Thoughts for Food #23

Potato Early Dying – Why Do Some Potato Fields Look Dead in August?  

As you drive through the Central Sands in August, it’s not unusual to see brown and wilted spots potatofieldin those potato fields that were so lush in July.  What causes potato crops that once looked so good to suddenly turn brown?  Ironically, it is the same problem that many homeowners have in their gardens.  It’s called potato early dying, and it results when two common soil inhabitants get together to prevent plants from taking in and transporting the nutrients needed for growth.

The two soil dwellers are a fungus and a nematode.  The fungal disease attacks roots and stems, blocking the cells that move nutrients. The microscopic nematode, which is invisible to the human eye, feeds on root hairs making it easier for the fungus to enter the plant. Mid-season conditions, such as heat and dry areas in fields, stress the rapidly growing plants in August; the seemingly innocuous critters then get together and stop the plant’s growth in its tracks at the very time when the plant is putting all its energies into enlarging its tubers.  Soil survey results show that the majority of Wisconsin’s potato fields and many home gardens are routinely infested by both the nematode and fungus; without proper management, plant growth can be reduced by 60% and result in potato yield losses of 20-50%.  Unfortunately, the only consistently effective management technique is chemical fumigation.

Dr. Ann MacGuidwin, Plant Pathologist from the University of Wisconsin, has been researching potential alternatives to chemical fumigation for Wisconsin potato growers since 2004.  MacGuidwin states, “Having effective alternative treatments is important for early dying management, and we are looking at a realm of options to counter the complex which can greatly limit yields and profits on farms”.  Dr. MacGuidwin’s research projects have uncovered a range of strategies that can be used for reducing the impact of early dying.  The cornerstone of this approach is to integrate several tools that work together to hold nematode and fungus populations below levels that cause economic damage.  Many strategies have the potential to reduce early dying impact, and growers are working on implementing these  strategies to ensure long-term reductions in pest populations and potential for yield loss. But, as with most biological systems, there is not a one-size-fits-all solution, and research must continually evolve.

Interestingly, a combination of approaches is most effective in both commercial fields and in home gardens. Crops in the potato family, such as tomato and eggplant, favor buildup of the fungus while certain crops, such as those in the cabbage family and some grasses, reduce nematode buildup. Using this information, growers and home gardeners have been able to devise rotations of crops and cover crops that hold early dying to manageable levels. The use of cover crops has been particularly effective in helping to manage early dying, and research has shown that incorporating green cover crops, such as Sudan Grass or certain brassicas, can significantly reduce nematodes and early dying.  In smaller scale operations, covering the soil and incorporated cover crops with black plastic—a process called soil solarization— essentially cooks the soil and mimics the effect of soil fumigants by killing both the fungus and the nematodes.  Killing weed seeds are an added bonus!  Another experimental approach is to remove the potato vines from the fields once the tubers have sized.  The fungus can survive in the vines over the winter and re-enter the soil.  This approach effectively breaks the disease cycle and is currently practical in smaller operations. Some growers are already experimenting with vine choppers to achieve the same end result.

Other approaches under investigation include:

  • Intensively sampling soil in a grid pattern, counting nematodes and fungus and developing maps that identify hot spots or heavily infested fields that can be avoided.
  • Growing varieties that are less sensitive to early dying.
  • Identifying and encouraging naturally occurring organisms in the soil that can attack the fungus or the nematodes.
  • Extending the rotation time between potato crops to allow early dying populations to decline. Some growers now rent non-potato land such as dairy pasture to avoid buildup of early dying.

Taking research based information and using their own ingenuity, many growers and home gardeners are conducting their own on-ground research by looking for options to manage this perennial problem.  Creativity has proven effective and putting together the best combination of approaches that are suited to individual situations is the best way to gain the upper hand in the fight to stay ahead of one of the hardest challenges to growing a tasty and profitable potato crop.

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