The successful storage of potatoes for processing or table stock requires the management of a healthy crop from planting to harvest and appropriate conditioning beyond. While environmental conditions can challenge the health of the crop by promoting the development of diseased or periderm-compromised tubers, management approached post-harvest can aid in limiting the advancement of disease and yield losses in storage.
Some of the most common and problematic potato tuber diseases in storages include diseases caused by fungi such as silver scurf and Fusarium dry rot, and diseases caused by fungus-like or "water mold" pathogens such as pink rot, late blight and Pythium leak. Bacterial soft rot should not go without mention; however, the focus of this article is on the management of diseases caused by fungi or fungus-like primary pathogens. Each disease is promoted by slightly different environmental conditions and each has key diagnostic features on tubers.
The Usual Suspects
Silver scurf, caused by Helminthosporium solani, produces symptoms of small, pale gray to brown spots that can darken in color and size as disease progresses. Lesions have definite margins and are typically circular, but they can coalesce as disease progresses. Infected tubers take on a silver appearance when lesions mature as a result of air pockets in dead periderm cells that may slough off. Infection never goes beyond periderm, and desiccation often accompanies severely infected tubers due to compromised periderm. Silver scurf is favored by more than 90 percent relative humidity and temperatures over 40 degrees F. Both good crop rotation and avoiding delay of harvest once skin is set can aid in management.
Fusarium dry rot is typically caused by Fusarium sambucinum. Symptoms begin as dark depressions on tuber surfaces in association with wounds. As lesions progress, the periderm shrivels in concentric rings as tissue desiccates. Internally, effected tissues are very dry and can vary in color from light brown to black. Severely effected tubers are all but hollowed out with fungal mycelia and spores lining the empty cavity. Fusarium dry rot is favored by more than 90 percent relative humidity and temperatures between 59 and 68 degrees F. Avoiding injuries to harvested and stored tubers by ensuring good skin set and harvest temperatures above 40 degrees F, as well as good airflow and high humidity in storage to promote healing, greatly aids in management. Selection of potato varieties with tolerance to Fusarium dry rot can aid in an overall management plan.
Phytophthora erythroseptica, the fungal-like pathogen causing pink rot, often causes tuber symptoms initiating from the stolon end that appear rubbery, yet firm. The infected areas of tubers are often delimited by a dark line visible through the skin. Buds, lenticels and underlying tissue are black and usually exude a clear liquid. When cut and exposed to air, pink rot infections turn pink-salmon in color after about 30 minutes. Pink rot is favored by high soil moisture, which promotes open lenticels and temperatures around 77 degrees F. Planting in well-drained fields with no history of pink rot, avoiding wounding at harvest and bin filling and lowering temperature and humidity in storage can aid in management.
Pythium leak, caused by the fungal-like pathogen Pythium ultimum, produces tuber symptoms that begin as light tan, water-soaked areas around a wound. As disease progresses, tissues can swell and periderm discolors with a dark line separating diseased tissue from healthy. Internally, the tissue is spongy and wet and may contain cavities. When squeezed, infected tubers exude a watery liquid. Over time, affected tubers in storage appear as empty, papery skins. This disease is favored by high temperatures, 77-86 degrees F. Avoiding harvesting in hot, dry weather and enhancing post-harvest conditions to promote wound healing can help manage Pythium leak.
Late blight, caused by another fungal-like pathogen Phytophthora infestans, produces tuber symptoms that can be both superficial and visible externally as dark brown to purple lesions and present in the interior as brown, dry and granular lesions. This disease is favored by temperatures from 64-75 degrees F and high relative humidity. Use of protectant fungicides in the production field while foliage is still viable, and use of fungicides with some systemic activity can aid in management.
While nature can be uncooperative in helping to limit post-harvest disease, there are chemical tools available to mitigate infection and spread.
True fungi: Fusarium dry rot is a challenge to control in storage and there are limited chemical tools available. For many years, thiabendazole was used to aid in storage management of Fusarium dry rot, but current populations of the pathogen have developed resistance, limiting its efficacy. While the mechanism of activity is unknown, the successful management of silver scurf with phosphorous acid compounds (such as Phostrol) has been demonstrated in university storage trials at the University of Idaho.
In a preliminary study at the University of Wisconsin (UW), Phostrol numerically reduced silver scurf by 5 percent, but the disease reduction was not statistically significant. Ozone was also evaluated in our UW study with no significant difference in silver scurf disease control when compared to untreated controls. We are continuing our evaluations of Phostrol and ozone treatments in Wisconsin. Azoxystrobin can also control silver scurf in storage; however, it is not registered for such use at this time.
"Water Molds": Because the water molds are not true fungi, only certain fungicides are effective in controlling pink rot, leak and late blight. In the past, single-site mode of action metalaxyl or mefenoxam fungicides (such as Ridomil) were very effective at limiting water molds. Metalaxyl and mefenoxam resistance in P. erythroseptica and infestans has been documented in various regions of the U.S. Recently, a fungicide resistance evaluation of P. erythroseptica isolated from tubers in storage in Wisconsin indicated that a portion of the isolates collected (about 25 percent) were resistant to mefenoxam.
Studies at the UI and North Dakota State University indicated that applications of phosphorous acid (H3PO3) on tubers entering storage or applied to foliage (two to three applications) can significantly limit late blight and/or pink rot. Phosphorous acid treatment cannot reverse the effects of field-infected tubers, but can limit the spread of disease during handling and storage. Field-applied phosphorous acid application has also been shown to provide residual control of pink rot to approximately 90 days after harvest.
In our UW inoculated storage trial, phosphorous acid (Phostrol) applied at bin-filling at rates of both 6.4 and 12.8 fl oz/ton significantly limited pink rot incidence and severity at about 30 and 60 days post treatment. Phostrol at both rates also significantly limited late blight in a separate UW inoculated study. Several newer active ingredients currently in the developmental pipeline toward registration have demonstrated excellent control of late blight when applied at bin-filling.
Hydrogen peroxide studies carried out by the Idaho research team showed application immediately following inoculation provided nearly 30 percent disease control when compared to untreated controls. However, when tubers were infected in the field and were treated post harvest, hydrogen peroxide did not provide adequate disease control. Our UW hydrogen peroxide study on pink rot control resulted in disease incidence and severity results that were not significantly different from our untreated control.
There are late-season and post-harvest treatments that are effective in minimizing infection and spread of potato storage diseases. However, they must be used in a multi-faceted management program including site selection, crop rotation, seed selection, irrigation management, preventative fungicide applications in production and good practices for harvest and storage with respect to temperature and relative humidity.