Potato Seed Piece Health Management

Reducing late blight and dry rot

Published in the April 2009 Issue Published online: Apr 30, 2009 Fungicide, Herbicide, Insecticide P.S. Wharton, U of I Potato Plant Pathologist

Seed-borne diseases of potatoes represent a significant constraint to potato production. 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. The first is the post-harvest control of seed piece decay in the tuber crop in the fall. The 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 transportation. 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.).

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. Seed tubers are maintained in storage at 37°, 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 pre-planting phase of potato production, seed tubers are warmed to about 54 degrees, 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.

Studies have shown that the effect of the timing of precutting 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 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 at 10, 5 or 2 days before planting significantly reduced the percentage of diseased sprouts per tuber and significantly 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 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.

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