Gary D. Franc, Associate Professor of Plant Pathology
University of Wyoming
Department of Plant Sciences
P.O. Box 3354
Laramie, WY 82071-3354
Asexual forms of the late blight fungus (Phytophthora infestans) overwinter only within infected, living plant tissue. For example, surveys conducted in Ireland over a 50 year period revealed that most late blight outbreaks resulted from inoculum that spread from infected seed, volunteer potatoes and cull piles. For the most part, late blight epidemics in North American production regions are believed to result from primary inoculum sources similar to those identified in Ireland, with the occasional tomato field targeted as the culprit. Ultimately, whether late blight spreads from cull piles, seed tubers or volunteer potatoes, 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 Berkley in 1846, so this is not a new concept. De Bary established in 1876 that sprouts produced by infected tubers may be invaded by P. infestans and that these sprouts occasionally survive long enough to reach the soil surface and produce sporangia. Thus, these Ainfector@ sprouts serve as infection centers from which late blight spreads. During attempts to repeat De Bary=s work, scientists rarely observed infector sprouts because they found that infected seed 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) was needed to initiate late blight epidemics in the Netherlands.
Recently, potato scientists started to rethink how late blight spreads among seed tubers. Since the fungus often produces spores on the surface of infected tubers, could these spores spread and infect healthy tubers in the seedlot during normal handling and cutting practices? Research in Maine, North Dakota, Oregon, Washington, Wyoming and at other locations revealed that U.S. isolates of late blight are readily spread from infected tubers 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 99% of these tubers decay before emergence, but also includes the otherwise healthy seed tubers that become infected shortly before planting.
Figure 1. Seed tubers are easily infected by the late blight fungus. Healthy Russet Norkotah seedpieces were tumbled in the presence of an infected source-tuber (P. infestans, US1). The source-tuber (left) was immediately removed after tumbling. Growth of the late blight fungus is visible on seedpieces photographed 10 days after tumbling (right).
Studies were done at the University of Wyoming to determine the effects of Aplanting-time@ late blight infection on seed performance and to also determine if these effects could be reversed by application of seed treatment fungicides. High elevation and low elevation field sites were established during the two year study to represent a range of environmental conditions present in the irrigated High Plains production area. Some observations from this research are:
1. Seed inoculated with the late blight fungus shortly before planting performs poorly. Inoculated seed emerged more slowly and final plant stands were reduced by 42% (year 1) and by 99% (year 2) compared to the non-inoculated healthy seed.
2. Beneficial fungicide effects were most obvious when fungicides were applied to seedpieces immediately after they were inoculated with the late blight fungus (year 2) versus when fungicide applications were delayed until several days after seed tubers were inoculated and late blight was allowed to become established (year 1). Treatments that included thiophanate-methyl, mancozeb and cymoxanil were most effective at restoring the ability of inoculated seed to establish plants.
Figure 2. Healthy seedpieces, exposed to late blight-infected source-tubers during cutting and handling, are readily infected and fail to emerge (foreground plot). However, if identical seed is treated with fungicide immediately after exposure to late blight inoculum, seedpieces emerge (background plots) and plants perform similar to those in noninoculated check plots.
During year 1, TOPS 5D and TOPS-MZ 2.5D improved final stands by an average of 7% while addition of cymoxanil to the fungicide increased final stands by an average of 21%. When seed was inoculated with the late blight fungus immediately before fungicide applications were made (year 2), TOPS MZ and fungicide treatments containing cymoxanil all resulted in emergence rates and final stands equivalent to those of non-inoculated healthy seed.
3. Once plants were established in the field, no late blight stem or foliar lesions resulted from the seedborne inoculum. However, these research plots received weekly applications of protectant fungicide which may have reduced our ability to detect infection, and conditions were not always favorable for foliar late blight development. Yield effects were a function of treatment effects on stand; if stand was increased then yields were also increased. No phytotoxicity from any treatment was detected in the plots.
In conclusion, several seedpiece fungicide treatments available to growers during 1999 are proven to reduce the late blight spread that occurs during seed handling and cutting. The specific formulations available are based on the labeling efforts for each State; label directions must be followed. It is important to realize that you cannot cure an infected seedlot by applying seedpiece fungicide treatments. Also, seedpiece treatments will not directly protect the foliage of emerged plants from above-ground inoculum sources.
Although most growers will agree that increased emergence is a desirable benefit of seedpiece treatment, don=t forget that the 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 a fungicide will provide an added layer of protection that prevents the inadvertent spread of late blight inoculum in your seed. In other words, since it is not possible to certify that a seedlot is late blight-free, labeled seedpiece fungicide treatments will ultimately reduce the risk of planting infected seed. Seed treatment fungicides will become an important component of an integrated late blight management program.
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