April 2007
Disease Control Measures Should Be Taken Right Up Until Harvest

After growers apply the desiccants that will kill their potato vines and set the stage for harvest, it’s not yet time to park the sprayer. They should plan to return within three or four days to make the first of perhaps two applications of fungicide depending on how fast the vines die.

You’ve worked all season to grow a disease-free crop,” said Walt Stevenson, plant pathologist at University of Wisconsin. “The application of vine desiccant is not the time to stop that effort.”

In past years, he said, growers were often advised to combine fungicide with the desiccant and do both jobs in one spray application. Now, he said, the recommendation is to apply the vine killer, wait a few days for it to work and then spray with fungicide when some of the foliage is gone, the canopy is more open and coverage will be better.

The major disease threat remains late blight, he said, but early blight can also infect plants late in the season. Late blight attacks leaves, stems and tubers.

“Disease control should continue right up until all potato plant tissue is dead,” he said.

That means returning to the field and evaluating how complete a job the desiccant did.

“In some years, vines may appear dead but resprouting of tiny leaves can occur at the soil line,” he said. “This fresh young tissue is wide open to attack by late blight if not protected by fungicide.”

Appearance of regrowth may call for a second application of fungicide and another application of desiccant as well.

If green tissue remains and is not protected, late blight inoculum can form and be washed into porous or cracked soil, infecting the tubers, Stevenson said.

“If you’re harvesting when there is still green tissue, you have the potential to inoculate your tubers going into storage with late blight,” he said.

Growers have an abundant choice of fungicides to use, Stevenson said.

Usually, growers dig tubers 14 to 21 days after vines are completely desiccated. This waiting period serves two functions: It allows the potato skin to set before harvest to avoid wounding during the harvest process, and it reduces potential tuber infection by early and late blights.

Periderm, or tuber skin, does not develop until tuber growth stops or slows significantly. The thickening and firming of the periderm, called “skin set,” and the loosening of tubers from the stolons are important reasons for killing vines well in advance of harvest. The tubers’ ability to stand up to the rigors of harvest, packing, shipping and storage greatly improves after skin set.

Tubers with thick, well-set skin that easily detaches from the vines are less susceptible to bruising and disease. And with less vine volume to go through a harvester, harvest is easier.

Disease problems in the field late in the season have the potential to carry into storage and create problems there, Stevenson said.

“It’s often much easier to control foliar diseases such as late blight in the field than to fight decay problems later in storage. Controlling diseases in the field through careful and timely application of fungicides and complete vine kill prior to harvest makes storage management a whole lot easier.”

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