Unexpected Disease Hits Michigan Tart Cherries

Thanks to an unusual combination of weather conditions in Michigan, European brown rot struck Montmorency tart cherries last year. (Photo credit: George Sundin)

Thanks to an unusual combination of weather conditions in Michigan, European brown rot struck Montmorency tart cherries last year. (Photo credit: George Sundin)

Michigan grows the largest percentage of tart cherries in the U.S, with approximately 95% of the crop planted in Montmorency variety, with the rest of the acreage in Balaton. In 2013, Michigan experienced a large outbreak of European brown rot (EBR) in Montmorency, a variety that is not typically prone to EBR outbreaks, while Balaton is very susceptible.

Research on EBR in tart cherries hasn’t been conducted since 1993, and researchers at Michigan State University (MSU) used that research to develop an EBR treatment strategy for Montmorency — based on what has been proven for Balaton — which includes Indar (Dow AgroSciences).

Advertisement

George Sundin, a professor in the Plant, Soil, and Microbial Sciences Department at Michigan State University, is quick to point out that research on EBR wasn’t a priority because the disease was confined to a minority of the tart cherry crop in the past, growers have been successful with their treatment plan so far, and funding for research is limited.

Sundin noted that EBR was likely more prominent because of the weather conditions in the spring last year.

Top Articles
Have a Plan For Climate Change? Why Fruit Growers Need To Act Now

“The European brown rot fungus (Monilinia laxa) prefers cool weather, even cold weather. Temperatures in the 30s and 40s are totally conducive to infection by that fungus. We see those conditions during bloom in Michigan every year,” says Sundin.

Why European Brow Rot Infected Montmorency
How EBR came to strike the Montmorency variety is a part of the research focus in 2014. Sundin sees unique weather events in 2013 as a direct link between the fungus and outbreak. EBR fungus attacks the flowers and then invades the shoot and kills both the flowers and the shoot.

“(Last year) during bloom we got rain, but we also had a very extended period of high humidity,” said Sundin. “That’s extremely conducive to infection. Then we had some orchards, especially on the old Mission peninsula, that were fogged-in in the mornings. That again leads to very high humidity and slow-drying conditions. This pathogen, we’re finding, is extremely sensitive to this kind of moisture requirement for disease.”

Sundin is also concerned because the American brown rot pathogen has started to show some resistance to Indar. American brown rot infects sweet and tart cherries, while EBR is confined to tart cherries.
“We’re worried that if we use more Indar at bloom to target European, even though we’re targeting European, there could be some selection on the American brown rot pathogen,” says Sundin. “We really don’t want to use Indar at bloom if we don’t have to, but we don’t have enough data yet on other fungicides, such that we can actually recommend using them yet.”

This year, Sundin and his research team will be recreating the weather events in order to pinpoint exactly what the requirements of infection are for European brown rot. The goal is to provide growers with the best timing for fungicide applications and a model to predict infection. Research will also be conducted on fungicides with different modes of action to provide more options for growers that experience an outbreak of EBR.

What To Do In 2014
As far as this year in terms of possible infection, Sundin says the advice to growers depends upon the amount of infection experienced on Montmorency.

“Where infection was not that high, we’re going to suggest pruning out those European strikes on the shoots first and removing that inoculum. Then we’re going to monitor the weather as we go into bloom,” says Sundin. “We need to have protection if the weather is telling us that we need to have protection on at that point, even before the flowers open. That protection will be needed through bloom.”

However, in blocks where there was an 80% rate of infection or higher, where pruning is not feasible, Sundin says a spray, regardless of weather conditions, will be necessary.

“Sometimes massive amounts of inoculum can cause disease even when the weather is telling us there shouldn’t be significant disease. In very heavily infected orchards, growers will have to put on a spray at the popcorn phase and one week later,” says Sundin. “In blocks of trees where there is poor air flow, that’s where the disease was heaviest. So those blocks are going to have to be especially cared for, and they’re going to have to be sprayed this coming year, just to try to clean this up.”

Sundin hopes that the European brown rot outbreak in Montmorency tart cherries was an exception for 2013 and that the rate of infection will be less in 2014.

“We’re hoping that this (was) a rare weather situation and that over the next couple of years, growers can clean up the disease,” says Sundin. “In the really heavily infected orchards, you have to spray this year and then if they don’t get a lot of infection, you can prune that out this year and hopefully be clean for 2015.”

0