Nov 25, 2016
Michigan trials spotlight carrot foliar diseases

Michigan is ranked second in the U.S. for the production of carrots, grown on 1,600 acres in 2014 with a value of $7.7 million.

Michigan carrot growers rely on fungicides for disease management, creating a tricky financial and practical balancing act regarding application schedules and efficacy.

Mary Hausbeck, Michigan State University (MSU) professor and Extension specialist, has pursued extensive research evaluating registered fungicide products for management of fungal foliar blights in commercial carrot fields in Michigan. Some of her lab’s work was discussed during the Oceana Research Tour in mid-September in Michigan’s Oceana and Mason counties.

High relative humidity and frequent rainfall/irrigation common during the growing season create a favorable environment for foliar fungal pathogens. Fungal foliar blights caused by Alternaria dauci and Cercospora carotae threaten yields yearly by reducing photosynthetic area and weakening leaves and petioles, interfering with harvest because tops break off in lifting.

These blights occur yearly, and fungi overwinter readily in carrot debris in soil. Hausbeck said the fungicides chlorothalonil (Bravo) and strobilurins (i.e. Cabrio or Quadris) have been typically used by growers for control of alternaria and cercospora blights, other than copper-based formulations, and may be applied as frequently as every seven to 10 days beginning in June and ending in mid-September.

Irene Donne, a Michigan State University graduate student, left, and Mary Hausbeck, MSU professor and Extension specialist, discuss foliar disease of carrot research.
Irene Donne, a Michigan State University graduate student, left, and Mary Hausbeck, MSU professor and Extension specialist, discuss foliar disease of carrot research.

Minimizing overall fungicide use and diversifying the fungicide active ingredient that is applied to the carrot crop is desirable so as to minimize/eliminate detectable residues on the harvested root. Disease management programs that reduce the total number of fungicide applications also reduce grower costs, potential residues on the produce and risk of development of fungicide resistance in the pathogens.

Hausbeck said one way to reduce the number of necessary fungicide applications without compromising disease control is through the use of disease warning systems that predict potential outbreaks, or increases in disease severity based on the weather and the use of biocontrol products.

One project has continued to look at the Tom-Cast disease forecasting system to time fungicide sprays.

“We’re most interested in timing sprays when the environment was most favorable for disease development,” she said. “There was a time when this
area was not much into the processing carrot industry. It was fairly new and the growers were getting caught with a lot of foliar plantings and they were unable to pull their carrots out. Spraying every week whether those sprays were needed or not doesn’t make a lot of sense.

“DSV’s (daily disease severity value) are the duration of the leaf wetness period and the temperature during the leaf wetness period,” Hausbeck told tour attendees. “You’ve been in a drought for much of the summer, however, you did have high dew points. You had significant dew periods where the temperature is really hot, so the DSV’s racked up rather quickly this year compared to last year when we were wet but quite cool. Those cool temperatures don’t drive the pathogens nearly as much as the warm temperatures of this year.

“We wanted to look at some of the newer products that are available today that were not available when we first looked at the Tom-Cast program,” she said. “This is really an important year to test the forecasting system because of the disease pressure. In a summer like this, you probably couldn’t put off spraying at all. In a year like this, the DSV’s would help remind you that spraying always has to be on top of the list.”

She said carrot cultivar Cupar seeds were put down in April. Foliar treatments were initiated on July 15 at “trace disease” and applied either on a seven- to 10-day schedule, or according to the Tom-Cast system at thresholds of 15 DSVs or 25 DSVs. As of Sept. 12, several product applications were still being evaluated.

Hausbeck said researchers were also looking at an evaluation of foliar applications of biorational products for their efficacy against fungal foliar blights in a commercial carrot field in Michigan.

Dan Brainard, associate professor at Michigan State University, shared findings about cover crop effects on carrots, weeds and nematodes.
Dan Brainard, associate professor at Michigan State University, shared findings about cover crop effects on carrots, weeds and nematodes.

In a previous year’s trial, Cupar seeds were planted in a grower-cooperator’s field on April 30 in Oceana County in sandy soil. Three fungicide programs were applied to the Tom-Cast trial, in accordance with three different spray schedules: a seven- to-10-day spray schedule; a Tom-Cast schedule based on the threshold DSVs of 15; and a Tom-Cast schedule based on a threshold DSV of 25.

Plants with one or more petiole lesions were counted, and plants were evaluated for petiole disease using the Horsfall-Barratt rating scale on Oct. 20.

The untreated control plants had the most disease. All other treatments limited plants with infected petioles and petiole disease severity.

In that previous study, applying the treatments according to the Tom-Cast disease forecaster saved six sprays when scheduled at 15 DSVs and 10 sprays when scheduled at 25 DSVs.

All treatments were significantly better than the untreated control for all parameters measured, said Hausbeck, who also shared updates on the evaluation of fungicide efficacy in a first-year asparagus seedbed trial. The work is investigating the efficacy of registered and experimental fungicides in controlling soilborne pathogens impacting newly seeded asparagus.

In a separate trial, Dan Brainard, associate professor in MSU’s Department of Horticulture, shared findings after researchers looked at cover crop effects on carrots, weeds and nematodes.

In a 2015 study, mustard family cover crops caused some problems in carrots, while oats fared well. In a 2016 trial, sorghum sudangrass, oats and oats-oilseed radish mixtures looked good.

Cover crops were planted either in June or mid-August in 2015. Carrots were planted in early May 2016.

“Cover crops suppressed both summer annual and winter annual weeds, but several mustard family cover crops seem to be suppressing the carrots,” Brainard said.

Irene Donne, an MSU graduate student, conducts foliar disease research on the Ralph and Ken Oomen farm in Michigan.
Irene Donne, an MSU graduate student, conducts foliar disease research on the Ralph and Ken Oomen farm in Michigan.

Lures target parasitoids

Asparagus research findings also were presented by Adam Ingrao and Zsofia Szendrei of MSU’s Department of Entomology.

Since 2010, they have been investigating volatiles of asparagus and the role they play in asparagus miner and common asparagus beetle behavior. In 2016, they deployed volatile lures in commercial fields to determine if parasitoid or other predator insects were attracted by volatiles of asparagus.

Ingrao said lures were deployed on field edges, attached to yellow sticky cards. Parasitoids were significantly attracted to ocimene lures at the low release rate. Parasitoids attracted to the lure include known asparagus miner parasitoids form the Eulophid, Braconid and Ptermalid families.

“Attract-and-kill technology can prevent colonization of asparagus fields by Japanese beetles,” Ingrao said. “Attract-and-kill devices use pheromone lures inside a pouch infused with contact insecticide (delamethrin-pyrethroid). Devices at the edge reduce Japanese beetle abundance within the field.”

He also said application of thlamethoxam (Platinum) to the soil – chemigation – reduces asparagus miner damage, and numbers of common asparagus beetle eggs, larvae and adults. However, Japanese beetles were more abundant after insecticide application.

— Gary Pullano, associate editor

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