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Expert advises on post-harvest treatment of common potato diseases

Despite best efforts at integrated disease management in the production field, the condition of tubers making their way to storage may require additional inputs to maintain quality in short and long term. 

In this article, Amanda J. Gevens, Extension Vegetable Pathologist and Associate Professor at UW-Madison addresses treatments of a few of the most common potato diseases that growers can apply at harvest or post-harvest during the storage of the harvested crop.

Water Molds 

Water molds are not true fungi, and 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. 

Internal symptoms of late blight infected tuber (click to enlarge)

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 (~25%) were resistant to mefenoxam.

Phosphorous acid

Applications of phosphorous acid (H3PO3) on tubers entering storage or applied to foliage (2-3 applications) can significantly limit late blight and/or pink rot.  Phosphorous acid treatment cannot reverse the effects of field-infected tubers, but it 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 approximately 30 and 60 days post treatment.

Phostrol at both rates also significantly limited late blight in a separate UW inoculated study.  Stadium (Syngenta; active ingredients fludioxonil, difenoconazole, azoxystrobin) has demonstrated excellent control of late blight when applied at bin-filling due to the azoxystrobin component. 

Hydrogen peroxide

Hydrogen peroxide studies carried out by the Idaho research team showed that application immediately following inoculation provided nearly 30% 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. 

Bacterial soft rot late season and post-harvest management

Bacterial soft rot symptoms
(click to enlarge)

•             Delay harvest up to 21 days post vine-kill to ensure complete skin set.

•             Avoid wet soil conditions at harvest to prevent soil from sticking to tuber skins.

•             Minimize cuts and bruises at harvest.

•             If soft rot is present in a portion of the field, this part of the field should not be harvested.  If infected tubers are stored, store them separately.

•             Harvesting equipment should be sanitized between lots.

•             Provide adequate ventilation to reduce conditions favorable to bacterial infection.  Check stored tubers regularly for temperature increase and odors.  Spot treat problem areas to minimize spread.

•             Reduce bacterial load on tubers as they enter storage and once in storage through use of post-harvest treatments such as chlorine dioxide, hydrogen dioxide, or ozone.

•             Dry potatoes before storage or shipping.

Pink rot infected tubers
(click to enlarge)

When to make fungicide applications to potato tubers post-harvest

The decision to make fungicide applications to potato tubers post-harvest is not trivial.  The addition of water to the pile, even in small volumes necessary for effectively carrying fungicides may create an environment favorable to disease under certain conditions (limited airflow, field heat interacting with cool storage condition). 

Typically, post-harvest fungicides are applied in ≤0.5 gal water/ton (2000 lb) of potatoes.  At this spray volume, an evenly emitted liquid will leave tubers appearing slightly dampened.  If tubers appear slick or shiny with wetness, the spray volume is likely greater than 0.5 gal/ton or the emitter may not be properly functioning. 

Under some circumstances, for instance when tubers come out of the field in excellent condition and field history includes little to no disease concern, additional tuber dampness may be unacceptable and seen as a bin risk that outweighs any fungicidal benefit. 

In other circumstances, tubers may come out looking rough or with harvest damage, and field history includes pink rot or late blight.  A scenario such as this may benefit from a post-harvest fungicide and resulting dampness should be mitigated by appropriate ventilation and temperature control. 

The 3 sides of the disease triangle

Inoculum, favorable disease conditions, and susceptible tubers provide the 3 sides of the disease triangle.  Avoiding or reducing inoculum on tubers as they enter storage will help considerably in preventing tuber infection in the bin.

Free water, high CO2, and warm storage temperatures will promote disease development in storage.  However, 2 of these 3 attributes are necessary in wound healing! 

The condition of the tubers themselves influences their susceptibility to infection by storage pathogens.

Key management factors

In summary, management tactics such as segregation of tubers from wet field areas, fungicide application in the field, harvest management (temperature, moisture, handling), post-harvest fungicides, and storage management, are crucial to successful storage of potatoes.

Author

Amanda J. Gevens, Extension Vegetable Pathologist, Associate Professor, UW-Madison, Dept. of Plant Pathology
Tel: 608-890-3072 (office)
Email:  gevens@wisc.edu
Vegetable Pathology website: https://wivegdis.wiscweb.wisc.edu

This article was first published in the February 2019 issue of the Global Potato News magazine.