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Monitoring Scheme Critical for Carrot Leaf Blights

Fungicides cut in half with scouting, proper timing

photo of a scout in a field of carrots

CCE Educator Lee Stivers (left) scouts a carrot field with her assistant Frances Tucker. They were a part of Abawi’s team on this project, as were CCE Educator Carol MacNeil; IPM Extension Educator Abby Seaman; Gilbert Scott, of Agrilink; Don Sweet, of Crop Advantage; Tim Widmer, of the Geneva plant pathology department; and four growers.

Photo by G. Abawi

When a 1997 plot comparison revealed no differences in carrot yield between a field receiving three fungicide treatments and one receiving eight, Cornell plant pathologist George Abawi recognized an urgent need for more work on carrot leaf blights. "I saw a tremendous opportunity to better control these diseases and also to save on fungicides," says Abawi. "There is a great need to educate growers about scouting and about withholding treatment until a certain level of disease severity has been reached."

This year’s work, led by Abawi, made good on the opportunity and the need. Five commercial carrot fields were split into "IPM plots" and "grower-managed" plots. The first treatment for leaf blight in the IPM plots was made only when sampling showed infection on 25 percent of the leaves. Subsequent treatments were applied at intervals of 10-14 days if the scouting reports and weather conditions showed a high probability of leaf blight development. The grower-managed plots were treated according to the growers’ standard practices.

The results were dramatic: 4 of the IPM plots received 0, 2, 3, and 3 fungicide applications, a total of 8, while corresponding grower-managed plots received 6, 4, 7, and 8 applications, totaling 25. Both the IPM plot and the grower’s plot received 6 sprays at the fifth site, which was planted to the highly susceptible variety ‘Eagle.’

Despite the much lower number of fungicide sprays applied in the IPM plots, incidence and severity of leaf blight was no worse in those plots than in the other sections of the fields. Furthermore, according to Abawi, "There was no detectable difference in the yield and marketability of carrots grown under the IPM scouting program and carrots grown under the regular spray schedule at the sites we harvested."

An added bonus came with the discovery that carrot varieties differed greatly in their tolerance of leaf blight. Some–particularly ‘Fullback’ and ‘Carson’–were highly tolerant; ‘Carson’ required no treatment at one site. Others (such as ‘Eagle’) were very susceptible to blight. Armed with this new information, growers can cut down on fungicides and increase profitability by choosing the right cultivars.