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UC Pest Management Guidelines

White or pink mycelium develops on the surface of tubers with Fusarium dry rot.


Fusarium Dry Rot and Seed Piece Decay

Pathogen: Fusarium spp.

(Reviewed 8/07, updated 5/08)

In this Guideline:


Fusarium causes a dry rot of infected tubers, although a moist rot may occur if secondary infection by soft rot bacteria is also involved. Initially, lesions appear as brown to black flecks on the tuber surface. Lesions later form large, hollow cavities. Frequently, the lesions appear wrinkled on the tuber surface with numerous white tufts of mycelium. Infected seed pieces may completely decay.


Fusarium spp. are present in all soils and are found on the surface of all tubers. Wounds are required for infection. Fusarium cannot penetrate intact tuber skin, lenticels, or suberized (healed) seed pieces.


Proper handling and curing is usually sufficient to give economic control of dry rot in storage. Allow tubers to mature before harvest and prevent bruising tubers during harvest and storage operations. Wound healing reduces infection by Fusarium; to speed the healing process, hold tubers at 50° to 60°F with good ventilation and a relative humidity of at least 95% for the first 2 to 3 weeks of storage. Fresh market tubers should be stored at 38° to 40°F with 90 to 95% relative humidity and adequate ventilation.

Seed piece decay is reduced when seed pieces are planted under conditions that favor rapid suberization; Fusarium cannot infect cut surfaces after they are suberized. Warm the seed tubers to 50°F before cutting, and keep cutting and handling equipment disinfected. Plant when the soil temperature is at least 45°F and when soil moisture is 60 to 80% of field capacity. If possible, avoid irrigation before emergence. When planting conditions are likely to favor seed piece decay, treat cut seed pieces with a fungicide.

Common name Amount to Use R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

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  (Maxim Potato Seed Protectant) 0.5 lb/cwt cut seed pieces 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Phenylpyrrole (12)
  (MonCoat MZ) 0.75–1 lb/cwt cut seed pieces 24 0
  (Tops MZ) 1 lb/100 lb cut seed pieces 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)/Multi-site contact (M3)
  (Mertect 340-F) 0.42 fl oz/2,000 lb of tubers 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
  COMMENTS: Mist unwashed tubers entering storage with designated rate in sufficient water for complete coverage. Note: Do not treat seed potatoes with thiabendazole after cutting as wound healing may be retarded. Strains of Fusarium spp. resistant to thiabendazole have been identified in California.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode of action Group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
R. M. Davis, Plant Pathology, UC Davis
J. Nuñez, UC Cooperative Extension, Kern Co.
B. J. Aegerter, UC Cooperative Extension, San Joaquin Co.
Acknowledgment for contributions to the disease section:
C. Smart, Plant Pathology, UC Davis

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