Jun 9, 2022
Tips to curb asparagus disease damage

Foliar disease damage from purple spot and soilborne disease damage from crown and root rots are still major threats to asparagus. That’s the message from Mary Hausbeck, Michigan State University vegetable disease pathologist.

Hausbeck talked about the diseases at the 2021 Great Lakes Fruit, Vegetable & Farm Market EXPO in Grand Rapids, Michigan.

Purple spot (Stemphylium vesicarium) can damage both the ferns in the fall and the spears the next spring.

Diseased ferns brown prematurely and defoliate, reducing the photosynthesis and yield the following spring. Heavy disease pressure in the fall results in more overwintering inoculum in Michigan’s no-till system which means more emerging spears may become infected and develop the purple spots.

vegetable disease pathologist Mary Hausbeck
Mary Hausbeck

“In the 1990s, we were primarily a processing industry, so spotting of the spears was not considered a big deal, but now that we have an important fresh market industry, the spots are a deal breaker,” Hausbeck said.

There is no chemical control available for purple spot during the spear stage. The only way to manage it is to reduce the amount of disease present in the ferns the year before.

“It’s important to keep the fern healthy during the summer and into the fall to have the best yield the next spring,” Hausbeck said. “It only takes one lesion to cause a needle to fall off and contribute to early defoliation.”

Bravo WeatherStik (chlorothalonil) and Roper (mancozeb) have been the core program used for disease control on the ferns. The sprays are timed using the TOMCAST computer model with the last treatment generally applied around Labor Day.

“Are fungicides needed in the fall?” Hausbeck asked. “The answer depends on the weather. We’ve had some warm falls. The purple spot pathogen will continue to progress if the weather allows; it doesn’t stop on Labor Day. There is the potential for a significant fall outbreak allowing a large amount of the pathogen to overwinter.”

MSU researchers have evaluated some new products for fern disease control. The treatments were made from July 8 through Sept. 8 and alternated with Bravo WeatherStik. Disease severity ratings were taken on Sept. 9.

“Miravis Prime was highly effective against purple spot,” Hausbeck said. “Statistically, it stood out from the other products. Merivon was also very promising.”

Although the spray treatments had stopped, evaluations of the amount of foliar purple spot disease continued into the fall, on Sept. 30, Oct. 13 and Oct. 20. By late October, the effectiveness of many of the tested products had faded and the ferns were showing disease severity and defoliation ratings close to three-fourths of the severity of the untreated plot.

The Oct. 20 evaluation showed both Miravis Prime and Merivon remained significantly more effective than the other products, although Merivon had disease severity ratings close to double that of Miravis Prime.

“We really need to have the Miravis Prime and Merivon as part of our control program because they’re so effective,” Hausbeck said. The study also showed that Luna Experience and Luna Tranquility hold promise.

The next step is for the industry to proactively work with the manufacturers gathering efficacy and crop safety data for new registrations and submitting a project clearance request through the IR-4 program.

Fusarium crown and root rot (Fusarium spp.) and phytophthora spear, crown and root rot (Phytophthora asparagi) are the main soilborne pathogens of asparagus.

Healthy crown and root tissue should be all white; the color of diseased tissue differs between the two diseases. Fusarium crown and root rot symptoms include a red-brown discoloration of lower stems and crowns, and red-brown lesions on the roots and lower stems. Phytophthora spear, crown and root rot symptoms include discolored storage and feeder roots that may appear to be gray and water-soaked.

“Phytophthora is a relatively new pathogen to this industry that is especially destructive,” Hausbeck said. “We have tested fungicides and biocontrol products for their effectiveness.”

Cannonball, Presidio, Ridomil Gold, Topsin and Serenade, a biological, have been evaluated in the asparagus crown nursery. With applications made every 21 days (July 1, July 21 and Aug. 11), and using crown health ratings, there were no observed differences in the products’ effectiveness, and all of the fungicides had good crop safety.

Crop safety is a major concern of the companies when considering a product registration for a new crop such as asparagus.

“Companies want to know there is no injury as a result of their product being used,” Hausbeck said.

Disease control data may already exist from research on a similar disease on another crop. The fungicides Presidio and Topsin are not registered for use on asparagus seedlings in a crown nursery, but are being tested. Presidio targets phytophthora and Topsin targets fusarium.

“We want to get more products registered for use in the seedbed and for the nursery industry,” Hausbeck said.

Cannonball is registered for use in Michigan as a crown soak. Ridomil Gold can be used after planting seedlings or after covering one-year-old crowns.

“They’re here; they can be used,” Hausbeck said.

Research will continue to expand tools that can be used in the crown nursery.

“We started with crown soaks,” Hausbeck said. “We soaked them after they were dug from the nursery and before they were planted in the production fields. It takes time to soak the roots. When you’re trying to disinfect something, time of contact is meaningful. Crowns are closely spaced in the nursery, and so treatment applied in the nursery has the potential to positively impact a high number of crowns that are planted across a lot of acreage.”

– Dean Peterson, VGN correspondent

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