Pair varietal resistance, chemicals and cultural practices to stem cucurbit diseases

By Marni Katz

Varietal resistance, fungicides and cultural practices all play a role in cucurbits for controlling foliar diseases, such as powdery and downy mildew. But as last year's downy mildew outbreak in Mid-Atlantic cucurbits illustrates, disease pathogens evolve, and growers must practice integrated management to maintain the control from fungicides and varietal resistance when conditions become favorable for these diseases.

Integrate varietal resistance with fungicides

Many cucurbit varieties have excellent inbred resistance against powdery mildew, says Margaret McGrath, plant pathologist at Cornell University's Long Island Horticultural Research and Extension Center. But it"s important to support that resistance with a reduced fungicide spray program as disease conditions or symptoms appear, she says. Otherwise, pathogens that survive the varietal resistance are left to multiply, and eventually the population overcomes the plant's resistance.

"Powdery mildew fungus has been able to develop resistance to several valuable classes of fungicides and has a strong potential to also overcome resistance in varieties, so having both fungicides and resistant varieties raises the bar for that pathogen developing resistance to current controls," McGrath says.

That means that even in cucurbit varieties with a high level of resistance to powdery mildew, growers should continue to scout and spray for the disease when symptoms appear. The advantage of having both is that varietal disease resistance greatly improves the economics of disease management by allowing you to stretch a seven-day spray interval to 14 days.

"Typically you will start to spray for powdery mildew a week or so later with resistant varieties. Then if you spray on a 14-day interval, you can further save on sprays and still get the control you need," McGrath says.

Downy mildew wake-up call

Gerald Holmes, Extension plant pathologist at North Carolina State University in Raleigh, says last year's Northeastern outbreak of downy mildew in cucumbers is a perfect example of how growers will need both disease-resistant varieties and preventative fungicide applications to control major diseases. The outbreak also showed that regular monitoring is important, regardless of the level of disease resistance in a field.

Spring and summer 2004 provided perfect conditions for downy mildew, a disease only seen sporadically on cucumbers in the mid-Atlantic and rarely at economic levels. While downy mildew commonly appears on squash and pumpkin, it typically has little impact on the cucumber crop, largely as a result of resistance built into most commonly grown varieties.

Downy mildew does not overwinter in areas where hard frosts occur, but migrates north from the southern vegetable growing regions of Florida and Texas. Every year is a whole new disease scenario. Last season, heavy dews in spring and a tropical storm in July created the cool, wet conditions favoring the development of downy mildew in cucumbers, and Holmes says pressures were so strong the disease became an economic problem in spite of inbred resistance.

Holmes says many growers were caught completely off guard with losses averaging 40 percent on cucumbers in the mid-Atlantic. Watermelons were also hit hard in certain regions.

"We were really devastated by the disease on cucumber. It was definitely the worst epidemic anybody can remember," he says. "Downy mildew is a serious problem all over the world, but our resistance has held up over the last several decades."

As a result of last year's epidemic, growers this season will be much more vigilant and have fungicides in the wing when the disease risk is high.

In addition, a University of North Carolina Web site provides a downy mildew tracking service, so growers throughout the country can see how the disease is progressing and at what time it might appear in their growing region.

The information will allow you to make preventative sprays in advance of disease symptoms but close to the period they are likely to occur. Controlling downy mildew before symptoms appear is a much better strategy--both for managing the disease and its potential resistance to fungicides--than waiting for an outbreak and attempting to control it curatively.

Pathogens evolve

Holmes says last year's epidemic was either due to favorable conditions or an evolving organism that may be overcoming some level of varietal resistance when the pathogen is in high enough numbers.

"We don't really know, but perhaps it's the two combined that contributed to this problem," he says.

The scenario at any rate provides a perfect case study for the importance of managing disease with resistance in mind to keep both fungicides and resistant varieties available for as long as possible.

Holmes says no one knows whether downy mildew will appear again in Northeastern cucumbers and other cucurbits as severely as it did in 2004. But an integrated program of monitoring the path of the disease, weather conditions and symptoms, and using resistant varieties and effective preventive fungicides as conditions warrant should help growers stay ahead of downy mildew.

Holmes is conducting fungicide trials in cucumbers to determine which sprays are most effective at controlling downy mildew. He says Curzate (cymoxanil) and Tanos (cymoxanil plus famoxadone) have been effective when applied preventatively in his trials. Previcure Flex (propamocarb hydrochloride) makes an excellent rotational product, along with Gavel, which has zoxamide and mancozeb. In addition, a new fungicide called Hero should be labeled soon to control downy mildew in cucurbits.

Managing resistant varieties is also important.

"As a general principle, disease management should include several tactics and these tactics should change over time. This is especially true for fungicide use as well as the type of plant resistance used," Holmes says.

That's why only an integrated approach will provide for the long-term management of foliar cucurbit diseases such as downy mildew in cucumbers.

"If everybody is on high alert, which I expect they will be, and watching the Web site to see where the disease is and applying systemic treatments in advance, based on how the disease is spreading, that should be sufficient to stay ahead of the disease," Holmes says.