The dramatic rise in downy mildew problems on a number of vegetable crops in the upper Southeast in 2004, followed by continuous problems since, is eerily similar to the spread of Asian soybean rust on beans.
The fungi that cause the two diseases are dramatically different. However, how downy mildew and soybean rust form and spread are not all that different. Downy, like soybean rust, being airborne most frequently begins on the upper leaves of vegetables from cucumbers to watermelon, spreads rapidly throughout the crop and robs the host plant of nutrients.
In vegetable crops, especially melons, flavor is also negatively affected.
Like soybean rust, downy mildew spores can be transmitted hundreds of miles in a relatively short period of time. One predictive model indicates downy mildew spores may move in windy, cloudy weather as much as 1,000 miles in 48 hours.
Since scientists have been closely tracking movement of the disease, it appears downy mildew spores move more rapidly from south to north than do soybean rust-causing spores.
In fact, late in the past few seasons, downy mildew has moved fairly dramatically from north to south, which is usually not characteristic of soybean rust.
Neither downy mildew nor Asian soybean rust can over-winter without a live host. Though live rust spores have been documented to survive winter as far north as Montgomery, Ala., central and north Florida are more likely the northern edge of winter survival for both diseases.
In 2007, downy mildew was first detected in vegetable crops in and around Immokalee, Fla. in early March. Located near the southwest Florida coast, Immokalee is a center for vegetable production. Cucumber and squash were affected around Immokalee with disease reaching moderate to high levels in some older fields. Incidence and severity were heavy in some locations.
At about the same time, the disease was reported on vegetable crops near Palm Beach on Florida's east coast and in cucumbers and melons near Homestead in south Florida. From these beginnings in 2007, the disease spread northward, though just how it did so isn't so clear.
In early June the disease was reported in Ohio and later in Michigan and mid-June it was detected in several fields in eastern, North Carolina. Unlike soybean rust there is no sophisticated system of sentinel plots to track movement of the disease. And, in the case of the North Carolina finds, it appeared the disease had been active for some time prior to being reported.
While downy mildew has not proven to over-winter further north than central Florida, some contend it has over-wintered in greenhouse crops as far north as Canada. And, there is some contention that the June outbreaks recorded in Michigan may have spread south from disease innoculum that came from over-wintering spores in Canadian greenhouses.
Clemson University Plant Pathologist Tony Keinath says efforts are under way nationwide to develop a tracking system, using a similar system to the one used to track movement of Asian soybean rust. “North Carolina is currently seeking funding to establish a nationwide sentinel system to track downy mildew movement. South Carolina was one of five states that used a small-scale model of the sentinel system for tracking downy mildew movement in 2007,” Keinath adds.
The network of sentinel plots to track soybean rust stretches from Florida to Canada and has been one of agriculture's most untold success stories. Modern technology has allowed scientists to communicate almost instantly when rust is found. In turn scientists have made this information available to growers almost as instantly.
In an ideal situation, vegetable and row crop pathologists could work together in side by side sentinel plots of soybeans and vegetables to monitor movement of both diseases. Both rust and downy mildew are very manageable with a number of proven fungicides. The key is to know when, or if, to treat without wasting the high cost of manpower and fungicides.
In soybeans, researchers have been highly successful in providing growers with plenty of advance warning to treat and protect soybeans from damage. In vegetable production, the communication system has not been so effective.
In 2004, downy mildew was primarily a disease problem on cucurbit crops in the Southeast. Why it spread dramatically northward in 2005, 2006, and again this past crop season has not been determined.
Neither has why the disease burst onto the scene with such voracity in 2004. There is some contention that a huge buildup of innoculum in the Southeast in 2004, led to spread of the disease northward in subsequent years, but the disease isn't known to over-winter further north than central Florida, so the correlation of more disease in the south to more disease in the north may not be the primary reason, unless downy disease spores travel much greater distances in much shorter time than other diseases, such as soybean rust.
In South Carolina, Keinath says some of the spread of the disease may be due to different strains of downy mildew. “I stop short of calling these new strains, because we don't have scientific evidence to confirm this,” Keinath says.
“For sure the disease can overcome resistance bred into pickling cucumbers and some of the newer disease isolates are resistant to some of the strobilurins and Ridomil that are commonly used to manage the disease,” he adds.
“We do still have fungicides that are effective against downy Mildew,” Keinath stresses. “In our tests Tanos, Previcur Flex and Ranman have all performed well against downy mildew. Tanos is a combination of a strobilurin-like material and a material outside the strobilurin family. The other two fungicides are new materials,” he says.
Initially, scientists thought the heightened problems with downy that occurred in 2004 was limited to cucumbers. Subsequent research has shown the same organism also attacks other cucurbit crops.
Keinath says exactly why the disease manifested in 2004 is not certain. Some contend it is a new strain that came in from Cuba or Mexico with a series of tropical storms and perhaps spread dramatically northward by megastorm Katrina. “We do know that since 2004, the disease has been more aggressive,” he adds.
“In South Carolina we had more problems with downy mildew in the fall than in previous years. We had a warmer than usual fall, and to me, downy mildew is a warm-season disease, so the late season infections were not a surprise to me, though some growers were either surprised by its occurrence late in the season or just hesitant to use expensive fungicides this late in the year,” Keinath concludes.
Keinath says it is a common misconception that downy mildew needs rainfall to form. The disease has to have moisture, but in the fall, heavy dew is common and sufficient to supply the moisture needed for the disease to form.
For 2008, growers are urged to either scout their crops carefully for downy mildew or to treat these high dollar value crops with fungicides. Hopefully, researchers in the Southeast will find funding to establish a monitoring system that will provide growers with ample warning, giving growers more flexibility in management strategy for downy mildew.