 2º F of the set temperature, it will be necessary to lower the set temperature at the rate mentioned above. The best time to measure the return air is during early morning hours because the pile would have gone through an extended period of cooling through the night. Ventilation should always be provided during cool down. Once the conditions inside the storage are stabilized, daily ventilation carried out should be long enough to maintain a 1 to 2º F differential between the bottom and the top of the pile. Increasingly, fans are being run in shorter cycles (at the rate of 2 to 4 hours per run and a break of at least 2 hours). The shorter cycles tends to reduce extreme pile temperature difference between the top and the bottom. The point to remenber is if  the fans are stopped for long periods, the pile tends to get warmer; therefore, it will require more time cool down. This recommendation is fairly new and therefore storage managers are advised to check the efficiency of the air system before making any changes.

PROCEDURES TO MANAGE SOME DISEASE PROBLEMS IN STORAGE
One of the toughest situation potato storage managers face is when they realize that they have a potato pile in storage seriously in danger of deteriorating due to diseases and disorders.  From a management standpoint anticipating such problems is half the job towards correcting the problem.  The information here is concerning anticipating storage diseases and the necessary steps to prevent or reduce the chances of the problem.

Pink Rot: Pink rot is a fungal disease caused by Phytopthora erythoseptica.
Disease characteristics:
1. This is a soil borne fungus.
2. When the plants and tubers are infected in the field the plants may show wilting symptoms and the leaves may appear chlorotic.
3. Potato tubers are infected through the eyes, lenticels and wounds and are usually infected in the field.
4. The affected external tissue on the tuber may show brown discoloration, especially around natural openings such as the lenticels and in the eyes.
5. Internally the infected area spreads almost in straight line across the potato tissue.
6. When affected tubers are cut the internal tissue is spongy and turns pink in 30 minutes.
7. Eventually the affected tissue can express a clear watery odorless fluid when squeezed.

Causes:
1. The infection of pink rot to the potato plant and the tubers is favored when subjected to water saturation in the fields.
2. Warm temperatures late in the growing season above 75°F will favor the disease.
3. Soft rot bacteria can eventually infect pink rot infected potatoes and deteriorate the tubers rapidly.  There is often a foul smell if this happens.

Remedies:
1. Avoid excess watering late in the growing season particularly if temperatures stay above 75°F.
2. Look for this disease in low lying water stagnated areas of the field especially around the pivot shaft.  If detected handle these potatoes separately after the rest of the field is harvested. Delay harvest of these potatoes and confirm the presence of pink rot. If confirmed disregard and avoid harvesting.  If harvested sort and discard.
3.  If potatoes in the field are affected in pockets and the choice is to harvest along with other healthy potatoes, pay extra attention in sorting and discarding the affected potatoes.  These potatoes should be placed last into storage closer to the access doors so that they can be removed first, or removed if and when they begin to deteriorate.
4. If the disease is detected after the potatoes are in storage provide adequate air flow through the pile.  The early curing condition should be 45-50°F for the the length of time it takes to dry the potatoes.  If only a small portion of the pile is affected the healthy potatoes should be cured at 50° F.  Subsequently, a rapid cooling to holding condition may be advantageous.  Continuous air flow is a must during this period.

Pythium Leak:  Pythium leak is a fungal disease caused by Pythium ultimum.
Disease chracteristics:
1. Pythium is a soil borne fungus.
2. The fungus normally infects through natural openings on the surface of the potato.
3. The internal affected areas can be clearly demarcated from healthy tissue by a dark boundary.
4. Rotted tissue is spongy and affected areas may deteriorate internally leaving the skin and cortical area intact.  This is often referred to as shell rot.
5. Cut tissue will turn white to gray to dark brown.
6. The disease can show within 2 to 3 weeks in storage.  The first sign of the problem areas appears as wet spots on the surface of the pile resulting from watery fluid from affected tubers.

Causes:
1. There is a high potential for this disease to occur under extremely wet conditions in the field followed by a short period of dryness during tuber maturation.
2. The disease is particularly troublesome when pulp temperatures exceed 70°F.
3. Pythium favors the infection of bacterial soft rot in storage.  Although pythium does not spread in storage, bacterial soft rot will.

Remedies:
1. Collect tubers from suspected areas in the field and place them in plastic bag under warm (room temperature) conditions. Affected potatoes will decay rapidly.  If confirmed avoid harvesting these potatoes.  If harvested, sort and discard.
2. Generally avoid harvesting potatoes under extreme warm condition.
3. Avoid mechanical injury to the potatoes during harvest.
4. If significant amount of potatoes are affected it is advisable to cure between 45°F to 50°F for minimum of 3 weeks.
5. If the disease persist consider rapidly cooling the potatoes to 40-45°F. Provide continuous forced air until the affected areas are dry.  Reducing the humidity during this process will facilitate drying.
6. Consider marketing early and place these potatoes closer to the access doors for easy removal.

Dry Rot: Dry rot is a fungal disease caused by Fusarium sambucinum.
Disease characteristics
1.This fungus can be both seed and soil borne.
2.The fungus enters tubers through wounds and bruises inflicted during harvest and handling operations.
3.Generally this disease can be detected under a bruised area in a tuber. Infected tuber areas internally are black and white with a crumbly decay.  The spread inside the tuber is irregular but there are distinct walled-off areas between the healthy tissue and the affected tissue.
4. The external surface of the affected areas can be sunken and wrinkled.
5. Occasionally white or pink fungal growth may be seen outside.
6. Secondary bacterial soft rot may eventually take over the dry-rot problem.

Causes
1. Temperature above 50°F generally favors the fungus.
2. This disease is usually seen if growing conditions are dry.
3. This disease can spread quickly if potatoes are improperly cured during the first 2 to 3 weeks.
4. Generally, this disease is a problem if potatoes are piled along with too much field soil.
5. Bruising of potatoes during harvest and handling will encourage infection of this fungus.

Remedies
1. Minimize bruising during harvest and handling.
2. Avoid harvesting potatoes when pulp temperatures are cold because cold potatoes are highly susceptible to bruising.
3. Assure proper skin set and maturity of the potatoes before harvest.
4. Remove excess dirt and clods during sorting and piling operation.
5. Post harvest treatments are recommended
6. A curing environment of 55°F with 95 % relative humidity encourages wound healing.  Wound healing is complete in 2 to 3 weeks.
7. When curing is complete gradually reduce the temperature at the rate of 0.5°F/day until holding conditions are reached.

Storage bacterial soft rot: Soft rot in storage is caused by the bacteria Erwinia caratovora
Disease characteristics
1. The soft rot infection that occurs in the field via infected stolon of the mother plant is designated as black leg.  There may be some vascular discoloration in the stem end and sometimes sunken black tissue extending deep into the tuber flesh.
2. In storage bacterial soft rot organism is opportunistic and can cause severe problems in association with other diseases.
3. Infection can enter through lenticels and wounds and external infected areas can be tan to dark brown with water soaked texture to the skin.
4. Internal soft rot tissue is wet, mushy or creamy associated with white to grayish-brown ooze.  The affected areas will show a black border separating it from the healthy tissue.
5. During the early stages of soft rot the decay is odorless, but eventually a foul odor will develop.  When large areas of the pile are affected there may be a characteristic ammonia like smell in the storage.  If the decay dries out the affected areas will be chalky white.
6. The decaying substance from affected potatoes can initiate soft rot in surrounding tubers by blocking respiratory passages in healthy potatoes.
7. A potato pile that is severely affected by this disease may show one or more of the following characteristics:
 a. may begin to sink
 b. a thick dark liquid may accumulate below in the ducts.
 c. could generate heat. Normally this heat can be detected on top of the affected  area of the pile. An infra-red heat detecting instrument can differentiate healthy  from the affected areas.
 d. Generally a severely affected pile will emanate a foul odor.

Causes:
1. Extreme wet conditions during growth and harvest.
2. Excessive weeds in the fields.  Weeds tend to harbor this organism.
3. Infected seed will increase the chances of infection in progeny tubers.
4. Harvesting immature tubers .
5. Harvesting when temperatures are above 70°F.
6. Excessive air leaks into the storage along with dysfunctional louvers.
7. Free moisture due to condensation or improper air distribution through the pile.
8. Adding excessive soil with the tubers into storage and failure to remove vines and clods.  These things will lead to pile compaction, impede air movement and cut-off oxygen supply.
9. Disease such as water rots and dry rots will facilitate soft rot infection.
10. Excessive bruising and improper wound healing will invite soft rot infection.

Remedies:
There is little information to support the use of bactericides or disinfectants through humidification to directly control bacterial soft rot in storage.  From a storage management stand point the following may be considered:
1. Storage floor soil from previous season severely affected with bacterial soft rot should be removed and these areas should be thoroughly cleaned.
2.  Encourage a healthy crop with good certified seed, appropriate watering and nutrition, good weed, insect and disease control during production.
3. Encourage good skin set and maturity before harvest.
4. Harvest with proper care and prevent bruising.
5. Good sorting procedures to remove vines, clods and soil during piling operation.
6. If only some loads are suspected with soft rot infection place these potatoes closer to the access doors so that they can be removed quickly if they begin to deteriorate.
7. If a high percentage of the disease is noticed during loading the storage or during early storage phase use little or no humidity with continuous air flow.
8. Proper curing of healthy potatoes for 2 to 3 week at 50-55°F and 95 % humidity.
9. If the disease is seen after the curing phase, the cool down to holding conditions should be rapid with a lot of air movement through the pile.
10. Prevent condensation in and on the potato pile.  Ventilation systems are available that can provide a continuous but low speed supply of air for better temperature equilibration within the pile. This prevents free moisture formation within the pile and also provides oxygen.
11. Use supplemental air in severely affected areas of the pile.