American Vegetable Grower: Late blight protection

Guarding spuds from this deadly disease starts with the seed.

PHYTOPHTHORA infestans, the fungus that causes late blight disease of potatoes, over-winters in infected tubers and on perennial weed hosts such as bittersweet and hairy nightshade. These infected tubers and plants can serve as resources of inoculum for starting late blight epidemics in the spring. A key to successful management is controlling potential sources of the fungus before epidemics get established.

CHANGING FOCUS

Programs to control sources of inoculum for late blight epidemics have focused on the destruction of cull piles and management of volunteer potatoes. In contrast, little attention has been given to the seed tuber, except to suggest that buyers purchase certified seed.

Recently, the importance of seed pieces in the establishment of disease has become more widely known thanks to research conducted by Dave Lambert and others at the University of Maine. When blighted tubers are pre-cut and allowed to sit, the cut surface becomes covered with spores of the pathogen. These spores are dispersed during the handling and planting operation, and can contaminate healthy seed pieces.

SEED PIECE ROULETTE

When blighted seed pieces are planted, most will immediately rot and will not germinate. In contrast, when contaminated seed pieces are planted, some will germinate but the sprouts will be killed before emergence; some will emerge with symptoms of late blight; and some will escape the disease entirely. Thus, blighted emerging sprouts are the springboard from which the pathogen is spread to the surrounding, healthy foliage.

If healthy seed pieces become contaminated with spores before planting, application of a seed dressing is an obvious preventive approach to managing this phase of the disease. But our research has shown that treatment of seed pieces is effective only when one or more of the components of the fungicide product has efficacy against P. infestans and only when the product is used before seed piece contamination by the pathogen.

FEW FUNGICIDES AVAILABLE

Until now, there have been no registered seed piece treatments that specifically target the late blight pathogen. Gustafson, Inc. has recently issued a label for Tops-MZ (thiophanatemethyl+mancozeb), which includes a statement for control of seed piece contamination by P. infestans. Other fungicide companies, in concert with U.S. potato research and extension specialists, are currently evaluating additional potato seed piece treatment products.

Treatment of seed pieces with fungicides that have activity against P. infestans can contribute to an increase in plant emergence and stand uniformity, and a decrease in the spread of the pathogen from the seed piece to the foliage. For maximum benefit, however, the fungicide must be in place before coming into contact with the pathogen. In contrast, treatment of seed pieces with fungicides with little or no activity against P. infestans will result in an increase in seed piece decay, translating into a reduction in stand and plant vigor. In such instances, the treatments may increase the risk of transmission of P. infestans from the seed piece to the emerging sprout.

Control of the seed tuber phase of late blight should be a central component of an integrated management program for potato late blight.

Powelson is a plant pathologist with the Oregon State University Vegetable Pathology Program and Inglis is a plant pathologist with the Washington State University-Mt. Vernon Vegetable Pathology Program.