Fusarium Dry Rot

Fusarium dry rot affects tubers in storage and seed tubers (whole or cut) after planting. The disease is caused by several species within the fungal genus Fusarium. On the tuber surface, external symptoms may first appear as non-descript flecks and expand to wrinkled lesions that may appear dark brown to black (Figure 1). Internally, hollow cavities with dry rotted tissue of various shades of brown to gray develop within tuber tissue, with tufts of fungal growth of white or various colors often visible within cavities and associated with lesions on the tuber surface (Figure 2). Disease can progress in storage until only mummified tubers remain. Cut seed can also develop symptoms prior to planting (Figure 3). Infected seed can completely decay after planting.

Biology

The pathogens that cause dry rot are ubiquitous and are present on tuber surfaces and in soil. Wounds are required for infection to occur; the pathogens cannot breach intact periderm, healed (suberized) wounds or cut surfaces, or natural openings like lenticels. The species most predominant in Idaho (F. sambucinum) grows and infects best at temperatures between 68-77 degrees Fahrenheit (20-25°C), but disease can still develop at the lowest temperatures used for potato storage. Common practices associated with potato harvest, handling and seed cutting induce wounds that provide the opportunity for infection. The pathogens can survive in soil for several years or from one season another in infected tubers or as inoculum on tuber surfaces. Inoculum can be moved any time tubers are handled.

Damage

• Lesions expand to hollow cavities, often with tufts of fungal growth visible within and on the outside of tubers
• Yield losses of 6-25% have been reported in storage
• Soft rot bacteria readily invade infected tubers in storage or that have been planted, especially if condensation occurs on tuber surfaces or relative humidity is high
• Infected seed pieces can lead to uneven emergence, poor stands and weakened plants

Management

Primary Management Tactics

Minimizing wounds and promoting wound healing (suberization) are the most important tools to manage Fusarium dry rot.

Cultural (changing the way the crop is grown to make it less suitable for the pest or to enhance its ability to withstand pest attack)

• Allow skin to fully set (mature) on tubers before harvest
• Adjust harvesting equipment to minimize impacts
• Promote wound healing after harvest, or after handling, transporting or cutting seed, by holding tubers at 50-55 degrees Fahrenheit (10-13°C) with good aeration and high relative humidity (at least 95%) for two-three weeks
• Warm seed to 50-55 degrees Fahrenheit (10-13°C) for one week prior to cutting and planting to promote sprout growth and wound healing. Such pre-conditioned seed tubers also bruise less readily during the cutting process, thus further reducing the risk of Fusarium dry rot and other types of seed decay.
• Plant in soils that promote rapid emergence, typically with temperatures at or above 45 degrees Fahrenheit (7°C) with soil moisture capacity at 60-80% (avoid planting seed in cold, excessively wet soils)

Chemical

• At harvest, fungicides are available for tubers going into storage, but be aware of restrictions that may exist in terms of maximum residue levels imposed by trade partners or for tubers destined for seed
• Treat seed (whether cut or kept whole) with a fungicide seed treatment applied directly to the seed with good coverage prior to planting, especially if soil conditions favor disease development or if the seed lot shows excessive bruising or Fusarium dry rot symptoms
• Thiabendazole and related fungicides (benzimidazoles) are not recommended in the Pacific Northwest since resistance to this group of fungicides is widespread in the predominant Fusarium dry rot pathogen (F. sambucinum) in the region
• Recommendations for pesticides to use in the management of Fusarium can be found on the PNW Pest Management Handbooks website 

Pesticide Warning

Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Trade Names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.

Kasia Duellman, Assistant Professor of Plant Pathology
2023