Getting Soft

Management and diagnosis soft rot diseases

Published online: Sep 06, 2019 Articles
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This article appears in the September 2019 issue of Potato Grower.

Dickeya and Pectobacterium are closely related bacteria that cause seed piece decay, blackleg, stem rot and wilt, and tuber soft rot. Blackleg results in stem death on sprouting tubers, poor emergence, stem rot during the growing season, and tuber soft rot late in the season and throughout the storage and marketing process. Early symptoms of infected seed tubers and on established plants from infected seed will start to show upper leaf curling and wilting.

Once infected seed tubers or seed pieces germinate, blackleg symptoms can occur at the base of the stem. Typically, only one stem from a tuber may show blackleg symptoms. The bacteria may decay the pith and cause the base of stem or the upper vine to be hollow and the vascular system will turn dark brown.

Pectobacterium and Dickeya cause similar symptoms, a fact that leads to significant challenges in detection, epidemiology and proper selection of varieties less susceptible to soft rot bacteria. Multiple Pectobacterium and Dickeya bacterial species are present in U.S. commercial and seed potato production. Only over recent growing seasons have significant sporadic losses been reported in some production. Beyond diligent sanitation measures, there are few management options available for these bacterial pathogens, and all commercial potato varieties are susceptible. Because Dickeya and Pectobacterium colonize the plant vasculature and no systemic antibacterial compounds are licensed for use in potato, these pathogens are well-protected from chemical management options once plants become infected.

Disease Management at Planting

It is important to use and plant certified seed and to the check health certificate tag. Whenever possible, use limited-generation seed. Seed should be stored at appropriate temperatures prior to planting (38 to 40 degree Fahrenheit) with storage airflow maintained (CO2 levels lower than 3,000 ppm). Seed lots should be kep separate and cutting and planting equipment sanitized between seed lots.

When using cut seed, allow seed to heal at least two days in storage with high humidity and proper air flow. Proper healing will reduce seed decay because bacterial pathogens are unable to penetrate wound barriers. Treat seed with a sanitizer while cutting, and warm seed prior to planting. Seed and soil should be approximately the same temperature (around 50 degrees). When loading trucks and planters, eliminate drops greater than a few inches to prevent bruising. If using particularly susceptible varieties (e.g., Lamoka), be diligent in proper disease management.

Disease Management During Growing Season

If possible, prepare the field to avoid excessive water. Over-watering can increase disease severity and may help in spreading the disease in the field because Dickeya and Pectobacterium thrive in water and low oxygen. Too much water, excessive rain or severe storms will spread Dickeya and Pectobacterium. Aerial stem rot and blackleg can occur during the growing season from mechanical and/or insect damage, high nitrogen levels or storm damage. Copper applications after severe rain or hail events may reduce disease severity by drying out existing infected plant tissue. In years when environmental conditions favor disease development and incidence is high, samples should be sent to state extension specialists or state and regional diagnostic clinics for confirmation and to rule out other more manageable diseases.


Disease Management at Harvest and in Storage

When harvesting potatoes, eliminate drops greater than a few inches to prevent bruising and wounds that favor soft rot. Harvest tubers when pulp temperatures are around 60 degrees. Avoid open foam rollers on equipment that can harbor and spread pathogens. Sanitize equipment between lots.

Proper healing (curing) is essential in the first few weeks of storage to reduce tuber soft rot as well as other diseases. Warm temperatures and low oxygen promote soft rot bacteria. Temperatures of 50 to 60 degrees should be maintained in storage to promote wound healing coupled; good airflow to promote adequate oxygen and low relative humidity will reduce disease spread and incidence. In the first few days in storage, cells below wounds suberize and form a corky layer that effectively stops infection by soft rot bacteria. Monitor storages daily for high-risk areas with elevated temperatures and/or moisture. Grade out rotten tubers prior to removal from storage bins and keep seed lots separate. If soft rot is diagnosed, determine if this is a primary or a secondary cause of infection. Tubers with other diseases or physiological disorders (ring rot, late blight, pink rot, leak, dry rot, black heart) are susceptible to soft rot bacteria.

Recently, an educational video series was developed in partnership with the Michigan Potato Industry Commission (MPIC), the Michigan Seed Potato Association, Michigan State University AgBio Research and MSU Extension. The purpose of this video series is to educate potato growers, stakeholders and the general public about bacterial diseases of potato and on how to better manage diseases of potato caused by soft rot bacteria. The first video is an overview of diseases caused by soft rot bacteria; the second focuses on disease management at planting; the third video is on management over the growing season; the fourth disease management at harvest; and the fifth on disease management in storage.

For more information on disease symptoms, the disease cycle, and post-harvest management strategies refer to the Michigan State University Extension bulletin E-3335 “Tuber Soft Rot, Blackleg and Aerial Stem Rot.”

 

Saltanat Mambetova is a research associate in Michigan State University’s Department of Plant, Soil and Microbial Sciences. Noah Rosenzweig is an assistant professor of plant pathology at Michigan State’s Department of Plant, Soil and Microbial Sciences.