Apr 7, 2007
Researchers Expect Downy Mildew Pressure In 2007

Cucurbit growers – especially cucumber growers aiming for either fresh market or pickling – should plan now to implement a full fungicide spray schedule this year and assume that the downy mildew epidemic of the last two or three years will continue.

That’s bad news, especially for pickling cucumber growers in Michigan and North Carolina, where acreage is large and they are not used to having to spray fungicides on their crop. The new approach can put 10 spray applications into the work schedule, each costing from $12 to $20 for a tank mix of two materials. That adds more than $200 per acre to production costs on more than 70,000 acres of cucumbers, 46,000 in Michigan and 23,000 in North Carolina.

For bad news, growers certainly seemed willing enough to hear it. About 140 of them turned out for the downy mildew/phytophthora workshop in East Lansing, Mich., Jan. 24, and there was a roomful in Columbus, Ohio, a week earlier.

For people interested in rotten fruit, the plant pathologists seemed pretty popular, too. Mary Hausbeck at Michigan State University put on the East Lansing workshop, bringing several graduate students from her laboratory. Plant pathologists Gerald Holmes and Kathy Ivors came from North Carolina State, bringing their graduate students, and Dan Egel was there from Purdue University in Indiana.

In Ohio, plant pathologist Sally Miller from Ohio State University was a double star at her meeting, presenting both her work and Mary Hausbeck’s. Hausbeck was barred from Ohio by too-lousy-to-fly East Lansing weather. Miller gave four presentations in a row, two of hers and two of Hausbeck’s.

The pathologists concur on the spray recommendations. Here’s the program, wherever you grow cucumbers:

Before the disease arrives, use a 7-day spray schedule. Use Previcur Flex or Tanos or Ranman or Gavel tank mixed with mancozeb or chlorothalinil. Alternate products from spray to spray to avoid selecting resistant strains of the disease.

After the disease arrives, tighten to a five-day schedule of the same products (except for Gavel, which is not recommended for the short schedule) and use the same alternating scheme.

In addition to using fungicides, destroy infected vines after harvest so they do not serve as a reservoir for the disease, which must have living host tissue to survive.

In more detail, a fungicide program might look like this:

Tanos 50WG (8 oz.) plus Manzate Pro-Stick 75DG (2 lb.) alternated every 5 to 7 days with Previcur Flex 6F (1.2 pt.) plus Bravo Weather Stik 6SC (2 pt.).

Manzate is a trade name. There are other labeled formulations of mancozeb that include Penncozeb and Dithane. Bravo is a trade name. Other labeled formulations of chlorothalonil include Echo and Equus.

The recommendations were set up to reduce the threat of the disease building resistance to the fungicides.

Tanos is a mixture of two fungicides, one from group 11 and one (Curzate) from group 27. Previcur Flex is a group 28 fungicide, Ranman belongs to group 21 and Gavel a mix of two from groups 22 and M.

Gavel is a trade name for a mixed product that contains mancozeb as one of the ingredients. Mancozeb and chlorothalonil are group M.

The grouping numbers and letters, called FRAC codes, indicate the mode of action or site where the fungicide works. Fungicides in the same group cannot be used as rotation herbicides. Group M fungicides work at multiple sites and are considered unlikely candidates for fungi to build resistance to. FRAC stands for Fungicide Resistance Action Committee.

You’ll also have to make choices based on preharvest application intervals. These are zero days for Ranman and chlorothalanil, two days for Previcur Flex, three days for Tanos and five days for mancozeb and Gavel.

“All products listed above must be used at full label rates,” Hausbeck said at the height of the problem last summer. “As a mixing partner, mancozeb is good against downy mildew but carries a five-day pre-harvest interval. This makes mancozeb use especially difficult for growers of hand-harvested cucumber, zucchini and summer squash. Remember, mancozeb is not registered for use on pumpkins, but maneb can be used. Some growers are using copper as a mixing partner with Previcur Flex or Tanos 50 DF when the pre-harvest interval prevents the use of mancozeb.”

Keep the spray interval short, she added.

“Fungicides must be applied to cucumbers every five days for effective results. Cucurbits grow rapidly, and the new growth must be protected. Growers near the hardest hit areas of the state must assume that the downy mildew spore load is high and so is the threat of disease.”

In her spore traps, she found as many as 70,000 spores per cubic meter of air.

The researchers ran a series of product tests in 2006 and found many that don’t provide control. These include Ridomil Gold (mefenoxam), Prophyt, Man-Kocide, Aliette, Pristine, Acrobat, Reason, Amistar and Sonata.

Some new fungicides, so far without labels, worked very well in test plots, so the battery of fungicides is well supplied.

There’s more to it

It’s not only about fungicides, and it’s not only about cucumbers.

Last year, downy mildew appeared in Michigan cucumbers June 9, the earliest ever, Hausbeck said. Since downy mildew persists only on living tissue and can’t survive freezing weather, its main overwintering place in eastern United States is southern Florida.

Holmes puts the line at 27 ˚ north latitude, and it was already in cucumber fields in southern Florida in January, he said. This reservoir is like a smokestack from which a plume of spores enters the atmosphere and moves north when weather conditions permit. In the past, those spores haven’t reached Michigan until August.

Hausbeck believes a different force was at work last year. Evidence shows the disease came into Michigan from plants grown in greenhouses in Ontario and into fields of growers planting slicing cucumbers in low tunnels in Monroe County in April. Infection spread across Michigan in the usual south to north manner, but Ohio and Indiana fields were infected from Michigan. For them, it came in from the north instead of the south.

Holmes, who runs a Web site at North Carolina State University and monitors weather and the movement of the disease, said there was no weather event associated with the early outbreak in Michigan.

“Beware of green bridges,” Hausbeck said. “Avoid plug plants from greenhouse where cucumbers are raised all year and from locations where downy mildew survives the winter.”

Greenhouses that harbor cucumbers throughout the year are sources of infection. Plants for transplanting should not come from such greenhouses, no matter where they are, she said.

Hausbeck and Holmes also are looking at the possibility that this strain of downy mildew is different – more virulent on cucumber. There are at least six known pathotypes that attack the 12 types of cucurbits, Holmes said, and this one might be number 7.

Holmes is looking for funding – and seeds for all 12 cucurbits -– and would like to coordinate a study with collaborators to map out the strains and susceptibility across the United States.

He is also looking for funding help to collect the data for his Web site. Cucumber growers should watch it for evidence of how close the disease is to them.

The Web site is www.ces.ncsu.edu/depts./pp/cucurbit. If you do a Web search for “cucurbit downy mildew,” it is the first entry that comes up.

Holmes encourages growers to be conservative. Spray early, not late, Even at the cotyledon stage, plants are susceptible – in fact, more so.

“You can’t play catch up with this disease,” he said. “You want fungicides there when the spores arrive.”

Infection takes place from four to 12 days before you see symptoms, but once in the field, plants can all be dead in 10 days, he said.

Other crops affected

By late summer last year, Hausbeck was reporting to growers on her Web site that the disease was being found on winter squash and muskmelon and on pumpkins in Ohio and New Jersey.

“Fungicides will also be needed for the duration of the growing season to protect these crops from downy mildew,” she said. “This disease is extremely destructive and can kill an unprotected crop within seven to 10 days.”

Disease behavior

The East Lansing workshop combined downy mildew and phytophthora on the program, so the two provided a point of contrast.

Both are water molds, Hausbeck said, but the relationship really ends there. Both need water to move and increase, but downy mildew:

∑ Does not infest the soil

∑ Travels by air, not water

∑ Infects the vines, but not the fruit

∑ Can’t survive winter

∑ Can’t survive away from cucurbit tissue

The stealthy disease develops during the cool of night, Hausbeck said. It needs four hours of darkness, temperatures from 59° F to 77° F and relative humidity of 95 percent to 100 percent. It likes foggy, cool, cloudy weather.

Under those conditions, spores invade the stomates on leaves and begin to grow into a sporangiosphore, a microscopic tree-like structure that grows lemon-shaped sporangia that, upon drying, move with wind to new infection sites on leaves.

“Downy mildew causes angular, yellow lesions on leaves,” Purdue’s Egel said. “On wet mornings or after a rains, the lesions on the underside of the leaf may be covered with a dark fuzz. This is the fungus growing on the leaf. The spores thus produced are easily spread by wind to additional leaves, plants and fields of cucurbits. Humidity of 100 percent for at leave six hours and temperatures of 59° F to 68° F will lead to the rapid spread of the disease. Lesions on the leaf surface may coalesce and turn brown. Large areas of the field may quickly be affected.”

Unlike phytophthora, it produces no oospores, the structures that allow that disease to overwinter in soil – and in fact persist in soil for many years.

Susan Colucci, a graduate student in Holmes’ lab at North Carolina State, said that downy mildew has a long history of attacking cucumbers, and the search for resistant varieties was important already in the 1940s. There are resistant cucumber varieties.

The disease, Pseudoperonospora cubensis, occurs worldwide, but in 2004 in North Carolina it came earlier than usual and seemed more virulent in cucumbers. Old standard fungicides, mefenoxam and the strobilurins, were not effective. In 2004, she said, North Carolina growers lost 40 percent of their crop and about $16 million. It was worse the next two years.

In 2005, it became a problem in Michigan, where it had never been a problem before. The epidemic continued last year, starting in June instead of August as it had in 2005.

Purdue’s Egel said the disease had been “mostly a nuisance” in the past, affecting mostly pumpkins. Coming in late and killing the vines, it made pumpkins easier to pick! But the last three years, it has arrived in Indiana earlier and affected watermelons in July, pumpkins in August and cucumbers in July.

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