Disease Thrives on Cool, Wet Weather in Midwest, East

Prevailing weather for the Midwest and Eastern states during the summer of 2004 can be summed up in two words cool and wet. It just so happens that the late blight pathogen we’ve all learned to recognize in the past few years loves this sort of weather regime.

Once you passed east of Chicago last summer, most states experienced some level of late blight that in many cases resembled the scenarios described in the mid-1990s. Midwestern states west of Chicago seemed to have escaped late blight and the elevated costs often associated with control efforts.

For those states confronting late blight in the face of inclement weather, fungicides played a major role in management last year. In areas where there was some cooperation from the weather (i.e. drier and warmer conditions for even short periods), control efforts were more successful than in areas where rain and cool weather persisted. It may be useful to look at some of the factors that were reported to be associated with late blight outbreaks in these areas during 2004.

Planting Infected Seed

While most educational programs begin with a discussion of the seed-borne nature of late blight, in reality, compromises are sometimes made as growers are caught in the middle of planting with unplanned shortages of their favorite variety of seed.

While it’s legal to sell seed potatoes with up to 1 percent late blight infection and it’s possible to cull out tubers with obvious symptoms, it’s virtually impossible to remove all seed with infection. My advice to growers: There are no bargains when you purchase seed. There is simply no such thing as just a little late blight infection.” Planting infected seed is a risk that is not easily overcome, and growers planting infected seed often are the first in the area with late blight problems.

Growers need to know about the seed they are planning to plant and they should do their best not to purchase and plant seed known to be infected with the late blight pathogen.

Another issue related to this is the planting of non-certified seed or seed left from last year’s crop where late blight was present in production fields. This can be risky business, not only for late blight considerations but for other diseases as well.

Cull Piles

Elimination of cull piles before planting or at least by the time the next crop emerges is one of the most critical components in late blight management. After preaching this gospel for too many years we still see many, if not most, local epidemics of late blight attributed to cull piles. Culls can be potatoes sorted out during grading operations, chips and slivers left after seed cutting, refuse from cleaning at the end of the storage year and potatoes that were not marketed or consumed. Low market prices and an over-abundance of potatoes in the last few years have helped create local disposal problems, and these potentially lead to exposed sources of late blight inoculum. It’s not easy disposing of thousands of tons of excess potatoes at season’s end, but careful attention needs to be paid to these mounds of potatoes as inoculum breeding grounds.

Volunteer Potatoes

A string of mild winters in the Midwest and Eastern states have contributed to mounting concerns about volunteer potatoes over the past several years. Subzero temperatures during the winter do wonders in reducing the number of viable potatoes overwintering in production fields after harvest. Freezing temperatures reduce the potential for volunteer potatoes appearing the next spring as sources of late blight inoculum.

In the absence of these cold temperatures or the presence of persistent heavy snow cover that insulates the soil and unharvested tubers, growers need to be prepared to deal with volunteers. This can be difficult, particularly when volunteers appear in plantings of rotational crops where fungicides are not registered or cultivation is not possible. Some of the states reporting late blight last season reported difficulty in controlling volunteer potatoes prior to disease outbreaks.

Termination of Fungicide Treatment

Growers in some production areas who do not store their crop seem to be depending on diseases such as early blight to kill vines and prepare their fields for harvest. Consequently, they are discontinuing fungicide sprays of green lush vines well in advance of harvest. The undesired consequence of this activity is to encourage the rapid movement of late blight through these production fields and thus contribute to local sources of inoculum. This level of disease management jeopardizes all growers in the area.

The appearance of late blight in a production area greatly increases the cost of disease management. Instead of a seven to 10 day spray interval, growers often treat their fields every five days when weather factors favor the development of late blight. In Wisconsin, where no late blight has occurred during each of the past two growing seasons, we calculate a savings of at least $12 million per year in reduced fungicide sprays, lower application costs and less use of fungicides specifically targeting late blight.

During the past growing season in Wisconsin, with only early blight to manage, we saw excess protectant fungicides and those fungicides with modes of action targeted at late blight shipped out-of-state to those areas experiencing difficulties in controlling late blight. The concern here is that if late blight had appeared in additional production areas last season, there may not have been sufficient plant protection chemicals to supply the control needs in all states.

This winter, grower education meetings focus on early identification of late blight symptoms, the need to plant certified seed, destruction of cull piles, control of volunteer potatoes and the choice of fungicide products for management of late blight.

For those states where late blight was not reported last year, the biggest worry is complacency. Late blight can be an opportunist in these situations, so rather than relaxing in the absence of late blight we are trying to highlight the importance of maintaining our vigilance. All states in our area are bracing themselves for the coming growing season, resolved to join the list of states where late blight did not appear last season.

Walter R. Stevenson is in the Department of Plant Pathology at the University of Wisconsin. He can be reached via e-mail at [email protected].