Open Wounds

This article appears in the April 2017 issue of Potato Grower.

Great care is taken to minimize wounding, bruising, and abrasions at harvest, in order to limit entry of the pathogens that cause Fusarium dry rot and other diseases. When seed is cut prior to planting, the greatest wounding just occurred to the potato tuber: a cut with a large surface area. Fortunately, potatoes have the ability to heal these wounds by creating their own Band-Aid: wound periderm. The wound periderm is the outermost layer of tissue on a potato tuber that forms after a wound is inflicted on the surface of the tuber.

The process of wound healing will depend upon several factors, including the type of wound, variety and wound healing conditions. After seed is cut, the objective is to protect the wound to stop any invasion by Fusarium dry rot or bacterial soft rot pathogens. This can be accomplished by winning the race between the wound healing process and the pathogen, and/or protecting the surface with a seed treatment.

The rate of wound healing from the cut surface is a function of variety and environmental conditions. Warmer temperatures will accelerate the wound healing process. General recommendations are to increase seed temperatures to at least 45 degrees Fahrenheit prior to handling to avoid bruising and shattering upon cutting. Wounds will heal much slower at temperatures of 45 degrees or lower than at 50 or 55 degrees. Therefore, it is best to plant into soil with favorable wound healing temperatures and conditions. Planting cold seed into warm, wet soil can produce conditions on the surface of the cut seed favorable for condensation and increase the risk of seed decay.

Varieties can respond differently to the wound healing process; some are faster and some are slower than the oft-grown Russet Burbank. There is also a wide range in susceptibility to Fusarium sambucinum dry rot decay between varieties, as seen in the table below. F. sambucinum is considered the pathogen that most commonly causes dry rot development in stored potatoes. Identifying and growing varieties that are less susceptible to dry rot development may provide a management strategy to begin the storage season. Unfortunately, you may not always have the market option to select less susceptible varieties. However, having awareness of the potential for dry rot development will help modify management strategies to lessen the impact of dry rot.

Fusarium seed piece decay can be caused by both Fusarium coeruleum and F. sambucinum, with F. coeruleum being more aggressive—especially after planting, thus reducing stand. Seed treatments containing mancozeb are highly effective against both Fusarium seed piece decay fungi. Additionally, seed treatments containing fludioxonil (e.g., Maxim MZ, Maxim 4FS, Spirato, CruiserMaxx Potato formulations), difenoconazole (CruiserMaxx Potato Extreme) or prothioconazole (Emesto Silver) are also effective against Fusarium dry rot pathogens.

Liquid seed treatments can provide more effective coverage of the cut surface if applied properly. Treating seed with effective fungicides is even more critical when seed is pre-cut and stored prior to planting. This practice gives the dry rot fungi a head start on decay if not properly carried out.

Many seed treatments mentioned above are also effective against Rhizoctonia canker and black scurf. Combining liquid seed treatments with a mancozeb dust is highly effective for Fusarium seed piece decay management. However, the use of dust is problematic from a worker safety standpoint.

The level of Fusarium seed piece decay in a seed lot prior to cutting can be predictive of the amount of Fusarium dry rot that a grower may experience in a field. Selecting seed lots with lower levels of Fusarium dry rot in storage, encouraging conditions favorable for wound healing, and treating seed with an effective fungicide will help ensure Fusarium seed piece decay does not develop in the field, which will in turn  reduce the threat of dry rot in storage.

Russet variety susceptibility to Fusarium dry rot (F. sambucinum) development in storage.