RHIZOCTONIA DISEASE OF POTATO Marc A. Cubeta, Extension Plant Pathologist Bryan R. Cody, Agricultural Research Technician Paulo C. Ceresini, Graduate Research Assistant

Vegetable Disease Information Note No. 26
Department of Plant Pathology
North Carolina State University

INTRODUCTION

Rhizoctonia disease of potato occurs each year in North Carolina, but usually is not recognized by growers because disease symptoms develop underground. Consequently, yield losses associated with Rhizoctonia disease usually do not become evident until after harvesting. Although yield losses associated with this disease are not well documented in North Carolina, losses of 10 to 15% are observed frequently in potato production areas outside of North Carolina.

DISEASE CYCLE AND SYMPTOMS

Rhizoctonia disease of potato is caused by the soil fungus Rhizoctonia solani and can be found on all underground parts of the plant at different times during the growing season. The fungus survives from season to season primarily as black structures called sclerotia which are present in soil and on potato seed tubers (Figure 1, left). Sclerotia that can be seen on potato seed tubers after washing with water are often referred to as the "dirt that won't wash off".

The disease begins when sclerotia present in soil and/or on planted potato seed tubers germinate to produce vegetative threads (hyphae). The hyphae attack developing sprouts when the soil is cool (50-59 °F) and moist. Symptoms on developing sprouts appear as reddish-brown, discolored areas, and the growing point of severely infected sprouts is often killed. As developing sprouts are killed, additional sprouts will form until a single sprout emerges from the soil. The repeated development of new sprouts to replace killed sprouts can deplete the potato seed tuber of important food reserves that are needed for future growth. Also, this "sprout nipping" results in delayed emergence and plant maturity, and poor and uneven plant stands. As the season progresses and the potato plant continues to grow, additional reddish-brown sunken areas called cankers may form on stems (Figure 2, upper left), stolons, and roots (Figure 3, bottom left). Cankers can reduce plant productivity and both the quality and quantity of newly formed daughter tubers by restricting the movement of water and nutrients within the plant. In many instances, infected plants with cankers appear healthy. However, aboveground symptoms may appear, such as the formation of aerial potato tubers and purplish-colored leaves, particularly when plants are severely infected and/or stressed (Figure 4, upper right). Late in the growing season during periods of cool, wet weather, a white fuzzy growth of the fungus (which may or may not include spores) can often be observed on the base of potato stems near the soil surface (Figure 5, bottom right). These spores are usually not important in spreading the disease.

As potato plants begin to reach maturity and vines begin to die naturally or are killed by chemical desiccants, the fungus can infest daughter tubers to produce black sclerotia. This phase of the Rhizoctonia disease is referred to as the "black scurf" stage. Black scurf is very difficult to manage and can reduce potato yields by causing cracked, irregular shaped tubers with decreased specific gravity.

MANAGEMENT

The management of Rhizoctonia disease requires an integrated approach and knowledge of each stage of the disease. One of the most important initial management decisions that should be considered by North Carolina potato growers is to purchase and plant only high quality seed potatoes that are not infested with black sclerotia of the Rhizoctonia fungus. If black sclerotia are present, they should cover no more than 5% of the seed potato surface (Figure 6, lower right, courtesy of Clive James, Agriculture Canada, "A manual of assessment keys for plant diseases", 1973). A fungicide seed treatment may provide some relief from the sprout nipping stage, particularly when potato seedpieces are contaminated with the fungus. However, a fungicide seed treatment will usually not be beneficial if the soil is infested with high levels of the Rhizoctonia fungus (see Ag Chem Manual).

Cultural conditions that promote rapid sprout emergence, such as warming potato seed tubers (55-60 °F) prior to planting and planting seed shallow (2 to 3" deep) and in soil above 55 °F are recommended. Although the Rhizoctonia fungus primarily attacks potato, this organism can persist in soil for several years. Therefore, growers are encouraged to plant in fields without a history of severe Rhizoctonia disease and practice a 2- to 3-year rotation without potato. Some potato varieties differ in their susceptibility to Rhizoctonia; however, no resistant varieties are currently available.

This page (http://www.ces.ncsu.edu/plymouth/pubs/scurf.html) was created by Marc A. Cubeta, Ph.D. Extension Plant Pathologist and Bruce A. Emmons, Area Specialized Agent, Information Management. The authors would like to acknowledge Vera MacConnell for technical assistance; Drs. David Shew and Tom Melton, Department of Plant Pathology, North Carolina State University, Raleigh, NC and Dr. Don Carling, University of Alaska, Palmer, AK for reviewing this publication and their many valuable comments and suggestions.

Date Created 01/14/98.
Last revised on 08/09/01.