Apple scab overwinters in infected leaves on the orchard floor. Spores from the dead leaves are produced in the spring and can cause primary infection of fruit.
Apple scab overwinters in infected leaves on the orchard floor. Spores from the dead leaves are produced in the spring and can cause primary infection of fruit.

The Michigan growing season was wet last year. As a result, apple growers had a tougher-than-usual time controlling scab. But the weather was the lesser of their problems. The type of fungicide they have been relying on for the last decade is losing its punch. Venturia inaequalis, the fungus that causes scab infections, is rapidly developing resistance to the strobilurin fungicides in Michigan.

Michigan State University plant pathologist Dr. George Sundin discussed scab resistance and control during a well-attended session at the 2009 Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, Michigan, in December. The message was that growers would have some adapting to do to be equal to the adaptability of V. inaequalis.

The strobilurin fungicides Sovran (kresoxim-methyl) and Flint (tri-floxystrobin) made their way into Michigan in 1999. Their early effectiveness was clouded by ominous reports that resistance was already beginning in Chile in 2003 and in France soon thereafter. Then, in 2008, growers reported control failures in the Peach Ridge area north of Grand Rapids, the state’s largest apple region. And 2008 was a relatively dry year. Sundin noted that the problem had been building for several years, then reached the tipping point where mass strobilurin ineffectiveness became obvious. Nature didn’t help by making 2009 wet.

Attack mode

The weakness of strobilurin, he explained, is that it has a single-site attack mode. If the fungus can mutate at that site in its genetic makeup, it neutralizes the fungicide’s effect. Applying the fungicide then becomes a waste of money.

The site has been identified, Sundin said, as a particular gene in which the mutation causes an amino-acid change “from glycine to alanine at position 143 in the protein.” For simplicity’s sake, it’s called the G143A mutation.

In 2008, Sundin and his laboratory colleagues screened for the G143A mutation in spores scraped from leaf lesions sampled from eight orchards in the Peach Ridge area and eight orchards in eastern Michigan.

The results were disquieting. One hundred percent of the isolates analyzed from five of the Peach Ridge orchards were already resistant to strobilurin, and the records at the other three orchards were little better. In eastern Michigan, where apple production is more dispersed, four of the orchards were still resistance-free, but the other four had significant levels of resistance, and in one case, it was nearly total.

The follow-up testing last year, expanded to other concentrated apple regions along Lake Michigan’s eastern shore from south to central to north, delivered negative news. “It’s a big problem,” Sundin said. “This fungicide class is basically done for us in terms of scab control.”

He advises growers to abandon strobilurin fungicides if any degree of resistance is evident in an orchard. Growers may believe that if control is weakening, a stronger dose will solve the problem. It won’t because the G143A mutation has already occurred at the critical genetic site in the resistant fungi: “This is an all-or-nothing type of resistance.”

Furthermore, continued strobilurin applications will merely accelerate the rate to total resistance in an orchard. Once the halfway resistance point is reached, he said, the fungi isolates that are still sensitive to the fungicide will be killed off, and the mutated population will take full command.

Since the strobilurins are fading from the Michigan apple-scab arsenal, Sundin presented alternative control options. Some old standbys are still available, facing a resurgence after being eclipsed for a decade by the excellent scab control of the strobilurins.

Sundin recommends an aggressive approach based on a protectant strategy, hitting the fungus before it has a chance to proliferate and tightening spray intervals to enhance suppression. It’s important to spray before the first spring infection, which develops from infected leaves from the previous season.

Copper can play an early role, but he cautions against copper applications after about half an inch of green tip to avoid fruit russeting later. Captan and the EBDCs are excellent protectants for five or six days at full rates, whereas sulfur and ziram have somewhat weaker action. In all cases, Sundin stressed, spray intervals must be shortened accordingly.

All those compounds have the advantage of attacking the scab fungus at more than one site and thereby decreasing its ability to become resistant. By contrast, the anilinopyrimidines Scala (pyrimethanil) and Vangard (cyprodinil) like the strobilurins have a single-site mode of action so must be used prudently to reduce resistance risk. Sundin suggested tank-mixing with a three-pounds-per-acre rate of EBDC. He said the anilinopyrimidines are effective early in the season but less satisfactory in controlling scab on fruit.

The sterol inhibitors are mostly in the category of the strobilurins V. inaequalis has the upper hand. A range of sensitivities of fungal isolates exists in orchards, so some degree of control may yet be possible, particularly with special formulations of sterol inhibitors. Again, Sundin emphasized, overuse is the enemy of long-term utilization.

“We’re losing our modes of action,” he said. “Infection periods for scab occur every year in Michigan orchards. And 100-percent control is necessary.” The conclusion: Growers must be flexible and adaptive to stay ahead of the fungus.

Reducing the amount of inoculum that is available to proliferate in the spring can help. And get that protective barrier on vulnerable tissues throughout the orchard before the start of the primary infection. Keep spray intervals tight throughout the season. He said several of the fungicides may be applied in light rains in case of special urgency. He also reminded growers to spray all middles to get full coverage.

The effectiveness of the strobilurins for a decade made life easier on the scab-control front in Michigan’s moist-climate apple production. Now, growers have to be resilient to meet the new challenges.

—by Dan Hager

Dan Hager is a writer and consultant in Lansing, Michigan, with extensive experience in agricultural issues.