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.