Late Blights Seed Handling

Seed-Borne Late Blight

(prepared with Dr. Gary Franc, Univ. Wyoming)

Asexual forms of the late blight fungus (Phytophthora infestans) overwinter only within infected, living plant tissue. For example, surveys conducted in Ireland over a period of 50 years revealed that most late blight outbreaks could be attributed to inoculum that spread from infected seed, volunteer potatoes and cull piles. The most important sources of late blight in a production region are probably cull piles and seed tubers. These tubers may appear healthy, yet carry the late blight fungus that eventually spreads to your crop. Volunteer potatoes play a disease-development role in the irrigated High Plains, as well, depending on their ability to survive the winter. Ultimately, whether late blight spreads from cull piles, seed tubers or volunteers, it is the infected tuber that permits the fungus to survive between growing seasons.

Survival of the late blight fungus in tubers was proposed by Berkely in 1846, so this is not a new idea. De Bary established in 1876 that sprouts produced by infected tubers may be invaded by P. infestans and that these sprouts can survive long enough to reach the soil surface and produce sporangia. Thus, these infector sprouts serve as infection centers from which late blight spreads. In attempts to repeat De Bary’s work, scientists rarely observed infector sprouts because they found that infected tubers usually decayed before plants could emerge. Van Der Zaag determined that only about 1 percent of the infected seed tubers actually give rise to infected plants. However, he also determined that only one infected plant per square kilometer (245 acres) is needed to initiate annual late blight epidemics in the Netherlands!

Recently, potato scientists started to rethink how late blight spreads among seed tubers. Since the fungus produces spores on the surface of infected tubers, can these be spread to surrounding healthy tubers in the seedlot during normal cutting and handling practices? Research in Maine, North Dakota, Washington, Wyoming and other areas revealed that U.S. isolates of late blight readily spread from infected to healthy seed tubers during handling and cutting. Therefore, perhaps a major source of inoculum is not simply the seed tuber with late blight decay, since as much as 99% of these decay before emergence, but also includes otherwise healthy seed tubers that become infected shortly before planting.

Chemical Control -- Fungicide seed-piece treatments, such as thiophanate-methyl, mancozeb and cymoxanil, were most effective at restoring the ability of inoculated seed to establish plants. With weekly applications of a protectant fungicide, such as chlorothalonil-Zn or mancozeb, no late blight stem or foliar lesions resulted from the seedborne inoculum. Yield effects were a function of treatment effects on stand. If stand was increased then yields were also increased. Phytotoxicity from any treatment would usually detected. It is important to realize that you cannot cure an infected seedlot by applying seedpiece fungicide treatments and also that seedpiece treatment will not protect the foliage.

Although most growers agree that increased emergence is a desirable benefit of seedpiece treatment, don’t forget that plants that emerge from late blight-infected seedpieces may ultimately serve as a source of inoculum from which late blight spreads. Therefore, is it really desirable to increase emergence by applying fungicide to seed, or should every late blight-carrying seedpiece rot in the ground before it produces a plant? Although the answer is not immediately obvious, if you always start with a seedlot that has no known prior exposure to late blight, application of fungicide will provide an added layer of protection that prevents the inadvertent spread of late blight inoculum in your seed. Labeled seed-piece fungicide treatments should reduce the risk of planting infected seed. Seed treatment fungicides are an important component of an integrated late blight management program.

Sources of Information

  • Lambert, D.H., A.I. Currier, and M.O. Olanya. 1998. Transmission of Phytophthora infestans in cut potato seed. Amer J of Potato Res 75:257-263.
  • Boyd, A.E.W. 1980. Development of potato blight (Phytophthora infestans) after planting infected seed tubers. Ann. Appl. Biol. 95:301-309.
  • De Bary, A. 1876. Researches into the nature of the potato fungus Phytophthora infestans. J. Royal Agr. Soc., England, Series 2, 12:239-269.
  • Dowley, L.J., and E. O’Sullivan. 1991. Sporulation of Phytophthora infestans (Mont.) de Bary on the surface of diseased tubers and tuber to tuber spread during handling. Potato Research 34:295-296.
  • McKay, R. 1957. A retrospect of fifty years outbreaks of potato blight in Ireland 1907-1956. The Department’s Journal Vol. LIII 5-10, University College, Dublin, Ireland.
  • Melhus, I.E. 1915. Hibernation of Phytophthora infestans in the Irish potato. J.A.R. 5:71-102.
  • Van Der Zaag, D.E. 1956. Overwintering an epidemiologie van Phytophthora infestans, tevens enige nieve bestrijdingsmogelijkheden. Tijdschrift over plantenziekien 62:89-156.