New Light on the Fight Against Fire Blight
February 16, 2021 
When it comes to fighting fire blight, a combination Apogee and Actigard works well, according to Michigan State University Researcher George Sundin. Meanwhile, applications of Kasumin should be timed accordingly because of its light sensitivity.
Photo by George Sundin
Kasugamycin, the antibiotic used on pome fruit in the treatment of fire blight, is light-sensitive and should be applied in the evening to improve its control of the bacterial disease, according to Michigan State University (MSU) Researcher George Sundin.
That disease management recommendation was one of three that Sundin presented in December at the annual Great Lakes Expo, held online for the first time. Two of his three treatment updates involved Kasumin, the brand name for kasugamycin. The third reaffirmed the success of the combined use of Apogee and Actigard.
LIGHT SENSITIVITY
For several years Sundin and his team at MSU have tested the efficacy of various antibiotics against the fire blight pathogen, Erwinia amylovora. Meanwhile, pome growers in Michigan have come to rely on Kasumin to control blossom blight because their trees have turned resistant to another antibiotic, the otherwise successful streptomycin.
In 2017, which posed an exceptionally cloudy bloom period in Michigan, both oxytetracycline and Kasumin worked better than they had in previous years. Oxytetracycline was already known to be light sensitive to ultraviolet (UV) light.
“So that got us wondering: Is Kasumin light sensitive?” Sundin says. “We exposed Kasumin to UV, and we saw no effect to that. But we did a second round of tests (in 2020), where we exposed Kasumin to visible light.”
In vitro tests with visible light revolved around daily light integrals (DLIs) at rates depicting two different types of average 16-hour days in Michigan in May — 23 (cloudy) and 35 (sunny). A bioassay was then performed to determine the degradation of Kasumin at a field rate of 100 micrograms per million.
Sundin’s team found that if it exposed the solutions for two days at 23 or 35 DLI, the effective rate of Kasumin dropped from 100 micrograms/mil to 40 micrograms/mil. After three days of the same DLIs, the rate fell to 20 micrograms/mil. Of significance, 40 micrograms/mil represents the average sensitivity of the fire blight pathogen to Kasumin.
“So, within two days, we’re knocking the amount of usable Kasumin to about the limit to where we would expect to start losing control,” Sundin says. “At three days, we’re down to 20 micrograms per mil, so we would have lost control with that Kasumin.”
For growers, “this is another thing we’re going to have to start thinking about, especially if we’re very sunny during bloom,” Sundin says. “We need to keep track of daily light integrals and think about putting Kasumin on more frequently because it’s going to be broken down by visible light.”
MORNING OR EVENING?
A second 2020 study involving Kasumin targeted the most effective timing of its spray application. MSU experimented on ‘Fuji’ during a predicted rating of 145 on the epiphytic infection potential (EIP) risk index — “pretty high for May 24,” Sundin says.
“The fire blight pathogen predominantly grows at night on bloom. So, we’re thinking, ‘Should we be applying antibiotics in the evening to take advantage and attack the pathogen before overnight growth occurs?’” Sundin says. “Also, (there was) the thinking that we’d get the best activity from Kasumin with more time out of the sun if we’re putting it on the evening before a predicted 145 EIP.”
On the ‘Fuji’ trees sprayed with water, the result was 55% blossom blight — “a really high level,” Sundin says.
On the trees sprayed the evening before with Kasumin or streptomycin, the rates of blossom blight were 15% to 20%, respectively — “not significantly different” and “pretty good control,” Sundin says. Expecting diminished control after waiting until the next morning to spray, the results were nonetheless similar to the evening before.
Where the experiment turned was with sprays that occurred on the evening of the 145 EIP day. Streptomycin, because it is partially systemic, remained at around 15%. But with Kasumin, the rate of blossom blight rose to around 35%.
“Streptomycin will go into the flower even if the pathogen has already gone into the flower. It can go in there and still knock back disease,” Sundin says. “But Kasumin cannot. Kasumin does not have any post-infection capability.”
The results illustrate that, if Kasumin is applied too late on a high EIP day, then growers will lose control because the infection has already occurred.
“The best times of applying Kasumin are the evening before that day or morning of that day,” Sundin says. “If you wait too long, and you’re going on the afternoon or evening of that high EIP day, it’s going to be too late to get the excellent control that we normally get with Kasumin.”
APOGEE/ACTIGARD COMBINATION
In the process of “trying to reach the top wire relatively quickly,” young high-density trees are highly susceptible to fire blight, Sundin says. Apogee (prohexadione calcium, BASF) helps to control the disease, but it also does its job as a plant growth regulator (PGR).
On one hand, “nobody wants to use Apogee to shut down that growth for fire blight control,” Sundin says. On the other hand, no grower wants to gamble with such a potentially decimating disease. “Solutions are needed for shoot blight that do not put a huge constraint on growth,” he says.
Sundin and his team have been researching lower rates of Apogee in combination with Actigard, a plant stimulator (acibenzolar-S-methyl, Syngenta) that induces resistance in the host.
“We’re trying to see if we can either prevent shoot blight strikes from occurring or at least slow them down, such that we can catch them and prune them out before the bacterium reaches the central leader of the tree,” Sundin says. “What we’ve found over the last several years is that it is a combination of both of these materials that provides the control. If we use the same rates of either one alone, we don’t get the control that we get with the combination.”
Separate studies in 2018 and 2019 exemplified this. The latter trial was repeated in 2020, resulting in a similar conclusion (despite uneven infection rates last year): The best treatment is a per-acre mix of 2 ounces Apogee and 1 ounce Actigard, which, in 2019, resulted in a 4.5% shoot blight infection rate compared to the control of percentage of 18.8.
Sundin recommends starting at king bloom petal fall and applying weekly for three to four weeks depending on rainfall totals. “If it’s very dry, you probably don’t have to put on the last application or maybe the last two,” he says. “If we’re getting a lot of rain, though, I think you should increase the rate of Apogee an ounce or two because Apogee works a little bit less the more rainfall we’re getting.”
Until further notice, this combination of 2 oz Apogee/1 oz Actigard is the “best option for shoot blight control in high-density plantings,” Sundin says.
“I really hope we’re getting more adoption of this usage. It’s going to knock back shoot blight, and it won’t inhibit your growth to the extent that you’re worried about it. But the other thing is it will control fire blight, and that can prevent epidemics that could cause you to lose a lot of trees from these vigorously growing early-stage high-density plantings.”
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