MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Potato seed production in the north central United States has been impacted in recent years by unusual, but increasingly regular, meteorological conditions during the tuber development phase in the field. In seed production areas of Michigan (and other regions of north central United States), air and soil temperature during this phase have been elevated to daily means of greater than 24°C (75°F) and have persisted into September and October (Michigan Automated weather Network, MAWN, 2005 – 2007). It is likely that such elevated temperature exposures have significant effects on the physiological age and biochemical condition (sugar status in particular) of the seed tubers.

In Michigan, this has resulted in problems affecting the condition of seed the following season, including reduced suberization and healing of seed after cutting, reduction in plant stand and culminating in yield reduction and uneven tuber size distribution at harvest. The physiological age of potato seed-tubers is difficult to determine because of a lack of criteria for measurement.

Physiological age is comprised of three main attributes, the developmental, physiological and biochemical condition of a tuber at any particular point in time. Physiological age is therefore a function of the processes to which tubers have been exposed and that principally affect the meristematic regions of the tuber and the storage tissues. Seed-tuber physiological age influences emergence, stem number, growth rate, maturity, tuber size and ultimate yield of the crop and these process can be further influenced by cutting seed tubers into up to four pieces.

Sprouting characteristics are an indicator of the age of a seed-tuber and can be measured by various means, the most precise of which is achieved by summing the number of leaf initials and leaf primordia developing on sprouts. Physiological age advances with chronological time and apical dominance declines; this is manifested by development and growth of a single apical sprout on the tuber, followed by multiple sprout development with advancing age. Complex changes in meristematic development and tuber tissue sugar content can also occur during the aging process. Key seasonal periods that influence physiological age include the stage immediately before seed tubers are harvested in the summer then cooled for storage; dormancy from cooling (3°C; 37°F) until the end of storage when physiological age progression is minimal; then from the end of dormancy when tubers are removed from cold storage prior to warming when physiological age progresses again prior to cutting and planting.

At this time complex interactions between and among sprouts occur that involve hormones such as auxins. These interactions influence the breakdown in dominance of the apical sprout. This breakdown in dominance triggers meristematic development and growth in previously dormant sprouts. Prior to cutting, seed tubers resume aging following warming to about 10 to 15°C (49 to 59°F). Changes in physiological age that occurred prior to storage of seed tubers could result in different outcomes from the cutting process. We have observed that seed that has been exposed to thermally enhanced conditions the previous season have healing problems in that cut surfaces do not suberize and bacterial rotting ensues. Simple starch tests have indicated that there is a reduction in starch content at the cut surface. In addition, tubers cut once tend to suberize more efficiently despite reduced starch content. We hypothesize that this effect is the result of enhanced physiological age of seed tubers at harvest after exposure to thermal conditions experienced by the seed tubers during development and growth the previous season. Given these conditions the management of seed is further complicated by the presence of other diseases.

Seed-borne diseases of potato represent a signifi­cant constraint to potato production in the United States. Pathogens such as Phytophthora infestans (late blight) and Fusarium sambucinum (Fusarium dry rot) are major pathogens of potato, affecting tubers in storage and seed tubers and sprouts after planting. In severe outbreaks, the pathogens may kill developing sprouts outright, resulting in delayed or non-emergence. Reduction in crop vigor then results from expenditure of seed energy used to produce secondary or tertiary sprouts to compensate for damage to primary sprouts. The use of an effective seed treatment in combination with good manage­ment practices during cutting and seed storage prior to planting is essential to reducing late blight and Fusarium dry rot, as well as secondary bacterial soft rot in cut seed prior to planting.

The potato crop cycle offers two main opportunities to control seed-borne diseases such as tuber late blight and Fusarium dry rot. First is the posthar­vest control of seed piece decay in the tuber crop in the fall. Second is the control of seed piece decay and sprout infection prior to planting the crop in the spring. The pathogens causing tuber rots and seed piece decay generally all infect tubers through wounds produced during harvest and transport. Fungal pathogens such as P. infestans and F. sambucinum are usually the first pathogens to infect tubers. These are followed by the bacterial soft rots (Pectobacterium spp. also known as Erwinia soft rots). The first symptoms of Fusarium dry rot are usually dark depressions on the surface of the tuber. As lesions increase in size, the skin becomes wrinkled in concentric rings as the underlying dead tissue desiccates.

During recent growing seasons in Michigan, three factors have enhanced seed-borne disease problems: lack of information on effective fungicides for both postharvest and preplanting use against seed-borne disease causing pathogens (e.g., P. infestans and F. sambucinum); an increase in the area of potatoes grown by fewer growers leading to management issues such as timing of pre-cutting of seed; climatic factors such as increased frequency of rain events during planting. In combination, these factors can delay planting and increase the impact of seed-borne diseases during the early portion of the growing season, and subsequently may affect yield and quality of the crop. Current recommendations for seed cutting describe some guidelines for the cutting process, but do not indicate a time period or management strategy for storage of cut seed. Potato seed tubers are main­tained in storage at 37°F which is approximately the temperature at which F. sambucinum is dormant. Consequently, there is minimal development of dry rot in storage.

However, some level of Fusarium dry rot is almost always present in commercially available seed. During the preplanting phase of potato produc­tion, seed tubers are warmed to about 54°F, then cut into seed pieces prior to planting. Tubers infected with F. sambucinum are particularly susceptible to the development of seed piece decay during this phase, and in cases of severe disease, seed pieces may rot completely before planting.

Alternatively, after planting, over 50 percent of sprouts developing on infected tubers may become diseased and may be killed outright before emergence. Damage at this stage results in delayed or non-emergence and is usually expressed as poor and uneven stands with weakened plants. Reduction in crop vigor then results from expenditure of seed energy used to produce secondary or tertiary sprouts to compensate for damage to primary sprouts.

Studies at Michigan State University have shown that the effect of the timing of pre-cutting potato seed and timing of application of seed piece fungicides prior to planting on seed piece decay, plant establishment, subsequent vigor and early crop development is complex and can be affected not just by the presence of inoculum, but also by seed storage conditions after seed cutting and prior to planting. The most effective control of seed-borne fungal pathogens is achieved by the application of an effective seed treatment, such as fludioxinil (Maxim-based products), prior to planting. Thus, the use of an effective seed treatment in combination with good management practices during the cutting process and storage of cut seed prior to planting is essential to reducing Fusarium dry rot and secondary bacterial soft rot in cut seed prior to planting.

Treatment of infected seed pieces with Maxim MZ at 10, five or two days before planting signifi­cantly reduced the percentage of diseased sprouts per tuber and reduced seed piece decay in cultivars Pike and FL1879. Some level of Fusarium dry rot is almost always present in commercially available seed. Even though it is not possible at present to be 100 percent sure that a seed lot is completely free of dry rot, it is sensible to plant seed that meets established seed certification standards. Although it may not seem cost-effective to apply seed treatments to healthy seed, these results suggest that applying a seed treatment up to 10 days prior to planting can provide effective control of dry rot and increase rate of emer­gence, rate of canopy closure and final plant stand. In addition, broad-spectrum seed treatments containing mancozeb may suppress other seed borne diseases such as Rhizoctonia stem canker and black scurf, silver scurf, black dot, and early blight.

Management recommendations

The following practices to minimize seed piece decay and maximize early plant development and vigor have been compiled from recommendations devel­oped at MSU, Cornell, North Dakota State University, and the universities of Minnesota, Idaho, Maine, and Wisconsin.

Plant only certified seed

• Varietal purity and disease standards are regu­lated.
• Historical aspects of seed, such as generation source, year, grow-out tests and field observations (e.g., late blight), are recorded.
• Develop personal relationships between suppliers and customers.
• Assurance about growing and storage conditions, e.g., fungicide programs, storage treatments.
• Assurance about conditions after seed has been received.
• Home-saved, over- or undersized or generally non-certified seed will cause problems later in the season.

Cultural practices

• Arrange a mutually acceptable delivery time, taking into account seasonal temperatures at both locations.
• Do not use a storage facility where sprout inhibi­tors have been used unless it has been thoroughly cleaned.
• Clean and disinfect seed storage facilities. Ventilation system, plenums, ducts, etc.
• Brush down walls and floors.
• Wash walls and floors (detergent and high-pressure washer).
• Recover surfaces with disinfectant (QA, bleach, ClO2, H2O2) for at least 10 minutes.
• Steam clean (in excess of 150°F).
• Rinse and allow to dry (hot or cold water).
• Do not store seed near potential sources of inoculum (e.g., cull piles).
• Keep seed lots as separate as possible.
• On receipt, check certification documents.
• Check for signs of damage during transit (odors and liquefaction).
• After careful unloading, store seed at 40°F to 42°F and 85 to 90 percent relative humidity and keep it well ventilated.
• Prior to cutting seed, slowly raise the storage temperature to 50°F to 55°F.
• After cutting (and treating), seed should be piled no more than six feet high, stored at 50°F to 55°F and ventilated to promote wound healing (REI after seed treatment is normally 24 hours).

Seed cutting

• Clean and disinfect seed cutters regularly.
• Use water-impermeable seed cutters. Closed-cell sponge rollers are recommended.
• Keep the blades sharp and adjusted to deliver an average seed piece weight of about two ounces.
• Clean and disinfect cutting equipment, preferably each day and definitely between seed lots.
• Enforce sanitation practices for workers.

Determining of potential for dry rot and seed piece decay

Seed lot should be visibly free of tubers with symptoms of Fusarium dry rot.

• Federal regulations allow one percent dry rot at shipping; by planting, two percent level may develop (reasonable).
• Determine Fusarium inoculum on tubers visibly and also cut symptom-free tubers (about 50) in half, place in a large paper bag and shake them for about two minutes. Incubate at about 50°F in high humidity and examine for symptoms of seed piece decay after 10 to 14 days.
• If greater than two to 10 percent of seed pieces have symptoms of Fusarium seed piece decay, a seed treatment containing mancozeb, such as Maxim MZ or Moncoat MZ, should be used.
• If greater than 10 percent of pieces are showing symptoms, consider planting whole seed, removing clearly diseased seed tubers, applying a seed treatment or rejecting the seed lot.